Microwave radiances from precipitating clouds containing aspherical ice, combined phase, and liquid hydrometeors

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Abstract
Microwave radiances have been computed as a function of rainfall rates from precipitating clouds containing ice, combined phase, and water hydrometeors. The ice at the top of the precipitating cloud depresses the brightness temperatures by reflecting radiances that emerge from the liquid hydrometeors at lower altitudes. Because the density of ice hydrometeors is related to the rainfall rate at the surface, the brightness temperatures are sensitive to large rainfall rates. The solution of the transfer equation considers the vertically inhomogeneous structure of the precipitation as well as the polarized emissivity of the underlying surface. The altitudes from which radiances derive their energy are presented for several rainfall profiles and frequencies. It is shown that the lower‐frequency radiances are sensitive to liquid precipitation at low altitudes while the higher‐frequency radiances are more sensitive to the ice hydrometeors at the cloud tops. The extinction coefficients of aspherical hydrometeors are presented as a function of rainfall rates. Measurements at 37.0‐GHz microwave radiances from Nimbus 7 suggest that the vertically polarized radiances are 12±2°K warmer than the horizontally polarized radiances at high rainfall rates. This difference diminishes at lower frequencies. The present model reveals that such effects can be traced to aspherical ice hydrometeors in the upper regions of precipitating clouds.