Sensitivity of AGCM simulations to modifications in the ozone distributionand refinements in selected physical parameterizations

The sensitivity of the stratospheric circulation simulated by an atmospheri c general circulation model (UCLA. AGCM) to modifications in the parameteri zation of several key physical processes is investigated. The focus is on t he northern hemisphere winter circulation simulated by the troposphere-stra tosphere version of the model. The parameterization of ozone mixing ratio is first investigated by compari ng multi-year simulations with distributions of ozone mixing ratio that are either prescribed according to an observed climatology, or predicted using a scheme with a simple photochemistry formulation. The prescribed ozone mi xing ratios produce a considerably more realistic circulation, although the results are degraded in some regions of the stratosphere. This leads to a closer look into the radiation parameterization. Several simulations for th e northern winter season are performed with adjustments to obtain more real istic radiative cooling rates mainly in the upper stratosphere. It is found that the simulation of the stratospheric polar night jet is very sensitive to these adjustments. Additional experiments performed by adding Rayleigh friction and Newtonian cooling reveal a similar sensitivity. The results of these experiments emphasize a need for caution when ad-hoc adjustments are applied in the model. They also confirm that the performance of an AGCM in the stratosphere can depend on several different model aspects, and that t his dependence is not always straightforward. In addition, the authors argue that their results support the notion that p arameterizations of orographic gravity wave drag that neglect or underestim ate the drag in the troposphere may overestimate the drag in the stratosphe re.