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Characterizing Particle Size, Water Path and Photon Tunneling in Ice and Water Clouds

Global information of ice water path (IWP) in ice clouds is urgently needed for testing of global climate models (GCMs) and other applications, but satellite retrievals of IWP are still in the developing stages, and tend to have large uncertainties (e.g. factor of 3 or more). Results of a new retrieval method are presented here that may have relatively low uncertainties, using several thermal-radiance channels. Recent research indicates that ice-cloud radiative properties depend on more than effective diameter (DEFF) and ice water content (IWC). The size distribution shape, or dispersion, such as the degree of bimodality, is also an important factor at thermal wavelengths. On the other hand, based on new information on ice particle mass- and area-dimensional relationships, ice particle shape uncertainties now appear to have minimal adverse influence on IWP retrievals. Consequently, we have developed a retrieval scheme that accounts for the size distribution (SD) shape in mid-latitude and tropical cirrus. By using a channel centered near 3.9 µm, absorption is largely a volume-dependent radiative process. This reduces particle shape uncertainties, which are associated with area-dependent absorption. Radiation-transfer model simulations that vary particle size, IWC and cloud depth suggest our IWP retrieval scheme is accurate to about ±15% over an IWP range of 1 to 100 g m -2 , and about ±25% over an IWP range of 100 to 200 g m -2 , except for very thin cirrus having unusually large crystals or multiple cloud layers. Retrieval results with ground-based AERI data are presented for ARM cirrus IOPs of 9 and 13 March 2000. IWP retrievals are compared with in-situ aircraft data, and coincident GOES retrievals are presented as well.