Vertical Sizing of Cirrus Cloud Using 1.38μm Spectral Reflectance and MODIS Data

Based on the sensitivity of 1.38μm spectral reflectance to ice crystal optical properties and water vapor absorption, we have developed a conceptual approach to infer the vertical distribution of ice crystal mean effective size. We have selected 17 channels between 6600 and 7500cm-1, 12 of them are located between 6600 and 6800cm-1, where ice crystal single-scattering albedo is sensitive to cloud particle size, and the water vapor absorption optical thickness varies from 1.8 × 10- 5 to 1.8 for a typical cirrus cloud located between 9 to 10km in a US standard atmosphere. The remaining 5 channels are located between 7200 and 7500cm-1, where the cirrus cloud absorption is minimal and reflectance is dependent on cloud optical thickness. Channels with relatively stronger water vapor absorption in this region also bear information of cloud temperature. We simulate the 1.38μm bidirectional reflectance spectrum in cirrus cloudy conditions on the basis of an adding-doubling radiative transfer program coupled with the correlated k-distribution approach for sorting absorption lines using 0.01cm-1 spectral interval. A library of reflectance at the 17 channels for a variety of two-layer cirrus clouds has been built and a three-step retrieval scheme based on look-up table approach has been developed. Synthetic retrievals for a number of cases involving small ice crystals aloft and large particles at the cloud bottom illustrate some success. We also apply the principle of the multi-layer multi-channel retrieval scheme to the MODIS visible and near-IR channels (0.645, 1.375,1.64, 2.13, and 3.75 μm) data. We have selected two cirrus cases over the ARM-SGP site on March 6,2001 and March 30, 2002, with significant presence of cirrus clouds, and inferred the vertical stratification of mean effective sizes. Results are validated using independent ground-based radar measurements.