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This paper describes a methodology to use the passive microwave measurements of the 6.9 GHz bandwidth of the AMSR-E sensor which is the most sensitive to surface soil moisture, to constrain satellite-based rainfall estimates over a semi arid region in West-Africa. The paper focuses on the aptitude of AMSR-E measurements to inform if rain occurs or not. The study was conducted over a 125 x 100 km(2) region located in Niger where a dense recording raingauge network is available to build an accurate ground-based 3-hour rainfall product at the 25 x 20 km(2) resolution. A satellite-based rainfall product (EPSAT-SG), based on both infrared and microwave measurements, was compared to the ground-based rainfall product. It was shown that EPSAT-SG overestimates by about 30% the total number of rain events during the 2004 and 2006 rainy seasons. A simple methodology based on the AMSR-E polarization ratio variations related to the surface soil moisture leaded to suppress a large amount of the wrong rain events.