Implementation of a Discrete Dipole Approximation Scattering Database Into Community Radiative Transfer Model

The Community Radiative Transfer Model (CRTM) is a fast model that requires bulk optical properties of hydrometeors in the form of lookup tables to simulate all‐sky satellite radiances. Current cloud scattering lookup tables of CRTM were generated using the Mie‐Lorenz theory thus assuming spherical shapes for all frozen habits, while actual clouds contain frozen hydrometeors with different shapes. The Discrete Dipole Approximation (DDA) technique is an effective technique for simulating the optical properties of non‐spherical hydrometeors in the microwave region. This paper discusses the implementation and validation of a comprehensive DDA cloud scattering database into CRTM for the microwave frequencies. The original DDA database assumes total random orientation in the calculation of single scattering properties. The mass scattering parameters required by CRTM were then computed from single scattering properties and water content dependent particle size distributions. The new lookup tables eliminate the requirement for providing the effective radius as input to CRTM by using the cloud water content for the mass dimension. A collocated dataset of short‐term forecasts from Integrated Forecast System of the European Center for Medium‐Range Weather Forecasts and satellite microwave data was used for the evaluation of results. The results overall showed that the DDA lookup tables, in comparison with the Mie tables, greatly reduce the differences among simulated and observed values. The Mie lookup tables especially introduce excessive scattering for the channels operating below 90 GHz and low scattering for the channels above 90 GHz.

社区辐射传输模型（CRTM）是一种快速模型，它需要以查找表形式的水汽凝结物的总体光学特性来模拟全天空卫星辐射。CRTM当前的云散射查找表是使用米氏 - 洛伦兹理论生成的，因此假设所有冻结形态都是球形的，而实际的云中包含不同形状的冻结水汽凝结物。离散偶极子近似（DDA）技术是一种模拟微波区域非球形水汽凝结物光学特性的有效技术。本文讨论了在CRTM中针对微波频率实现和验证一个综合的DDA云散射数据库。原始的DDA数据库在计算单次散射特性时假设完全随机取向。然后根据单次散射特性和与含水量相关的粒径分布计算出CRTM所需的质量散射参数。新的查找表通过使用云水含量来表示质量维度，从而消除了向CRTM提供有效半径作为输入的要求。使用了欧洲中期天气预报中心综合预报系统的短期预报和卫星微波数据的并置数据集来评估结果。总体结果表明，与米氏表相比，DDA查找表大大减少了模拟值和观测值之间的差异。米氏查找表尤其对90GHz以下的通道引入了过度散射，而对90GHz以上的通道引入了低散射。