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) for mation is presented which is appropriate for use in two-dimensional (2 -D) photochemical models of the stratosphere. The calculation of PSC f requency of occurrence and surface area density uses climatological te mperature probability distributions obtained from National Meteorologi cal 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 chan ges in PSC formation and heterogeneous processing due to perturbations of stratospheric trace constituents. It is therefore useful in assess ing the potential effects of a fleet of stratospheric aircraft (high s peed civil transports, or HSCTs) on stratospheric composition. The mod el calculated frequency of PSC occurrence agrees well with a climatolo gy based on stratosphepic aerosol measurement (SAM) II observations. P SCs are predicted to occur in the tropics. Their vertical range is nar row, 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 bette r agreement with the total ozone mapping spectrometer (TOMS)-derived O -3 trend than otherwise. However, significant discrepancies in the nor thern midlatitudes remain. The overall changes in model O-3 response t o standard HSCT perturbation scenarios produced by the parameterizatio n are small and tend to decrease the model sensitivity to the HSCT per turbation. However, in the southern hemisphere spring a significant in crease in O-3 sensitivity to HSCT perturbations is found. At this loca tion and time, increased PSC formation leads to increased levels of ac tive chlorine, which produce the O-3 decreases.