Abstract
The features of the time variation of cloud amount with time scale longer than 15 days, in the western Pacific region (50°N to 46°S and 90°E to 172°W) were studied using 5-day averaged cloud amount data of Geostationary Meteorological Satellite (GMS I or II) and one-day averaged cloud amount data which were made from infrared images of GMS microfilms.
Following conclusions were attained. Besides the seasonal variation which is dominant north of 40°N, in the zones of 20°N to 10°N and 5°S to 20°S, the time variation of cloud amount with period of 20 to 40 days takes place almost all over the western Pacific. Though this 30-day period variation does not continue to be dominant throughout the whole year, it is predominant for several months. The domain where cloud amount varies simultaneously with about 30-day period has the size of about 2000km in the E-W direction and 1000 to 1500km in the N-S direction. Neither the regular propagation of 30-day period variation in the E-W direction nor that in the N-S direction the propagation of it is clear. The variance or amplitude of 30-day period variation is the largest in the zone on the north or south of the equator where mean cloud amount is small, and these zones move in the N-S direction with season. Over the ocean south of Japan the maximum anomaly of 30-day period variation appears as the continuation of cloud amount near 10 for several days or the appearance of maximum cloud amount at intervals of 20 to 40 days.
