The Impact of Polar Stratospheric Clouds on the Heating Rates of the Winter Polar Stratosphere

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  • 1 Space Science Division, NASA Ames Research Center, Moffett Field, CA 94035
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Abstract

We have computed the perturbation to the infrared radiative heating rates of the lower stratosphere due to the occurrence of polar stratospheric clouds (PSCs) during the winter season in the Antarctic and Arctic regions. The calculations were made with a multispectral radiative transfer code that allows for scattering, absorption, and thermal emission by particles and gases. We investigated perturbations due to both particulate opacity of the PSCs (direct effect), and to the partial condensation, and hence, decrease of H20 vapor accompanying their formation (indirect effect).

For plausible values of model parameters, the direct effect is always one of increased radiative cooling, while the indirect effect is always one of decreased cooling. On a synoptic time scale of a single PSC event (∼ days), the net effect is probably one of enhanced cooling, with its magnitude having an important impact on stratospheric heating rates only for the most optically thick PSCs (extinction coefficient > 10−1 km−1). On the time scale of the winter season (∼ mouths), the cumulative radiative effect of multiple PSC formation may be significant for the heat budget and temperatures in the Antarctic region, but probably not so in the Arctic, where PSCs are both optically thinner and less frequent. In cases where the short- and long-term radiative effects of PSCs are significant, they act to alter the growth rate of individual PSC events and the frequency of occurrence of PSCs, respectively.

Abstract

We have computed the perturbation to the infrared radiative heating rates of the lower stratosphere due to the occurrence of polar stratospheric clouds (PSCs) during the winter season in the Antarctic and Arctic regions. The calculations were made with a multispectral radiative transfer code that allows for scattering, absorption, and thermal emission by particles and gases. We investigated perturbations due to both particulate opacity of the PSCs (direct effect), and to the partial condensation, and hence, decrease of H20 vapor accompanying their formation (indirect effect).

For plausible values of model parameters, the direct effect is always one of increased radiative cooling, while the indirect effect is always one of decreased cooling. On a synoptic time scale of a single PSC event (∼ days), the net effect is probably one of enhanced cooling, with its magnitude having an important impact on stratospheric heating rates only for the most optically thick PSCs (extinction coefficient > 10−1 km−1). On the time scale of the winter season (∼ mouths), the cumulative radiative effect of multiple PSC formation may be significant for the heat budget and temperatures in the Antarctic region, but probably not so in the Arctic, where PSCs are both optically thinner and less frequent. In cases where the short- and long-term radiative effects of PSCs are significant, they act to alter the growth rate of individual PSC events and the frequency of occurrence of PSCs, respectively.

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