Active and Passive Vector Radiative Transfer Model for Three-Dimensional Studies of Clouds and Precipitation

As observations of clouds and precipitation increasingly rely on combinations of multiple radars and radiometers, a forward modeling framework that is capable of consistently accommodating a range of sensing geometries for both active and passive instruments is integral for correctly interpreting and synergistically combining multi-sensor measurements from ground-based, airborne, and spaceborne platforms. To support such studies, we extended the Atmospheric Radiative Transfer Simulator (ARTS), version 2.3, to produce radar reflectivities for three-dimensional cloud and precipitation input fields. This active radiative transfer model uses Monte Carlo sampling to efficiently sample Gaussian antenna beams and to fully account for multiple scattering, and the module complements the passive three-dimensional solver that has been included in ARTS since versions 1.1. By relying on common ray tracing tools, gaseous absorption models, and scattering properties, the model reproduces accurate and consistent radar and radiometer observables. In particular, the flexibility to include scattering parameters computed from a variety of sources, e.g., T-Matrix and DDSCAT, as well as the ability to solve the vector (polarized) radiative transfer equation makes this model well-suited for microphysical studies. An overview of the model will be presented, including details for handling preferentially-aligned hydrometeors. Examples highlighting the performance of the model for space-based and airborne observations will be given, and some case studies showing the response to particle type and orientation will be offered. Simulations of radar quantities such as reflectivity factor, linear depolarization ratio, and dual-frequency ratio, and radiometer observables, such as brightness temperature intensity and polarization difference, will be used to demonstrate the capabilities of the model.