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Si-Chee Tsay and Kolf Jayaweera

Abstract

Observations of the physical properties of Arctic status clouds (ASC) over the Beaufort Sea area were made by the NCAR Electra aircraft during June 1980. The cloud morphology and microstructure observed during these experiments are described here. Arctic stratus clouds were formed under various meteorological conditions, but not when the axis of the Beaufort Sea ridge was zonal and the airflow into the region was continental.

The mean drop diameter in clouds observed under all conditions remained near 10 μm, while the mean liquid water content (LWC) was characteristic of the air mass forming the clouds and essentially determined by the mean drop concentration. Clouds showed considerable horizontal homogeneity but significant vertical changes occurred within them. The vertical profiles of LWC show that the values generally increased with height, as a result of an increase in droplet size rather than concentration. The drop size distribution near the base was monomodal, characteristic of condensation on a nucleus spectrum, but changed to a bimodal distribution near the top of the cloud.

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Takeshi Ohtake, Kolf Jayaweera, and Ken-Ichi Sakurai

Abstract

Clear sky ice crystals or diamond dust displays are observed in polar regions, both remote and populated; when the temperature falls to −20°C and where abundant sources of water vapor are present. In remote areas of the Arctic, these ice crystals are confined to the lowest 1000 m, leeward of open leads in the sea ice. The crystals always occur in air between ice and water saturation, and air dryer than ice saturation normally overlies the display. In populated regions, water vapor is supplied from man-made sources such as heating plants. Morphological studies of these ice crystals suggest that they are formed first by condensation into water droplets and then by freezing.

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Si-Chee Tsay, Knut Stamnes, and Kolf Jayaweera

Abstract

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 m−2. 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 10–12 W m−2. By comparison the greenhouse effect of doubling the carbon dioxide amount increases the downward flux at the surface by about 4–7 W m−2 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 em day−1) than does a cloudy sky condition (6 cm day−1). 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.

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