CHARACTERISTICS OF STRATOSPHERE-TROPOSPHERE EXCHANGE IN A GENERAL-CIRCULATION MODEL

Air and trace gases are exchanged between the stratosphere and the troposphere on a variety of scales; but general circulation models (GCMs) are unable to represent the smaller scales. It would be useful to see how a GCM represents stratosphere-troposphere exchange (STE), both to identify possible model deficiencies which would affect other studies and to see how important the smaller-scale physics might be in the atmosphere itself. Our understanding of observed STE depends largely on inferences from tracer distributions. In this study we examine mass exchange, water vapor exchange, and the behavior of idealized tracers and parcels to diagnose STE in the National Center for Atmospheric Research GCM, the Community Climate Model (CCM2). The CCM2 correctly represents the seasonality of mass exchange across 100 hPa, but values are uniformly too strong. Water vapor, however, indicates that tropical STE is not well represented in the CCM2; even though mean tropopause temperatures are colder than observed, the lower stratosphere is too moist. Most net mass flux occurs at water vapor mixing ratios of about 4-5 parts per million by volume (ppmv), about 1 ppmv too moist. Vertical resolution has little impact on the nature of tropical STE. In midlatitudes, CCM2 more successfully represents STE, which occurs in developing baroclinic waves and stationary anticyclones. Exchange from troposphere to stratosphere does occur but only influences die lowest few kilometers of the extra-tropical stratosphere, even for tracers with large vertical gradients.