Polarized scattering by frozen hydrometeors is investigated for the first time up to 157 GHz, based on the passive microwave observations of the Microwave Analysis and Detection of Rain and Atmospheric Structures (MADRAS) instrument on board the Indo-French Megha-Tropiques satellite mission. A comparison with time-coincident Tropical Rainfall Measurement Mission Microwave Imager records confirms the consistency of the coincident observations collected independently by the two instruments up to 89 GHz. The MADRAS noise levels of 1.2 K at 89 GHz and of 2.5 K at 157 GHz are in agreement with the required specifications of the mission. Compared to the 89 GHz polarized channels that mainly sense large ice particles (snow and graupel), the 157 GHz polarized channel is sensitive to smaller particles and provides additional information on the cloud systems. The analysis of the radiometric signal at 157 GHz reveals that the ice scattering can induce a polarization difference of the order of 10 K at that frequency. Based on radiative transfer modeling the specific signature is interpreted as the effect of mainly horizontally oriented ice cloud particles. This suggests that the effects of the cloud particle orientation should be considered in rain and cloud retrievals using passive radiometry at microwave and millimeter wavelengths.

Concurrent passive and active microwave measurements onboard the Tropical Rainfall Measurement Mission (TRMM) show that under cloudy conditions, when a melting layer is detected by the precipitation radar, a polarized scattering signal at 85 GHz in passive mode is often observed. Radiative transfer simulations confirm the role of large horizontally oriented non‐spherical particles on the polarized scattering signal and assess the effect of changes in particle phase, from solid ice to dry snow to melting snow, on the radiative properties. We conclude on the necessity to account for this polarization generated by the clouds in passive microwave rain retrievals and to use this specific signature to help diagnose the precipitation type and derive more accurate algorithms. In addition, analysis of the passive microwave polarized scattering is a unique way to get insight into microphysical properties of clouds at global scale, and this potential should be explored at millimeter and submillimeter frequencies that are more sensitive to the scattering generated by smaller particles.