基于双参数云微物理方案的快速微波散射辐射计算方法研究

Recently, the application of satellite observations in NWP model has improved the accuracy of weather prediction and microwave data is superior to others dramatically. However, cloud and precipitation affected microwave radiances are eliminated in the assimilation system because of many difficulties. The fast calculation of microwave radiation is one of the important reasons. Limited to cloud model, traditional fast radiative transfer models adopt one-moment cloud microphysics scheme. Encouragingly, cloud model based on two-moment cloud microphysics scheme has been developed rapidly, which can simulate atmospheric parameters and cloud microphysical parameters including hydrometeor water contents. So this research is promoted.. The idea of the research is as follows: 1) to construct Mie lookup tables based on a two-moment cloud microphysics scheme. Lookup tables include four scattering coefficients (single scattering albedo, extinction coefficient, and asymmetry parameter), which are dependant on microwave frequency, hydrometeor phase, atmospheric temperature, hydrometeor water content and hydrometeor number concentration; 2) to interpolate the optical properties of clouds from Mie lookup tables for use in the scattering radiative transfer and achieve the goal of fast calculation. A fast model is developed then; 3) to validate the fast model based on a common microwave radiative transfer model and satellite microwave observations. The research is basic for assimilation of cloud and precipitation affected microwave radiances.

近年来，数值天气预报的精度大大提高，这主要得益于卫星资料的同化应用，其中微波资料的贡献明显高于其它资料。然而，受云雨影响的卫星微波资料却由于在同化应用中存在较多困难而被剔除，快速辐射传输计算是其中一个重要原因。受限于云模式的发展，传统的快速微波辐射传输模式采用单参数云微物理方案，但近年来基于双参数云微物理方案的云模式正在快速发展，可以模拟包含各种水凝物粒子数浓度在内的大气参数和云微物理参数。本项目的研究思路是基于双参数云微物理方案，构造依赖于微波频率、水凝物相态、大气温度、水凝物含量和粒子数浓度的Mie散射系数查找表（散射系数包括单次散射反照率、衰减系数和不对称因子）；然后利用插值方法耦合查找表与微波辐射传输计算模块，建立适用于云雨大气的快速散射辐射计算模式；最后利用常规微波辐射传输模式与卫星实际观测资料对快速模式进行验证分析。本研究是面向云雨卫星微波观测资料的数值同化应用的基础性研究。

卫星微波资料的同化应用大大改进了数值天气预报的精度。受限于多种因素的困扰，云雨大气微波资料的同化应用一直是发展数值预报模式的难点，精确且快速的辐射传输模式则是同化卫星资料的关键所在。本项目针对云雨大气的快速微波辐射传输计算方法开展了如下研究：利用WRF中尺度天气模式和中国气象科学研究院研发的CAMS双参数云微物理方案模拟了台风“天兔”的大气和云雨环境场，为建立双参数快速散射系数查找表提供参考依据并为辐射模拟提供输入场；以微波成像仪TMI为例，分别建立了适用于其通道特征的单参数和双参数散射系数查找表，提供依赖于频率、温度（仅限于液态水成物）、含水量以及数浓度（仅限于双参数查找表）的散射衰减系数、单次散射反照率和不对称因子；分别针对液态/固态水成物和单参数/双参数查找表采用不同的插值方案与Eddington辐射传输模式相耦合，实现了对云雨大气的快速微波辐射计算。与不采用查找表的显式计算方法相比，该方法在计算速度上有极大的提升，中高频通道两种快速方法模拟效果与显式模拟效果相当，低频通道采用单参数快速计算方法效果更佳。该项研究成果可应用于风云三号气象卫星微波仪器的数据仿真，也可为我国风云气象卫星快速辐射传输模式的开发提供参考依据。