The Berlin troposphere–stratosphere–mesosphere GCM: Sensitivity to physical parametrizations

The sensitivity of a troposphere–stratosphere–mesosphere general‐circulation model to changes in the radiation transfer and subgrid‐scale drag parametrizations is investigated. The large interannual variability of the middle atmospheric circulation necessitated a methodological approach. A set of 1260–day, perpetual‐January integrations was performed; this enabled significant signals to be extracted from the model variability at a reasonable cost. A Rayleigh‐friction scheme was used for the mesospheric drag. Reducing its strength led to changes in the zonal‐mean structure of the model which resembled the leading mode of variability. Increasing its depth led to significant changes throughout the stratosphere and extending into the troposphere. Including the diurnal cycle induced no significant changes in the zonal‐mean state below the stratopause. Radiation‐transfer calculations are not performed fully every time‐step; decreasing the frequency of computation from 2 to 12 hours caused marked changes in the mean structure and variability. Including a topographic gravity‐wave drag parametrization (TGWD) restored the zonal‐mean structure of the 2‐hour radiation frequency in the lower stratosphere at the expense of changes in both the temporal variability and the planetary‐wave structures. The sensitivity to the frequency of radiation calculations arose from the highly coupled nature of tropical cloud–radiation interactions: the modified structure of the upper tropospheric divergence led to changes in the Rossby‐wave source term in the extratropics. The major conclusions are: (1) the natural variability must be properly included in interpretations of the model sensitivity; (2) the adequacy of the tropospheric simulation can profoundly affect the stratosphere; (3) changes in the mesospheric drag can modify the tropospheric circulation; and (4) deficiencies in one parametrization (in this case radiation transfer) can be compensated by other changes (TGWD), but such ‘improvements’ may not apply to all aspects of the simulation.