Diurnal Variation of Tropical Ice Cloud Microphysics: Evidence from Global Precipitation Measurement Microwave Imager (GPM-GMI) Polarimetric Measurements

Modeling the Radiative Effect on Microphysics in Cirrus Clouds Against Satellite Observations

The radiative effect on microphysics (REM) plays an important role in the dew/frost formation near the surface. How REM impacts cirrus clouds is investigated in this study, using bin microphysical model simulations and coincident data of the CloudSat and Global Precipitation Measurement (GPM) satellites. REM affects ice crystal spectrum with two types: radiative cooling and warming. Radiative cooling, as predicted by the bin-model simulations, favors the formation of horizontally oriented ice crystals (HOICs), but radiative warming does not. Hence, a test of REM can be transformed to a test of HOICs, because HOICs can be measured by the microwave polarization observations of the GPM Microwave Imager (GMI) at 166 GHz. To analyze the GMI data for their HOIC distribution, clouds are sorted into four groups with different optical depth and altitude, based on the radiative cooling/warming ratio (or eta) computed with satellite-retrieved ice water content. Their HOIC distributions (e.g., the midlevel thick clouds have more HOICs than the high-level ones) agree well with those predicted by the bin-model simulations. The general agreement between the GMI observations and bin-model simulations suggests that REM is common in cirrus clouds and impacts cirrus clouds significantly.

Radiatively Induced Precipitation Formation in Diamond Dust

Radiative cooling leads to the formation of dew and frost. This process is extended into a numerical model to simulate the ice crystal characteristics of diamond dust. The model replicates the low ice crystal concentration of diamond dust and the precipitation in stationary air. Its results are consistent with the arctic observations that large ice crystals grow while small ones sublimate and partly explain the geographic and seasonal distributions of diamond dust such as the high frequency of diamond dust in the arctic regions and winter. Furthermore, its results show that plate/column-like ice crystals with radiative cooling grow in expense of quasi-spherical ice particles, partly explaining the ice crystal shapes observed in diamond dust.