Polar stratospheric cloud impacts on Antarctic stratospheric heating rates

The impact of polar stratospheric clouds (PSCs) on the stratospheric radiat ive heating rate is analysed by including a nominal PSC in heating-rate cal culations that incorporate realistic atmospheric variables including tropos pheric clouds. The use of realistic atmospheric conditions constrains the p ossible radiative effects of PSCs, which previous studies have shown to be very sensitive to such variables as temperature, tropospheric clouds, and s olar zenith angle. Over the pole, winter heating rates within the stratosph eric polar vortex are decreased substantially by the presence of a PSC, whi le a PSC increases the heating rates equatorward of 75-85 degreesS. Althoug h the PSC always increases the short-wave heating, the effect in the long-w ave region depends on the ground temperature, the stratospheric temperature , and presence of a tropospheric cloud. For the thickest PSCs (Type II), th e effect in August 1994 varies from cooling by 0.25 K d(-1) (potential-temp erature difference Delta(theta) over dot = 0.5 K d(-1)) at the pole to heat ing by 0.3 K d(-1) (Delta(theta) over dot = 0.6 K d(-1)) at 65 degreesS to slight cooling equatorward of 57 degreesS. September 1994 results are simil ar. Calculated beating rates over the pole including PSCs are near -0.5 K d (-1) ((theta) over dot = -1 K d(-1)) for both months, and positive heating rates of up to 0.25 K d(-1) ((theta) over dot =0.5 K d(-1)) occur near the vortex edge. Thinner PSCs (Type 1) have less of an effect; for example, hea ting rates of 0.375 K d(-1) ((theta) over dot = 0.75 K d(-1)) occur over th e pole in August when a Type I PSC is included. These results should be vie wed as an upper bound to the effect of PSCs since the calculations specify 100% PSC cover; satellite results show that this assumption is not unreason able within the vortex during winter and early spring, however. The increas ed latitudinal gradient in descent rates in the presence of a PSC is consis tent with the behaviour of long-lived trace-gas observations, and strengthe ns the vortex relative to a PSC-free case.