1 A Revised Geophysical Model Function for A Revised Geophysical Model Function for the Advanced Scatterometer (ASCAT) at the Advanced Scatterometer (ASCAT) at NOAA/NESDIS NOAA/NESDIS Seubson Soisuvarn, Zorana Jelenak, Paul S. Chang, and Qi Zhu NOAA/NESDIS/STAR-UCAR IGARSS 2010, July 25-30, Honolulu, Hawaii, USA
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TU4.L10 - A REVISED GEOPHYSICAL MODEL FUNCTION FOR THE ADVANCED SCATTEROMETER (ASCAT) AT NOAA/NESDIS
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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
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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?
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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
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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)
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Measured and Model Sigma0 dependence on Measured and Model Sigma0 dependence on
wind vector at 30 deg incidencewind vector at 30 deg incidence