Cloud Ice Crystal Orientation Inferred from Global Precipitation Measurement (GPM) Microwave Imager
Abstract:Ice crystal orientation can produce significantly different scattering in vertically (V) and horizontally (H) polarized microwave radiances and affect the accuracy of cloud ice measurement. Designed to observe the precipitable-sized particles, GPM Microwave Imager (GMI) is used in this study to infer ice crystal orientation inside ice clouds. By identifying ice cloud scenes using the 183.3±3 GHz channel, we compare the 89 and 166 GHz radiances for their V-H differences. Ice cloud crystals are found highly polarized with V-H > 0 throughout the tropics and the mid-latitude jet regions. The V-H difference can be as large as 10% (5%) of the mean radiance at 166 GHz (89 GHz). The largest values generally occur over convective outflows, but decreasing in the vicinity of deep convective cores and remote thin cirrus regions. The negative V-H values prominently happen in the equator side of the winter hemisphere storm track regions.
A polarized radiative transfer model is employed to interpret the observed polarization. Simulations with systematically oriented non-spherical ice particles can reproduce the observed V-H differences, while spherical or randomly oriented non-spherical particles cannot. This finding suggests that accurate cloud ice retrievals must take into account ice crystal orientation. The observed V-H relationship with cloud regimes may relate with vertical velocity, in-cloud turbulence, lightning, and other physical processes, which will be briefly discussed in this presentation.