小西 啓之

Long-term observations of precipitating clouds were carried out by a vertical pointing radar and plan-position indicator (PPI) radar at Syowa station (69.0 degrees S, 40.5 degrees E), Antarctica, in 1989. The observations revealed that there were three abundant snowfall seasons at Syowa station and the amount of snowfall was uniform in all seasons except summer. The amounts of precipitation in autumn, winter and spring were 74, 74 and 53 mm (liquid water equivalent), respectively. The amounts of precipitation in autumn and winter were large at Syowa station in 1989. However, the amount of precipitation on the inland ice is expected to be small in winter, indicated by the distributions of precipitation measured by the PPI radar. The occurrence frequencies of cloud vortices which brought snowfall to Syowa station increased in the autumn and spring seasons, corresponding to the activity of the circumpolar trough. However, the activities of cloud systems that bring precipitation weaken in spring when the sea-ice area expands to low latitudes, because of a lower supply of heat and vapor. Thus the activities of cloud vortices are also weakened above the sea ice.

Snow clouds were observed by vertically-pointing radar and microwave radiometer at Syowa Station in Antarctica in 1989 to measure vertically integrated ice water content (IWC) and liquid water content (LWC) in the clouds. Most of the water-rich clouds, which are defined here as those of more than 40 mg/cm2 in LWC, appeared in autumn, when the area of sea ice was at its annual mininum. On the other hand, the water-poor clouds, which are defined as those less than 10 mg/cm2 in LWC, appeared in almost all seasons, especially frequently in winter and early spring when the sea ice area was at its annual maximum. The occurrence frequency of these clouds seems to correlate with the area of sea ice rather than with air temperature. The convective activity to produce supercooled water droplets becomes suppressed during their passage above the sea ice with less supply of water vapor from the sea. There was a difference in the amount and area of snowfall among the clouds. The water-rich clouds brought much more snowfall within 50 km of the coastline than the water-poor clouds. This localization of snowfall would stem from the orographic effect caused by production of water condensate in clouds due to lifting of air along the slope of the continent. The water-poor clouds brought less condensed water after lifting.