Sensitivity of ozone and temperature to vertical resolution in a gcm with coupled stratospheric chemistry

Abstract Results are presented from a general‐circulation model with comprehensive stratospheric photochemistry and which includes the coupling between radiative heating and simulated ozone. Each model integration covers the 60‐day period beginning 15 January during which episodes of polar stratospheric clouds (PSCs) are normally observed in the Arctic. Results from two versions of the model with different numbers of atmospheric levels near and above the tropopause are compared with observations. In the 19‐level model the ozone transport is poorly simulated and, in particular, there is a significant increase in the tropospheric column. In contrast, in the 49‐level model the simulated ozone distribution is in good general agreement with observations and reproduces well the steep vertical gradients in ozone mixing ratios in the tropical lower 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 distribution is well simulated, including this ozone in the radiation calculation has only a moderate influence on the results. However, with the 19‐level model, it substantially increases the global stratospheric temperature error. Increasing the number of levels improves the simulation 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 observed 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 stratosphere to ensure a realistic meridional circulation and a more accurate representation of ozone transport across the tropopause.