The effect of rain on retrieval of C- and Ku-band scatterometer surface winds during Hurricanes Lili (2002) and Isabel (2003)

The Imaging Wind and Rain Airborne Profiler (IWRAP) and Simultaneous Frequency Microwave Radiometer have been flown aboard the NOAA WP-3D "Hurricane Hunter" aircraft for the past five (2002-2006) hurricane seasons. IWRAP is a conically scanning, high resolution C- and Ku-band Doppler radar that profiles from the aircraft to ocean at four incidence angles. The microwave return from the ocean is used to infer ocean surface vector winds via scatterometry. The UMass Simultaneous Frequency Microwave Radiometer (USFMR) is a C-band radiometer that measures integrated rainfall rate and ocean surface wind speed. The combination of these two instruments allows for the determination of the effect of rain on scatterometer derived surface vector winds. Our focus is on scatterometer retrievals at an incidence angle of 50 degrees during Hurricanes Lili (2002) and Isabel (2003). The measurements were carried out under the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) program and are the highest resolution radar measurements of the lower atmospheric boundary layer in powerful hurricanes. The retrievals at C-band use the CMOD5 geophysical model function (GMF) while those at Ku-band use one which is a synthesis of the QSCAT-1 GMF and one developed using IWRAP data. As has been reported, the effect of rain at Ku-band is a dramatic decrease, even for wind speeds greater than 20 m s(-1) The atmospheric attenuation dominates these results. At C-band the primary effect is a decrease in azimuthal modulation which is especially important at high wind speeds where this modulation decreases in the absence of rain.