Development and Application of Microphysics Scheme-Consistent Cloud Optical Properties for CRTM in Microwave

The Community Radiative Transfer Model (CRTM) forward model is modified so that cloud and precipitation single-scattering properties at microwave wavelengths are consistent with the particle properties and size distributions of the WSM6, Goddard (2007 single-moment version), Morrison, Milbrandt-Yau and Thompson microphysics parameterization schemes.

One method uses cloud single-scattering property lookup tables that are constructed for every species over the respective particle size distribution, and so is called Distribution-Specific (CRTM-DS). In a second method, called Bin-Generalized (CRTM-BG), the CRTM internally builds the single-scattering properties for clouds of any species using liquid/ice particle single-scattering property lookup tables. CRTM-DS is the more accurate method with high-resolution of particle sizes used in constructing cloud lookup tables, and it preserves the fast whole-cloud treatment of the unmodified CRTM. CRTM-BG has significant differences in structure thus it can be slower to run, and at reasonable particle radius resolution of size distribution integration (tens of discretizations/bins between 1 and 10000 microns) is generally accurate to within a few percent of CRTM-DS. CRTM-BG offers easier flexibility in cloud particle properties and size distribution parameter values compared to constructing a CRTM-DS cloud lookup table for every variation.

Using microphysics-consistent cloud scattering properties generates much greater variety in the simulated brightness temperature fields across the different microphysics schemes and reduced correlation of brightness temperature to hydrometeor content. With respect to the same microphysics scheme, simulations with the modified CRTM significantly differ from simulations with the unmodified CRTM using either spatially-uniform effective radii or the standard distribution-based definition of effective radius. Simulated brightness temperatures with the modified CRTM are substantially different (generally colder) than observations.

For most species, the microphysics scheme specifies a spherical mass-volume relationship for hydrometeors, for which case we apply single-scattering properties of spheres by Mie theory and use Maxwell-Garnet mixing of ice and air. Pristine ice particles in Milbrandt-Yau is stated as bullet rosettes, so Discrete Dipole Approximation (DDA) simulations of this shape are used instead of Mie theory, and these particles are assumed randomly oriented in clouds. We are experimenting using the single-scattering properties of non-spherical particles for correcting the cold bias of simulated to observed brightness temperature, which may be caused in part by the use of physically-unrealistic single-scattering properties of spheres.