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NCAR/TN-301+STRNCAR TECHNICAL NOTE
March 1988
INTERCOMPARISON OF
NMC AND ECMWF GLOBAL ANALYSES:1980-1986
KEVIN E. TRENBERTH
JERRY G. OLSON
.D
E
L,
an
LatitudeJUL 86
CLIMATE AND GLOBAL DYNAMICS DIVISION
NATIONAL CENTER FOR ATMOSPHERIC RESEARCHBOULDER, COLORADO
-
II-
Table of ContentsPreface . . . . . . . . . . . . . . . . . . .
Acknowledgments ...............
1. Introduction . . .. . . . .. . . . . .. .
2. The data sets ...........
3. The intercomparison of data sets .......
3.1 Time series of zonal mean differences . . .
3.2 Meridional cross sections ... ...
3.3 Geographical distributions .......
3.4 Poleward eddy heat and momentum fluxes
3.5 Surface differences over the oceans . ...
4. A case study in June 1985 .........
4.1 The South Pacific low .........
4.2 The South Atlantic low .........
4.3 The Antarctic Peninsula ........
5. Discussion and conclusions ..........
5.1 Divergent wind ............
5.2 Relative humidity ..........
5.3 Surface fields . ...........
5.4 The Southern Hemisphere .....
5.5 Heat and momentum fluxes ......
5.6 Concluding remarks . ........
References . . . . . . . . . .. . . . . . . .
Appendix I Acronyms .............
v
. .. . . . . V1·. .. . . . 1·
... . . . . 4
. 5
.· · · · · · 6
... . . . .20
...... .32
.... . . . . 48
....... 58
....... 68
....... 68
....... 72
....... 72
....... 73
....... 73
....... 74
....... 75
....... 76
....... 77
....... 77
... . . . . 79
. . . . . . . 81
111
Preface
This report is the third in a series of three as part of a project to comprehensively
evaluate data sets of global analyses from the U.S. National Meteorological Center (NMC)
and the European Centre for Medium Range Weather Forecasts (ECMWF). Our partic-
ular purpose is to establish the climate record from global analyses for use in compiling
climatologies which may then be used to validate the National Center for Atmospheric
Research (NCAR) Community Climate Model or as a base for examining anomalies such
as occur in association with El Ninio-Southern Oscillation events.
In order to make optimal use of these data sets, it is necessary to know of any
problems that might exist and the effects of any changes in the operational system that
produced the analyses. These issues were extensively documented in the first two Technical
Notes which separately evaluated the NMC and ECMWF data sets of global analyses. This
third Note describes results from a detailed intercomparison of the two data sets and a
case study which compares the analyses with each other and with observations. In this
way we provide a measure of the accuracy of individual and mean analyses and the relative
biases in the two data sets, and a documentation of how these have changed with time. It
therefore provides a measure of how much confidence should be placed in the fields and,
in particular, in resulting general circulation statistics.
The main results shown are monthly mean and root mean square (RMS) differences
between the analyses. Geopotential heights or sea level pressures, wind components, and
relative humidity are the main state variables examined, but we also consider the eddy
poleward heat and momentum fluxes. Both the mean and RMS differences have been
zonally averaged as a function of latitude and plotted as time series from 1980 to 1986 at
single levels and as cross sections. The differences have also been mapped geographically
for each month. At the surface, separate fields are presented for the oceans as a whole,
and for each ocean separately, to show how well the fields important for programs such
as the World Ocean Circulation Experiment (WOCE) and the Tropical Oceans Global
Atmosphere (TOGA) Program are known.
v
Results show fairly widespread agreement between the analyses from the two centers
over the NH extratropics. However, in general, the quality of the analyses, as revealed by
their differences, is much less in the tropics and SH. This is reflected in much greater
differences in wind fields south of 20 0 N, with RMS differences in both components often
exceeding 5 m s - ' above ^500 mb throughout most of this region. It is further revealed
in the much greater differences in geopotential height south of .30°S. In part, this is due
to the poorer traditional observational synoptic network in the SH. But it also appears, at
least in part, to be due to problems in assimilating observations that are available. Both the
mean and RMS differences over and around Antarctica reveal major problems in that area
at both centers, and especially at NMC prior to May 1986. In the tropics, there are major
disagreements in the divergent wind field and associated vertical motions. These fields
have become more intense and realistic with time but still appear to be poorly known. The
relative humidity field, however, is the poorest known and has undergone major changes
with time at both centers. The possible reasons for these results are discussed and some
implications and recommendations are given in the conclusions section.
Acknowledgments
This research was partially supported by the Tropical Oceans Global Atmosphere
Project Office under grant NA86AANRG0100. We thank Grant Branstator for helpful
comments.
vi
1. Introduction
There is a compelling need for increasingly accurate knowledge about the state of
the atmosphere, both at any instant, for weather forecast-type studies, and for longer-term
means. A reliable climatology of the general circulation of the atmosphere is required for
many purposes, ranging from everyday use of mean conditions for planning air transporta-
tion routes to the validation of climate models. However, accurate means on shorter,
say monthly, time scales are also essential along with the basic climatology in order to
monitor ubiquitous climate variations and determine anomalies in the circulation, such as
those associated with the El Niniio-Southern Oscillation phenomenon. The latter can cause
widespread devastation through excessive rains in some parts of the world while droughts
pervade other parts with pronounced economic and political consequences.
One goal of our current research is to determine climatologies of the global atmo-
spheric circulation and establish how' reliable they are and their suitability for various
uses. A second goal is to examine the extent to which the monthly anomalies, defined
as departures from the mean climatology, can be reliably determined. We are interested
in not only the mean state variables of the atmosphere, but also the second moments,
such as variances, and covariances that determine the fluxes of quantities of interest. In
order to address these goals we have comprehensively evaluated two sets of global daily
analyses from the U.S. National Meteorological Center (NMC) and the European Centre
for Medium Range Weather Forecasts (ECMWF) for the period 1979-1986. This is the
third of a three-part project. In Trenberth and Olson (1988a) we reported on the NMC
data set and in Trenberth and Olson (1988b) the ECMWF analyses were examined.
At both centers, the analyses are produced using a four-dimensional data assimila-
tion system that uses a set of first guess fields as the base for integrating the observations
into an analysis. At NMC the analyses are produced under what is referred to as the
Global Data Assimilation System (GDAS). The first guess at both centers is either a 6
or 12 hour forecast using a numerical weather prediction (NWP) model from a previous
analysis and it therefore carries the information from all the previous analyses foreward
1
in time. However, it also means that the first guess is dependent upon the veracity of
the NWP model and it can be biased as a consequence. The subsequent analysis proce-
dure itself is based upon some sort of statistically optimum interpolation scheme which
makes use of the statistical mean errors expected in both the first guess and in the data.
Then the results are initialized using nonlinear normal mode initialization (NNMI) which
is a procedure designed, using the normal modes of the model equations, to ensure that
the fields are dynamically consistent with each other while appropriately emphasizing the
relatively slow meteorologically significant components and damping the spurious gravity
waves. Such a procedure greatly influences the vertical motion field and associated diver-
gent wind components, with the result that the latter have undergone spurious changes
with time as the initialization procedures have been improved.
In Trenberth and Olson (1988a), which dealt with the NMC data set, and in Tren-
berth and Olson (1988b), which considered the ECMWF analyses, we primarily focused
on evaluating each data set alone. We compiled a detailed chronology of the changes in
the operational systems at each center that produced the analyses and assessed the impact
of each change on the analyses. We documented any missing data and, through a series
of tests for the internal consistency of the data set, mainly through examination of daily
time series of various statistics, we compiled lists of "bad" analyses. These are days when
something clearly went wrong in the operational analysis/forecast cycle and/or erroneous
observations greatly impacted the analyses. Such analyses generally should not be used, in
compilations of climatological statistics, in forecast experiments, or for any other purpose.
Results of these studies showed that missing data was a problem in the NMC set
with more than 13% of the analyses missing in 1982 and 1983. Relatively few (only four
from 1980-86) analyses are missing in the ECMWF set. In addition, bad analyses are more
common in the NMC set and we classified an additional 6.6% of the analyses in 1984, for
instance, as bad. At ECMWF bad analyses occur only in the earlier years.
We have also shown that many of the changes in the operational systems used
to produce the analyses at both centers had a substantial impact on the analyses and
resulted in either spurious trends or discontinuities in several quantities. The most sensitive
2
quantities are the divergent wind component and associated vertical motion fields, and the
moisture fields.
In this third part of the project, we attempt to obtain a measure of the accuracy of
the analyses and the relative biases in the two data sets by systematically comparing the
NMC and ECMWF global fields. We will focus on the differences between the analyses and,
accordingly, this does not provide an absolute measure of accuracy. It is also important to
recognize that a decrease in the differences between the analyses does not necessarily mean
improved accuracy. This is because changes that improve NWP model forecast performance
at one center are likely to be adopted at the other center with resulting parallel changes
in the analyses. In particular, the relative humidity (RH) fields are evidently dominated
by input from the NWP model at the expense of observations, so that values are apt to
change whenever the model is altered.
The interpretation of the differences is complicated by the spurious discontinuities
due to changes in the two analysis systems, but even such a relative measure of how well
we really know the state of the atmosphere at any time is a useful guide on how much
faith to put in the resulting climatologies. Further, it shows where more effort is needed
in enhancing the observations and in improving data analysis-assimilation methods.
It is possible to attempt to define a more fundamental measure of accuracy by
comparing the analyses with observations. This has also been done to a limited extent but
it is necessary to recognize that station observations are also flawed. In particular, many
station records have data missing which can bias mean statistics (Trenberth, 1987a). A
not uncommon cutoff criterion for computing statistics has been to require a minimum of
10 observations per month. Kidson and Trenberth (1988) examined the effects of missing
data on estimates of monthly mean general circulation statistics and found, for the zonal
wind at 300 mb for example, that 11 randomly distributed observations in a month would
result in standard errors in the monthly mean wind of up to 4.1 m s-.
In this report we intercompare the NMC and ECMWF analyses at selected levels.
A previous comparison of the ECMWF analyses with those from the Geophysical Fluid
3
Dynamics Laboratory (GFDL) for parts of the year of the First GARP Global Experiment
(FGGE) has been reported by Lau (1984, 1985). We have therefore not included the FGGE
year (December 1978-November 1979) in this comparison, in part also because for that
year the two analyses are on different grids. The comparison focuses on the 1980-86 period
when both sets of analyses are available on a 2 ° grid.
The comparison was first done on a daily basis by differencing individual analyses
and producing time series of various summary statistics for each month. Such series were
helpful in identifying the bad days in the two previous studies. For this note, we have
summarized all the statistics into either monthly means or time series of 15-day means.
Horizontal maps and cross sections will be shown for individual months to illustrate the
complete fields associated with the time series difference fields. In addition we report on a
case study of one major difference in June 1985 in which both analyses are compared with
the original available data.
A list of acronyms used in this report is given in Appendix I.
2. The data sets
The data sets consist of twice daily, at 0000 and 1200 GMT, global analyses at
multiple levels of the fields of geopotential height z, temperature T, zonal and meridional
components of wind u and v, and relative humidity RH. Other parameters were not
considered. For the ECMWF data set there are only seven levels in the vertical at 1000,
850, 700, 500, 300, 200 and 100 mb, so that the intercomparison was limited to these levels.
For 1980-86 both data sets are available on the same 2 ° latitude-longitude grid and it
was on this grid that the fields were differenced.
Trenberth and Olson (1988a and b) describe many more details about each data
set including lists of the missing days, days determined to be bad, documentation of all
the changes in the analysis systems and the impacts of these changes on the analyses. We
have excluded the bad days from the intercomparison, except where noted in Section 3.5.
4
3. The intercomparison of data sets
Fields have been compared on a daily basis and results summarized in several ways.
For each month from 1980 to 1986 the mean and root mean square (RMS) temporal differ-
ences have been computed at each grid point. The mean difference shows the systematic
component which may arise through a bias in one or other analysis system. The RMS
differences include the mean differences but also include the time dependent uncertainties
in the individual analyses. Generally, we have produced horizontal maps at selected levels
in the vertical to reveal the geographic distribution of the differences and zonal mean cross
sections of the two differences as a means of concisely summarizing the results.
In addition to the monthly means, for each latitude we have computed zonal mean
RMS and mean differences averaged over non-overlapping 15-day periods. These have been
plotted as time series as a function of latitude, but in order to cut down on clutter, it was
found desirable to further smooth the results using a (1 - 2 - 1) binomial smoother, so
that the time series are more representative of running monthly means.
The main detailed comparison was between the following fields: z, u and v at 1000,
500 and 200 mb, and RH at 850, 700 and 300 mb. T is related to z through the hydrostatic
equation and was not compared directly. However, in order to gain an appreciation of the
impacts of differences in the primary state variables, time series were constructed of daily
values of [v*T*] and [u'v*], where the bracket refers to the zonal mean and the ( )*
the departure from the zonal mean. These quantities represent the poleward heat and
momentum fluxes by the eddies and are probably the most important second moment
general circulation statistics. Comparisons will be shown of [v*T*] at 700 mb, and [u*v*]
at 200 mb, both near the respective level at which the poleward flux of each quantity
reaches its maximum.
For some purposes, the more familiar sea level pressure field is preferred over the
1000 mb height. In particular, in order to examine how well the surface fields agree over
the oceans, we have converted the 1000 mb heights to sea level pressures making use of a
fairly accurate conversion factor that depends latitudinally on temperature. The oceanic
5
surface fields are of particular interest to scientists involved in the Tropical Oceans Global
Atmosphere (TOGA) program for assessing how well surface fluxes of sensible and latent
heat and momentum (wind stress) are determined. We have therefore compared the surface
pressure and wind fields over the oceans, both as a whole and individually.
For the most part, we do not present fields of the state variables themselves and
only show the differences. The mean statistics for each month of the year are given for the
ECMWF data in Trenberth and Olson (1988b).
3.1 Time series of zonal mean differences
In order to provide an overall measure of the differences between the analyses and
how they have changed with time, we first present a series of latitude-time sections. These
are 1(1 - 2 - 1) smoothed 15-day averages in time (roughly monthly means) and zonal
means of the differences as a function of latitude. For each 15-day period, values were set
to missing only if there were less than 7 out of 30 good analyses available and all missing
cases are due to lost NMC data. All differences are computed as NMC-ECMWF and
both the mean and RMS differences are presented for selected levels. On all plots negative
values are dashed and stippling has been used to highlight the larger differences. Periods
with bad or missing data have been left blank. Note that in most cases, the contour levels
are uneven and have been chosen to specifically show different thresholds of the differences.
When uneven contours are used, the complete list of all contours plotted is shown on the
side of each plot.
Several of the fields are related, most notably the geopotential heights and winds
through the geostrophic relation. Consequently, we will defer most discussion until after
the related fields have been presented. We will comment on the differences and, wherever
possible, suggest causes for the differences and attribute them to one or other center. The
background for this is in the extensive documentation of the changes made at each center
and the impacts these had on the analyses, as described by Trenberth and Olson (1988a,
1988b). Reference to both of these Technical Notes is implied in much of the following
discussion.
6
Figure 1 shows the 1000 mb z differences, the 200 mb z differences are given in Fig. 2
and the corresponding u differences in Figs. 3 and 4. The top panel in each figure shows
the zonal mean differences and the lower panel shows the zonal mean RMS differences.
At 1000 mb the mean z differences are mostly less than 5 m after 1980 north of 35°S
and these correspond to RMS z differences of 10-20 m and for u of less than 4 m s- 1. The
exceptions, as will be seen from the geographic distribution shown later in Figs. 19-22, are
mainly associated with orography (e.g., near 350 N in Fig. 1) and result from differences in
the way the two centers extrapolate below the ground. However, this does not account for
the large differences from 30-65°S although it is a factor in the enormous differences over
Antarctica in Figs. 1 and 3. There is some reduction in mean differences in Fig. 1 over
Antarctica after changes were made to the NMC GDAS system on 28 May 1986, but the
reduction in RMS differences elsewhere was small.
A major part of the huge differences over the Southern Hemisphere (SH) can be
traced to problems at NMC. Trenberth and Christy (1985) compared sea level pressures
from NMC with those from Australian analyses. NMC pressures were too high until late
1981 from 35-65°S, both relative to the Australian and ECMWF analyses (see Fig. 1). They
were corrected by introducing the Australian PAOBs (Pseudo Australian Observations)
and some other changes. More insight on the nature of the problems at both centers in
the SH is shown by the case study reported in Section 4.
At 200 mb (Figs. 2 and 4) problems in handling orography cannot be blamed for
the differences which remain huge over Antarctica and which are generally higher south
of 20°S than over the Northern Hemisphere (NH). There is an annual cycle to the RMS u
differences from 20-40°N in association with the wintertime development of the strong NH
subtropical jet. But in the tropics and SH RMS u differences exceed 5 m s - 1 year round
although with notable reduction in late 1985 and again in late 1986 (Fig. 4).
At NMC there were chronic problems with analyses over Antarctica until changes
were made in May 1986. The impact of that change is documented by Bonner et al. (1986)
who confirmed that the analyzed heights at NMC had been much too high over Antarctica.
7
[ZINM - [Z]EcMWF
SMOOTHEb
(M)
1000 MB
1979 1980 1981 1982 1983 1984 1985 1 o5
15-DAY AVERAGES
RMS DIFF. (ZNMC/ECMWF
SMOOTHED
M)
1000 MB
12.1 11.E
L
0
11.0
· . ~ O.%~ ';:.: : - - ...
O L10.3
. . . . . . .
...... . .......... i...
(
1979 1980 1981 1982 1983 1984 1985 1986
15-DAY AVERAGES
Fig. 1. Differences in z at 1000 mb as a function of latitude and smoothed 15-day
averages in time. The contour interval is uneven as given at left. Negative
contours are dashed. Top: [Z]NMC - IZ]ECMWF- Values greater than ±20
m are stippled. Bottom: RMS z differences. Values greater than 20 and 50
m are stippled in different densities.
8
90N
60N
30N
305
605
905
CONTOURS:102050
100150200
60N -
30N -
3ON -
)
j 0-
305 -
605
905
I I I I - I r . L , t I - I 0 .x II 1 .. IL '!
I . . I I I I l y f - "'- - I-I· -· 1- I -Il-
C ... ...yt~.
[Z]NMC - [Z]ECMWF
SMOOTHED
90N
60N
30N
' o
305
605
90S
(M)
200 MB
15-DAY AVERAGES
RMS DIFF. (ZNMC/ECMWF
SMOOTHED
90N
60N
30N
Q)
.I-b-3
(t
0
305
605
905
M)
200 MB
15-DAY AVERAGES
Fig. 2. Differences in z at 200 mb as a function of latitude and time.Top: [Z]NMC - [Z]ECMWF. Values greater than ±50 m are stippled.Bottom: RMS z differences. Values greater than 20 and 50 m are stippledin different densities.
9
[U]NMC - [U]ECMWF
SMOOTHED
90N
60N
30N
1
305
605
905
(M/S)1000 MB
15-DAY AVERAGES
RMS DIFF. (UNMC/ECMWF
SMOOTHED
M/S)
1000 MB
15-DAY AVERAGES
Fig. 3. Differences in u at 1000 mb as a function of latitude and time.
Top: [u]NMC - [U]ECMWF. Values greater than ±2 m s- 1 are stippled.
Bottom: RMS u differences. Values greater than 4 m s-1 are stippled.
10
90N
60N
30N
O.->
305
605
905
cIC
..m-
,aT:C*
[U]NMC - [U]ECMWF
SMOOTHED
(M/S)
200 MB
30 L. I 1LZ -3.09-3; 06 - -3. 09
CONTOURS:-15-10-5-225 H
1.55
C 5 ' °o 0,0'0
..-, .
" ^..~ ·,,,
.' ); , , ,, ;, ,' ,5- '
'JR L,^ o _, i , ,
1! I,, i :" , ';.:",l ,":';I:-,'~"' i;,J ·"~ ' ':i:;.':;i:.,: ,v, ::,:.~-:::::,:,
H1.59
H2.29
,. : ,--'2 \'
I I . I . - I " .. .. .. ' ....'" ,'1979 1980' 1981 1982 1983 1984
15-DAY AVERAGES
1985 1986
H.784
RMS DIFF. (UNMC/ECMWF
SMOOTHED
1979 1980 198T'" 1 98
15-DAY19A 3 1'V
AVERAGES
_ . . . .I . .
Fig. 4. Differences in u at 200 mb as a function of latitude and time.
Top: [u]NMC - [U]]ECMWF Values greater than ±5 m s- 1 are stippled.
Bottom: RMS u differences. Values greater than 5 and 10 m s- 1 are stippled
in different densities.
11
90N
60N
30N
>i0
305
60S
905
M/S)
200 MB90N
60N
30N
0._
305
605
905
I) L2.01
. ..·..'''.. . . . . . . . . .'·'. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .:: : : i
.. .. . . . . . .
...... ...... 4. . . . . . .. . . . . . . . . . . . . . . . . .~:·:·:. . . . . . . . . .. . . .. . . . . ............ ........... ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ .i;·iY' · ·'.1111 ~~~~~~~ ;J.............. .... .. .. .... ~~~~~~~~~~~~~::~ii it~ii
- I ..... .I.- I . Ii -,.- ', .....- .. . . .. . .- - is ......... lpII- I
II T. . . .. . . . .
Q.
IC
.j
D04 1 to AI o
A comparison of both NMC (not shown) and ECMWF (see Trenberth and Olson, 1988b)
analyzed heights with station data at the South Pole (Amundsen-Scott) and McMurdo
Sound has been carried out for days when station data existed. For NMC at 200 mb,
monthly mean RMS differences between station and analyzed values averaged s50 m at
McMurdo and ~80 m at the South Pole during 1979. But from 1980 to 1982 NMC values
were usually biased high with RMS differences typically >100 m and at times >200 m.
From 1983 to May 1986 at the South Pole both mean and RMS differences increased to
>300 m in the winter months and resulted in a small polar high rather than the polar
vortex shown by the observations. As a result there was a spurious easterly component
to the zonal winds of >10 m s -1 near 80°S in the winter half year. After May 1986, the
NMC RMS differences at 200 mb from NMC diminished to -50 m at both stations.
Trenberth and Olson (1988b) present results of the comparison and discuss the
difficulties in Antarctic analyses at ECMWF. They note huge negative biases in z (<-200 m
at 200 mb) compared with station values over Antarctica in spring months of 1980 and
1981. There is a marked improvement in 1982 and a further gradual improvement with
time, so that by 1986 the monthly mean RMS differences are 40 to 50 m. The main
difficulty at both NMC and ECMWF is suspected to be communication problems with the
result that data are not received for input to the analyses, although this is compounded
by a total absence of data at times.
Figures 5 and 6 show the differences in the v field at 1000 and 200 mb. The RMS
values for v are dominated by the rotational component and consequently tend to be quite
similar to those for u (Figs. 3 and 4) with biggest uncertainties revealed over the SH.
However the zonal mean v, given by [v], is zero for the rotational component and thus the
top panels of Figs. 5 and 6 show the differences in the lower and upper branches of the
mean meridional circulation corresponding entirely to the divergent wind component. In
the tropics these reveal the Hadley circulation differences as seen in the initialized fields
from the two centers. Trenberth and Olson (1988a and 1988b) noted that this quantity
was extremely sensitive to changes in the model and data assimilation and there were
major discontinuities in the records at both centers. Fig. 7, adapted from Trenberth and
12
[V]NMC - [V]ECMWF
SMOOTHED
0CONTOURS:-2.0-1.0-0.5
0.51.02.0
(M/S)
1000 MB
.�3'14 O
,· o0( ooO,,,.I 3 .1 ·!·:·:� *�
" ;
r:·:·:·:·:·�
r·� I.·.'.·.(C;.t, r.i·t,I
.Ic��� �------���- Ir, C t :·�4:,, �J3i5'107
7�t,:::::It
�· * ,r.· ·�·',, r\'
,-·
;rr �,'·· g,, b I-, ,, ,� ,,, 4-- I:--·- --- c,-r .O =;,�,,··-· .· .··� i� T ·;� :I·'r ...................'..'.`h'......)
.·�L.··'·�·�)·' r� T·:·:·* s''·V·' ·i�·�:·:·:-:::·l·:· :·;·:·:·:·:·�ii: � r:·-:·:·;� ·r3 h I·fC=�"� LL......''.: .....�;.......�..' )'��;�.i;:CL� .,�����;;,,· iIf , rEio',=, 1...,,,��iqjt��'� C��O �I o f o oh h:,IrClli.; tit; -, Ly _UjC cl��i��i·ul, '. .i L �·-� -�---r=·���·.=������=- 'r"
n �-· .· ···�.il;·-·li .Ci�J,·� L�rA , I , L. -* m -7. A *- - .. . I
1979 1980 1981 1982 1983 1984 1985 1986
15-DAY AVERAGES
RMS DIFF. (VNMC/ECMWF M/S)
SMOOTHED 1000 MB
CONTOURS:2
812
YH H3.76 L 3.60
2.34
H_ .4 '1 * H 3.73
3.67L
2.57
II ,. - ' .. .1..... '; j tX : W *, cfi: :U.... ...
.... ............. ......~iiiii~~i : ~ · 81ijii:i~;1~::1::1
1979 1980 1981 1982 1983
5-DAY AVERAGI1984 1985 1986
Fig. 5. Differences in v at 1000 mb as a function of latitude and time.Top: [V]NMC - [V]ECMWF. Values greater than ±1 m s - 1 are stippled.Bottom: RMS v differences. Values greater than 4 m s - 1 are stippled.
13
90N
60N
30N
0
305
605
905
90N
60N
30N
O0
305
605
905
1 - T V-~~~~~~~~~~~ ~~~L~~~ r ~ ~ I
-^ k
1
[V]NMC - [V]ECMWF
SMOOTHED
90N -
60N -
30N -
30
305 -
605 -
905
90N
60N
30N
0O
305
605
905
I . . .. . . . . .. I I, I......
1979 1980 1981 1982 1983 1984
15-DAY AVERAGES
RMS DIFF. (VNMC/ECMWF
SMOOTHED
(M/S)
200 MB
1985T - -I I 198
M/S)
200 MB
1979 1980 1981 I l8z 1o3 i 9o4 1io I ou
15-DAY AVERAGES
Fig. 6. Differences in v at 200 mb as a function of latitude and time.
Top: [V]NMC - [V]ECMWF. Values greater than ±1 m s- 1 are stippled.
Bottom: RMS v differences. Values greater than 5 and 10 m s-1 are stippled
in different densities.
14
CONTOURS:-5.0-2.0
-0.5 HL t
0 -. 429 .0'701.02.0
::::~:.::' ,,: . ::::: :::::.::::~:;~:.:,'' 1 C·'''',::::..:', .,
· : ... ,-*. ,,~:, 'i'I (ft1d';
'^'ry ^'..(M I\^\ ^*>\u\~
oi··Iiiii . .S-i r : ~ o `
6- "~~~~~~~~~~~~-l
I I I~~~~~~~Jl'rL( ,X ,4Wb51 ,+ j * & %4 -
VW ·-I , to - :61 Y 9 · CC4 to .no r - I WC , I 6&oLjkg--
QIaT"s&6;
.I,,..0-
(t:
1986
3
z
0tC
_
2
0
-1
-2
-3
REGIONAL MEAN OF v
15-DAY PERIODS1 JRN 79 OZ - 19 DEC 86 12Z
Fig. 7. Time series of 15-day averages of regional means of [v] averaged over 20°Nto 20°S at 200 mb. The NMC values are in solid and the ECMWF valuesare dashed. Values are in m s- 1.
15
(M/S) 200 MB
Olson (1988a and 1988b), shows the temporal changes for the 200 mb [v] averaged from
20°N to 20°S and thus depicts the upper branch of the main winter hemisphere Hadley
cell. Changes in [v] were especially pronounced at ECMWF, and this is also reflected in
Figs. 15 and 16, presented later.
Differences in the upper branch of the Hadley cell are best illustrated by Fig. 6 and
become more extensive with time as the meridional circulations were increased at both
centers with the introduction of diabatic effects into the NNMI. They became largest after
1 May 1985 when ECMWF invoked other improvements associated with the introduction
of shallow convection into their model. The differences lessened again after 28 May 1986
when shallow convection and related changes were implemented at NMC.
It is noteworthy that there is a strong annual cycle to the differences in the top
panels of Figs. 5 and 6 and this is directly linked to the pronounced annual cycle in the
Hadley cells themselves. Also of note is that the differences commonly exceed 1 m s - 1
which is comparable to the strength of the Hadley circulation itself. Consequently, it
should be recognized that there will be major uncertainties in any diagnostic studies,
such as analyses of atmospheric energetics, which depend upon the verisimilitude of the
divergent wind and associated vertical motion fields.
Figures 8, 9 and 10 present differences in RH at 850, 700 and 300 mb. RH is another
quantity that is extremely sensitive to changes in the NWP model and data assimilation
systems at both centers and, although the changes typically have an impact at all three
levels at the same time, the direction and magnitude of the impact at each level is often
quite different. Trenberth et al. (1987) and Trenberth and Olson (1988b) discuss the
changes in RH at ECMWF in detail. A number of changes occurred in 1980 and 1981
that are reflected in Figs. 8-10. However the most notable change occurred on 1 May
1985 at ECMWF with the introduction of shallow convection in the model that resulted
in a 22% drop in the analyzed tropical RH at 700 mb. This is reflected in Fig. 9. A
similar change occurred at 300 mb (Fig. 10), and an opposite change occurred at 850 mb
(Fig. 8). Then the introduction of shallow convection in the NWP model at NMC in May
1986 brought the tropical differences back to more modest levels. At 300 mb (Fig. 10)
16
[RH]NMC - [RH]ECMWF
SMOOTHED. . . I I . . . . .
(7o)
850 MB
CONTOURS:-30-20
'1020304050
,L'-14 '1
- l r. 8 ;. .. C'? 0
. A.-'~'/ "' :::::.'':: _ '," ,
Y IL ) )�) Ir ( I I r Ir · r'
I -(r-- (II ((( U· '
r
(rI I , \ r �.rr r I
Ccr rI (,r
IrrI
Ir ·�·r
,r ·�, , I
I 'I (II ,I I II r ( Ir( .I r It ·'·'·�·�--I I .·.·..-..-.-. ;;-rI )· ··.. ·.......'('�� :·:·:·:·):·:·:·:·:·:·:��I �·.·;·.·�i ;·.·:·;·:·;(�
1.·I:�;Lai:E·�Sj......;':r
�·;�·�·�� IrrI
'.rrI
I ,
I r.·.\ tI r I ('·· '·� ') ·- · i"rI ( rr,L;a 1 r?·j .·: C
' ' I i ·, .. I · I ·, - . q-
1979 1980 1981 1982 1983 1984
15-DAY AVERAGES
RMS DIFF. (RHNMC/ECMWF %)
SMOOTHED
1985 1986
850 MB
1979 1980 1981 1982 1983 1984 1985 1986
15-DAY AVERAGES
Fig. 8. Differences in RH at 850 mb as a function of latitude and time.Top: [RH]NMC - [RH]ECMWF. Values greater than ±20% m are stippled.Bottom: RMS RH differences. Values greater than 30 and 40% are stippledin different densities.
17
90N
60N
30N -
,)
-DO-
305 -
605 -
90S i
90N
60N
30N
,)
3 O
30S
605
905
I . I- '. I 1 . - I . . . L... . .1I-I I " , , , . I I . , I ,
........._ _ ' _%.t r ' ' .. r. I
0A
I.,:
I
-.-. :-
. I
[RH]NMC - [RH]ECMWF
SMOOTHED 700 MB
CONTOURS:-30-20-10
1020304050
- 0 , ; 'H "
-21*. ,6 -" . ,,
·. 0^U^ .
-214.76,
U
1979 1980I , .. I .- . .
1981 1982 1983 1984 1985 1986
15-DAY AVERAGES
RMS DIFF. (RHNMC/ECMWF %)
SMOOTHED 700 MB
1979 1980 1981 1982 1983 1984 1985 1986
15-DAY AVERAGES
Fig. 9. Differences in RH at 700 mb as a function of latitude and time.Top: [RHNMC - [RH]ECMWF. Values greater than ±20% m are stippled.Bottom: RMS RH differences. Values greater than 30 and 40% are stippledin different densities.
18
^; .
.o
, ,. : ',
.- *,o
.... ."
' * '
,. * ' ,;
' .', *%', I
* vv
. , , ° '
..':, .'::]:-.
*, ., , . .·
; ., , 5 .
90N
60N -
30N -
0 -
305 -
605 -
905
90N
60N
30N
0O
305
605
90S
. I I - I -- .. . i . I I I 1 2 , , I . . I .
I1
Or�r·1Lr, r)I r r r rr r
r"· r c��:_ � .ciir ·
I I I I . I . . .. I ,, " Q 4..
I
,:·r'·'·'·'·'·'·�·� �:::::::::·i4 �·;·:·:·:·:r
;·'·'·'·'/
r Ir (
I·
·r
Co
SON
60N
30N
305
60S
90S
[RH]NMC - [RH]ECMWF
SMOOTHED
(%)
300 MB
15-DAY AVERAGES
RMS DIFF. (RHNMC/ECMwF
SMOOTHED
90N
60N
30N
)
O
3
305
605
905
300 MB
15-DAY AVERAGES
Fig. 10. Differences in RH at 300 mb as a function of latitude and time.Top: [RH]NMC - [RH]ECMWF. Values greater than ±20% m are stippled.Bottom: RMS RH differences. Values greater than 30 and 40% are stippledin different densities.
19
--
differences are also very large at high latitudes.
It is emphasized that the huge changes in analyzed RH at both centers have resulted
primarily from changes in the NWP model, not in the available data or data assimilation.
It is apparent that the analyzed fields in the tropics and subtropics (Trenberth and Olson,
1988b) have tended to be dominated by the NWP model and the associated convection
parameterization schemes. Observations of RH apparently have minimal impact. Conse-
quently there is little comfort in the recent reductions in mean differences analyzed at the
two centers. We note that even though the mean differences dropped to 10%% in RH, the
RMS differences still exceed 20% which is regarded as an unacceptably large amount for
a quantity whose total range is 0 to 100%.
Further time series of differences are presented in Section 3.4 for the poleward eddy
heat and momentum fluxes, and in Section 3.5 for the surface fields over the oceans.
3.2 Meridional cross sections
In order to better appreciate the nature of the zonal mean differences, we present a
number of monthly mean differences as latitude-height cross sections. Figure 11 presents
the meridional cross sections of the zonal mean and RMS z differences for January, April,
July and October of 1980. The mean z differences for 1983 are given in Fig. 12 and the
mean and RMS differences for 1986 are given in Fig. 13. The mean fields mainly illustrate
the huge differences over Antarctica which extend well into the SH. The differences are
greatest in the winter half year. By 1983 the systematic differences are as big as at any
time (over 400 m) but confined to south of 60°S. These enormous differences continue until
May 1986, as seen in the April panel of Fig. 13, but are much less in July and October of
1986 following the improvements at NMC discussed earlier.
The RMS z differences in Figs. 11 and 13 include the mean differences but reveal
other aspects of the analysis uncertainties. In 1980, there were big differences at all levels
south of 40°S quite aside from Antarctica problems, but also with differences becoming
larger with height and exceeding 25 m at 300 mb even in the NH in January and April
20
.0E0L.
0.J-CL
I.
CU -(lCMWF (M)
LatitudeJAN 80
.0EwCl,0
L.Q.
LatitudeAPR 80
.0
E0co5..
0Cl,Cl,wL
0.
.0Ew
IIV5..
0.
LatitudeJUL 80
LatitudeOCT 80
Fig. 11a. Meridional cross sections of [Z]NMC - Z]ECMWF for January, April, July
and October 1980. Negative values are stippled. The contour interval is 25
m.
21
SE
9,Iwm4,wa.
LatitudeJAN 80
S1..E
InL
01
LatitudeAPR 80
S
:3
toC)
a.
LatitudeJUL 80
EIL.C)
a)I
LatitudeOCT 80
Fig. lib. Meridional cross sections of zonal mean RMS z differences for January, April,July and October 1980. Values greater than 100 m are stippled and thecontour interval is 25 m.
22
i
0
.0
E
..
0.L
LatitudeJAN 83
., : :.: : l ........ .. . ........ ... ..,,,,,.... .... .... .;.;. .. ...... ,,, .{\ ................
K .. .... ........................
......., ...
..,,,.......... :
w.:,,,,.............. :' s ' '''
... . .. . .. . ... . ...:~~~~~~~~~~~~~~~~~~~~~~~~ .., ... ........
\ \ .: ..... .. ... :::M ::::........... ............... .....\\
::t l ::,:::.:.,,:: ::::::: :: :m:::: ::::: \\\R
W ,., .... ,. , ....... ~~~~~. ... .. .... ....:
:: :,: . :. ,::. ... :. . :. .... :. . :: ..: : : : :: :::: ...: . .
s , , ~~~~~~~~~~~~~~~~~~~~~....... ..... .,..,,. ̂s
" I....A....................................
............................·''·'·'....... . ;;I).~~~~~~~.......
...·~~:~ ··. ~ ~ ~ ....r.................... ................ ... . ... :~. ·· ·· _.············ · · ··
S30s os 90SN fON 30ON 0
LatitudeAPR 83
LatitudeJUL 83
LatitudeOCT 83
Fig. 12. Meridional cross sections of [z]NMC - [Z]ECMWF for January, April, Julyand October 1983. Negative values are stippled. The contour interval is25 m.
23
100
o-
E
5-3) 0 0
-
to
C70
700-
1$Oa 1I ............. - -7--- ---------------
1
.0E
S.
w,
0.
.0E
S.
I,0.S.
0.
~ I ... .. ... .. .. I , l
*
.0EvL.
A.
I,
LatitudeJAN B6
.0Ev3)
L.
0.
.0E
L.
U,6)
I.
LatitudeAPR 86
LatitudeJUL 86
.0E
U,
U,InA.0.
LatitudeOCT 86
Fig. 13a. Meridional cross sections of [z]NMC - [Z]ECMWF for January, April, Julyand October 1986. Negative values. are stippled. The contour interval is25 m.
24
.0E
I-
0.on
S.
LatitudeJAN 86
.0E6)
026v&IC.
LatitudeAPR 86
.0E
S.
0,
I..
LatitudeJUL 86
E
0.
U,IVa.l.
4ON ON 30oN O OS «S
LatitudeOCT 86
Fig. 13b. Meridional cross sections of zonal mean RMS z differences for January, April,July and October 1986. Values greater than 100 m are stippled and thecontour interval is 25 m.
25
1980. These may be associated with start-up problems at the ECMWF. By 1986 such
large RMS differences are confined to 100 mb and/or south of .40°S and with further
reductions after May 1986. But the large discrepancies remain south of -40°S.
Mean cross sections of [u] differences are not shown since they merely geostrophically
reflect the gradients in Figs. 11-13. However, sample RMS u differences for 1982 and the
most recent year, 1986, are shown in Fig. 14. Aside from Antarctica problems, RMS
differences over 5 m s - 1 occur mainly south of 20°N and above 500 mb, although they also
occur near 30°N and 200 mb in the subtropical jet stream in the NH winter. This figure
depicts, better than Figs. 11-13, the uncertainties in the analyses in the tropics. It also
shows that only minor reductions in differences have occurred with time.
We discussed the problems with [v] earlier but the differences actually have a rich
vertical structure which has varied with time, as seen for January 1981, 1983, 1985 and 1986
in Fig. 15 and for July of the same years in Fig. 16. Much of this complex structure is due
to problems at ECMWF as noted in Trenberth and Olson (1988b) who presented the total
[v] fields for July for the same months (see their Fig. 13). In 1981 [v] was generally weaker
in the ECMWF fields (see also Fig. 7), but became stronger after September 1982 following
the introduction of diabatic NNMI at ECMWF. But then [v] at NMC became stronger
after May 1986 following changes at NMC. Meanwhile the strange vertical structure in the
ECMWF [v] fields, which was associated with the number of vertical modes initialized,
has been gradually eliminated over the years.
In Figs. 17 and 18 we present the mean and RMS differences in RH for January
and July of 1985 and 1986. The pattern in January 1985 was fairly typical of that from
about 1981 to 1 May of 1985. The main change in the differences between January 1985
and January 1986 in Figs. 17 and 18 is due to the ECMWF model changes on 1 May 1985
which dried out the tropics at and above 700 mb. The changes in the difference patterns
in July 1985, compared with July 1986, reflect the impact of subsequent model changes at
NMC in May 1986. It is clear that the RMS RH differences exceed 15% and often 20%
over most of the domain.
26
.0E
S.
to0)
D.
V..
.0E
I)v
1.
a.
.0E
10
g)
to
L. -CL
Cl)
C)S.
0.
RMS DIFF. (Uwuc/rcur M/S) ZONAL MEAN
1000- oa.^. , - *< . -o - .
300ON 30N 0 305
7 00 L ..........
2.05 40 5
1000a---i ii. i-' , -.--
90N 60N 30N 0 30$ bOS 90S
:! : ...:.... .::::::i:::i:::::::i!i.i::!::: : :..:~., .:
i ·· · : * -.- w _;.: :-- :.-:-:-: .......
100 2.0S 0 :'ffN 6ON 3ON 0 30$ *0S -0S
LatitudeJUL 82
LatitudeJAN 86
LatitudeJUL 86
Fig. 14. Meridional cross sections of zonal mean RMS u differences for January andJuly of 1982 and 1986. Values greater than 5 m s - 1 are stippled and thecontour interval is 2.5 m s-.
27
.0EC)
01wC.-U)(6
5
.0E
-
go
P2
LatitudeJAN 81
.0E
S..
5-I..
LatitudeJAN 83
.0
I
so
S.
I-
LatitudeJAN 85
.0
P2
E
S..soma.
LatitudeJAN 86
Fig. 15. Meridional cross sections of [V]NMC - [V]ECMWF differences for January of1981, 1983, 1985 and 1986. Negative values are stippled and the contourinterval is 0.2 m s- 1.
28
i
i
LatitudeJUL 81
90N 6m 30W 0
LatitudeJUL 83
LatitudeJUL 85
LatitudeJUL 86
Fig. 16. Meridional cross sections of [V]NMC - [VECMWF differences for July of 1981,1983, 1985 and 1986. Negative values are stippled and the contour-intervalis 0.2 m s- 1.
29
.0E
S..
U..I
too-
.0 -
E6) 300-
w-
a . c-
a.
........ 1.. 1
I'""'' ." ti;. : t ; -: ;l.. .......: .,,-.:'':,1 ::-.::,, j j -,1,;A, .... .:.:.,:.
r. .... \ [. .......7W .;.., ........ ,...,.
, ~~~~~~~.....; ... .. ,F_,......
ZEL'l '' C'
DsS . oS
.0Ew5-
5..
I-
0t
L..SU
, lI_dl,_ _ [_ _ _ _ _- .. l_- I-__. ...___ _F
I
I.I UL"[' J''' i--- " I. --.--- .. -EN1W
Ew
0.
0.
LatitudeJAN 85
.0E
3,
L.0
a.
LatitudeJUL 85
.0
E0)
CE,3-
0)5.
Q.
LatitudeJAN 86
.0E0)
a.C.
LatitudeJUL 86
Fig. 17. Meridional cross sections of [RH]NMC-[RH]ECMWF differences for Januaryand July of 1985 and 1986. Negative values are stippled and the contourinterval is 5%.
30
i
E
L.
0.Q~
LatitudeJAN 85
O9N 6ON 30N 0
LatitudeJUL 85
LatitudeJAN 86
LatitudeJUL 86
Fig. 18. Meridional cross sections of RMS RH differences for January and July of1985 and 1986. Values greater than 20% are stippled and the contour intervalis 5%.
31
.C
EC)
a.
100-
300-
700-
1000-
·········~·\··~~·~ ··~····A *···I??J/ANK\.4····· ·······K~~~~~~~~~~~~~~~~~~~~····;·~·
· · ··. ~ ~ ~ ·SO
E
ad
tocnw
5.
5.
E.w9.00ww0.
t30S 605
3.3 Geographical distributions
A fairly small sample of the individual monthly mean maps of the NMC-ECMWF
differences is presented in this section as a guide to how representative the zonal means
are. Figure 19 shows the 1000 mb mean and RMS differences for z for January 1983.
The corresponding fields for July 1983, and January and July 1986 are shown in Figs. 20,
21 and 22. The most striking aspect of these four figures is that the RMS differences
are all quite similar to first order, with the major RMS differences occurring where there
is high orography. In this case, as noted earlier, the zonal mean differences tend to be
strongly influenced by the differences over land. One might hope that a large component
of the RMS differences, which occur because of different procedures at the two centers
for extrapolating below ground, might be systematic. It can be seen from Figs. 19-22
that, indeed, there are often large mean differences in the same places. Unfortunately,
such differences are not so consistent from month to month, since they vary in magnitude
and even in sign, so that there is a significant additional random component in all areas.
An example of contrasts in a similar environment is shown in Fig. 21 for January 1986
over Antarctica, where the mean difference of -80 m near 30°E contrasts with a +86 m
difference near 100°W.
Also evident in Figs. 19-22 are the large RMS differences over the oceans south of
-30°S; regions where limited numbers of observations are available and the daily variance
is large. As can be deduced from Fig. 1, differences are much larger prior to 1983 . Further
discussion of these aspects is contained in Section 3.5.
At 200 mb there tends to be more zonal symmetry so that the previously presented
zonal mean sections are more meaningful. Nevertheless, there are some features that
emerge in the geographic distribution that are of interest. Figures 23 and 24 present the
200 mb differences for z and u for January 1983. In the NH, mean z differences >20 m and
RMS differences up to 50 m occur over the central North Pacific Ocean, a region where the
daily standard deviation is ~150 m. They are reflected in Fig. 24 in u by RMS differences
over 6 m s - l. In the tropics u is a more sensitive indicator than z of the meaningful
differences, and in January 1983 the mean differences exceed 6 m s- 1 in the central and
32
nnnI ilnMUV U D ZNMC - Z ECMWF
150W 120o 90W 60W 30o 0
(M)30E 60E 90E 120E 150E 180
90N
60N
30N
0
305
60S
90S150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180
00 MB oFRMS DIFF. (ZNurrCruWF M)
150W 120W 90W 60W 30W0 , 30E
0 30E 60E 90E 120E 150E 18090N
60N
30N
0
30S
60S
90530
JAN 83
Fig. 19. Monthly mean 1000 mb z difference fields for January 1983. Top:ZNMC - ZECMWF. Negative values are stippled and the contour intervalis 20 m. Bottom: RMS z differences. The contour interval is 20 m andvalues greater than 20 m are stippled.
33
IU
18090N
60N
30N
0
30S
60S
90S180
10
18090N
60N
30N
0
30S
60S
90S1
I
1000 MBZNMC - ' CWF
150W 120W 90W 60W
(M)30W 0 30E 60E 90E 120E 150E 180
150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180
00 MB RMS DIFF. (ZkurCrur. M)
90N
60N
30N
0
30S
60S
90S
150W 120W 90W 60W 30W
150W 120W 90W 60W 30W
0 30E 60E 90E 120E 150E 180
0 30E 60E 90E 120E 150E 180
90N
60N
30N
0
30S
60S
90S
JUL 83Fig. 20. Monthly mean 1000 mb z difference fields for July 1983. Top:
NMC - ZECMWF. Negative values are stippled and the contour intervalis 20 m. Bottom: RMS z differences. The contour interval is 20 m andvalues greater than 20 m are stippled.
34
90N
6ON60N
30N
0
30S
60S
90S
180
180
10
90N
60N
30N
0
30S
60S
90S
180
180
1000 MB Z NMC -
150W 120W 90u 60b 30u
Z ECMWF
0 30E
(M)60E 90E 120E 150E 180
1000 MB
150W 120W
RMS DIFF. (ZNMC/ECMWF
90W 60W 30o 0 30E bOE
M)90E 120E 150E 180
90N
bON
30N
0
30S
60S
90S0
JAN 86
Fig. 21. Monthly mean 1000 mb z difference fields for January 1986. Top: ZNMC -
ZECMWF. Negative values are stippled and the contour interval is 20 m.Bottom: RMS z differences. The contour interval is 20 m and values greaterthan 20 m are stippled.
35
1800ON
>ON
30N
'Os
bOS
90S
18090N
60N
30N
0
30S
60S
I
1000 MB ZNMC - ZECMWF
150W 120W 90W 60W 30W 0 30E
150W 120W 90W 60W 30W
(M)60E 90E 120E 150E 180
0 30E 60E 90E 120E 150E 180
RMS DIFF.
150W 120W 90W 60W 30W
(ZNMC/ECMWF
0 30E 60E
M)90E 120E 150E 180
90N
60N
30N
0
30S
60S
90S0
JUL 86Fig. 22. Monthly mean 1000 mb z difference fields for July 1986. Top:
ZNMC - ZECMWF. Negative values are stippled and the contour intervalis 20 m. Bottom: RMS z differences. The contour interval is 20 m andvalues greater than 20 m are stippled.
36
9ON
60N
30N
0
30S
60S
90S
180
180
90N
60N
30N
0
30S
60S
90S
1000 MB
18090N
60N
30N
0
30S
60S
90S1
I
200 MB
150W 120W 90W
ZNMC -Z ECMWF
60W 30W 0
(M)30E 60E 90E 120E 150E 180
200 MB
9ON
60N
30N
0
30S
60S
90S180
RMS DIFF. (ZNMC/ECMWF M)
150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180
JAN 83
Fig. 23. Monthly mean 200 mb z difference fields for January 1983. Top:ZNMC - ZECMWF. Negative values are stippled and the contour intervalis 20 m. Bottom: RMS z differences. The contour interval is 20 m andvalues greater than 20 m are stippled.
37
180901
601
301
30!
60
90!
200 MBU NMC - U ECMWF
180 1SOW 120W 90U 60W 30W 0% P L. ,.
(M/S)30E 60E . 90E 120E 150E 180
Io -0 . W 120U1o -5 '*W 1 20W 90. 60, 30
90W 60W 30W 0 30E 60E 90E
LivV .,. U RMS DIFF.
150W 120W 90W 60W 30W
(UNMC/E CWF
0 30E
M/S)60E 90E 120E 150E 180
90N
60N
30N
0
30S
90S0s
JAN 83
Fig. 24. Monthly mean 200 mb u difference fields for January 1983. Top:UNMC - UECMWF. The contour interval is 4 m s - 1 and all values greaterthan ±2 m s - 1 are stippled. Bottom: RMS u differences. The contourinterval is 4 m s - 1 and values greater than 6 m s- 1 are stippled.
38
60N
30N
0
30S
60S
90S18
·............ ...
;;;Lk';'C,. ..,,,,,,~ ~ ~ ~~.........." ""' ··' ··· ~ ~ ~ ~ ~ ~ ~ ~ ~ . .....
. . . . . .. .. . .. . .. . .. .. .. . .. .
60N
30N
O0
30S
60S
90Sit
dry%
180
120E 150E -4S1
90N
60N
30N
0
305
60S
90S1
.-
. ......... -- .4. .i . K--. I - . . . . . . . . I .;-, �-� � . . . � . I I
urn .J L~~~~~~~~~~~~~~~~~~~~~~~ .....a..............-WV"- - l % Li
tUNr
Pnn upR
eastern Pacific, across South America and into the Indian Ocean. These are regions where
the daily standard deviation is also -6 m s-1 (Trenberth and Olson, 1988b). The RMS
differences are over 10 m s - 1 in several places.
Figures 25 and 26 show the z and u differences for 200 mb in July 1983. The July
z differences are shown mainly to emphasize the nature of the huge differences over and
near Antarctica which result in u differences of up to 26 m s-1. In the weaker gradients of
the NH summer, differences over the NH are small in July. Differences in u in the tropics
and over most of the SH are significant, the main exception being the Australasian sector
which has the most reliable data base.
Figures 27, 28, 29 and 30 show the corresponding z and u differences at 200 mb
for January and July of 1986. In January problems are still evident in the vicinity of the
NH subtropical jet over the Pacific with RMS u differences up to 11 m s- and exceeding
6 m s - 1 over a broad region. Even mean differences locally exceed 4 m s-1. RMS height
differences are bigger than in January 1983 over the north Atlantic and continue to be
large over Africa and south of 20°S while RMS differences in u over 6 m s-l are common
in the tropics and subtropics.
In July 1986 large differences >30 m in 200 mb z are present over the tropical
Atlantic and Indonesia and these are the rule through to December 1986. In December
1986 (not shown) the -28 m differences over India in Fig. 29 were reversed to be +43 m.
Problems with u differences >6 m s-l continue south of -20°N in Fig. 30.
Of particular note in Figs. 24, 26, 28 and 30 are the substantial differences that
exist throughout the tropical Pacific, an area of considerable interest for studies of the
El Nfino and Southern Oscillation phenomena. In the tropical east Pacific monthly mean
u differences commonly exceed 5 m s - ', which is comparable to the expected real inter-
annual variability, and RMS differences exceed 10 m s-1. Such differences could seriously
undermine results of diagnostic studies focussing on that area.
There are huge variations in the RH differences with time and we therefore show
only samples from the last year, 1986, in Figs. 31 and 32 at 850, 700 and 300 mb. In January
39
120W 90W
Z NC - Z ECMWF
60W 30W 0
180 150W 120W 90W 60W 30W "''
(M)30E 60E 90E 120E 150E 180
)N
)N
30E 60E 90E 120E 150E 18L
u u MtD RMS DIFF. (ZNMC/ECMWF M)
180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 18090N
6ON
30N
0
30S
60S
90S
180 150W 120W 90W 60W 4S 0 30E 60E 90E 120E 150E 180
JUL 83
Fig. 25. Monthly mean 200 mb z difference fields for July 1983. Top: ZNMC -
ZECMWF. Negative values are stippled and the contour interval is 20 m.Bottom: RMS z differences. The contour interval is 20 m and values greaterthan 20 m are stippled.
40
200 MB
180 150W90N
60N
30N
0
30S
60S
90S
90N
60N
30N
0
30S
60S
90o
)N
)S
OS
3SA l ~11
d^lf^ tr,
zi
200 MB
1190N
60N
30N
0
30S
60S
90S1
UNMC U ECMWF
'I\ Ju n D RMS DIFF. (1
150W 120W 90O 60 30W
UNMC/ECMWF
0 30E
M/S)60E 90E 120E 150E 180
90N
60N
30N
0
30S
60S
905130 150W 120W 90 60W 30V 0 30E 60E 90E 120E 150E 180
90S
JUL 83Fig. 26. Monthly mean 200 mb u difference fields for July 1983. Top: UNMC -
UECMWF. The contour interval is 4 m s - 1 and all values greater than ±2m s- 1 are stippled. Bottom: RMS u differences. The contour interval is 4m s - 1 and values greater than 6 m s - 1 are stippled.
41
(M/S)
ornn tID
18090N
60N
30N
0
30S
60S
90N
6ON
3ON
30S
60S
90S
90N
30N
0
30S
60S
90S
180
180
180
200 MB Z NC -
150W 120W 90W 60W 30W
Z ECMWF
0 30E
200 MB RMS DIFF. (ZNMC/ECMWF
150W 120W 90W 60W 30W 0 30E 60E
(M)60E 90E 120E 150E 180
M)90E 120E 150E 180
9UN
60N
30N
0
30S
60S
90S150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180
JAN 86Fig. 27. Monthly mean 200 mb z difference fields for January 1986. Top:
ZNMC - ZECMWF. Negative values are stippled and the contour intervalis 20 m. Bottom: RMS z differences. The contour interval is 20 m andvalues greater than 20 m are stippled.
42
II
200 MBiAn 1 snu 12nu Ow
U NMC - U ECMWF
60W 30W 0
(M/S)30E 60E 90E 120E 150E 180
200 MB RMS DIFF. (UNMC/ECMWF
150W 120W 90W 60W 30W 0 30E
150W 120W 90W 60W 30W
M/S)60E 90E 120E 150E 180
0 30E 60E 90E 120E 150E 180
9ON
60NbON
30N
0
30S
60S
90S
JAN 86
Fig. 28. Monthly mean 200 mb u difference fields for January 1986. Top:UNMC - UECMWF. The contour interval is 4 m s - 1 and all values greaterthan ±2 m s - 1 are stippled. Bottom: RMS u differences. The contourinterval is 4 m s-1 and values greater than 6 m s - 1 are stippled.
43
90N
60N
30N
0
30S
60S
90S
180
180
200 MB ZNMC -
150W 120W 90W 60W 30W
Z ECMWF
0 30E
(M)60E 90E 120E 150E 180
200 MB RMS DIFF. (ZNMC/ECMWF M)
90N
60N
30N
30S
60S
90S905180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180
JUL 86Fig. 29. Monthly mean 200 mb z difference fields for July 1986. Top: ZNMC -
ZECMWF. Negative values are stippled and the contour interval is 20 m.Bottom: RMS z differences. The contour interval is 20 m and values greaterthan 20 m are stippled.
44
18090N
60N
30N
C
30S
60S
90S
)ON
SON
)ON
30S
bOS
90S
U NMC - U ECMWF
60W 30W 0
(M/S)
30E 60E 90E 120E 150E 180
200 MB150W 120W
RMS DIFF. (UNMC/ECMWF
90W 60W 30W 0 30E
M/S)60E 90E 120E 150E 180
150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180 -
90N
60N
30N
0
305
60S
90S
JUL 86Fig. 30. Monthly mean 200 mb u difference fields for July 1986. Top: UNMC -
UECMWF. The contour interval is 4 m s- 1 and all values greater than ±2m S- 1 are stippled. Bottom: RMS u differences. The contour interval is 4m s - 1 and values greater than 6 m s - 1 are stippled.
45
200 MB15ow 1180 20W 90W
90N
60N
30N
0
30S
60S
90S
tON
>ON
3ON
3OS
9OS~OS
90N
60N
30N
0
30S
60S
9OS
180
180
4i
. .
RH Mc - Hi £CMWF (') 850 MB
ury
?ON
W0N
3ON
~0
30S
bOs
90s
RH NMC - RH ECMF ) 700 MB
180 150W 120W 90W 60o 30o 0 30E 60E 90E 120E 150E 180
RHNuc - RH ECMWF (70) 300 MB180 150W 120W 90o 60W 30W 0 30E 60E 90E 120E 150E 180
9ON
6ON
3ON
30S
90S
9ON
6ON
3ON~0
30S
b0s
JAN 86Fig. 31. Monthly mean RH difference fields for January 1986 RHNMC -RHECMWF
at 850, 700 and 300 mb. Negative values are stippled and the contour intervalis 20%.
46
9o.
6ON
30
30S
60S
90N
60N
30N
0
30S
60S
90S
90N
60N
30N
0
30S
6OS
9051
r A L
I
RH NUC - RH ECMWF
180 150W 120W 90o 60o 30W 0 30E
(7.) 850 MB60E 90E 120E 1SOE 180
RH NMC - RH ECMWF (7') 700 MB
90N
60N
30N
0
30S
60S
90S1
RH NMC - RH ECMWF (7) 300 MB180 150s 120W 90W 60W 30o 0 30E 60E 90E 120E 150E 180
JUL 86Fig. 32. Monthly mean RH difference fields for July 1986 RHNMC - RHECMWF at
850, 700 and 300 mb. Negative values are stippled and the contour intervalis 20%. 47
90N
60N
30N
0
30S
60S
90N
60N
30N
0
30S
60S
90SI go
._O
1986, biggest differences occur over the oceans but the magnitude of these differences-over
60% in the eastern tropical Pacific at 850 mb-is astounding. At 700 mb in January the
tropical differences reverse in sign but still exceed 50% over the tropical Pacific and Indian
Oceans, and over Antarctica. Locally differences of N40% continue in July 1986 over the
tropical eastern Pacific at all three levels and over the Indian Ocean at 300 mb. The zonal
mean differences in RH shown earlier (Figs. 8, 9, 10 and 17) are representative of the
overall differences but, if anything, underestimate the huge differences that exist locally.
The RMS differences for RH are not shown as geographical distributions since they are
dominated by the mean differences.
3.4 Poleward eddy heat and momentum fluxes
The poleward eddy heat flux [v*T*] and the poleward eddy momentum flux [u*v*j
are fundamental quantities in maintaining the general circulation of the atmosphere. In
addition to their importance to the heat and momentum budgets themselves, they are
also the main quantities that go into the Eliassen-Palm (E-P) flux of wave activity (e.g.,
Trenberth, 1987b). The vertical wave activity component is proportional to [v*T*] and
the meridional component is proportional to [u*v*j. Note that we are dealing with the
total wave flux here including contributions from both the stationary and transient waves.
As background for interpreting the differences in the fluxes from NMC and ECMWF,
we first present estimates of these quantities from ECMWF for January and July 1986 in
Figs. 33 and 34. The strongest poleward eddy heat flux occurs in winter with maxima be-
tween 850 and 700 mb and a secondary maximum in the lower stratosphere near 200 mb.
The latter is associated with the vertical propagation of waves into the stratosphere which
is greatest in winter in the NH. The maximum poleward transport of westerly momentum,
Fig. 34, occurs near the tropopause and again the values are largest in the NH winter. The
annual cycles for both fluxes have much larger amplitudes in the NH.
Time series of the smoothed 15-day averaged [v*T*] at 700 mb and [u*v*] at 200 mb
are shown in Figs. 35 and 36, while time series of the differences are given in Fig. 37. The
latter are also given as meridional cross sections for January and July of 1982 and 1986 in
48
.0
E
Q)S.. I,
[" T ] ECWF (M-KS- )
LatitudeJAN 86
.0E
Q)
V)
a.
90N 60N 30N 0 305 60S 90S
LatitudeJUL 86
Fig. 33. Meridional cross sections of [v*T*] from ECMWF in K m s- 1 for Januaryand July 1986. The contour interval is 5 K m s - 1 and negative values arestippled.
49
.0Ea)S..
a)a.
LatitudeJAN 86
.0
E
(U0)
EnQ)
Q.I,
90N 60N 30N
Fig. 34. Meridional cross sections ofand July 1986. The contourstippled.
0 30S 60S 90S
LatitudeJUL 86
[u*v*] from ECMWF in m 2 s - 2 for Januaryinterval is 10 m 2 s - 2 and negative values are
50
[v T ]NMc (M-KS- 1 )
SMOOTHED
90N
60N
30N
Q5
-d
(t
0
305
60S
905
700 MB
15-DAY AVERAGES
[Iv T ]ECMWF (M-KS- )
SMOOTHED
90N,
60N
30N
0
305
605
905
700 MB
15-DAY AVERAGES
Fig. 35. Smoothed 15-day averages of [v'T*] at 700 mb as a function of latitude and
time. Values greater than ±10 K m s- 1 are stippled and the contour interval
is 5 K m s- 1. Top: from NMC; and bottom: from ECMWF.
51
0
._.-D
[u' V]NMC (M2 S- 2 )
SMOOTHED 200 MB1 I * ' I . .I I 1 r I I r I ' I , I I i I I t I I I t I
1979 1980 1981 1982 1983 1984
15-DAY AVERAGES
1985 1986
[UI V ]ECMWF
SMOOTHED
60Nt
30N4
C!
305
605
905
(M2 S- 2 )
200 MB
15-DAY AVERAGES
Fig. 36. Smoothed 15-day averages of [u*v*] at 200 mb as a function of latitude andtime. Values greater than ±40 m 2 s- 2 are stippled and the contour intervalis 20 m 2 s- 2. Top: from NMC; and bottom: from ECMWF.
52
9Er, -
30N -
0I -
30M -
A r I-
90S
v'J ~ ,.-v<.. .
)>i@6 / z~
, i I I 1 I II I I - I I I I I , I I i - 1 ) l , I I t
Iv T* ]NMC-ECMWF
SMOOTHED
90N
60N
09
305
60S
90S
(M-KS-' )
700 MB
15-DAY AVERAGES
U[ 'V ]NuC-_ECMW
SMOOTHED
60N
30t.
C
Q)
3
cl6-
._-
305
605
905
(M2 S- 2 )
200 MB
15-DAY AVERAGES
Fig. 37. Differences (NMC - ECMWF) in smoothed 15-day averages of Iv'T*] at
700 mb (top) and [u*v*] at 200 mb (bottom) as a function of latitude and
time. Values greater than ±5 K m s-1 (top) and ±10 m 2 s-2 (bottom) are
stippled.53
Figs. 38 and 39.
For [v*T*], the differences in Fig. 37 are mostly relatively small, less then 2 K m s 1,
except for the region south of 40°S and especially near Antarctica. As can be seen in
Fig. 35, very large and unrealistic values occur there in the ECMWF analyses. However,
the differences from 40-65°S are likely to be due to general uncertainties in the analyses
although no doubt with the SH problems at NMC exacerbating the problem, e.g. see
Section 4.
It turns out that the biggest discrepancies in [v*T*] are not near its maximum at
700 mb but instead occur at the surface and in the upper troposphere and lower strato-
sphere (see Fig. 38). At the surface, pronounced differences are seen in January 1986 and
are due to stronger poleward fluxes there in the NMC data. Such differences are also
found in most other months and evidently arise from the boundary layer wind properties.
Inclusion of data extrapolated below ground may also be a factor. In the lower strato-
sphere, differences of over -5 K m s- 1 are the rule, especially south of 30°S, usually with
a stronger ECMWF poleward flux (negative values, positive NMC-ECMWF difference).
Such differences, 30-40% of the total, are disconcerting and have implications for infer-
ences drawn about lower stratospheric dynamics of the SH. Evidently the wave activity
propagates vertically much more as seen from the ECMWF analyses than from the NMC
analyses.
A major concern at present is the "ozone hole", the observed trend toward lower
ozone amounts in the lower stratosphere over Antarctica in the southern spring. In order
to help unravel the role of dynamics in contributing to this phenomenon it is essential to
know the components of the E-P flux in detail. Fig. 40 therefore shows the total ECMWF
[v*T*] and the NMC-ECMWF differences for October 1986. Similar differences are found
in the previous two years in September and/or October. Clearly caution is called for in
drawing any conclusions about the P flux and the role of dynamics in the heat and ozone
budgets with uncertainties such as those that Fig. 40 presents.
The differences in [u*v*] (Figs. 37 and 39) are somewhat variable in time. Differ-
54
.0
EvLi
eL
LatitudeJAN 82
H 4ON 3ON 0
LatitudeJUL 82
3os eOS 90S
LatitudeJAN 86
.aE
S.
An5.
0.
LatitudeJUL 86
Fig. 38. Meridional cross sections of [v'T*] differences (NMC - ECMWF) in K m s-for January and July of 1982 and 1986. The contour interval is 2.5 K m s-1and negative values are stippled.
55
100-
-.0E
L3
700-
q
E5-
C,,I.
L
0.
...... I I.... I _............. mA . ....m ..... m .-~ - .-2~ ....m _ m_ __ _. m _ _
r-r
·rar ! - -. ' -_ - -. .. - . ....._~~~~~~~~~~~~~~~ .IWVV-Ia
-r
5
·::f~~l·:·~·:.........................
.............................. ................... . .....·:·:·:·...................... ·. ··. ··.·
... ... .... ... ... .. .... ..... .. ... .. ..... .
.. .. . . .. .. . .. . .. .
B litiltlirtltttt~ i~............................. ......... ..~~~~~~~~~~~i~lii~i
.......................... .. . .. . .. . .. .
.. ............ ....... ~ ~ :·:·.... ..... ... .... ... ... . .. ... ... ... ...
... .. .. .. ....... .. .. ..... .. ..... .. ...........·.·.·.·;.......... ............ ... ............::::~.r~.'.' · ~
I)
L.
Co
2 ,A.
E
4)L.co
E
3
L I.
E
S..
U)03U
5.
LatitudeJAN 82
LatitudeJUL 82
LatitudeJAN 8b
LatitudeJUL 86
Fig. 39. Meridional cross sections of [u'v*] differences (NMC - ECMWF) in m 2 s - 2
for January and July of 1982 and 1986. The contour interval is 5 m 2 s - 2
and negative values are stippled.
56
i
I
Iv' T' ] ECWF
LatitudeOCT 86
LatitudeOCT 86
Fig. 40. Meridional cross sections for October 1986 of [v*T*] from ECMWF anddifferences (NMC - ECMWF) in K m s- 1 . The contour interval is 5 (above)and 2.5 (below) K m s - 1 and negative values are stippled.
57
,0
EQ)
S)
.0
coUr)a)
CL.
E
1.0(n
to
(M-KS-' )
;._
ences are mostly small (< 5 m 2 s- 2) in the NH but often become large (>10 m 2 s- 2 ) in
the subtropical NH jet and in the tropics. For the latter region, the differences are 100%
of the total in January 1982 and in the NH jet differences can be .20O%, although they
are usually much less. Once again, the biggest differences arise in the SH and are quite
unacceptable (over 30%) in 1982 (Fig. 39) and still problematical in 1986. Such differences
have profound implications for the momentum budget (e.g. Trenberth, 1987b).
3.5. Surface differences over the oceans
Measures of how well the surface atmospheric fields are known are of interest because
of their implications for interactions of the atmosphere with the surface. In particular, in
this section we pursue how well the NMC and ECMWF analyzed fields agree over the
oceans. This has implications for the TOGA Program and the World Ocean Circulation
Experiment (WOCE). As well as having a bearing on the atmospheric forcing by the oceans,
the surface fluxes of sensible and latent heat and the surface wind stress (or momentum
fluxes) are of vital importance for driving the oceans. We have not computed the surface
fluxes themselves, since the lowest level information available from ECMWF is 1000 mb.
Rather, we have computed sea level pressures and used the 1000 mb analyzed winds to
compute differences over each major ocean.
Snapshots for individual months of these or related quantities were presented in
Figs. 19, 20, 21 and 22 for z. Here we present time series of the monthly means and
temporal RMS differences zonally averaged across each of the three main oceans and for
the oceans as a whole. In this way we can fairly concisely summarize the characteristics
of how well the analyses agree. Some "bad" days were not screened out when these
computations were made but their impact was either minimal or their effects have been
subsequently removed from the plots.
In order to average over the oceans, we have designated each point on the globe as
either land or ocean, using a mask with 2.5° resolution. The mask is conservative with
respect to the oceans by designating marginal regions and areas dominated by islands, etc.
as 'land' in order that the results will be meaningful for the open oceans. The mask is
58
shown in Fig. 41 and the non-shaded area is open ocean for the purposes of summarizing
statistics. In order to delimit the individual oceans, we have used the divisions, also shown
in Fig. 41. The ocean region from 120°E to 800W is referred to as the Pacific, from 70°W
to 20°E as the Atlantic, and from 20°E to 1200 E the Indian Ocean. Note that the region
from 80 to 700 W is in the Atlantic in the NH but in the Pacific in the SH and therefore,
for simplicity in averaging, we have omitted it from both.
Figure 42 shows the zonally averaged mean and RMS differences in sea level pressure
P for the global oceans as latitude-time sections. The corresponding u sections are shown
in Fig. 43. As the RMS differences in v are similar to those in u and the zonal means of v
are not very meaningful they are not presented. Zonal mean P differences exceed 1 mb over
the tropical oceans, where the standard deviation of the monthly means are only about
0.6 mb, until the end of 1980. Mean differences of ~0.5 mb continued through 1984. These
uncertainties in the P field are reflected in the zonal mean u field as differences commonly
exceeding 1 m s - 1, which is to be compared with the mean trade wind values of 4 to
6 m s-~. After 1980 mean P differences in the NH are small, but in the SH the previously
discussed problems with the NMC analyses prior to 1982 are again clearly evident.
The RMS differences in Figs. 42 and 43 are perhaps more meaningful in terms of the
likely uncertainty in flux quantities. RMS differences in P are rarely less than 1 mb and
exceed 2 mb in the Arctic Ocean and south of s35°S. For u the result is RMS differences
exceeding 3 m s- 1 (corresponding to the total wind speed of ~5 m s - 1) over the NH
oceans in the winter half year, in the equatorial belt and south of 30 0S. In the equatorial
belt, the differences have been due in large part to problems at NMC which were at least
partly corrected in May 1986. As shown by Bonner et al. (1986) the equatorial winds
were unrealistic and too weak prior to that date (see also Leetmaa, 1987). Harrison et al.
(1988) have compared ocean model responses to different surface wind fields for 1982-83,
including those from NMC and ECMWF, and show that there are substantial differences
among the products, apparently partly due to an inadequate data base.
The mean differences in P over the individual oceans are quite similar to those
shown in Fig. 42 and are not presented. Fig. 44 shows the RMS differences over the three
59
(2.5 X 2.5 RESOLUTION)
180 150W 120W
180
90W 60W 30W 0 30E 60E 90E 120E 150E
150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E
Fig. 41. Map showing the unmasked area designated as open oceans. The dividinglongitudes for the different oceans are also marked in.
180- 0 Pt&
180
90N
60N
30N
0
30S
60S
90S
90N
60N
30N
00I
0
305
60S
qnS
LAND/OCEAN FLAGS
0 o
I··~~ .'
GLOBAL OCEANS
p*& OQII£ (a i
ri 6'Bs ' ':;
RrJ
c?5 :...;
hss ' ,,;.- · ·// s'^ .' *';
. ,. *'**.' 1. <^ ; :' '" '
-. 739
I:: 458 ",
;..··.;5r--i'j j 5.'~~-. *. .* 5~ 5
H.149
^ ̂/ ;: -- ((n w
* *. .................................*
,~ ..
.' ·.S ,. S ,.
CONTOUP5:-2.0-1.0-0.5
0.51.02.05.0
1980 1981 1982 1983 1984 1985 1986
Months
RMS DIFF. (PN- .'ECMWF mb) GLOBAL OCEANS
1980 1981 1982 1983 19 1985 19861980 1981 1982 19B3 1984 1985 1986
CONTOURS:1.02.05.0
10.0
MonthsJPN 80 - DEC 86
Fig. 42. Latitude-time series of the monthly mean differences in sea level pressureszonally averaged over the total oceans. The uneven contours are given atright. Top: PNMC-PECMWF, negative values are dashed and values greaterthan ±1 are stippled. Bottom: RMS P differences with values greater than2 and 5 mb stippled.
61
9OJ
9 I U J
I:
305 -
605 -
905
9C'0
60:';
30rJ
-)IC
-6 .
625 -
905
./,,y
_L
: ;: : ':';;:i : .-.'. ......... :.:::.......... ........................................
1.32S~' ' ,''' '"'. '..: ::;i ''' '" " ', " "'i. ' "2 '....:..
i - , - t,,,- __ I " 4 , -1 , I tm L ·t I , " ' I ,I , ' o · -
- i I I I I Il
i I I I I
I I~ ~~~~~ .I .t I I I I.1 I . , i I I I I ..1 . . . m m ~ i -
-' r
[Phmrc -- [PIECMWF- (mb)
(
I
I .. ·
,a ':. ;.
'...1 - -
[U]NNC - [UJECVwF
90N
6 Or
30N
30
605
605
RMS DIFF. (U,t,.C/ECMWF
3C'!
305
605
9C'5
(ms- 1 ) GLOBAL OCEANS
1000 mb
Months
ms-' )
CONTOUPS;-4.0-3.C-2.0-1.0-0.5
0.51.02.03.04.05.0
GLOBAL OCEANS
1000 mb
CONTOURS:2.C3.C4.05.06.07.08.0
MonthsJRN 80 - DEC 86
Fig. 43. Latitude-time series of the monthly mean differences in 1000 mb u zonallyaveraged over the total oceans. The uneven contour intervals are given atright. Top: UNMC - UECMWF, negative values are dashed. Values greaterthan ±1 m s-1 are stippled. Bottom: RMS u differences with values greaterthan 4 and 6 m s - stippled. 62
oceans and it further illustrates the similarity of the patterns over each ocean. The biggest
uncertainties of over 2 mb RMS difference are south of 30°S over all oceans, from 40 to
50°N in the Pacific in winter, and north of 50°N in the Atlantic in winter.
Figures 45, 46 and 47 present the mean and RMS u differences over the three oceans.
Here there are more notable distinctions between them. Problems are evident near the
equator in all three oceans. The tropical differences have more seasonal character in the
Indian Ocean and are largest in the NH summer monsoon from 1980 to 1983. Stronger
equatorial easterlies have been the rule in ECMWF data in the Atlantic, but the same
applies in the Pacific only after late 1982. In the extratropics, largest RMS u differences
of over 3 m s- 1 are found in winter months ~40 to 50°N in the Pacific and Atlantic, and
south of 30°S in all three oceans with even larger differences in winter.
In closing this section, we note that sensible and latent heat fluxes at the surface,
through bulk formulae, depend on the wind speed and the covariations of wind with air-sea
temperature differences and air humidity - saturation humidity differences, respectively.
In the tropics, the latent heat flux dominates and climatological values are -200 W m -2 .
A mean wind error of 1 m s - 1 is apt to result in flux errors of ~40 W m - 2 . RMS errors in
u of 4m s - are likely to lead to 15 to 25% errors in surface wind stress in mid latitudes, or
over 40% errors near the equator. Such uncertainties are not acceptable for driving ocean
models (e.g. see Harrison et al., 1988). There has been a recent increase in attention
focussed on surface fields as a consequence of the TOGA program and fortunately there
are signs that the surface products can be and are being improved.
63
RMS DIFF. (Pwc/[cwr rnb) PACIFIC OCEAN
CONTOURS:1.02.0S.0
10.0
1980 1981 1982 1983 1984 1965 1986IMS DIFF. (PNUC/ECMuF mb) ATLANTIC OCEAN
* COT"OURS:1.02.05.0
10.0
1980 1981 1982 1983 1964 1985 1986RMS DIFF. (PWC/Cuwr mb) INDIAN OCEAN
1980 1981 1982 1983 1964 1985 1988
Months
CONTOR5:1.02.05.0
10.0
Fig. 44. Latitude-time series of the monthly mean RMS differences in P zonally av-eraged over the three oceans in mb.
64
00
Lj. 8 lO·s1.22
....... ....i·. · ..··.. ··... ..................... ....., .... ........... .... . .... .... .... ..... .... ..... .... ....:40 :~z r~s ·~:·:··~r · · ··............ . ........ ......;·:··:··:··: ·... ....... . " "
90
60SN
30SN
C)
3 o-
.3
305
605
905 -
90N -
60N
305
605
905
60N
30N
O)
.. 0
305
60S .
Io
Oii.::.
N b.J:2 1 ... .b . . .
918 ~ ~ ~ ~ ~ .42'
..A. ..... ..... ... ....ri~-~ ·~t ·: :::~'~.....
"-w .~ -- . "Z Y · ·
I Wo - -^ ^08 1 - *(
..- . , . ....!.: i: . :,:' :'-': - ; :.--.......... .. ..-.. :.-.-..-. -. : ....... , ...... , .-, ; - ... ,...F: :..-:.-.*.... .-s*.1 - **..* -
I - 1 - - - - I . I - 1 - - . - - I - - i
I . . . . . . . . . . . . . . . . . . . I . . . I � . � I4-
I- -. -. I - - 1 I - - I - -~ - 1 I . · . i-.
. . - I - - . . . . . I . . . I . .i I -- -- -- --- A -- - - --- -
[U]NMC - [U]ECMWF
60tJ
Q)
-I
I-I
M,
i C
3C S
605
9C5
RMIS DIFF. (Ul4t,/ECMwF
3CJ
'Ccl)
605
90S
(ms- 1 ) PACIFIC OCEAN
1000 mb
Months
ms-' )
CONTOURS:-5.0-4.0-3.0-2.0-1.0-0.5
0.51.02.03.04.05.06.0
PACIFIC OCEAN
1000 mb
CONTOUPS:2.03.04.05.06.07.08.0
MonthsJRN 80 - DEC 86
Fig. 45. Latitude-time series of the monthly mean differences in 1000 mb u zonallyaveraged over the Pacific. Top: UNMC - UECMWF, negative values aredashed. Bottom: RMS u differences in m s-1.
65
�t'.-··�l': 1C� �(`·\r· ijCII
G?' iOii r i%7I I
/f c..r .·r L·\
.rr. ·· C'' ' ' . ·· :i t ·''\· ··.' �·r 'r;" .1\'·' ' ''L�··? �� '' .�· r ·?�QO .. it·( ;·�·.CL I�·)
· ·I·K�� ··.-, ···II-·rI c.;Q-·,.Y�% ri....5··�r j�' I·\ *
r C.' �- 3·�
[U]NMC - [U]ECMWF (ms-' ) ATLANTIC OCEAN
1000 mb
CONTOURS:-4.0-3.0-2.0-1.0-0.50.51.02.03.04.05.06.0
Months
ms- 1 )
0
305
605
90S
ATLANTIC OCEAN
1000 mb
CONTOUPS:2.03.04.05.06.07.08.0
1980 1981 1982 1983 1984 1985 1986
MonthsJRN 80 - DEC 86
Fig. 46. Latitude-time series of the monthly mean differences in 1000 mb u zonallyaveraged over the Atlantic. Top: UNMC - UECMWF, negative values aredashed. Bottom: RMS u differences in m s-1.
66
IR t-S DIFF. (ut,,ui/E Cm W
[U]NMC - [U]ECMWF
6ON
3CKS
KS$
(ms-' )
.Months
RMS DIFF. (UIJC/ECMWF ms-' )
QE'-'
3: .
305
605
90S
INDIAN OCEAN
1000 mb
CONTOURS:-4.0-3.0-2.0-1.0-0.50.51.02.03.04.05.06.0
INDIAN OCEAN
1000 mb
CONTOURS:2.03.04.05.06.07.0e.0
MonthsJRN 80 - DEC 86
Fig. 47. Latitude-time series of the monthly mean differences in 1000 mb u zonallyaveraged over the Indian Ocean. Top: UNMC - UECMWF, negative valuesare dashed. Bottom: RMS u differences in m s-1.
67
4. A Case Study in June 1985
In this section we present the results of one case which was chosen because of impres-
sions gained through the examination of synoptic charts received over the routine facsimile
transmissions from NMC. On the day in question it was noted that several observations
were plotted on the charts over the South Pacific, but they were not compatible with the
analysis.
Fig. 48 shows the 1000 mb analyses from NMC and ECMWF for 20 June 1985
at 0000 GMT and Fig. 49 presents the differences at both 1000 and 300 mb. In the NH,
differences at 1000 mb are -50 m in several places. But by far the biggest differences of
310 m at 1000 mb and 218 m at 300 mb are found near 45°S 1000 W. Large and significant
differences are also found near 40°S 10°W (-191 m) and over the Antarctic Peninsula
(-271 m). Huge differences appear to be standard fare over Antarctica. We have therefore
examined the information available to NMC at this and adjacent synoptic times as archived
on the NMC data tapes.
4.1 The South Pacific Low
Although TOGA buoys are available and can influence the analyses, they were not a
major factor in the South Pacific. Instead, there was apparently a fleet of ships in the area,
moving very slowly, centered near 45°S 110°W. For the region 35°-50°S 900 -115°W from
0000 GMT 19-21 June 1985 there were 45 synoptic reports from 12 ships with different
call signs. The incidence of reports varied from nine at 0000 GMT 19 June to two at the
1200 GMT hours. The ships clearly reveal the development and movement southeastwards
of an intense storm that deepened to ~950 mb at 1200 GMT 19 June and sustained those
low pressures until at least 1800 GMT 21 June. From 0600 on the 19th near 45°S 109°W
pressures dropped from 970 mb to 952 mb (verified by two ships) and winds reversed
from 23 m s- 1 northeasterlies to 24 m s - 1 southwesterlies. Subsequently southwesterlies
of 29 m s-1 were recorded. Another ship downstream at 48°S 99°W recorded 25 m s-1
northeasterlies and 959.8 mb at 1800 on the 19th but the wind backed through northwest
68
90N
60N
30N
0
305
60S
905
90N
bON
30N
0
30S
b60
90S
1000 MB ZNMC (M) 20 JUN 85 OZ
180 150W 120W 90W 60b 30W 0 30E bOE 90E 120E 150E 180
180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180
1000 MB ZECMWF (M) 20 JUN 85 OZ
180 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180
)UN
)ON60N
30N
30S
bOS
90S
90N
bON
30N
0
30S
60S
90S180 150W 120u 90W b6W 30W 0 30E 60E 90E 120E 150E 180
Fig. 48. 1000 mb analyses at 0000 GMT 20 June 1985 from NMC (top), and ECMWF(bottom). The contour interval is 60 m and negative values are dashed.
69
1000 MB ZNMC-ECMWF (M) 20 JUN 85 OZ
180 150W 120W qoW 6bO 30W 0 30E 60E 90E 120E 150E 180
90N
6ON
30N
0
305
60S
90S1
300 MB ZNMC-ECMWF (M) 20 JUN 85 OZ
180 150W 120W 90W W 30W 3 30E 60E 90E 120E 150E 180............... ,,., ,...
Fig. 49. Differences in analyses ZNMC - ECMWF at 0000 GMT 20 June 1985 at 1000mb (top), and 300 mb (bottom). The contour interval is 60 m at 1000 mband 100 m at 300 mb and negative values are dashed.
70
I
I
I
I
Table 1. Values of 1000 mb geopotential height 1) from NMC analyses; 2) from
ECMWF analyses; 3) from observations for the period 19-21 June 1985. Given are
values estimated for 45°S 1100W, the depth of the low center, and the maximum
differences between the NMC and ECMWF analyses in the area.
19 June 20 June 21 June
0000 1200 0000 1200 0000
45°S 110°WNMC 30 20 -10 00 15
EC -85 -175 -173 -122 -120
'observed' -100 -370 -200 -152 -120
Low CenterNMC x x -135 -223 -253
EC -125 -241 -361 -303 -257
'observed' -200 -400 -400 -380 -360
Max. Difference
NMC-EC 224 248 310 171 158
71
and then to southwest a day later with lowest pressures recorded of 954.3 mb. Pressure
tendencies generally exceeded 6 mb/3 hours for up to 12 hours both prior to and following
the disturbance.
A summary of the deduced 1000 mb heights at 450S 1100 W and in the low center is
given in Table 1 along with the values interpolated from the NMC and ECMWF analyses.
In spite of the abundance of observations, NMC failed to capture the system at all on the
19th and 20th. ECMWF generally placed the low very realistically but underestimated its
strength. Neither analysis was very good at 1200 GMT on the 19th when errors of -400 m
occurred in the NMC analysis and -160 m in the ECMWF analysis.
4.2 The South Atlantic Low
In the South Atlantic the low centers in Fig. 48 are near 40°S 10°W in both
analyses, near Gough Island. Also, two buoys were in the area and should have influenced
the analyses. In this case the NMC system is more intense by 191 m. At Gough Island
(40.35°S, 9.90 W) the sea level pressure at 0000 GMT 20 June was 1005.1 mb with a
southeasterly wind of 17 m s - 1. The NMC analysis is too low at Gough by -100 m and
there are no observations that indicate a center lower than -80 m. The ECMWF analysis
fits the available observations quite well.
4.3 The Antarctic Peninsula
A similar pattern emerges over the Antarctic Peninsula. Differences between ECMWF
and NMC were large throughout, exceeding +264 m on the 19th and -289 m on the 21st.
There were abundant observations available, not only on the 20th at 0000 GMT but at all
synoptic times for the days before and after. At 0000 GMT on the 20th between 44 and
70°W, 50-70°S there were 25 synoptic observations from surface stations plus some buoy
observations. The observations fit the ECMWF analyses reasonably well throughout the
19-21 June period but the NMC analyses are seriously in error.
72
5. Discussion and conclusions
We have presented mean and RMS differences between the NMC and ECMWF
global analyses of a number of fields. Maps, zonal mean meridional cross sections, and
latitude time sections of the differences were shown, along with a case study for a few days
in June 1985.
It is pleasing to see the fairly widespread agreement between the analyses from
the two centers over the NH extratropics. Of course, there are local differences in detail
which result in discrepancies in z of -50 m at 1000 mb and -80.6 m at 300 mb in the
north Pacific in Fig. 49, for instance. However, in general it seems that the quality of
the analyses, as revealed by their differences, is much less in the tropics and SH. This is
reflected in much greater differences in wind fields south of 200N, with RMS differences
often exceeding 5 m s- 1 above -500 mb in this region. It is further revealed in the much
greater differences in geopotential height south of n^30°S. In part, this is due to the poorer
traditional observational synoptic network in the tropics and SH, but it also appears, at
least in part, to be due to the traditional emphasis and experience gained by the centers in
dealing with the more familiar NH circulation so that procedures may have been somewhat
tuned to produce this result. Other problems are revealed by both the mean and RMS
differences over and around Antarctica. Below we comment more specifically on several
aspects.
5.1 Divergent wind
In the tropics, a major problem is related to the intense diabatic heating that
occurs with latent heat release in organized tropical convection over large areas and the
associated vertical motions and divergent winds. We showed that the zonal mean analyzed
divergent wind fields have changed enormously over the seven years from 1980 to 1986,
always increasing in intensity as NNMI procedures have been developed further to include
diabatic heating effects. It is therefore important to recognize that there have been major
discontinuities with time, more so at ECMWF, and that there is still great uncertainty as
73
to the true values. We have used [v] as a measure of the Hadley cell and differences in its
strength in the two analyses have typically been 30 to 100% of the total. This will result
in major uncertainties in any diagnostic analysis for which the divergent wind or vertical
motion fields are important. Generally, these fields appear to have become more realistic
with time (Trenberth and Olson, 1988b), in spite of sometimes larger differences, and thus
the most recent analyzed values are probably best.
5.2 Relative humidity
The RH field is one that has generally received little attention. Regardless of
the values analyzed, the NWP models typically adjust very quickly to make the model
RH values compatible with the large scale dynamics of the synoptic situation and the
cumulus parameterization scheme. Thus the analyzed fields have not been very important
for large-scale long-term forecasting. In addition, the inherent small scales in precipitation
and RH fields means that observations may not be very representative of the larger scales
and thus are given less weight in the analysis. Consequently, especially in the tropics,
it seems that the RH analyses have been dominated by the information coming from
the first guess in the NWP model rather than from the observations. The evidence for
this comes from the huge changes in RH that have occurred when changes have been
made in the model but without any change in the observations available or in the analysis
procedure. The resulting discontinuities are especially pronounced in the ECMWF RH
analyses. Even after somewhat similar changes have been made in the models at both
centers, local differences in monthly mean analyzed RH exceed 40%, so it appears that the
moisture fields are very poorly known. Although there have been some recent reductions
in the zonal mean differences in RH, there is little comfort in this fact in view of the above
discussion and it is evident that the moisture field is not known to a satisfactory degree
for almost any purpose. These problems therefore impact directly on attempts to assess
the atmospheric energetics and hydrological cycle. The main hope lies in increased use
of satellite data on the atmospheric moisture content. For instance, at ECMWF, satellite
precipitable water observations have been used in the analyses since March 1986.
74
5.3 Surface fields
Over the oceans, the surface atmospheric fields are important because of their im-
plications with regard to surface fluxes and thus ocean driving. Differences in P in the
tropics have typically been about as large as the expected signal associated with interan-
nual variability, both in a mean and RMS sense. Uncertainties in the mean westerly wind
component have been -20% of the total, and this would be reflected in uncertainties in the
latent heat flux of as much as 40 W m-2. Over the extratropical oceans, RMS differences
in wind speed exceed -5 m s-1 over the NH oceans from 40 to 50°N in winter and south of
30°S over all the southern oceans, thereby resulting in uncertainties of -25% in the surface
wind stress. A goal of TOGA and WOCE is to markedly reduce these uncertainties and
it is pleasing to see that analysis differences have been reduced during 1986.
Sea level pressures and 1000 mb heights must often be determined using artificial
information to extrapolate below the surface of the earth. It is clear that the procedures for
doing this are quite different at both centers with the result that there are major differences
in the fields where there is orography. This emphasizes the need for an accurate surface
pressure field so that the region below ground can be recognized and the information
screened. Generally such a surface pressure field is not available and the surface pressures
that are archived are of no use. This is because the NWP model surface does not correspond
to the real surface. Quite aside from resolution problems is the fact that the model surface
is usually represented nowadays in spectral space, so that there are spurious ripples in
the surface field. In addition, both centers use an enhanced orography ("envelope" or
"silhouette") as a means of compensating for the fact that the free atmospheric circulation
generally is not influenced by the air that exists in valleys within rough topography. The
envelope orography produces improved dynamical effects in the model but at the expense
of degraded physical effects. We suggest that all models, or post processed representations
of the models, should include a true representation (not spectral, not envelope) of the
orography of the surface of the earth and that this should be a standard archival level for
all data sets.
75
5.4 The Southern Hemisphere
Results have clearly shown, at least prior to May 1986, that the analyses at NMC
have been seriously in error over and near Antarctica. The quality of the analyses south
of ~30°S has also left much to be desired. Sea level pressures and 1000 mb heights were
analyzed to be too high in the vicinity of the circumpolar trough at NMC prior to 1982
relative to both Australian and ECMWF analyses. Moreover, the case study for June
1985 shows that the analyses took little notice of the available observations in this region.
There has always been a serious problem over the SH with the sparsity of observations,
but the implication from this research is that the first order problem has been the inability
to properly assimilate the observations that have been available. Changes at NMC in May
1986 have at least partly corrected the situation over Antarctica, but it remains to be seen
whether the available data are being fully utilized.
RMS differences between NMC and ECMWF continue to be large over the southern
oceans even though, since 1985 as part of the TOGA Program, a drifting buoy program
has been in place to help fill the huge data gaps. Together with the satellite soundings,
these should enable a reasonable analysis to be produced. While the ECMWF analyses
are to be preferred in the SH, the case study also shows that they too are not perfect and
they tend to be quite conservative. The evidence suggests that there is too much weight
placed upon the first guess field from the NWP model at both centers.
The reason for this, of course, is the problems in dealing with isolated station data
from ships and buoys. One problem is that the data are single level data and therefore
hard to properly assimilate. But a major problem is with quality control and the need to
catch the not uncommon errors and biases present in the data. Because of the isolation
of many ship and buoy platforms, it is often not possible to check the data by comparing
values with nearby observations since there is insufficient redundancy. Instead, alternative
kinds of checks must be made. Foremost among these is the need for time series checks on
the position of the platform and the observation sequence. It is also necessary to recognize
that NWP model forecast accuracy is not as good over the SH as in the NH and rapid
deepening rates and very intense low pressure systems are common over the southern
76
oceans (e.g. see Guymer and Le Marshall, 1981; Trenberth and van Loon, 1981).
Isolated observations, by their very nature, are more difficult to handle in any data
assimilation system, yet if correct their value is much greater. The challenge is to ensure
that these data are received, properly checked, and assimilated.
5.5 Heat and momentum fluxes
It was pleasing to see that the differences in the poleward heat fluxes near 700 mb
from the two centers were relatively small. However, larger differences were apparent at
the surface and in the upper troposphere and lower stratosphere, especially south of 30°S.
Stronger poleward heat fluxes in the SH lower stratosphere in the ECMWF analyses, even
in recent months such as October 1986, imply that the dynamics entering into the problem
associated with the depletion of ozone in the SH spring, the "ozone hole", are not well
known. Similarly, uncertainties in the poleward momentum fluxes are also largest in the
SH upper troposphere and are too large to allow definitive studies of wave dynamics or
the momentum budget.
5.6 Concluding remarks
All of the NMC and ECMWF fields are available from the Data Support Section
of NCAR. In addition, all of the ECMWF fields and summary statistics are available on a
Gaussian grid on-line at NCAR via the Community Climate Model (CCM) processor, as
described by Trenberth and Olson (1988b).
There is fairly good agreement between the analyses over the extratropics of the
NH so that some variables, including the rotational wind components, geopotential height
and temperature, can be considered to be reasonably well known there. Other variables,
including the divergent wind, vertical motion, and humidity, are not as well known any-
where. In the tropics we found substantial disagreements in the total wind and all variables
are most uncertain in the SH.
Interannual variability and climate change can only be explored to a limited extent
with these data sets and only if proper account is taken of the impacts of changes in analysis
77
procedures. Consequently a strong case can be made for a reanalysis of the original data,
preferably enhanced with further observations (either delayed or non-real time) that did
not arrive before the operationally imposed cut-off, using the same state of the art analysis
system.
In any case, there is a need to clearly document all changes in the analysis system
that have an impact on the analyses, and thus on the implied climate record. This is now
mostly done at both centers. There is also a need to document the impact of these changes
on the analyses, and this aspect could be improved.
The main approach taken in this note can only show how different the analyses are
and it does not reveal their overall fidelity. Nevertheless, the global analyses are very useful
for many purposes, especially if account is taken of the information in this report and in
Trenberth and Olson (1988a and 1988b) which provide a measure of their uncertainty.
78
References
Bonner, W. D., G. H. White, M. S. Tracton, and V. E. Kousky, 1986: Global analysis
and prediction at NMC Washington. Proc. of Second International Conference on
Southern Hemisphere Meteorology, Wellington, New Zealand 1-5 Dec. 1986. Amer.
Meteor. Soc., Boston, MA, pp. 1-9.
Guymer, L. B., and J. F. Le Marshall, 1981: Impact of FGGE buoy data on Southern
Hemisphere analyses. Bull. Amer. Meteor. Soc., 62, 38-47.
Harrison, D. E., W. S. Kessler and B. S. Giese, 1988: Ocean circulation model hindcasts
of the 1982-83 El Ninio: Thermal variability along the ship of opportunity tracks.
J. Phys. Oceanogr., submitted.
Kidson, J. W., and K. E. Trenberth, 1988: Effects of missing data on estimates of monthly
mean general circulation statistics. J. Climate, submitted.
Lau, N-C., 1984: A comparison of circulation statistics based on FGGE Level III-b analyses
produced by GFDL and ECMWF for the special observing periods. NOAA Data
Rep. ERL GFDL-6, 237 pp.
1985: Publication of circulation statistics based on FGGE Level III-b analyses pro-
duced by GFDL and ECMWF. Bull. Amer. Meteor. Soc., 66, 1293-1301.
Leetmaa, A., 1987: Progress towards an operational ocean model of the tropical Pacific at
NMC/CAC. In "Further progress in Equatorial Oceanography", E. J. Katz and J.
M. Witte Editors, Nova University Press. 439-450.
Trenberth, K. E., and J. R. Christy, 1985: Global fluctuations in the distribution of atmo-
spheric mass. J. Geophys. Res., 90, 8042-8052.
1987a: The zonal mean westerlies over the Southern Hemisphere. Mon. Wea. Rev.,
115, 1528-1533.
_ 1987b: The role of eddies in maintaining the westerlies in the Southern Hemisphere
winter. J. Atmos. Sci., 44, 1498-1508.
_ , J. R. Christy and J. G. Olson, 1987: Global atmospheric mass, surface pressure and
water vapor variations. J. Geophys. Res., 92, 14815-14826.
79
and J. G. Olson, 1988a: Evaluation of NMC global analyses. NCAR Tech. Note
NCAR/TN-299+STR. 82 pp.
and , 1988b: ECMWF Global Analyses 1979-1986: Circulation statistics and
data evaluation. NCAR Tech. Note NCAR/TN-300+STR. 94pp plus 12 fiche.
,and H. van Loon, 1981: Comment on "Impact of FGGE Buoy data on Southern
Hemisphere analyses". Bull. Amer. Meteor. Soc., 62, 1486-1489.
80
Appendix IAcronyms
CCM Community Climate Model
ECMWF European Centre for Medium Range Weather Forecasts
E-P Eliassen-PalmFGGE First GARP Global Experiment
GARP Global Atmospheric Research Program
GDAS Global Data Assimilation System
GFDL Geophysical Fluid Dynamics Laboratory
NCAR National Center for Atmospheric Research
NH Northern HemisphereNMC National Meteorological CenterNNMI Nonlinear Normal Mode Initialization
NOAA National Oceanic and Atmospheric Administration
NWP Numerical Weather PredictionPAOBs Pseudo Australian OBservations of SLP
RH Relative HumidityRMS Root Mean SquareSLP Sea level pressure
SH Southern HemisphereTOGA Tropical Oceans Global Atmosphere
WOCE World Ocean Circulation Experiment
81
Recommended