Window channel measurements from the NOAA-15 Advanced Microwave Sounding Unit (AMSU) are used to retrieve rain rates. This study focuses on the rain rate retrievals over land and uses measurements at 89 and 150 GHz to detect a scattering signal by millimeter-sized ice particles under precipitating atmospheres. An operational algorithm using the AMSU-A module (89 GHz) implemented in August 1999 is also described. The algorithms are calibrated with hourly surface rain observations. It is shown that identification of stratiform rain can be dramatically improved using measurements at 150 GHz from the AMSU-B sensor. Improved spatial resolution of AMSU-B is a major factor contributing to this algorithm. ¿ 2000 American Geophysical Union