Polarized Microwave Radiances due to Cloud Ice and Floating Snow Scattering

Process-scale observations are required to better understand the coupling between clouds, precipitation and underlying thermodynamics. Remote sensing of clouds and precipitation from space has long relied on passive microwave sensors to provide global rapid updates and the interior structure of liquid and ice hydrometeors. Scattering from frozen hydrometeors induces polarimetric differences in the satellite radiances observed at a slant angle, which can be used to infer hydrometeors' microphysical properties (e.g., size, shape and orientation). Analyzing the vertically (V) and horizontally (H) polarized radiances from the Global Precipitation Measurement (GPM) Microwave Imager (GMI) 89 and 166 GHz channels, we find that the cloud-induced V-H difference (PD) appears to be universally positive throughout the tropics as well as in the wintertime storm-track regions. The PD has a peak amplitude of 10-20 K at 89 and 166 GHz in the satellite measurements. More interestingly, GMI observations show that the PD diurnal cycle apparently leads that of deep convection intensity, or total ice mass aloft to the upper troposphere, by 2 h. The new polarimetric information from NASA's GMI and airborne measurements highlight the value and need for more advanced mm/submm-wave instruments in future remote sensing. These satellite observables can provide key contributions to a better understanding of clouds, convection and precipitation processes as targeted by the 2017 Earth Science Decadal Survey.