MEASUREMENT OF RAINFALL PATH ATTENUATION NEAR NADIR - A COMPARISON OFRADAR AND RADIOMETER METHODS AT 13.8 GHZ

Rain profile retrieval from spaceborne radar is difficult because of t he presence of attenuation at the higher frequencies planned for these systems. One way to reduce the ambiguity in the retrieved rainfall pr ofile is to use the path-integrated attenuation as a constraint. Two t echniques for measuring the path-integrated attenuation have been prop osed: the radar surface reference technique and microwave radiometry. We compare these two techniques using data acquired by the Airborne Ra in Mapping Radar (ARMAR) 13.8-GHz airborne radar and radiometer during the Tropical Ocean-Global Atmosphere Coupled Ocean Atmosphere Respons e Experiment (TOGA COARE) in the western Pacific Ocean in early 1993. The two techniques have a mean difference close to zero for both nadir and 10 degrees incidence. The RMS difference is 1.4 dB and is reduced to 1 dB or less if points where the radiometer was likely saturated a re excluded. Part of the RMS difference can be attributed to variabili ty in the ocean surface cross section due to wind effects and possibly rain effects. The results presented here are relevant for the Tropica l Rainfall Measuring Mission, which will include a 13.8-GHz precipitat ion radar.