Studies on the characteristics and seasonal variations of precipitation phenomena obtained by radar observations at Syowa station, Antarctica 南極昭和基地のレーダー観測からみた降水の特徴と季節変動に関する研究

博士論文

Long-term observations of precipitating clouds were carried out by a vertical pointing radar, PPI radar and a 37 GHz microwave radiometer at Syowa Station (69°00'S, 40°35'E), Antarctica in 1989. The results obtained include the characteristics and seasonal variations of snowfalls. The former revealed the cause of precipitation near Syowa Station, Antarctica. It is concluded from the observations that precipitation near Syowa Station, Antarctica is mainly brought by cloud vortices associated with extratropical cyclones which advance to high latitude while developing to a mature stage. It is proposed that the outer side of the cloud vortex is composed of layer clouds above the warm front formed by warmer air advancing southward from low latitude. The warmer air is considered to be raised above the colder and drier air circulating westward around the polar high along the coast near Syowa Station, Antarctica. The inside of the vortex is compesed of convective clouds formed in colder air rotating around the low center along the welldeveloped spiral. Therefore the cloud vortex is concluded to be created by the occluded front composed of the cold front upgliding above the warm front. Precipitation from an occluded front crossing over Syowa Station was often observed when the low pressure center was stationary northwest of Syowa Station. The time-height cross sections of radar echoes in this case are considered to be normal cross sections of cloud vortex. It is concluded from the observations that the necessary conditions to bring large amount of precipitation to the coastal region near Syowa Station, Antarctica are that the low pressure system is in its mature stage, locates northwest of the region, and is stationary. Especially, a low pressure in its mature stage northwest of the station would bring a large amount of precipitation for a long time when the low pressure center moves westward in accordance with the orientation of the cloud band of cloud vortex. The seasonal variations of clouds and precipitation were analyzed corresponding to the seasonal changes of air temperature and sea ice area. The echo top height of stratified deeper clouds accompanied by the warm front changed with season corresponding to the change of height of -40℃. On the contrary the occurrence frequency of convective lower clouds decreased in winter and spring seasons when the air temperature is the lower and the sea ice area is the larger. The seasonal variations of the amount of vertically integrated liquid water content obtained by microwave radiometer also showed a similar trend corresponding to the seasonal change of sea ice area. The clouds having large amount of water content of liquid phase were frequently observed in fall when the open sea ice area expanded to Syowa Station. However, clouds in liquid phase were seldom observed in the austral winter and spring seasons when the sea ice area expanded to lower latitudes. The air temperature and the sea ice area influence not only activities of cloud systems but also activities and trajectories of low pressure system accompanied by cloud vortex. The occurrence frequencies of cloud vortex which brought snowfall to Syowa Station increased in fall and spring seasons corresponding to activity of the circumpolar trough. However, the activities of cloud systems that brings precipitation are weaken in spring when the sea ice area expands to low latitudes, because of less supply of heat and vapor. Thus the activities of cloud vortex are also weakened above the sea ice. In comparison with fall and spring, precipitation occurred frequently in fall associated with low pressures, whereas, precipitation occurred occasionally in spring associated with low pressures. Therefore if the amount of precipitation caused by each low pressure system varies, the amount of precipitation in spring can be expected to change more often from year to year than that in fall. In 1989, the amount of precipitation in spring brought by a few snowfall events was as large as the amount of precipitation in fall brought by frequent snowfall events. Radar observations revealed that there were three abundant snowfall seasons at Syowa Station and the amount of snowfall was uniform in all season except summer. The amounts of precipitation in fall, winter and spring seasons were 74, 74 and 53 mm respectively. The amounts of precipitation in fall and winter were large at Syowa Station in 1989. However, it is expected that the amount of snow in the continent would be small in winter from the distributions of precipitation measured by the PPI radar. Deeper clouds having much liquid water content caused a large amount of precipitation in the coastal region near Syowa Station, Antarctica. Therefore the season which contributes the most to the snow accumulation around Syowa Station, Antarctica should be fall because there are many deeper clouds with much liquid water content in that season.