太平洋上における雲量のスペクトル解析

Abstract

Spectrum analysis is applied to satellite observed cloudiness data over the northern and southern Pacific. Power spectra with periods up to 50 days are computed by using continuous data series during the four year period from October, 1965 through September, 1969. They reveal the existence of four pronounced peaks in the period ranges of 33.3-50.0 days, 16.7-25.0 days, 7.1-10.0 days, and 4.0-5.0 days. Spectra in the long period end (33.3-50.0 days) exhibit a significant feature of two maxima (one in the vicinity of 20 N and the other between 5-10 N) over the northern Pacific, while they are not in zonal alignment in the southern Pacific. The shorter period fluctuation (4.0-5.0 days) appears to be significant in low latitudes only.Upon eliminating annual mean variations from the original data series (Jones, 1964, 1971), the daily filtered data are further separated into two groups : four winter half year data and four summer half-year data. An attempt is then made to compute the 4-year mean cloud spectral power with periods up to 25 days in both seasons. This computation proves the existence of three marked peaks with periods 16.7-25.0 days, 7.1-10.0 days and 4.0-5.0 days. The analysis of coherence and phase difference with respect to a reference point (22.5 N, 155 W) reveals the zonal wavelength of cloud disturbances (16.7-25.0 day period) to be about 12, 200km (30, 000km) in the winter (summer) half year. Attempts to obtain wavelength and phase speed for both the 7.1-10.0 day period and the 4.0-5.0 day period were unsuccessful because of a low coherence.