PARAMETERIZATION OF THE RADIATIVE PROPERTIES OF CIRRUS CLOUDS

A new approach for parameterization of the broadband solar and infrare d radiative properties of ice clouds has been developed. This paramete rization scheme integrates in a coherent manner the delta-four-stream approximation for radiative transfer, the correlated k-distribution me thod for nongray gaseous absorption, and the scattering and absorption properties of hexagonal ice crystals. A mean effective size is used, representing an area-weighted mean crystal width, to account for the i ce crystal size distribution with respect to radiative calculations. B ased on physical principles, the basic single-scattering properties of ice crystals, including the extinction coefficient divided by ice wat er content, single-scattering albedo, and expansion coefficients of th e phase function, can be parameterized using third-degree polynomials in terms of the mean effective size. In the development of this parame terization the results computed from a light scattering program that i ncludes a geometric ray-tracing program for size parameters larger tha n 30 and the exact spheroid solution for size parameters less than 30 are used. The computations are carried out for 11 observed ice crystal size distributions and cover the entire solar and thermal infrared sp ectra. Parameterization of the single-scattering properties is shown t o provide an accuracy within about 1%. Comparisons have been carried o ut between results computed from the model and those obtained during t he 1986 cirrus FIRE IFO. It is shown that the model results can be use d to reasonably interpret the observed IR emissivities and solar albed o involving cirrus clouds. The newly developed scheme has been employe d to investigate the radiative effects of ice crystal size distributio ns. For a given ice water path, cirrus clouds with smaller mean effect ive sizes reflect more solar radiation, trap more infrared radiation, and produce stronger cloud-top cooling and cloud-base heating. The lat ter effect would enhance the in-cloud heating rate gradients. Further, the effects of ice crystal size distribution in the context of IR gre enhouse versus solar albedo effects involving cirrus clouds are presen ted with the aid of the upward flux at the top of the atmosphere. In m ost cirrus cases, the IR greenhouse effect outweighs the solar albedo effect. One exception occurs when a significant number of small ice cr ystals are present. The present scheme for radiative transfer in the a tmosphere involving cirrus clouds is well suited for incorporation in numerical models to study the climatic effects of cirrus clouds, as we ll as to investigate interactions and feedbacks between cloud microphy sics and radiation.