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Persistent and extensive layers of stratiform cloud occur over the Arctic basin from late spring until early fall. The frequency of low cloudiness is greater than seventy percent, and these high values are found in all regions of the Central Polar Ocean and the peripheral seas. The clouds occur frequently as two or more well-defined stratus layers approximately 300 m thick that are separated by distinct clear regions of the same approximate depth. We attempt to find self-consistent radiative-diffusive models that will predict the observed multiple stratus layers. We consider the formation of Arctic stratus clouds in the relatively warmer and moister continental air as it flows over the pack ice. The initial profiles of temperature and relative humidity as well as their values at the surface are specified in advance, and the modification of the continental air is calculated as a time marching problem, Turbulent transport in the surface layer is parameterized by the Monin-Obukhov similarity formulation. Above the surface layer a simple mixing-length formulation is used, except in the unstable cases when a convective adjustment is used. Solar radiation is treated with Chandrasekhar's "First Approxi-mation". The water vapor spectrum in the NIR is represented with two grey absorption coefficients, and the cloud is treated as a grey scatterer and absorber. Thermal radiation is computed with emissivities. The cloud is a grey absorber and emitter in the thermal spectrum, and a "mixed-emissivity" is computed for the cloud-gas mixture. We find that condensation is induced in an initially unsaturated air mass due to diffusive cooling to the colder surface and longwave emission to space. Intense mixing within the cloud is generated by strong cooling due to droplet emission at the cloud top. A bi-layered structure develops, and we attribute this to a greenhouse mechanism whereby solar radiation penetrates to the interior of the cloud and causes evaporation there, while at the same time the top remains cold due to emission to space, and the lowest layer remains cold since the surface temperature is fixed at 0 0 C. A mechanism is also suggested for the lifting of a cloud top due to longwave emission in the clear atmosphere directly above the cloud. It is suggested that the persistence of stratus clouds in the Arctic is due to the absence of dissipative mechanisms that are ordinarily encountered at lower latitudes. Condensation occurs initially over a fairly rapid time scale, but convective …

Radiative-diffusive models of the artic boundary layer