Cirrus cloud infrared scattering and absorption in a GCM radiative model

The simulation of certain dynamical features by General Circulation Models (GCMs) has been shown to be sensitive to the treatment of cirrus cloud infrared (IR) radiative properties in these models. To address this, a new cirrus cloud IR parameterization, which accounts for scattering and absorption of IR radiation by cirrus particles assumed to be randomly oriented hexagonal ice crystals, is incorporated into the Community Climate Model version 3.6 (CCM3.6) and evaluated. This scheme furthers the ability to study the important role of cirrus clouds in radiative-hydrologic-dynamic feedback processes at low latitudes, particularly within the context of global change simulations.

A comprehensive set of cloud, radiation and meteorological properties that were measured in the equatorial region of the tropical Western Pacific from surface-, space- and aircraft-based platforms are used to evaluate a CCM3.6 radiative column model with the new parameterization. The data come from the ARM Pilot Radiation and Observation Experiment (PROBE), and the Pilot Tropical Cirrus Experiment (PTCE), which were held in or near Kavieng, New Ireland, Papua New Guinea as part of the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. Model-to-observation comparisons are conducted with the observed data and results from single time step calculations of the model. The comparisons specifically focus on top of the atmosphere and surface fluxes, as well as flux divergences and associated cooling rates during convective and cirroform episodes.

Global calculations are also conducted to determine the change in cloud radiative forcing and, as a result, estimate the potential impact of new scheme in climate simulations.