Toward an Explanation of the Annual Cycle of Cloudiness over the Arctic Ocean

The annual cycle of low cloud amount over the Arctic Ocean is examined using climatological data and a radiative-turbulent column model. Three hypotheses for the annual cycle are formulated, compared with climatological data for consistency, and then tested using the numerical model. The hypotheses identify three factors that might explain the difference in arctic low cloud amount between summer and winter: 1) the difference in surface specific humidity between the pack ice and the surrounding continents, 2) evaporation at the surface of the ice pack, and 3) the temperature-dependent formation and precipitation of atmospheric ice. Qualitatively, the hypotheses all appear to be consistent with available climatological data, except that the transition between the winter and summer cloudiness regimes occurs one month before the influx of atmospheric moisture increases from its wintertime level, which is inconsistent with the first hypothesis. The model, which includes a turbulence-closure cloud scheme and ice-phase microphysical processes, simulates the summer and winter cloud regimes when forced with the boundary conditions corresponding to those seasons. The model results suggest that the temperature dependence of ice-phase microphysical processes is an essential factor in explaining the annual cycle of low cloud amount. When the growth of atmospheric ice by vapor deposition is suppressed in the model, the winter case is as cloudy as the summer case. It is shown that, among general circulation models participating in the Atmospheric Model Intercomparison Project, those including ice microphysics produce more accurate simulations of the mean difference between summer and winter cloud amount over the Arctic Ocean than models without atmospheric ice processes. The results of this study suggest that the duration of the summertime cloudy season over the Arctic Ocean would be longer in a warmer climate and shorter in a cooler climate.