Measurement of rainfall path attenuation near nadir: A comparison of radar and radiometer methods at 13.8 GHz

Abstract

Rain profile retrieval from spaceborne radar is difficult because of the presence of attenuation at the higher frequencies planned for these systems. One way to reduce the ambiguity in the retrieved rainfall profile is to use the path‐integrated attenuation as a constraint. Two techniques for measuring the path‐integrated attenuation have been proposed: the radar surface reference technique and microwave radiometry. We compare these two techniques using data acquired by the Airborne Rain Mapping Radar (ARMAR) 13.8‐GHz airborne radar and radiometer during the Tropical Ocean‐Global Atmosphere Coupled Ocean Atmosphere Response 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° incidence. The RMS difference is 1.4 dB and is reduced to 1 dB or less if points where the radiometer was likely saturated are excluded. Part of the RMS difference can be attributed to variability in the ocean surface cross section due to wind effects and possibly rain effects. The results presented here are relevant for the Tropical Rainfall Measuring Mission, which will include a 13.8‐GHz precipitation radar.