This project will extend the work on millimeter-wave scattering by ice crystals that was initiated in a previous NSF Grant, ATM-9225116. The objective is to evaluate the scattering characteristics of ice crystals having many different shapes and sizes to aid in the interpretation of polarimetric radar observations of ice clouds. The approach is to calculate the scattering from ice crystals and to compare the results with experimental measurements using mm-wave radars and airborne cloud-sampling equipment. The calculations will be for three different radio frequencies, corresponding to those in use on short-wavelength radars: 35, 94, and 220 GHz (corresponding to wavelengths of 8.6, 3.2, and 1.4 mm). Because the wavelengths are comparable to the size of ice crystals, the Rayleigh or the geometric optics approximations for scattering cannot be used. Instead, the Waterman T-matrix approach will be used for crystals that have no dimension larger than 0.1 mm and the computationally intensive finite-difference time domain (FDTD) method for all crystals larger than this size. The scattering properties of clouds of ice crystals will be computed by integrating over a range of crystal sizes approximated by either gamma distributions or bimodal distributions, both of which have been found to describe cloud populations. The calculations will include all measurable polarization parameters, such as vertical and horizontal reflectivity factors, differential reflectivity, differential phase shift, and copolarized correlation coefficient. Comparison of the computed results with radar observations will be made through collaboration with groups at the University of Wyoming, the University of Massachusetts at Amherst, and the meteorology department at Penn State. The goal of the work is to develop a method to estimate crystal habit and cloud ice content by radar. The results would have application to both remote sensing and the parameterization of radiative transfer by ice clouds in large scale numerical models of the atmosphere.
