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Polar stratospheric cloud impacts on Antarctic stratospheric heating rates

The impact of polar stratospheric clouds (PSCs) on the stratospheric radiative heating rate is analysed by including a nominal PSC in heating‐rate calculations that incorporate realistic atmospheric variables including tropospheric clouds. The use of realistic atmospheric conditions constrains the possible radiative effects of PSCs, which previous studies have shown to be very sensitive to such variables as temperature, tropospheric clouds, and solar zenith angle. Over the pole, winter heating rates within the stratospheric polar vortex are decreased substantially by the presence of a PSC, while a PSC increases the heating rates equatorward of 75–85°S. Although the PSC always increases the short‐wave heating, the effect in the long‐wave region depends on the ground temperature, the stratospheric temperature, and presence of a tropospheric cloud. For the thickest PSCs (Type II), the effect in August 1994 varies from cooling by 0.25 K d−1 (potential‐temperature difference Δθ = 0.5 K d−1) at the pole to heating by 0.3 K d−1 (Δθ = 0.6 K d−1) at 65°S to slight cooling equatorward of 57°S. September 1994 results are similar. Calculated heating rates over the pole including PSCs are near –0.5 K d−1 (θ = –1 K d−1) for both months, and positive heating rates of up to 0.25 K d−1 (θ = 0.5 K d−1) occur near the vortex edge. Thinner PSCs (Type I) have less of an effect; for example, heating rates of 0.375 K d−1 (θ = 0.75 K d−1) occur over the pole in August when a Type I PSC is included. These results should be viewed as an upper bound to the effect of PSCs since the calculations specify 100% PSC cover; satellite results show that this assumption is not unreasonable within the vortex during winter and early spring, however. The increased latitudinal gradient in descent rates in the presence of a PSC is consistent with the behaviour of long‐lived trace‐gas observations, and strengthens the vortex relative to a PSC‐free case.