On the characteristics of the 1993/1994
Geostationary Meteorological Satellite high cloud amount
Ae-Sook Sub, Kyung-1a Ha*, Sung-Euii Moon* and Seung-Hee Sohn
Remote Sensing Lab., Meteorological Research Institute, Seoul, Korea
Dept. of Atmospheric Sciences, Pusan National University, Pusan,
Korea
Abstract
The characteristics of the GMS high cloud amount have been investigated for summer monsoon period of 1993/1994, the contrasting years in view of the summer monsoon precipitation. In order to investigate the monsoon features over the eastern Asian monsoon region, the cloudiness ( using derived by the geo stationary meteorological satellite), the condition of underlying surface including sea-surface temperature, and the summer rainfall are analyzed and some comparisons with 1993 and 1994 are also made and the characteristic differences are discussed.
An analysis of the 2-degree latitude-Iongitude gridded 5-day mean high cloud amount data shows the detailed movement and persistence of the convective activities. In order to describe the spatial and time structure of the intraseasonal oscillation for the movement and evolution of the monsoon cloud, the extended empirical orthogonal function analysis with the twenty-day window size is used for the each year .Also, in order to find out the periodicity of the equatorial convective cluster , Fourier harmonic analysis is applied to the each year .
The most prevailing intraseasonal oscillations of high cloud amount are 61day mode and 15 day mode in the equatorial and the subtropical oceans. However it was found that the most.. prevailing modes over the equatorial western Pacific and Indian ocean were different for each year , hence raising the possibility that the contrasting monsoon precipitation may be more fundamentally related to the interaction of intraseasonal oscillations and seasonal variation of convective activities over the lower latitude ocean.
1. Introduction
During the 1993/1994 summer over east Asia, we suffered from the contrasting climatic characteristics of precipitation such as a prolonged intense changma rainfall in 1993 and a weak changma with hot temperature in 1994. This paper, this contrasting characteristics will be analyzed from connection to the monsoonal structure of high cloud amount over east Asia and the equatorial ocean.
The intensity and duration of east Asia monsoon precipitation were thought to be connected with the onset and retreat dates of monsoon convective band, and its variabilities were characterized by sea surface temperature (SST) anomaly, the amount of snow mass over Eurasia, the equatorial flow of tropospheric low level, the process of flux convergence of moisture and the dynamic process of upper atmosphere. 'Specially, it has been well recognized by recent observational and theoretical studies that the summer precipitation over east Asia is largely affected by the SST and convective activity in the tropical Pacific (Nitta, 1987: Shen and Lau,1995). This relationship between the tropical western Pacific and the summer climate over east Asia has been clarified by the interaction of annual and intraseasonal variation of convective activity in the tropical region(Li and Wang, 1994). In the key studies of this nature, the 30-60 day oscillation was analyzed by many studies(Madden and Julian, 1971; Lau and Chan, 1986), the quasi-biweekly oscillation has been recognized (Krishnamurti and Bhalme, 1976).
In a more recent study, Tanaka(1994) have used the high cloud amount derived by the ..geostationary meteorological satellite(GMS) to analyze the summer monsoonal convection over west Asia. Maruyama et al (1986) showed also that the GMS high cloud amount can be used to estimate the rainfall in the tropical Pacific. Nitta(1987 ) and Tanaka(l99i, 1994) analyzed this data set and investigated the onset and retreat dates, and seasonal cycle in the summer monsoon clouds and its interaction with the intraseasonal oscillation. These results let it confirm that GMS high cloud amount is useful to analyze the summer monsoon activity .
In the present study, the extended empirical orthogonal function is used to obtain the evolutional structure of the principal spatial function of high cloud amount. And the intraseasonal oscillation is analyzed by the Fourier harmonic analysis, and its anomalous oscillation is established by means of the comparison of 1993 and 1994.
2 Data
The major data used in the present study is the five-day mean 2-degree latitude-Iongitude grid data of the GMS high cloud amount produced by the Japan meteorological satellite center . The data is constructed by time series from 1 April to 2 October in 1993 and 1994 respectively. The GMS high cloud amount is expressed as non negative integer below 10 and defined as the clouds having a cloud top temperature below the 400 hp a level climatological temperature based on observation (for example, Tanaka(1992) defined the summer monsoon cloud by the regions with more than 3 of the mean high cloud amount). This data is useful to estimate the precipitation for the reason of no missing data over land and ocean.
3. The characteristics of the 1993/1994 east Asian high cloud amount
Time-Iongitude sections of latitudinally averaged high cloud amounts for 32-38N and 6-12N are presented by Figure 1 and Figure 2, respectively. In case of 1993, the convective activity is situated strongly during summer monsoon period (June- July) over east Asia. Specially, one cloud branch moves to the northward toward Korea and Japan, while the inter tropical convergence zone(ITCZ) in the Northern Hemisphere appears lastingly in 5N-10N and extends northwestward in summer monsoon. However , during June and July in 1994, the northward branch is not appeared and the large cloud amount is accumulated at 15N-20N and the convective activity around Korea becomes noticeable in August.
Figure 1. Time-longitude section of latitudinally (32-38N) averaged high cloud amount (a) 1993 and (b)1994 The dotted regions indicate the area larger than 3.0 of high cloud amount fraction.
Figure.2 Same as in Figure except for 6-12N.
Specially, characteristics over eastward of 120E is situated contrastly. The convective activity moves eastward in time for May- June in 1993. That is the horizontally extended convective cloud band is appeared from July to August. In 1994, this development of convective activity is separated by two band with relatively small cloud area along northward movement and the strong convective cloud from 130E to 150E is lasted for July at lower latitude.
Namely, in case of 1993, the strong activity at the lower latitude is appeared for whole changma period. Whereas in 1994, for July the convective activity exists slightly around Korea and Japan, and It IS distinctly lasted at the lower latitude.
