Scattering From Ice Crystals for Millimeter-Wave Radar Remote Sensing of Clouds

该项目将扩展冰晶对毫米波散射的研究，该研究是在之前的NSF拨款中启动的，编号为ATM-9225116。目的是评估具有许多不同形状和大小的冰晶的散射特性，以帮助解释极化雷达对冰云的观测。方法是计算冰晶的散射，并将结果与使用毫米波雷达和机载云采样设备的实验测量结果进行比较。计算将针对三种不同的无线电频率，对应于短波雷达上使用的频率：35,94和220 GHz（对应于波长8.6,3.2和1.4 mm）。因为波长与冰晶的大小相当，所以散射的瑞利近似或几何光学近似不能使用。相反，Waterman t矩阵方法将用于尺寸不大于0.1 mm的晶体，而计算密集型有限差分时域（FDTD）方法将用于尺寸大于0.1 mm的所有晶体。冰晶云的散射特性将通过在由伽马分布或双峰分布近似的晶体尺寸范围内积分来计算，这两种分布都已被发现用于描述云群。计算将包括所有可测量的偏振参数，如垂直和水平反射率因子、微分反射率、微分相移和共偏振相关系数。通过与怀俄明大学、马萨诸塞大学阿默斯特分校和宾夕法尼亚州立大学气象系的合作，将计算结果与雷达观测结果进行比较。本研究的目标是开发一种利用雷达估计晶体习性和云冰含量的方法。研究结果可应用于遥感和大尺度大气数值模式中冰云辐射传输的参数化。

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.