Upgrading the CRTM Hydrometeor Scattering Properties: Toward Fully Polarized Radiative Transfer

The purpose of this presentation is to provide an overview of the current efforts to improve the hydrometeor scattering look-up tables (LUTs) in the Community Radiative Transfer Model (CRTM) of the JCSDA. In particular, the effort is focused on the ICE, SNOW, and GRAUPEL data types of the CRTM.

Important updates include the transition to ice optical properties based on the MODIS Collection 6 habit model and adding a temperature dependence to the ICE and SNOW types in the same way as the already existing dependence for the RAIN type. It is important to note that the MODIS C6 scattering properties are based on a single habit comprised of an aggregate of roughened hexagonal columns. Previously, the impact of the temperature-dependent refractive index of ice on the optical properties of the frozen hydrometeors had already been considered. Improving on this basis, the temperature sensitivity of the normalized particle size distributions is also considered in the new LUT iteration.

In a parallel effort, Mie calculations are conducted that will provide a baseline reference for the MODIS C6 scattering properties and any future model updates. Unprecedented flexibility in terms of the scattering particle mass density is enabled through the recently proposed Bicontinuous Random Medium (BRM) model, however the application of this model is still under scrutiny and a corresponding full scattering database is not available yet. Finally, the ongoing effort to extend the CRTM into a fully polarized RT solver is supported by providing the full Müller matrix information in the updated LUTs.

Biases caused by the modified scattering properties between observation and forecast are quantified using GSI and compared to the RTTOV radiative transfer model.