SENSITIVITY OF THE CLIMATE RESPONSE OF AN ATMOSPHERIC GENERAL-CIRCULATION MODEL TO CHANGES IN CONVECTIVE PARAMETERIZATION AND HORIZONTAL RESOLUTION

Three equilibrium doubled CO2 experiments have been performed using th e Bureau of Meteorology Research Centre atmospheric general circulatio n model. These experiments used identical versions of the model, apart from changes in the convective parameterization and the horizontal re solution. The penetrative convection parameterizations used were varia nts of the Tiedtke (1989) mass flux and Kuo (1974) schemes. The shallo w convection was also varied in strength. In the control climate the m ass flux scheme produces a warmer, moister troposphere, with substanti ally more high cloud than the Kuo scheme. The precipitation distributi ons agree reasonably with observations overall, although the mass flux scheme gives improvements in some regions. The increase in resolution is generally found to have a smaller impact upon the climate than the change in convection. Under a doubling of atmospheric CO2, the equili brium responses of all three experiments were extremely similar in sur face and tropospheric temperatures and humidity changes. The model res ponse is at the low end of the scale of simulated climate change, cons istent with a strong negative feedback found due to clouds. This feedb ack is similar to that found in earlier fixed season experiments. It a ppears to be insensitive to differences in cloud cover simulated in th e control climates of the present experiments.