Developing a Radar Signal Simulator for the Community Radiative Transfer Model

Active radar instruments provide vertically resolved clouds and precipitation measurements that cannot be provided by the passive instruments. These active measurements are not conventionally assimilated into the data assimilation systems because of the lack of fast forward radiative transfer (RT) models and also difficulties in the error modeling of the measurements. This article describes the development, evaluation, and sensitivity analysis for a forward radar model implemented in the community RT model (CRTM). The scattering properties required by the forward model are provided by the hydrometeor lookup tables that were generated using the discrete dipole approximation (DDA). The model is able to calculate both the reflectivity and the attenuated reflectivity for any given radar instrument at any given zenith angles as long as CRTM instrument-specific coefficients are available. The evaluation using CloudSat measurements shows a very good agreement between the simulations and measurements as long as the input profiles of hydrometeors are consistent with the measured reflectivity profiles. Major sources contributing to the differences between the measured and simulated reflectivities are input hydrometeor profiles, scattering lookup tables, lack of melting layer in the forward model, CRTM scattering solvers, and attenuation calculations. In addition to the forward model, both tangent linear (TL) and adjoint (AD) of the model are also implemented and tested within CRTM. These components may be required by some data assimilation systems for the assimilation of radar measurements.

有源雷达仪器提供垂直分辨率的云量和降水测量数据，这是无源仪器无法提供的。由于缺乏快速正向辐射传输（RT）模型，以及测量误差建模方面的困难，这些有源测量数据通常不会被同化到数据同化系统中。本文介绍了在通用辐射传输模型（CRTM）中实现的正向雷达模型的开发、评估和敏感性分析。正向模型所需的散射特性由使用离散偶极子近似（DDA）生成的水凝物查找表提供。只要有CRTM特定仪器的系数，该模型就能计算任何给定雷达仪器在任何给定天顶角下的反射率和衰减反射率。使用CloudSat测量数据进行的评估表明，只要水凝物的输入廓线与测量的反射率廓线一致，模拟结果和测量结果就非常吻合。导致测量反射率和模拟反射率之间存在差异的主要因素包括：水凝物输入廓线、散射查找表、正向模型中缺少融化层、CRTM散射求解器以及衰减计算。除了正向模型，该模型的切线性（TL）和伴随（AD）形式也在CRTM中得以实现和测试。一些数据同化系统在同化雷达测量数据时可能需要这些组件。