An interactive cirrus cloud radiative parameterization for global climate models

An interactive cirrus cloud radiative parameterization is developed for global climate models from recent observations and analytical results that more accurately characterize cirrus cloud optical and microphysical properties. The radiative properties are based on the assumption that cirrus clouds are composed of hexagonal crystals. For the infrared component, a new mass absorption coefficient is parameterized to calculate emissivity, and for the solar, single-scattering properties from an existing parameterization are modified and employed. The solar and infrared optical properties are given as a function of ice water content and effective particle size. Aircraft observations are used to parameterize the microphysical properties in terms of temperature, thus allowing the radiative properties to interact with the local model climate. The interactive cirrus radiative parameterization is evaluated in model-to-observation comparisons with a comprehensive set of cloud and radiation measurements obtained during the spring 1994 and fall 1995 Intensive Observation Periods of the Atmospheric Radiation Measurement program. It is shown that the model with the new parameterization calculates realistic infrared radiation and improved solar radiation incident at the surface. Specifically, biases in calculated solar direct and diffuse fluxes are reduced by 60 and 40%, respectively. Further, the shortwave flux is shown to be more sensitive than the longwave flux to variability in the ice water content and in the base and top heights of observed clouds replicated in model calculations. The potential effect of the new parameterization on climate simulations is investigated in the context of initial radiative forcing. The new parameterization calculates a significantly different ice water path distribution from an existing parameterization that has been used for global climate change studies. For example, in the high latitudes of the summer hemisphere the new ice water path is larger by more than 7.7 g m^-2 (>100%), and in the tropics it can be smaller by as much as -3.5 g m^-2 (∼80%). These differences lead to an increased solar albedo effect in the high latitudes of the summer hemisphere and a decreased greenhouse effect in the tropics, both of which contribute to a smaller, 2.26 W m^-2, global- and annual-mean forcing of the surface-troposphere system.