Formation and Persistence of Summertime Arctic Stratus Clouds

We have developed a numerical model which can explain the principle observed properties of the summertime stratus clouds occurring over the Arctic Basin. Warm, moist air from the surrounding land masses is modified as it passes over the melting pack ice. Motions are calculated on the basis of a fixed pressure field, but all other relevant fields (temperature, mixing, solar and terrestrial radiation, water in the liquid and vapor phase) are calculated on the basis of an internally consistent thermodynamic model which is considered to be complete in all essential physical processes. The ubiquitous occurrence of stratus clouds is explained in terms of the relative unimportance of cloud dissipative processes over the Arctic Ocean. The commonly observed layering results from the trapping of solar radiation inside a cloud which is opaque to thermal radiation. The absence of similar layering in mid-latitude stratus is probably due to the diurnal variation of solar radiation, which is largely absent in the Arctic. The seasonal variation of the occurrence of stratus cloud is explained in terms of the changing surface temperature, but other factors may also be involved. Two appendices present simple heuristic models for two of the most interesting features of Arctic stratus: the growth of the cloud top and the development of layers.