Towards an Open Access Scattering Library for Realistically Shaped Ice and Snow Particles at Microwave Frequencies

Description

In recent years an increasing number of studies showed that spherical or spheroid “soft sphere” approximations for the scattering properties of snowfall at higher microwave frequencies (>30 GHz) are insufficient to describe the scattering properties of real snowfall (e.g. observed by multi-frequency cloud radars). On the other hand, computational expensive techniques to simulate the scattering properties of more realistic snow particle models with e.g. the Discrete Dipole Approximation (DDA) have become quite popular since computational resources are increasingly available. The fundamental question which scattering model is most realistic is a question which so far neither in-situ observations nor remote sensing studies can fully answer. Due to the large natural variability of snowfall it will probably always be necessary to apply appropriate mixtures of different habits and densities (degrees of riming). However, there are currently only a few datasets for realistic ice/snow scattering publicly available although many groups are performing extensive DDA calculations. This contribution describes an ongoing effort to collect scattering computations of realistically shaped ice and snow particles including snow aggregates converted into a standardized data format (like netcdf). The library will include several data products starting from basic scattering properties (e.g. cross sections, phase function, asymmetry parameter), meta-data describing the particle’s dipole structure and information about the numerical calculations as well as more comprehensive scattering (if available) properties like the full Mueller matrix, polarization or scattering properties of oriented particles. This open-access library could be an important resource for active and passive MW remote sensing studies and would also perfectly complement the existing ASR observational dataset. The data library will also help to investigate and test novel scattering approximations (e.g. Raleigh-Gans theory) that would be of great importance for e.g. data assimilation of passive and active MW observations of ice and snow clouds.