This document discusses combining space-based active microwave (CloudSat) and passive microwave (PMW) observations to improve global snowfall estimates. CloudSat uses radar to detect snowfall but has limitations without additional data. PMW provides constraints by relating brightness temperature depression to ice water path. The authors aim to use optimal estimation to relate CloudSat radar reflectivity measurements to snowfall rate, incorporating prior particle properties from ground observations. Combined CloudSat and PMW data can help determine key retrieval parameters like snow size distribution and fall speed.