A polarimetric scattering database for non-spherical ice particles at microwave wavelengths

Abstract

The atmospheric science community has entered a period in which electromagnetic scattering properties at microwave frequencies of realistically constructed ice particles are necessary for making progress on a number of fronts. One front includes retrieval of ice-particle properties and signatures from ground-based, airborne, and satellite-based radar and radiometer observations. Another front is evaluation of model microphysics by application of forward operators to their outputs and comparison to observations during case study periods. Yet a third front is data assimilation, where again forward operators are applied to databases of ice-particle scattering properties and the results compared to observations, with their differences leading to corrections of the model state. Over the past decade investigators have developed databases of ice-particle scattering properties at microwave frequencies and made them openly available. Motivated by and complementing these earlier efforts, a database containing polarimetric single-scattering properties of various types of ice particles at millimeter to centimeter wavelengths is presented. While the database presented here contains only single-scattering properties of ice particles in a fixed orientation, ice-particle scattering properties are computed for many different directions of the radiation incident on them. These results are useful for understanding the dependence of ice-particle scattering properties on ice-particle orientation with respect to the incident radiation. For ice particles that are small compared to the wavelength, the number of incident directions of the radiation is sufficient to compute reasonable estimates of their (randomly) orientation-averaged scattering properties. This database is complementary to earlier ones in that it contains complete (polarimetric) scattering property information for each ice particle – 44 plates, 30 columns, 405 branched planar crystals, 660 aggregates, and 640 conical graupel – and direction of incident radiation but is limited to four frequencies (X-, Ku-, Ka-, and W-bands), does not include temperature dependencies of the single-scattering properties, and does not include scattering properties averaged over randomly oriented ice particles. Rules for constructing the morphologies of ice particles from one database to the next often differ; consequently, analyses that incorporate all of the different databases will contain the most variability, while illuminating important differences between them. Publication of this database is in support of future analyses of this nature and comes with the hope that doing so helps contribute to the development of a database standard for ice-particle scattering properties, like the NetCDF (Network Common Data Form) CF (Climate and Forecast) or NetCDF CF/Radial metadata conventions.