Retrieval of ice cloud properties using hyperspectral infrared channel observations

Atmosphere Infrared Sounder (AIRS) provides a unique opportunity to infer cloud properties using high spectral resolution infrared data under both daytime and nighttime conditions. In this study, we use the AIRS data acquired over a tropical oceanic region to retrieve ice cloud properties (cloud top pressure, effective cloud amount, cloud phase, cloud optical thickness, and effective particle size). Ice clouds are classified in terms of the cloud-top pressure similar to the approach used by the International Satellite Cloud Climatology Project (ISCCP). The classical CO₂ slicing method is used to infer cloud top pressure and effective cloud amount (cloud emittance multiplied by cloud fraction). The cloud thermodyamic phase is determined from a bispectral method involving the 8.5 and 11- μm channels. Using the information about cloud top pressure, effective cloud amount, and cloud phase, we further retrieve cloud optical thickness (τ) and effective particle size (D_e) on the basis of the sensitivities of brightness temperature to τ and D_e in the region of 760-990 cm^-1 and 1060-1130 cm^-1, respectively. These bands are in the window region. Thus, water vapor or other minor atmospheric gases are not important for these channels. For the retrieval of cloud optical thickness and effective particle size, the simulated annealing method is used, which finds the global minimum function value in two (or more) dimensions. Furthermore, the present retrievals are compared with similar products from the Moderate Resolution Imaging Spectroradiometer (MODIS).