An Accurate Parameterization of the Solar Radiative Properties of Cirrus Clouds for Climate Models

Abstract

An accurate parameterization of the solar radiative properties of cirrus clouds is developed based on improved light scattering calculations. Here 28 ice crystal size distributions from in situ aircraft observations in both tropical and midlatitude regions are employed. In the single scattering calculations, the most recent measurements of the imaginary refractive indices of ice are used, thereby eliminating a large existing uncertainty. The single scattering properties of hexagonal ice crystals are calculated by using an improved geometric ray-tracing program that can produce accurate results for size parameters larger than 15. A generalized effective size, Dge is defined to account for the ice crystal size distribution in the radiative calculations. Based on physical principles, the single scattering properties have been parameterized in terms of ice water content (IWC) and Dge. This allows the cirrus cloud single scattering properties to respond independently to changes in IWC or Dge. The generalized effective size can be related to the total cross-sectional area of ice particles per unit volume, a quantity directly measured by the 2D optical probe in in situ microphysical observations of cirrus clouds. The present parameterization of the extinction coefficient and the single scattering albedo in terms of IWC and Dge can be properly applied to cirrus clouds that contain various nonspherical particles, such as plates, columns, bullet rosettes, and aggregates, etc. The present parameterization of the single scattering properties of cirrus clouds is evaluated by examining the bulk radiative properties for a wide range of atmospheric conditions. Compared with reference results, the typical relative errors due to the parameterization are ?1.2%, ?0.3%, and ?2.9% in reflectance, transmittance, and absorptance, respectively. The accuracy of this parameterization guarantees its reliability in applications to climate models. Cloud absorption plays an important role in cloud-radiation interactions and therefore in climate systems. Because of the large variation in the co-albedo of ice near the wavelength of 1.41 ?m sum, one of the spectral divisions is chosen at 1.41 ?m to predict cloud absorption properly. Furthermore, the averaging technique for single scattering albedo in spectral intervals associated with absorption bands is important for the parameterization of radiative properties of ice clouds.