Simulation of polar stratospheric clouds in the specified dynamics version of the whole atmosphere community climate model

Abstract

We evaluate the simulation of polar stratospheric clouds (PSCs) in the Specified Dynamics version of the Whole Atmosphere Community Climate Model for the Antarctic winter 2005. In this model, PSCs are assumed to form instantaneously at a prescribed supersaturation, with a prescribed size distribution and number density. We use satellite observations of the Antarctic winter 2005 of nitric acid, water vapor, and PSCs to test and improve this PSC parameterization. Cloud-Aerosol Lidar with Orthogonal Polarization observations since 2006 show that in both hemispheres, the dominant PSC type throughout the entire polar winter is a mixture of Nitric Acid Trihydrate (NAT) and Supercooled Ternary Solutions droplets, but typical assumptions about PSC formation in the model at a given supersaturation do not produce such a population of particles and lead to earlier removal of HNO3 from the gas phase compared to observations. In our new PSC scheme, the formation of mixed PSCs is forced by only allowing a fraction of total available HNO3 to freeze to NAT and the remaining part to form STS. With this approach, a mixture of both is present throughout the winter, in agreement with observations. This approach yields good agreement with observations in terms of temperature-dependent removal of gas-phase HNO3 and irreversible denitrification. In addition to nitric acid containing PSCs, we also investigate ice PSCs. We show that the choice of required saturation ratio of water vapor for ice formation can significantly improve the calculated vertical distribution of water vapor and is required to produce good agreement with observations.