Radiative Transfer to Space through a Precipitating Cloud at Multiple Microwave Frequencies. Part I: Model Description

Abstract

In a two-part study we investigate the impact of time-dependent cloud microphysical structure on the transfer to space of passive microwave radiation at several frequencies across the EHF and lower SHF portions of the microwave spectrum in order to explore the feasibility of using multichannel passive-microwave retrieval techniques for the estimation of precipitation from space-based platforms. A series of numerical sensitivity experiments have been conducted that were designed to quantify the impact of an evolving cumulus cloud in conjunction with a superimposed rain layer on the transfer to space of microwave radiation emitted and scattered from the cloud layers, rain layer and the underlying surface. The specification of cloud microphysics has been based on the results of a time-dependent two-dimensional numerical cumulus model developed by Hall (1980). An assortment of vertically homogeneous rain layers, described by the Marshall-Palmer rain drop distribution, has been inserted in the model atmosphere to simulate the evolution of rainfall in a precipitating cumulus cell. The effects of ice hydrometeors on upwelling brightness temperatures have been studied by placing several types of ice canopies over the cloud and rain layers. Both rough ocean and land backgrounds have been considered. The top-of-atmosphere brightness temperatures have been computed by means of a vertically and angularly detailed plane-parallel radiative transfer model for unpolarized microwave radiation. Part I describes the modeling framework. In addition, it provides a detailed description of the single-scattering properties of the hydrometeors (model-cloud water drops, ice crystals and rain drops) in order to evaluate each component's role in influencing the upwelling radiation to space. We demonstrate that cloud water can have a major impact on the upwelling microwave radiation originating from both the surface and a rain layer placed below cloud base. The radiative properties of the model cloud are shown to be significantly different from those of an equivalent Marshall-Palmer treatment. It is the appearance of the large-drop mode (r> 100 ?m) of the cumulus cloud drop distribution function that denotes the point at which cloud drops begin to attenuate the microwave signals, even at the lower frequencies, which are normally considered to be mostly unaffected by purely cloud processes. It is shown that at the early stages of cloud evolution, the model cloud acts mainly through absorption/emission processes. As the cloud develops, however, scattering plays an ever-increasing role. It is also demonstrated that the relative contribution by the small drop mode (r<100 ?m) of the cloud to absorption/emission is always significant. It is concluded that the vertical variation of the microphysical structure of the rain-cloud plays an important role in the interpretation of passive microwave rainfall signatures and thus should be considered in precipitation retrieval algorithms.