EFFECTS OF A POLAR STRATOSPHERIC CLOUD PARAMETERIZATION ON OZONE DEPLETION DUE TO STRATOSPHERIC AIRCRAFT IN A 2-DIMENSIONAL MODEL

A parameterization of Type 1 and 2 polar stratospheric cloud (PSC) formation is presented which is appropriate for use in two-dimensional (2-D) photochemical models of the stratosphere. The calculation of PSC frequency of occurrence and surface area density uses climatological temperature probability distributions obtained from National Meteorological Center data to avoid using zonal mean temperatures, which are not good predictors of PSC behavior. The parameterization does not attempt to model the microphysics of PSCs. The parameterization predicts changes in PSC formation and heterogeneous processing due to perturbations of stratospheric trace constituents. It is therefore useful in assessing the potential effects of a fleet of stratospheric aircraft (high speed civil transports, or HSCTs) on stratospheric composition. The model calculated frequency of PSC occurrence agrees well with a climatology based on stratosphepic aerosol measurement (SAM) II observations. PSCs are predicted to occur in the tropics. Their vertical range is narrow, however, and their impact on model O-3 fields is small. When PSC and sulfate aerosol heterogeneous processes are included in the model-calculations, the O-3 change for 1980 - 1990 is in substantially better agreement with the total ozone mapping spectrometer (TOMS)-derived O-3 trend than otherwise. However, significant discrepancies in the northern midlatitudes remain. The overall changes in model O-3 response to standard HSCT perturbation scenarios produced by the parameterization are small and tend to decrease the model sensitivity to the HSCT perturbation. However, in the southern hemisphere spring a significant increase in O-3 sensitivity to HSCT perturbations is found. At this location and time, increased PSC formation leads to increased levels of active chlorine, which produce the O-3 decreases.