Corrections to scatterometer wind vectors—Removing the effects of rain

The presence of rain results in the erroneously high radar cross section measurements, in most cases, by polar orbiting scatterometers, such as QuikSCAT. This is due to the rain producing volumetric backscatter as well as surface rain impacts that modify the sea surface roughness. In addition, one must also consider attenuation by the rain. Thus, wind vectors derived from scatterometer reflectivity in the presence of rainfall are often seriously erroneous.

Using near simultaneous collocated measurements of atmospheric precipitation from remote sensing systems are being analyzed to develop a method for removing the effects of rain backscatter and signal attenuation from the total signal received back at the scatterometer. Data from the Tropical Rainfall Measuring Mission (TRMM), Advanced Microwave Scanning Radiometer (AMSR) and surface based Weather Surveillance Radar - 1988 Doppler (WSR-88D) are being utilized. A mathematical model is being developed, which represents the principal electromagnetic mechanisms of attenuation and scattering, using the polarization and illumination details for the QuikSCAT scatterometer. The data products being used include QuikSCAT level 2A normalized radar cross section data, TRMM microwave imager (TMI) volumetric profile for the precipitation water and the WSR-88D reflectivity archive level II data products. The TMI products provide both stratiform and convective rain fractions, which allows for some evaluation of raindrop size distribution.

The presentation will show results from a case along the east coast of the United States for Hurricane Isabelle in September 2003. Additional application of the results of the study to other portions of the world, where WSR-88D data is not available will be presented.