気象衛星写真より見た冬季の日本周辺の雲分布の特徴
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概要
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In order to understand a general view of the cloud distribution around Japan in winter, the monthly-mean cloud distributions in the winter, 1977-1978 were made, using the visible photographs taken by the meteorological satellite NOAA-5. The cloud amount were measured on photographs on each intersection of latitudinal and longitudinal lines by 0.5 degrees over the region from 28°N to 52°N, 128°E to 152°E, only over the sea, for the period from December, 1977 to February, 1978. The cloud amount on each point is taken to be as 1 (or 0), if the cloud exists on the point (or not).The seasonal-mean cloud distribution was obtained by averaging for the period of three months. Six axes of the cloudy areas can be set up over the Japan Sea, and four axes off the Pacific side of the Japan Islands. These axes of the cloudy areas seem to relate the upwind topography. The clear areas with lower frequency extend to the lee of the mountains and islands which prevent northwesterly wind of the winter monsoon. The clear areas to the lee of the Japan Islands were very narrow and vanish in a short distance.In February, the band-shaped structure of the cloud distribution is striking, and the cloud amount is generally larger than those in December and January. Cloud pattern in February shifts to the south from that in January, especially in the northern part of the Japan Sea.From the investigation of relationship between the cloud amount and synoptic situation, it is shown that the large cloud amount is associated with the lows and fronts in a synoptic scale. However, the large cloud amount which is not associated with the lows and fronts, appears when the cold air outbreak occurs. The mean cloud distribution during the cold air outbreak shows the band-shaped structure of the cloud distribution clearly.In conclusion, the band structure of the cloud distribution appears prevalently around Japan in winter, especially at the time when the cold air outbreak occurs. The axes of the cloudy areas can be divided into four types in relationship to topography. That is to say, i) axes from the open sea surrounded by the cold land and sea ice area, ii) those continuing from the cloudy areas over the Japan Sea, iii) those to the lee of the gaps of the mountain ranges, and iv) those to the lee of the mountains.