Modern data assimilation frameworks require sophisticated physical and radiative models to guide assimilation and interpretation of satellite-based observations. To date, satellite-based infrared and passive microwave radiances, in various scenarios, are being assimilated operationally at multiple centers around the world (e.g., ECMWF, NOAA), however precipitating/cloudy radiances assimilation is still under development for most observation streams. Additionally, with the advent of

5 space-based precipitation radars (e.g., TRMM, GPM, CloudSat), active microwave scatterometers (e.g., RapidScat), and radar altimeters (e.g., JASON), interest in directly assimilating satellite-based active microwave observations is increasing. This paper describes the development of the Community Active Sensor Module (CASM), which is designed to simulate active microwave sensor observations, consistent with current and future sensors. This paper presents the forward modeling component of CASM, providing a model description, key physical elements, and sensitivity to the various inputs and implicit / explicit

10 assumptions. CASM is also evaluated against the the Global Precipitation Measurement Mission Dual-Frequency Precipitation Radar (GPM DPR) observations in both a targeted case study and a global, year-long analysis.