Parameterization of Infrared Absorption in Midlatitude Cirrus Clouds

Abstract

Employing a new approach based on combined Raman lidar and millimeter-wave radar measurements and a parameterization of the infrared absorption coefficient σa (km?1) in terms of retrieved cloud microphysics, a statistical relation between σa and cirrus cloud temperature is derived. The relations σa = 0.3949 + 5.3886 ? 10?3T + 1.526 ? 10?5T2 for ambient temperature T(°C) and σa = 0.2896 + 3.409 ? 10?3Tm for midcloud temperature Tm(°C) are found using a second-order polynomial fit. Comparison with two σa-versus-Tm relations obtained primarily from midlatitude cirrus using the combined lidar?infrared radiometer (LIRAD) approach reveals significant differences. However, it is shown that this reflects both the previous convention used in curve fitting (i.e., σa ? 0 at ??80°C) and the types of clouds included in the datasets. Without such constraints, convergence is found in the three independent remote sensing datasets within the range of conditions considered to be valid for cirrus (i.e., cloud visible optical depth less than ?3.0 and Tm less than ??20°C). Hence, for completeness, reanalyzed parameterizations for a visible extinction coefficient σe-versus-Tm relation for midlatitude cirrus and a data sample involving cirrus that evolved into midlevel altostratus clouds with higher optical depths are also provided.