Description and performance of the UIUC 24-layer stratosphere/troposphere general circulation model

This paper describes the 24-layer stratosphere/troposphere general circulat ion model (24-L ST-GCM) developed by the Climate Research Group of the Univ ersity of Illinois at Urbana-Champaign (UIUC). The model's dynamical and ph ysical processes, similar to those of its ancestors, are briefly described. The newly implemented parameterizations of longwave and solar radiation, c loud-radiation interaction, subgrid-scale orographic gravity wave drag, and aerosol radiative forcing are described in more detail. Sensitivity tests using simplified one-dimensional column models and the 24-L ST-GCM are carr ied out to evaluate these implementations. A 15-year simulation with prescr ibed climatological sea-surface temperatures and sea-ice extents has been p erformed. To determine the model's strengths and weaknesses, the simulated results are compared with observations. The model simulates well the geogra phical distributions of surface-air temperature and precipitation and their seasonal variations. The simulated cloud cover and cloud radiative forcing have the observed magnitudes and latitudinal variations, except near 60 de grees S where the model underestimates the cloud cover by similar to 20-30% . It is found that the large-scale cloud distribution and the cloud-top alt itude depend on the respective critical relative humidities for the onset o f large-scale precipitation and penetrating convection. The model captures reasonably well the observed features of atmospheric temperature and zonal wind in both the stratosphere and troposphere in all seasons, with the exce ption of the northern stratospheric polar-night jet. The simulated Transfor med-Eulerian-Mean residual circulation in the stratosphere has comparable m agnitudes and distributions to those obtained by data assimilation and othe r general circulation models (GCMs). The two-cell Brewer-Dobson circulation is captured. The use of an orographic-type gravity wave drag parameterizat ion is responsible for an abnormally warm northern polar stratosphere in wi nter, which is contrary to most other GCMs.