Effect of Cloud Vertical Inhomogeneity on the Retrieval of Cirrus Cloud Temperature and Infrared Optical Depth Using the ASTR

Abstract

A method is described for the retrieval of cirrus cloud temperature and optical depth using thermal infrared data from the Along-Track Scanning Radiometer. The method utilizes above-cloud and nearby clear-sky thermal infrared data at a single wavelength and two different viewing angles and assumes that the cirrus cloud is nonscattering, isothermal, and semitransparent. The sensitivity of the method to small uncertainties in the input parameters is calculated. The effect on the retrieval of vertical inhomogeneity is investigated using idealized models of cirrus cloud vertical structure. It is shown that a vertical temperature structure within the cloud, alone and in conjunction with vertical inhomogeneity in absorption coefficient, can cause large errors in the retrieved quantities for a wide range of cloud types. However, these investigations show that retrieved quantities remain within usable limits for the majority of expected cirrus clouds. For example, for clouds with a lapse rate of 9 K km?1 and a linear absorption coefficient profile with gradient ranging from ?2 to +2, optical depth can be retrieved to an accuracy of better than 20% and temperature to within 10 K of the midcloud temperature, for clouds of thickness 2 km or less and optical depths between 0.8 and 4.