The climatic effects and requirements of arctic clouds
Beesley, John Anthony
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The climatic effect and requirements of clouds over the Arctic Ocean are examined using a pair of one-dimensional models. A coupled ice-atmosphere column model with prescribed cloud properties is used to examine the effect of clouds on the central arctic climate. When forced with boundary conditions representative of the Central Arctic, the model produces a realistic simulation of the seasonal cycle of arctic air temperature, ice thickness, and radiative fluxes. Unlike previous studies, the results suggest that the annual net effect of clouds is to increase ice thickness slightly. This can be attributed to the fact that previous studies have not accounted for the effect of clouds on the atmospheric temperature profile, which is an internally-computed model variable in the present study. The results indicate that the response of sea ice thickness to a perturbation in clouds can be predicted by estimating how the cloud perturbation affects the radiative budget at the top of the atmosphere.The annual cycle of low-cloud amount over the Arctic Ocean is examined using climatological data and a time-dependent atmospheric column model. Three hypotheses for the annual cycle are formulated, compared with climatological data for consistency, and then tested using a numerical model. The model, which includes a turbulence-closure cloud scheme and ice-phase micro-physical processes, simulates the summer and winter cloud regimes when forced with the boundary conditions corresponding to those seasons. Results from the model experiments suggest that the main factor in determining the annual cycle of low cloud amount is the effectiveness of ice processes in eliminating liquid water clouds and preventing the water vapor mixing ratio in clear air from reaching saturation with respect to liquid. It is shown that, among general circulation models participating in the Atmospheric Model Intercomparison Project, those including ice microphysics produce qualitatively more realistic simulations of the mean annual cycle of cloud amount over the Arctic Ocean than models without atmospheric ice processes. The results suggest that the duration of the summertime cloudy season should be longer when the Arctic Ocean climate is warmer, and shorter when it is cooler.
- Atmospheric sciences