LARGE-SCALE ICE CLOUDS IN THE GFDL SKYHI GENERAL-CIRCULATION MODEL

Ice clouds associated with large-scale atmospheric processes are studi ed using the SKYHI general circulation model (GCM) and parameterizatio ns for their microphysical and radiative properties. The ice source is deposition from vapor, and the ice sinks are gravitational settling a nd sublimation. Effective particle sizes for ice distributions are rel ated empirically to temperature. Radiative properties are evaluated as functions of ice path and effective size using approximations to deta iled radiative-transfer solutions (Mie theory and geometric ray tracin g). The distributions of atmospheric ice and their impact on climate a nd climate sensitivity are evaluated by integrating the SKYHI GCM (dev eloped at the Geophysical Fluid Dynamics Laboratory) for six model mon ths. Most of the major climatological cirrus regions revealed by satel lite observations appear in the GCM. The radiative forcing associated with ice clouds acts to warm the Earth-atmosphere system. Relative to a SKYHI integration without these clouds, zonally averaged temperature s are warmer in the upper tropical troposphere with ice clouds. The pr esence of ice produced small net changes in the sensitivity of SKYHI c limate to radiative perturbations, but this represents an intricate ba lance among changes in clear-, cloud-, solar-, and longwave-sensitivit y components. Deficiencies in the representation of ice clouds are ide ntified as results of biases in the large-scale GCM fields which drive the parameterization and neglect of subgrid variations in these field s, as well as parameterization simplifications of complex microphysica l and radiative processes.