Radiative Transfer to Space Through a Precipitating Cloud at Multiple Microwave Frequencies

[摘要] By the use of a vertically and angularly detailed, plane-parallel microwave radiative transfer model, we have conducted a series of numerical experiments in conjunction with a cloud model simulation to investigate the impact of time-dependent cloud microphysical structure on the transfer to space of passive microwave radiation at several frequencies across the EHF-SHF spectrum. Our overall objective is to explore the detailed physics of using multi-channel, passive-microwave retrieval techniques for the estimation of precipitation from space-based platforms. This paper is a continuation of a previous sensitivity study, which we have published in two-parts (Mugnai and Smith, 1988; Smith and Mugnai, 1988).The impact of large ice particles on passive microwave brightness temperatures over an evolving model rain cloud are examined at 10 separate frequencies in the EHF/SHF spectrum. Three separate cloud model designs are considered for both hard ice and low density ice freezing modes. The results emphasize how the range of frequencies between 10.7 and 231GHz differentially respond to the various ice models and freezing modes.It is shown how frequency-dependent, vertically distributed generalized emission/scattering weighting functions, which we have introduced to vertically resolve the contributions by individual cloud and precipitation layers to the brightness temperatures, can be used to identify the specific layers responsible for regulating the magnitude of top-of-atmosphere brightness termperatures. The weighting function is also coupled to a fractional contribution by scattering function which will exhibit the relative magnitude of the scattering source within the generalized weighting function itself. This enables a thorough understanding of how brightness temperatures are modulated by hydrometeors in individual layers.Additional salient results of the study are the identification of the importance of incorporating a mixed layer at intermediate and higher frequencies (37GHz and higher), and quantifying the influence of freezing mode at 85GHz. It is also shown that the vertical scale of the cloud column, for a fixed equivalent liquid water path, is a major determining factor in generating brightness temperatures consistent with actual observations. This result is based on a case study of a severe thunderstorm monitored with a 2-channel airborne microwave radiometer during the 1986COHMEX experiment in northern Alabama.

[学科分类] 大气科学