SENSITIVITY OF OZONE AND TEMPERATURE TO VERTICAL RESOLUTION IN A GCM WITH COUPLED STRATOSPHERIC CHEMISTRY

Results are presented from a general-circulation model with comprehens ive stratospheric photochemistry and which includes the coupling betwe en radiative heating and simulated ozone. Each model integration cover s the 60-day period beginning 15 January during which episodes of pola r stratospheric clouds (PSCs) are normally observed in the Arctic. Res ults from two versions of the model with different numbers of atmosphe ric levels near and above the tropopause are compared with observation s. In the 19-level model the ozone transport is poorly simulated and, in particular, there is a significant increase in the tropospheric col umn. In contrast, in the 49-level model the simulated ozone distributi on is in good general agreement with observations and reproduces well the steep vertical gradients in ozone mixing ratios in the tropical lo wer stratosphere, and the weak vertical gradients in the high-latitude middle stratosphere. This version also maintains a virtually constant tropospheric ozone column. Since in the 49-level model the ozone dist ribution is well simulated, including this ozone in the radiation calc ulation has only a moderate influence on the results. However, with th e 19-level model, it substantially increases the global stratospheric temperature error. Increasing the number of levels improves the simula tion of stratospheric temperatures but the zonal-mean temperatures in both versions of the model are generally lower than observed. Despite this, the modelled frequency of occurrences of PSCs is less than obser ved because of the underprediction of the zonally asymmetric component of the temperature distribution. The results suggest that if coupled chemistry-climate simulations are to proceed, it is important to have both a high upper boundary and good vertical resolution in the lower s tratosphere to ensure a realistic meridional circulation and a more ac curate representation of ozone transport across the tropopause.