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1
A Revised Geophysical Model Function A Revised Geophysical Model Function
for the Advanced Scatterometer (ASCAT) for the Advanced Scatterometer (ASCAT)
at NOAA/NESDISat NOAA/NESDIS
Seubson Soisuvarn, Zorana Jelenak, Paul S. Chang, and Qi Zhu
NOAA/NESDIS/STAR-UCAR
IGARSS 2010, July 25-30, Honolulu, Hawaii, USA
2
IntroductionIntroduction
• ASCAT measures radar
backscatter (sigma0) using C-band
vertically polarized fan-beam
antennas - multiple incidence
angles (~25°-65°)
• Operational ASCAT winds
retrievals is based on CMOD5.n
model function and it shows clearly
underestimates high wind speeds
• Hurricane force wind detection
from ASCAT is very rare Can ASCAT high wind speed retrievals be improved?
3
What do we knowWhat do we know
• Scatterometer wind retrievals are dependent on Geophysical Model
Function (GMF) that relates backscatter (Sigma0) to ocean surface
wind fields
• Satellite C-band V-Pol Sigma0 collocation data at high winds are not
currently sufficient to build empirical geophysical model function
– Winds are not quite uniformly distributed in all directions
• C-band V-Pol Sigma0 measurements for high winds are available
from Airborne scatterometer (IWRAP) but at limited incidence
angles and much higher resolution than satellite measurements
4
Study ApproachStudy Approach
• The ASCAT operational GMF (CMOD5.n) is written in harmonic terms:
• ASCAT/QuikSCAT collocations show B0 is biased high at high wind
speeds.
• Replace B0 term with a new B0 and assuming B1 and B2 are modeled
correctly
– Not a new idea (Jones et. al – TRMM PR GMF, IGARSS’03, ’04)
• New B0 is determined based on a linear characteristic of an airborne
scatterometer GMF (IWRAP)
– Replace B0 only at higher winds
6.12100 ]2cos),(cos),(1[),( BBB
Hersbach, et al., J. Geophys. Res. (2007)
5
Measured and Model Sigma0 dependence on Measured and Model Sigma0 dependence on
wind vector at 30 deg incidencewind vector at 30 deg incidence
Relative wind direction (Degree)
Sig
ma0
ASCAT/QuikSCAT matchu-ups @ 30° incidence
CMOD5n B0
“True” B0
5m/s 9m/s 12m/s
15m/s 17m/s 20m/s
6
cmod5n is not saturate beyond 40° incidence
IWRAP saturation wind speed
New SaturationWind speed
C-Band V-pol Saturation wind speedsC-Band V-pol Saturation wind speeds
• IWRAP GMF is valid at 29°, 34°, 40° and 50° incidence and 25-65 m/s
• The B0 is written in the form:
• U10Nsat is defined as the wind speed where B0 reaches its maximum value:
2
1
1010
2
10
0 100
NsatN UUNU
B
21021010 )][log()log((10 NN UUB
Fernandez, et al., J. Geophys. Res. (2006)
Fernandez, et al., J. Geophys. Res. (2006)
7
High wind speed BHigh wind speed B0 0 ; Incidence: 50 deg; Incidence: 50 deg
B0 vs wind speed
U10Nsat =61.5 m/s
IWRAP
New B0
NU
B
10
0
vs wind speed
U10Nsat =61.5 m/s
New B0
IWRAP
8
CMOD5n vs New BCMOD5n vs New B00
CMOD5n B0
New B0
Wind Speed (m/s)
B0
25° 30° 40°
50° 60° 65°
9
Sigma0 Model vs. Measurements
Measured Sigma0 (dB)
Cal
cula
ted
Sig
ma0
(dB
)
CMOD5.n
New GMF
Fore-beam Mid-beam Aft-beam
Fore-beam Mid-beam Aft-beam
10
ASCAT vs QuikSCAT speed (before)ASCAT vs QuikSCAT speed (before)
Cumulative distributioncurve
QuikSCAT Wind Speeds
AS
CA
T W
ind
Spe
eds
11
ASCAT vs QuikSCAT speed (after)ASCAT vs QuikSCAT speed (after)
Cumulative distributioncurve
QuikSCAT Wind Speeds
AS
CA
T W
ind
Spe
eds
12
ASCAT vs QuikSCAT direction (before)ASCAT vs QuikSCAT direction (before)
QuikSCAT Wind Directions
AS
CA
T W
ind
Dire
ctio
ns
13
ASCAT vs QuikSCAT direction (after)ASCAT vs QuikSCAT direction (after)
QuikSCAT Wind Directions
AS
CA
T W
ind
Dire
ctio
ns
14
Across Swath Speed Biases (before)Across Swath Speed Biases (before)
2 m/s reference line
15
Across Swath Speed Biases (after)Across Swath Speed Biases (after)
2 m/s reference line
16
Across Swath Speed RMS errors (before)Across Swath Speed RMS errors (before)
2 m/s reference line
17
Across Swath Speed RMS errors (after)Across Swath Speed RMS errors (after)
2 m/s reference line
18
Operational ASCATQuikSCAT
Mean Wind Speed FieldMean Wind Speed Field2007-2009 Winter Seasons - North Pacific2007-2009 Winter Seasons - North Pacific
19
Mean Wind Speed FieldMean Wind Speed Field2007-2009 Winter Seasons - North Pacific2007-2009 Winter Seasons - North Pacific
New ASCATQuikSCAT
20
1000km2000km
3000km
Mean Wind Speed FieldMean Wind Speed Field2007-2009 Winter Seasons - North Pacific2007-2009 Winter Seasons - North Pacific
QuikSCAT New ASCAT
21
Gale Force Wind Frequency
Storm Force Wind Frequency
QuikSCAT
Gale Force Wind Frequency
Storm Force Wind Frequency
Operational ASCAT
Hurricane Force Wind Frequency Hurricane Force Wind Frequency
22
Gale Force Wind Frequency
Storm Force Wind Frequency
New ASCAT
Hurricane Force Wind Frequency
Gale Force Wind Frequency
Storm Force Wind Frequency
Hurricane Force Wind Frequency
Operational ASCAT
23
OperationalASCAT
Sonde Wind Speed 28.8m/s
SFMR 28 m/s
Operational ASCAT 22 m/s
24
New ASCAT
Sonde Wind Speed 28.8m/s
SFMR 28 m/s
New ASCAT 25 m/s
25
ConclusionsConclusions
• New High wind ASCAT GMF was developed
– B0 was modified in CMOD5.n
• Matching IWRAP GMF high wind trend and correcting for observed B0
biases
• New ASCAT high wind speed retrievals are improved– Bias and RMS wind speed errors are compared better to QuikSCAT and
SFMR
– New ASCAT wind speed retrievals are still biased low but resultant biases are smaller
– Improve storm and hurricane force wind detection
Next Step:
• Transition new ASCAT winds product into operations (Soon)
26
BACK UP
27
Operational wind speed histogram
28
New wind speed histogram
29
Operational ASCATOperational ASCAT
KNMIKNMI
Mean bias = -3.77 m/sMean STD = 1.01 m/s
30
New ASCATNew ASCAT
KNMIKNMI
Mean bias = -2.38 m/sMean STD = 0.98 m/s
31
Model vs. Measurement @ 30° incidence
CMOD5.n
New
32
Measured vs CMOD5.n 30 deg
33
Measured vs CMOD5.n 40 deg
34
Measured vs CMOD5.n 50 deg
35
Measured vs CMOD5.n 60 deg
36
Hurricane Force ObservationsHurricane Force ObservationsOct’07 ~ May’08Oct’07 ~ May’08
0
20
40
60
80
100
120
140
160
180
200
QSCAT-Hi QSCAT-p ASCAT GFS ECMWF Surf. Obs.
195
168
2
38
415
• Loss of QuikSCAT data could possibly result in 80-
90% reduction in detection of hurricane force winds
• Necessary to investigate improvement in ASCAT
wind retrievals are in order to support wind warning
and forecast products for extreme winter ocean
storms
•ASCAT underestimate high wind speeds in comparison to QuikSCAT
•Hurricane force wind detection from ASCAT is very rare
Can ASCAT high wind speed
retrievals be improved?
37
Hurricane Force ObservationsHurricane Force Observations( Oct’07 ~ May’08 )( Oct’07 ~ May’08 )
0
50
100
150
200
250
QS -12.5 km QS - 25 km AS - opr AS - new GFS OBS
212188
2 39 38 19
38
Operational ASCAT wind product detects STORM force wind. One wind warning category lower than the actual winds.
North Atlantic Extratropical Storm ExampleNorth Atlantic Extratropical Storm Example
QuikSCAT & New ASCAT wind products detects HF wind.
New ASCAT Operational ASCAT
QuikSCAT
39
40
Resolution effect
Ernesto RodriguezOVWST meeting 2009
41
Smoothing
N
nn
N
nnnew
xB
xBBB
new
10
10
00
)(
)(),(),(
42
B0 vs wind speed