Authors: Timothy J. Garrett, Melissa M. Maestas and Chuanfeng Zhao
It has recently been shown that aerosol have the capacity to increase low cloud longwave emissivity in the Arctic. Increased thermal emission from arctic clouds is interesting because of its potential to augment the rate of seasonal sea-ice melt. Given pollution aerosol are most abundant in the Arctic during winter and spring, the seasonality of the warming is in phase with the melting. Of course, aerosol also indirectly cool the Arctic surface in summer, provided the surface is black, and the sea-ice or snow has melted. However, measurements at Barrow indicate that, averaged over the year, the net indirect forcing of the surface is strongly positive. Polluted clouds emit 10 to 15 W/m2 more longwave radiation than clean clouds. Notably, however, measurements and model results suggest that the magnitude of the effect appears to be influenced not only by increased density of droplet absorption cross-sections, but also by a positive feedback associated with aerosol-cloud-radiation interactions. Using a LES model, with idealized microphysics, but coupled radiation and dynamics, we create stratus from a moist boundary layer. What we find is that, in a polluted environment, aerosol pollution enhances cloud-top radiative cooling in thin stratus by making cloud thermally more opaque. Then, compared to clean cloud, the higher cooling makes dynamic motions more vigorous and the stratus layer deepen more rapidly: effectively the cloud evolution is accelerated. The combined effect of emissivity perturbations and radiatively induced feedbacks is to substantially amplify the effect of aerosol pollution on cloudy thermal emission and Arctic surface warming.
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