ARCUS Student Award | 1st Annual Award

1st Annual ARCUS Award for Arctic Research Excellence

Submitted by:

J. A. (Tony) Beesley


J. A. Beesley


Towards an Explanation of the Annual Cycle of Low-Cloud Amount Over the Arctic Ocean


Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA


The annual cycle of low cloud amount over the Arctic Ocean is examined using climatological data and a time-dependent atmospheric column model. Low stratiform clouds are a ubiquitous feature in the Arctic summer and are much less prevalent in the winter, with swift transitions in the spring and fall. They have a strong influence on the climate by interacting with long- and shortwave radiation and by modifying profiles of temperature and humidity in the lower troposphere. Three hypotheses for the annual cycle are formulated, compared with climatological data for consistency, and then tested using the model. The hypotheses identify three different factors that might produce the annual cycle in cloud amount over the Arctic Ocean: (1) water vapor advection into the Arctic, (2) evaporation at the surface of the ice pack, and (3) the temperature-dependent formation and precipitation of atmospheric ice. The hypotheses all appear to be consistent with available climatological data. A small but interesting exception is 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 weakens the first hypothesis.

The model used in this study includes processes necessary to simulate low stratiform cloudiness observed at lower latitudes plus ice phase physical processes that may be important over the Arctic. The boundary conditions consist of incoming solar radiation at the top of the model; profiles of divergence, temperature advection, and moisture advection at the sides; and open water fraction and ice thickness at the bottom. The model simulates the summer and winter cloud regimes when forced with the boundary conditions corresponding to those seasons. In an experimental simulation of winter climate, a parameter representing the triple point temperature of water is reduced from 0 °C to -20 °C. The low cloud amount in this wintertime experiment is greater than 90%. Results from the full set of 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 that represent atmospheric ice processes produce qualitatively more realistic simulations of the mean annual cycle of cloud amount over the Arctic Ocean than models without ice processes. It is predicted that the duration of the summertime cloudy season should be longer when the Arctic Ocean climate is warmer, and shorter when it is cooler.