THE BERLIN TROPOSPHERE-STRATOSPHERE-MESOSPHERE GCM - SENSITIVITY TO PHYSICAL PARAMETRIZATIONS

The sensitivity of a troposphere-stratosphere-mesosphere general-circu lation model to changes in the radiation transfer and subgrid-scale dr ag parametrizations is investigated. The large interannual variability of the middle atmospheric circulation necessitated a methodological a pproach. A set of 1260-day, perpetual-January integrations was perform ed; this enabled significant signals to be extracted from the model va riability at a reasonable cost. A Rayleigh-friction scheme was used fo r the mesospheric drag. Reducing its strength led to changes in the zo nal-mean structure of the model which resembled the leading mode of va riability. Increasing its depth led to significant changes throughout the stratosphere and extending Into the troposphere. Including the diu rnal cycle induced no significant changes in the zonal-mean state belo w the stratopause. Radiation-transfer calculations are not performed f ully every time-step; decreasing the frequency of computation from 2 t o 12 hours caused marked changes in the mean structure and variability . Including a topographic gravity-wave drag parametrization (TGWD) res tored the zonal-mean structure of the 2-hour radiation frequency in th e lower stratosphere at the expense of changes in both the temporal va riability and the planetary-wave structures. The sensitivity to the fr equency of radiation calculations arose from the highly coupled nature of tropical cloud-radiation interactions: the modified structure of t he upper tropospheric divergence led to changes in the Rossby-wave sou rce term in the extratropics. The major conclusions are: (1) the natur al variability must be properly included in interpretations of the mod el sensitivity; (2) the adequacy of the tropospheric simulation can pr ofoundly affect the stratosphere; (3) changes in the mesospheric drag can modify the tropospheric circulation; and (4) deficiencies in one p arametrization (in this case radiation transfer) can be compensated by other changes (TGWD), but such 'improvements' may not apply to all as pects of the simulation.