Microweve Radiative Transfer through Clouds Composed of Realistically Shaped Ice Crystals. Part II. Remote Sensing of Ice Clouds

This paper presents the results of polarized microwave radiative transfer modeling of cirrus clouds containing five different particle shoes and 18 Gamma size distributions. Upwelling brightness temperatures for tropical and midlatitude winter atmospheres are simulated at 85.5, 157, 220, and 340 GHz using scattering properties computed with the discrete dipole approximation (described in Part I). The key parameter for the results is the sensitivity (ΔTb/IWP), which relates the modeled brightness temperature depression to the ice water path. It is shown that for the higher frequencies or distributions of larger particles (i.e., in the scattering regime) the sensitivity is nearly independent of cloud temperature and details of the underlying atmosphere. As expected from the single-scattering results, the characteristic particle size has a large effect on the sensitivity, while the distribution width has only a minor effect. The range in sensitivity over the five particle shapes is typically a factor of 2. The sensitivity for a size distribution of solid columns with a median of the third power of the dimension of 250 µm is about 0.1 K/g m−2). Ratios of ΔTb's at adjacent frequencies can determine the characteristic size of the distribution, though the relationship is double valued for the most sensitive frequencies considered here. Ratios of ΔTb at horizontal to vertical polarization contain information about particle shape primarily via the aspect ratio. Ideas concerning the development of a specific cirrus retrieval algorithm are discussed.