MODULATION OF SSM/I MICROWAVE SOIL RADIANCES BY RAINFALL

This article presents a detailed case study of the observed impact of rainfall and soil moisture on microwave brightness temperatures of the lower Great Plains of the United States using SSM/I satellite data. The SSM/I channels provide better spatial resolution and are at higher frequencies than typically used for studies of this type. Examination of the SSM/I observations during a month and a half period in Oklahoma in 1987 indicated large spatial temperature variations due to both vegetation cover and soil moisture. The brightness temperatures of the land background were strongly modulated by rainfall during the study period. Areas of lowest brightness temperatures coincided with regions of bare soil which had received significant rainfall. Examination of the time history of brightness temperatures in the bare soil regions showed that rainfall events caused significant temperature drops, which persisted for at least several days. Time history plots of brightness temperature and the antecedent precipitation index during an extremely large rain event indicated a slow recovery period (approximately 15 days) back to the dry soil state. On the other hand, vegetated regions showed smaller temperature drops and much weaker correlation with rain events, thus questioning the feasibility of using these SSM/I measurements for soil moisture estimation.