Intercomparison of ERA-40, ERA-15, and NCEP-NCAR Reanalyses Diagnosed Diabatic Heating

STEVEN C. CHAN, AND SUMANT NIGAM

Department of Atmospheric and Oceanic Science,

University of Maryland,

College Park, Maryland

----------------------------- Corresponding author address: Steven C. Chan, Department of Atmospheric and Oceanic Science, University of Maryland, College Park, Maryland 20742 E-mail: scchan@atmos.umd.edu Keywords: Reanalysis, Heating, Data Assimilation, Climate

ABSTRACT

Diabatic heating has been diagnosed from the newly available European Centre

for Medium-Range Forecasting (ECMWF) ERA-40 reanalysis. ERA-40 heating is then

compared with ECMWF ERA-15 and U.S. National Center for Environmental

Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalyses heating.

ERA-40 tropical heating is found to be stronger in the tropics (especially in the summer)

than both ERA-15 and NCEP-NCAR reanalyses, which is consistent with the stronger

Hadley Circulation that is found in ERA-40. All three reanalyses have similar Asian

monsoon and winter storm track heating. Both ERA-15 and ERA-40 reanalyses show a

significantly stronger ITCZ than NCEP-NCAR Reanalysis in the East Pacific and the

Atlantic, and this is more consistent with CMAP latent heating estimates. Over Africa,

there is large disagreement between all three reanalyses and observational estimates of

diabatic heating.

1. Introduction

Atmospheric diabatic heating has long been known to play a key role in the large-

scale atmospheric general circulation. A significant fraction of the atmospheric diabatic

heating is caused by water condensation (latent heating) in the tropics, mid-latitude storm

tracks, and regional/seasonal convergence zones like the boreal summer Indian monsoon

and the South Pacific Convergence Zone (SPCZ). The largest latent heating rates are

observed in the tropics, where moisture is abundant in the lower troposphere. Tropical

Rainfall Measurement Mission (TRMM)-estimated mid-tropospheric tropical Pacific

latent heating is up to 10ºC/day (Tao et al. 2003).

In the deep tropics, latent heating is mostly balanced by adiabatic cooling from

large-scale ascent. This is because, the tropics has weak horizontal temperature gradients,

and temperature advection is not sufficient to adjust temperature perturbations (Hoskins

1996). In the extra-tropics, horizontal temperature advection becomes dominant in

adjusting to temperature perturbations. This divides the tropics and extra-tropics into two

different thermodynamic regimes.

Other important sources of diabatic heating include time-mean and transient

horizontal and vertical heat flux convergences. Sensible heat flux in the lower

troposphere and the absorption of radiation (radiative flux convergences) also contribute

to heating, and the Sun is the fundamental source of the heating. Latent heating in clouds

is nothing more than evaporation of wet surfaces into water vapor and subsequent

condensation and latent heat release in the atmosphere. On the planetary scale, mean and

transient heat flux convergences redistribute solar energy from the warmer tropics to the

cooler poles.

Long wave radiation cools down the atmosphere. Globally, on time scales shorter

than a few decades, annual-mean global long wave cooling is in close radiative energy

balance with the annual-mean global incoming solar short wave heating.

The above discussion concentrates in the horizontal distribution of diabatic

heating, but the vertical structure of diabatic heating is just as important. The average

vertical diabatic heating profile is the large-scale manifestation of mesoscale cloud

processes. Nimbostratus and stratus have latent heating that are concentrated in the lower

troposphere, but deep convective clouds often have latent heating maximums in the

middle and upper troposphere. Clouds also have high albedo to visible light, and are

optically thick to infrared red radiation. Mapes and Houze (1993), and Houze (1997)

show convective clouds of different ages have very different diabatic heating vertical

profiles. Schumacher et al. (2003) shows the large-scale responses to tropical latent

diabatic heating are sensitive to the vertical structure of the diabatic heating.

Both divergent flow and diabatic heating are difficult to quantify as there are no

direct ways to measure them. Rotational flow is usually well-constrained in reanalyses

and analyses due to its large horizontal scale and its independence from diabatic heating

(Sardeshmukh 1993). However, divergence is often a small difference between

meridional and zonal wind gradients, and is strongly coupled with the diabatic heating

field. Errors in estimating either diabatic heating or divergence often lead to large errors

in the other. Geostrophic wind balance indicates tropical flow is driven mainly by

divergent circulation due to the low Coriolis parameter. The tropics are also more poorly

observed. Therefore, the quantification of tropical diabatic heating and divergent

circulation is naturally a difficult problem.

Reanalyses and analyses, which are widely used to represent the climatological or

daily/hourly dynamical state of the atmosphere, have questionable divergent circulation,

vertical motion, and diabatic heating fields. As divergent flow itself is hard to separate

from diabatic heating itself, divergent flow and vertical motion fields depend on model

diabatic heating parameterization (like cumulus parameterization) and data assimilation

schemes. Such parameterizations are often simplistic and questionable (Sardeshmukh

1993). Vertical interpolation during post-processing is also known to introduce errors to

diabatic heating fields; this is because the interpolation introduces errors to vertical

temperature advection (Hoerling and Sanford 1993). Obviously, different analyses and

reanalyses use different diabatic heating parameterizations and data assimilation schemes.

There are potentially “large” differences in the 3-D divergent circulations, vertical motion

fields, and diabatic heating in between different reanalyses and analyses. Considering

how widely used these reanalyses and analyses are in current atmospheric science

research and operations, there are strong motivations to quantify these differences.

They are no direct ways to measure diabatic heating, and the validation of

diabatic heating is another difficult problem. Diabatic heating comparisons are often

made among diabatic heating fields from different reanalyses and post-processed in-

situ/remote-sensing data. The latter is sensitive to sampling (e.g. the degree of

representation of the time and location of the in-situ data is taken) and algorithm (e.g. the

inversion of satellite radiances into diabatic heating rates) issues. Precipitation and

outgoing long wave radiation (OLR) offer hope in reasonably estimating latent heating

horizontal distribution and rates in the tropics, because of the close relationship between

vertical motion, cloud development, and diabatic heating.

There are many climate research applications for an “accurate” diabatic heating

dataset. Planetary standing waves, which are often excited by tropical divergences and

diabatic heating in the vortex stretching term of the vorticity equation, play a key role in

determining both the mean state and variability of the atmosphere (Webster 1972). The

El Niño-Southern Oscillation (ENSO) is marked with significant anomalies in tropical

Pacific diabatic heating from the climatological state. With the excitation of

teleconnection patterns from the tropics, ENSO has profound climatological impact

globally. Subtropical highs and deserts have been linked to boreal summer monsoon-

forced diabatic heating and divergences (Hoskins 1996, Hoskins and Rodwell 1996). A

good understanding of both the climatology and variability of the diabatic heating and

divergent circulation is necessary for climate prediction, dynamics, and variability

(Nigam 1983, Ting 1994).

The European Centre for Medium-Range Forecasting (ECMWF) has recently

released the ERA-40 reanalysis (Simmons and Gibbons 2000), and this reanalysis is

made available to US researchers thru the National Center for Atmospheric Research

(NCAR). In this paper, the ERA-40 diabatic heating diagnosis is presented, and is inter-

compared with the ERA-15 (Gibson et al. 1999) and the National Centers for

Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR)

Reanalysis (Kalnay et al. 1996) diabatic heating diagnoses. ERA-40, being released later

than NCEP-NCAR and ERA-15 reanalyses, has more modern data assimilation and NWP

system. It is hoped that ERA-40 represents the state-of-art dynamical representation of

the atmosphere, and the diabatic heating diagnoses and divergent circulations of ERA-40

are hoped to be the most accurate to date.

A discussion of the reanalyses and observations that are used in this study is

presented in Section 2. The heating diagnosis methodology is discussed in Section 3.

The intercomparison between different reanalyses is in Chapter 4. Diabatic heating

validation is discussed in Section 5.

2. Data

a. ERA-40

The ERA-40 is ECMWF’s latest 6-hourly global atmosphere reanalysis. Diabatic

heating diagnoses are completed for all the years that are available from NCAR (1957 -

2002) when this paper is written. The monthly-mean ERA-40 diagnosed diabatic heating

and reanalysis are archived in a 2.5º longitude x 2.5º latitude horizontal grid, and have 23

vertical isobaric levels from 1000-hPa to 1-hPa. Between the surface to the 150-hPa

isobar (the approximate tropical tropopause location), there are a total of 12 isobaric

levels.

b. ERA-15

ECMWF ERA-15 is the forerunner of ERA-40. The archived 5.0º longitude x

2.5º latitude global monthly-mean ERA-15 reanalysis and diagnosed diabatic heating

have 17 isobaric levels from 1000-hPa to 10-hPa. Due to the difference in horizontal

resolution, ERA-40 is re-gridded to a 5.0º longitude x 2.5º latitude grid for comparison.

The archived ERA-15 data share the same isobars with ERA-40 up to 150-hPa isobar.

Only diabatic heating diagnoses below the 150-hPa isobar are discussed in this paper.

The reanalysis and diabatic heating data span 15 years from 1979 to 1993, and these 15

years are chosen to be the base years for the climatology. There are certain known ERA-

15 issues including artificial trends that are caused by assimilation of satellite data, and

these issues are corrected in ERA-40 (Simmons and Gibbons 2000, Trenberth et al. 2001).

c. NCEP-NCAR Reanalysis

The achieved 1949-2002 global 5.0º longitude x 2.5º latitude monthly-mean

NCEP-NCAR Reanalysis and diagnosed diabatic heating have 17 vertical isobaric levels

from 1000-hPa to 10-hPa. Since ERA-15 and ERA-40 reanalyses have the 775-hPa

isobar that the NCEP-NCAR Reanalysis does not have, the 775-hPa isobar is vertically

interpolated for the NCEP-NCAR Reanalysis for inter-comparison purposes. Diabatic

heating fields are available from the original reanalysis. For the sake of consistency,

diabatic heating diagnoses are carried out for the NCEP-NCAR Reanalysis, and the

diagnoses are used instead of the original diabatic heating fields. The original reanalysis

diabatic heating is used to validate the accuracy of the diagnoses.

d. Validation Datasets

The NOAA Climate Prediction Center (CPC) Merged Analysis of Precipitation

Version II (CMAP) (Xie and Arkin 1996) and the NOAA-Cooperative Institute for

Research in Environmental Science (CIRES) Climate Diagnostic Center (CDC)

Interpolated OLR data (NOAA-CIRES CDC 2005) are used to validate the diabatic

heating diagnoses. The archived global monthly-mean CMAP and OLR data have a

horizontal resolution of 5.0º longitude x 2.5º latitude.

3. Methodology of Heating Diagnoses

The ERA-15 and NCEP-NCAR diabatic heating diagnoses are carried out in the

methods that are described in Nigam (1994). There is a slight change to the

methodologies for ERA-40 diabatic heating diagnoses, and the changes are described in

the Appendix. The changes only affect the diabatic heating diagnoses near the poles. A

brief description of the methods of the diabatic heating diagnoses is presented here.

Diabatic heating is diagnosed as a residue quantity, and the governing equation of

the residue diabatic heating diagnosis is:

( ) ( )

∂∂

+⋅∇

+

∂∂

+∇⋅+

∆∆

=•

ppp

pppT

tTtPyxQ ''''vv,,,

00

θωθθωαα

(1)

(a) (b) (c) (d) (e)

( )tPyxQ ,,,•

(on the LHS) is the diagnosed monthly diabatic heating rate (in

temperature per unit time) at a specific spatial grid point on a specific isobar, vector v is

horizontal wind vector, ω is vertical pressure speed, θ is potential temperature, T is

temperature, p is pressure, and 0p is a constant set to 1000-hPa. Over-bars indicate

monthly-means, and primes indicate sub-monthly departures from the monthly-mean. No

time filtering is used in calculated primed variables. Primed variables include both

synoptic transients and “lower-frequency variabilities” like the Madden-Julian Oscillation

or the Pacific-North American Pattern. Beginning from the first term on RHS of the

equation (1), they are: (a) the local time rate of change of temperature that is estimated

from the first and last analysis time (00Z 1st day of the month, and 18Z last day of the

month, respectively) of the month; (b) the horizontal monthly-mean temperature

advection by the monthly-mean horizontal winds; (c) the vertical monthly-mean

temperature advection by the monthly-mean ω; and the eddy (d) horizontal and (e)

vertical heat flux divergences due to sub-monthly variability.

As discussed already, ω and divergences in reanalyses must be treated with

suspicion. The original ERA-40 ω is used to diagnose the diabatic heating. Nigam (1994)

has re-estimated ECMWF un-initialized analysis ω for diabatic heating diagnosis, but the

resultant diagnosed heating fields do not change significantly.

In order to carry out the diabatic heating diagnosis properly, 6-hourly data are

used. The resultant diagnosed heating fields are stored as a monthly mean. Apart from

diabatic heating, monthly eddy heat and momentum covariances (i.e. '','' Tuvu ) are

diagnosed along with wind, temperature, and geopotential variances (i.e. 22 ',' zu ).

4. Intercomparison of Diabatic Heating Diagnoses

a. Global distribution of heating

1) Zonal-mean Heating

Zonal-mean diabatic heating rates not only represent the most basic description of

the global distribution of diabatic heating, it also gives an integrated view of the mean-

meridional circulation and precipitating latitudes. Shown in the two panels in Figure 1

are the January and July 1979-1993 climatological zonal-mean surface-to-125-hPa mass-

weighted vertically-averaged diagnosed diabatic heating rates of the three reanalyses.

For both months, one sees distinct tropical heating maximums between 10ºS-5ºN

(January) and 5ºN-15ºN (July). Those two maximums represent the well-known Inter-

Tropical Convergence Zone (ITCZ) – the ascending branch of the Hadley Circulation.

All three reanalyses capture the broader and double diabatic heating maximum during

January (0.5-1.25 K/day) and the sharper and stronger diabatic heating maximum during

July (1.3-2.5 K/day). All three reanalyses show a region of diabatic cooling in the winter

subtropics (10º-35ºN/S). This is the descending branch of the Hadley Circulation.

Poleward into the winter mid-latitudes, diabatic heating rates begin to increase again.

This is the mid-latitude storm track. Compared to the tropics, the zonal-mean diabatic

heating rates over the mid-latitude stormtracks are much more moderate.

ERA-40 has stronger ITCZ diabatic heating and subtropical diabatic cooling than

the two other reanalyses. For January, ERA-40 ITCZ diabatic heating rates are about

0.25-0.5 K/day more than ERA-15 and NCEP-NCAR reanalyses. Tropical heating

differences between ERA-40 and the two other reanalyses are even larger in July. For

July, ERA-40 ITCZ heating is about 1.2 K/day (about double) more than both ERA-15

and NCEP-NCAR reanalyses. Consistent with stronger ITCZ diabatic heating,

subtropical ERA-40 diabatic cooling is about 0.25 K/day and 0.2K/day stronger in

January and July respectively. It will be shown later that ERA-40 has a stronger

meridional Hadley Circulation than the two other reanalyses, which is consistent with

stronger tropical diabatic heating and subtropical diabatic cooling. The zonal-mean

ERA-15 tropical diabatic heating and subtropical diabatic cooling rates for both January

and July are stronger than the NCEP-NCAR Reanalysis, but the differences between

ERA-15 and NCEP-NCAR are much less than the difference with ERA-40.

In the northern extra-tropics (poleward from 30ºN) during boreal winter, ERA-40

diabatic heating rates are much closer to the two other reanalyses. ERA-15 shows more

diabatic cooling in the midlatitude stormtracks (35º-50º N/S). ERA-40 and NCEP-

NCAR Reanalysis are in good consensus with each other in those latitudes. Due to the

lack of observations, the quality of all three reanalyses in the Southern Hemisphere extra-

tropics is uncertain. During July (austral winter) when the Southern Hemisphere

stormtracks are strongest, ERA-40 has more diabatic heating from 35ºS to 55ºS. The

difference is quite moderate (less than 0.2 K/day) when compared to the differences in

the tropics.

2) Regional Distribution

Clearly, diabatic heating is not distributed in a zonally uniform manner. Due to the

orography and land-sea contrast, standing waves (stormtracks, monsoons, and the Walker

Circulation) organize diabatic heating and cooling to only certain parts of the globe.

Figure 2 and 3 are the January and July 1979-1993 climatological surface-125-hPa global

vertically-integrated diabatic heating rate for the three reanalyses. The diabatic heating

rate differences between the three reanalyses for the same two months are shown in

Figure 4 and 5 respectively.

For January, all three reanalyses show diabatic heating over Indonesia, Amazon-La

Plata Basin, Northwest Pacific and Atlantic (winter storm tracks), equatorial oceans

(ITCZ), Southwest Pacific (SPCZ), and southern Africa. Diabatic cooling can be seen

over continental Eurasia, North Subtropical and Southeast Pacific and Atlantic, and the

Sahara.

For July, all three reanalyses are also in close agreement in the areas with diabatic

heating and cooling. Diabatic heating is observed in equatorial oceans (ITCZ), Pacific

Warm Pool, South and East Asia (Asian Summer Monsoon), Southwest Pacific (SPCZ),

Central America (Central-North American Monsoon), the African Sahel, and Southeast

United States. Diabatic cooling is observed in the extra-tropical East Atlantic and Pacific,

south Indian Ocean, and over Australia.

In January, ERA-40 has stronger diabatic heating over Indonesia than both ERA-15

(up to 1.75 K/day) and NCEP-NCAR (up to 2.25 K/day) reanalyses. The difference

between ERA-15 and NCEP-NCAR reanalyses over Indonesia has no clear spatial

structure. ERA-40 and NCEP-NCAR reanalyses show greater (storm track) diabatic

heating (by about 0.25-0.75 K/day) than ERA-15 over Japan and Eastern US and Canada

Significant differences of the magnitude of July diabatic heating between the two

ERA reanalyses and NCEP-NCAR Reanalysis are seen in the tropical East Pacific and

Atlantic, and the African Sahel. In those regions, both ERA reanalyses show a much

stronger diabatic heating than the NCEP-NCAR Reanalysis. ERA-15 diabatic heating for

those areas is weaker than ERA-40, but is still stronger than NCEP-NCAR Reanalysis.

ERA-40 diabatic heating is up to 2.25K/day and 1.75 K/day stronger than NCEP-NCAR

Reanalysis in the tropical East Pacific and Atlantic respectively. There is also more

diabatic cooling poleward for both ERA reanalyses. The largest cooling differences are

found on the northwest / southwest of the East Pacific/Atlantic diabatic heating centers.

The largest diabatic heating differences between the reanalyses seem to all occur in

poorly observed “wet” regions (Africa and East Pacific). With the lack of observations,

divergent flow becomes more NWP model-dependent. Therefore, it should not be

surprising that diabatic heating differences are largest there. For the same reason, it

should hardly be surprising that there are fewer differences over and downstream of

major North Hemisphere land masses, which are well observed.

b. Divergent circulation and its relationship with heating

1) Meridional Divergent Circulation

It has been noted from other past studies (Nigam et al. 2000) that ERA-15

meridional divergent circulation is stronger than NCEP-NCAR Reanalysis. The analyses

from the past sections imply ERA-40 has an even stronger divergent circulation than

ERA-15, and this is found to be true when the divergent circulations of the three

reanalyses are inter-compared.

The divergent and rotational components of climatological winds are spectrally

decomposed. Zonal-mean meridional wind already represents the divergent meridional

flow. The calculated divergent flow is also used to inter-compare the Walker Circulation

between different reanalyses. Shown in Figure 6 and 7 are the climatological zonal-mean

meridional wind (v), pressure vertical velocity (ω), and the diagnosed diabatic heating for

January and July. The differences between the three reanalyses are shown in Figure 8

and 9 respectively.

The Hadley Cell is strongest in the winter hemisphere in all three reanalyses. The

latitudes of maximum ascent and descent are in good agreement among the three

reanalyses. Maximum ITCZ heating is found between 300- and 600-hPa. Same as

Figure 1, the Hadley ascending branch is wider in January than in July. The narrower

latitude band of tropical diabatic heating during July is consistent with the narrower

latitude vertical ascent latitude band.

ERA-40 has a stronger Hadley Cell for both seasons than the two other reanalyses.

ERA-15 Hadley Circulation is stronger than NCEP-NCAR Reanalysis Hadley

Circulation, but the difference between ERA-15 and NCEP-NCAR reanalyses are less

than with ERA-40. January ERA-40 and ERA-15 middle-troposphere ITCZ ascent are

about -1 and -0.5 Pa/sec stronger than NCEP-NCAR Reanalysis respectively. For July,

ERA-40 and ERA-15 middle-troposphere ITCZ ascent are about -2.5 and -1.2 Pa/sec

stronger.

The Hadley Circulation in both ERA reanalyses also appears to be shallower than

both NCEP-NCAR Reanalysis. In both January and July, both ERA reanalyses show

stronger winter poleward flow between 200- and 300-hPa isobars with opposite-direction

differences further aloft in the 150- and 100-hPa isobars. This indicates both ERA

reanalyses have a shallower upper-troposphere poleward flow than NCEP-NCAR

Reanalysis.

The wind vector differences in the Hadley Circulation are consistent with

diagnosed heating differences. ERA-40, which has stronger ascent, also has stronger

ITCZ heating; while NCEP-NCAR Reanalysis, which has weaker ascent, has also weaker

ITCZ heating. The opposite is true for the subtropical descent and cooling.

The extra-tropics (30º-60º N/S) are marked by the weaker Ferrel Cell. The flow is

close to quasi-geostrophic, and temperature perturbations are balanced mainly by

horizontal temperature advection, and not by vertical motion. During January, the Ferrel

Cell has maximum ascent poleward of 55ºN and 50ºS in the Northern and South

Hemisphere respectively. Below 400-hPa, diabatic heating to close-to-neutral conditions

occur in the Northern Hemisphere Ferrel Cell descending latitudes in the Northern

Hemisphere around 35º-50ºN. During July, the Ferrel Cell is poorly defined in the

Northern Hemisphere, but a well-defined meridional circulation pattern can be seen in the

Southern Hemisphere with weaker ascent poleward of 50ºS.

In the Northern Hemisphere extra-tropics, the diabatic heating differences

between the three reanalyses in the extra-tropics is more decoupled with vertical motion

as stronger ascent/descent is not necessarily linked with stronger diabatic heating/cooling.

In January, stronger ERA-15/40 reanalyses ascent between 50ºS-60ºS does seem to be

linked with stronger middle/lower-troposphere (500- to 700-hPa) warming for those

latitudes. In the Northern Hemisphere, ERA-40 has stronger lower troposphere (under

600-hPa) ascent between 35º-50ºN than the two other reanalyses, but no clear pattern can

be seen with the diabatic heating field except ERA-40 has stronger lower-troposphere

heating poleward of 45ºN. For July, most differences between the three reanalyses

appears in the lower troposphere below 600-hPa. Both ERA reanalyses have stronger

heating in the boundary layer in both hemispheres. For the Southern Hemisphere, there is

more cooling on top of that boundary layer heating from the equator to about 45ºS.

2) The Pacific Walker Circulation

Zonal-asymmetric circulations over the equator are closely related to different

climate regimes that are observed in the deep tropics (the warm West Pacific vs. the cold

East Pacific). In the Pacific, this zonal-asymmetry circulation forms the Walker

Circulation. The inter-annual air-sea coupled variability (ENSO) of the Walker

Circulation is one of the most important global modes of climate variability. Therefore, it

is of scientific interest to quantify the mean state of the Walker Circulation. Shown in

Figure 10 and 11 are the divergent Walker Circulation and diabatic heating cross-section

of the three reanalyses. The differences are shown in Figure 12 and 13. All three

reanalyses show maximum ascent west of the Dateline and west of 160ºW for January

and July respectively. The diabatic heating maximum is found in the middle troposphere

between 600- and 300-hPa isobars. Maximum descent in the East Pacific is between

120ºW-100ºW. In all three reanalyses, January diabatic cooling in the East Pacific show

a west-to-east slant in the lower troposphere and an east-to-west slant in the upper

troposphere. East Indian Ocean and Indonesian (west of 120ºE) diabatic heating and

ascent is weaker (in NCEP-NCAR Reanalysis) or absent (in the two ERA reanalyses) in

July when compared with January. During July, Indonesian diabatic heating and

ascending motion move poleward when the Asian boreal summer monsoon strengthens.

The January differences from the Dateline to 75ºW show little difference between

ERA-15 and ERA-40, but ERA-40 has stronger ascent west of the Dateline. Except west

of 120ºE and east of 75ºW, NCEP-NCAR Reanalysis has generally weaker equatorial

ascent across the Pacific. This is consistent with the weaker zonal-mean Hadley

Circulation in NCEP-NCAR Reanalysis that has been discussed earlier.

For July, NCEP-NCAR Reanalysis shows stronger middle/lower-troposphere

ascent between the Dateline and 120ºW, when compared with the two ERA reanalyses.

This is related to the eastward expanded ascending branch of the Walker Circulation in

NCEP-NCAR Reanalysis. Associated with the eastward expansion, NCEP-NCAR

Reanalysis has a slanted east-to-west lower troposphere diabatic heating structure

between 180º-165ºW. West of 120ºE, NCEP-NCAR Reanalysis also shows ascent and

diabatic heating, which are absent in both ERA reanalyses.

5. Validation of Diagnosis

a) Separation of Radiative and Latent Diabatic Heating

The most important heat source in the tropics is the release of latent heat from

deep convection. Yanai and Tomita (1998) calculated the vertically averaged and mean

vertical profile of Yanai Q1 and Q2 (Yanai et al. 1973), and argue “most” tropical diabatic

heating is driven by condensation in deep convection. With actual field measured

sounding, precipitation rates and radar imagery, Johnson and Ciesielski (2000) use the

residue method to find that radiation has a net cooling effect in deep convection. The

amount of radiative cooling ranges from 0.3-0.5 ºC/day, and daily variability of daily

cooling is highly sensitive to the amount of high clouds.

The residue method that is used in our diabatic heating diagnoses contains no

information about the actual physical process that leads to the residue diabatic heating or

cooling. In numerical models and reanalyses, diabatic heating components (radiative,

sensible, and latent) are separated. It is a more a question whether that information is

achieved during post-processing. However, the numerical model separation of different

diabatic heating components depends on model parameterizations. Although individual

heating components are not constrained under geophysical fluid dynamics, the total

heating is, and that is what the residue method can offer. The physical radiative,

boundary layer, and cloud processes that leads to diabatic heating does go hand-in-hand

with geophysical fluid dynamics.

NCEP-NCAR Reanalysis condensational, radiative and sensible diabatic heating

rates are used to estimate the contribution from radiative and latent heating. Shown in

Figure 14 is the NCEP-NCAR Reanalysis climatological July monthly-mean vertically-

averaged condensational heating and total diabatic heating rates. In the NCEP-NCAR

Reanalysis climate, condensational warming in the tropics sets the upper bound of total

heating. The net diabatic heating is less than the condensational diabatic heating. Total

diabatic heating over Indonesia and Central America is about 1 ºC/day cooler than latent

heating, which is in on the order of magnitude as in Johnson and Ciesielski (2000). This

shows radiation dominates the atmospheric cooling. In the Northeast and Southeast

Pacific and Atlantic where there is little vertically-integrated latent heating, radiation

leads to net cooling. Radiative cooling and the lack of condensational warming in those

regions mark them as Earth’s “cooling window.” It is the safe to argue that diagnosed

diabatic cooling regions are dominated by the radiation effects.

b) CMAP-Estimated Latent Heating Rates

NWP and GCM cloud parameterizations are often questionable. Despite the high

social and economic relevance of precipitation, it is the one of the most difficult variables

to forecast in NWP, and the GCM hydrological cycle is often of suspect. Extreme care

must be taken to interpret reanalysis NWP and GCM latent heating.

CMAP precipitation analyses offer a better way to estimate the tropical latent

diabatic heating. Precipitation is the product of rained-out condensation in the

atmosphere, and non-precipitating clouds do not cause net column diabatic warming to

the atmosphere. In the extra-tropics where horizontal temperature gradients are larger,

mean and transient temperature advection contributes significantly to diabatic heating.

Therefore, the discussion is focused in the lower latitudes (30ºN-30ºS) where latent

heating is the most dominant.

CMAP monthly mean daily precipitation rate R (in mm/day) is converted to

vertically-averaged equivalent latent heat release (in ºC/day) ecipt

TPr∂

∂ by:

DryAirp

onVaporizatiOHlOH

ecip cmmm

lgRtT

,

,)(

Pr *1000

***22

ρ≈

∂∂ (4)

)(2 lOHρ is the density of liquid water (~1000 kg m-3), g is the acceleration due to gravity

near the surface of Earth (~9.81 m s-2), onVaporizatiOHl ,2is the specific latent heat of

vaporization (~2.5X106 J kg-1), DryAirpc , is the specific heat of dry air (~1004 J K-1 kg-1).

The calculated climatological latent heat releases from precipitation for January and July

are shown in Figure 15.

1) January

For January, all three reanalyses are spatially coherent with CMAP-estimated

latent diabatic heating. The ITCZ in the Indian and Pacific Oceans is well captured in the

three reanalyses. In certain parts of the tropical East and Central Pacific, ERA-40

diabatic heating actually exceeds CMAP estimates. This implies ERA-40 January

tropical Pacific diabatic heating may actually to be too strong. There is a disagreement in

diabatic heating distribution between the ERA-15/40 and CMAP estimates over Africa.

CMAP estimates show maximum latent heating in Southeast Africa, while ERA

reanalyses have strongest latent heating in Southwest Africa near Angola and Cameroon.

2) July

July ERA-15/40 diabatic heating rates are more consistent with CMAP in the

tropical East Pacific and Atlantic. CMAP shows East Pacific latent heating extends from

the Latin American coast to around 150ºW, which is in better agreement with the ERA

reanalyses. CMAP July Atlantic heating extends westward off the coast of Sahel Africa,

which again is in better agreement with the ERA reanalyses.

c) Consistency with OLR fields

Outgoing longwave radiation (OLR) is an important signature of deep convection

in the tropics. However, it cannot be compared directly with diabatic heating, as OLR

cannot be casually converted to diabatic heating. OLR is more a proxy for cloud top

height and deep convective activities. However, it is reasonable to argue that areas with

more deep convection are likely to have more latent diabatic heating. Unlike CMAP – a

dataset in which different types of rain gauge and satellite data are assimilated together,

OLR is directly observed with remote sensing. Therefore, OLR is less subject to data

assimilation issues, and is best estimate to deep convection centers.

Only 30ºS-30ºN is used for this comparison, because of the close relationship

between diabatic heating and vertical motion/deep convection. Shown in the two panels

in Figure 16 are the January and July 1979-1999 climatological monthly-mean NOAA-

CPC OLR.

1) January

For January, most active convection is observed over Indonesia (over Java and

Borneo), the Amazon Basin (near the center of the continent), and south central Africa.

All three reanalyses agree well with a maximum of heating over Indonesia. Over South

America, all three reanalyses have heating centered more near the coast of northeast

Brazil, but OLR has deep convection more centered to the west. Over Africa, ERA-15

shows the best spatial correspondence with OLR, with maximum heating/convective

activity in south central Africa.

2) July

For July, OLR data indicate that maximum convection is over the Bay of Bengal,

Pacific Warm Pool, Central America, nearby tropical East Pacific, Equatorial Pacific and

Atlantic, and African Sahel. All three reanalyses have similar spatial structure in diabatic

heating over the Indian monsoon region with a maximum of diabatic heating over

Bangladesh and northern Bay of Bengal. The East Pacific is where considerable

differences are seen among the three reanalyses. Peak convective activity is seen near

Panama and the west coastal waters of Mexico. This is in agreement with the three

reanalyses, but not with CMAP. CMAP has no latent heating maximum over land near

Panama. Lower OLR values can be seen to extend westward from Central America to

about 120ºW. This agrees well with CMAP, ERA-15, and ERA-40, and further supports

that there is not enough East Pacific convective heating in the NCEP-NCAR Reanalysis.

6. Summary and Conclusions

It has been argued that ERA-15 shows stronger divergent circulation (and its

associated diabatic heating) when compared with NCEP-NCAR Reanalysis. Our analysis

indicates ERA-40 shows even stronger heating and divergent circulations. All three

reanalyses are in better consensus in extra-tropical diabatic heating than in the tropics.

The most noticeable differences between ERA-15/40 and NCEP-NCAR Reanalyses are

in the tropics. ERA-40 shows a much stronger Hadley Circulation than both ERA-15 and

NCEP-NCAR reanalyses. Regionally, ERA-40 ITCZ diabatic heating rates may even be

too strong when compared with observational estimates. The largest regional differences

in diabatic heating are seen over the tropical East Pacific, Atlantic, and Africa.

The agreement in diabatic heating between ERA-40 and ERA-15 is better with

NCEP-NCAR Reanalysis. This is hardly surprising as ERA-15/40 data assimilation and

NWP systems are more similar than with each other. Comparing the three reanalyses

with CMAP and OLR data, it does seem ERA-15/40 have better representation in tropical

East Pacific and Atlantic diabatic heating. With the considerable amount of disagreement

among observation- and reanalyses-estimated diabatic heating over Africa, it is hard to

quantify the quality of the diabatic heating diagnoses that area.

This paper only inter-compares and discusses the realism of the heating diagnosis.

The causes for the differences can only be speculated. As discussed in the introduction,

the divergent circulation and heating of the tropics are sensitive to small differences in

wind and the model physics that generates the heating. Wind field differences that are

introduced due to differences in data assimilation and NWP schemes are likely to be

largest in poorly observed areas. It should not be surprising that the largest differences

are observed over tropical East Pacific, Atlantic, and Africa, because it is simply being

poorly observed. Tropical East Pacific, Atlantic, and Africa cover from about 150ºW to

40ºE, and that is more than half of all tropical and equatorial longitudes. Clearly, a more

comprehensive global surface and upper air observation can significantly improve the

understanding of the dynamical and thermodyamical state of the atmosphere.

From the global climate change and dynamics perspective, it is important to know

the differences between different realizations of global climate. Clearly, one does not

want one’s conclusion to be a data artifact. It is hoped that ERA-40 represents the

current best estimate of the state of atmosphere. The word “best” is vague; the

quantification of the accuracy of reanalyses is as hard as constructing the reanalyses.

Acknowledgements. This research was sponsored by the NASA Earth System Science

Fellowship ESSF/04. Many thanks to colleagues Renu Joseph and Alfredo Ruiz-

Barradas for their input.

APPENDIX

Treatment of Vectors in Polar Regions

Vectors near the poles must be done carefully as meaning of the zonal and

meridional wind become questionable there. There are differences in the numerical

algorithms in between ERA-40 and NCEP-NCAR/ERA-15 reanalyses in how vectors

( ''θv ) at the pole are interpolated. In the NCEP-NCAR and ERA-15 reanalyses all

vector quantities are forced to be to zero. In other words, no heat flux is allowed at the

pole. However in ERA-40, only the x component of the vector is forced to be zero. The

y component of the vector is set equal to the wavenumber #1 component at the first off-

polar regular grid.

For all three reanalyses, the polar correction only applies to the heat flux itself.

The original winds of the reanalyses are not modified.

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CAPTIONS

FIG 1. Shown above are the January (upper panel) and July (lower panel) 1979-1993

climatological zonal mean surface-to-125hPa mass-weighted vertical average diabatic

heating rates for the ERA-40, ERA-15, and NCEP-NCAR reanalyses. Units are in

ºC/day.

FIG 2. The above three panels are the January 1979-1993 climatological surface-to-

125hPa mass-weighted vertical-average diabatic heating for the ERA-40 (upper panel),

ERA-15 (middle panel), and NCEP-NCAR (lower panel) reanalyses. Contours are every

1ºC/day, and diabatic heating rates below ± 0.5ºC/day are not shaded. Dark shading

indicates heating for more than 0.5ºC/day, and light shading indicates cooling for more

than 0.5ºC/day.

FIG 3. Same as Figure 3, but it is for July.

FIG 4. Same as in Figure 3, but it is for diabatic heating differences between ERA-40,

ERA-15, and NCEP-NCAR reanalyses. Contours are every 0.5ºC/day, and diabatic

heating rates below ±0.25ºC/day are not shaded. Negative values have light shadings,

while positive values have dark shadings.

FIG 5. Same as Figure 5, but it is for July.

FIG 6. Shown above is the 1979-1993 January climatological zonal-mean diabatic

heating (contours and shadings) and divergent v-ω (vectors) for the ERA-40 (upper

panel), ERA-15 (center panel), and NCEP-NCAR (lower panel) reanalyses. Units for

divergent v and ω are m/s and -Pa/sec respectively. Diabatic heating are contoured every

0.5ºC/day, and heating below ±0.25ºC/day are not shaded. Negative values have light

shadings, while positive values have dark shadings.

FIG 7. Same as Figure 7, but it is for July.

FIG 8. Same as in Figure 7, but for the differences between the three reanalyses.

FIG 9. Same as Figure 9, but it is for July.

FIG 10. Shown above is the 1979-1993 5ºN-5ºS meridional-averaged January

climatological diabatic heating rate (contours and shades) and divergent u-ω (vectors) for

the ERA-40 (upper panel), ERA-15 (center panel), and NCEP-NCAR Reanalysis (lower

panel). Units for divergent u and ω are m/s and -Pa/sec respectively. Diabatic heating

are contoured every 1ºC/day, and heating below ±0.5ºC/day are not shaded. Negative

values have light shadings, while positive values have dark shadings.

FIG 11. Same as Figure 10, but it is for July.

FIG 12. Same as in Figure 10, but for the differences between the three reanalyses.

FIG 13. Same as Figure 13, but it is for July.

FIG 15. Shown above are the NCEP-NCAR Reanalysis 1979-1993 climatological

surface-125hPa mass-weighted vertical-averaged condensational (large-scale +

convective + shallow) (upper panel) and total (condensational + radiative + diffusive)

(lower panel) heating rates. Contours are every 1ºC/day, and diabatic heating rates below

± 0.5ºC/day are not shaded. Negative values have light shadings, while positive values

have dark shadings.

FIG 16. Shown above are the latent heating rates converted from precipitation (in ºC/day)

from the 1979-1993 climatological CMAP precipitation between 30ºS and 30ºN. Upper

panel is for January, and the lower panel is for July. Contours are every 1ºC/day, and

diabatic heating rates below 0.5ºC/day are not shaded. The area between 30ºN/S to 70º

S/N is left blank, so all maps have the same latitude ranges.

FIG 17. Shown above is the 1979-1993 January (upper panel) and July (lower panel)

climatological CPC outgoing long wave radiation. Contours are every 20 W/m2, and

radiative flux above 260 W/m2 not shaded. The area between 30ºN/S to 70º S/N is left

blank, so all maps in this paper have the same latitude ranges.

FIGURES

FIG 1

FIG 2

FIG 3

FIG 4

FIG 5

FIG 6

FIG 7

FIG 8

FIG 9

FIG 10

FIG 11

FIG 12

FIG 13

FIG14

FIG 15

FIG 16