A radiation model is constructed that includes radiative interactions with atmospheric gases, as well as parameterized treatments of scattering and absorption/emission by cloud droplets and haze particles. A unified treatment of solar and terrestrial radiation is obtained by using identical cloud and haze parameterization procedure for the shortwave and longwave region. The influence of the relative humidity of the haze particles is also considered. Snow conditions of the arctic region are simulated in terms of snow grain sizes and soot contamination in the surface layers. Data from the Arctic Stratus Cloud Experiment collected in 1980 are used for model comparisons and sensitivity studies under cloudy and hazy sky conditions.

During the arctic summer, stratus clouds are a persistent feature and decrease the downward flux at the surface by about 130-200 W/m2. Arctic haze in the summertime is important if it is above the cloud layer or in air with low relative humidity, and it decreases the downward flux at the surface by about lo-12 W/m2. By comparison the greenhouse effect of doubling the carbon dioxide amount increases the downward flux at the surface by about 4-7 W/m2 and can be offset by the background haze or by an increase in cloudiness of about 4%.

Assuming steady microstructures of stratus clouds, we find that in late June a clear sky condition results in more available downward flux for snow melt (yielding a melting rate of 9.3 cm/day) than does a cloudy sky condition (6 cm/day). This is because the increase of infrared radiation diffused back to the surface by the cloud can not compensate for the reduction of solar radiation. When the snow starts to melt, the decreasing snow albedo further accelerates the melting process.