Appearance of Stratiform Clouds over South China and Their Relation to Synoptic Fields during the Winter Monsoon Season

Geostational Meteorological Satellite imagery often shows the persistent presence of stratiform clouds over South China during the winter monsoon season. Two types of stratiform clouds, an enlarged and a diminished type, are clearly distinguished.<br> The purpose of this paper is to describe the relation between the cloud shapes of stratiform clouds and synoptic fields around the stratiform cloud area. Based on a case study for two ten-day periods representing the enlarged type and two ten-day periods representing the diminished type, a new model for the synoptic fields of two types of stratiform clouds is presented. This paper investigates the relation between the monthly frequence of two types of stratiform clouds and the seasonal variation of large-scale cold outbreaks in East Asia. The following findings are obtained:<br> (1) The cloud shapes of stratiform clouds<br> The enlarged type refers to the case in which the stratiform clouds cover an extremely wide area on the continent south of ca. 32°N. This type appears from the eastern edge of the Tibetan Plateau extending as far east as the Ryukyu Islands, with a uniform height of some 3, 000m. The northern periphery generally runs east-west along the parallel ca. 32°N. The southwestern end reaches the eastern coast of Vietnam (Photo 1).<br> In contrast, the stratiform clouds may appear only over the Szechwan Basin or the southeast side of the Tibetan Plateau; this I call the diminished type in this paper. When this type appears, the rest of the area on the continent is practically cloudless, and cloud clusters of irregular shape or low cloud bands are found over the South China Sea (Photo 2).<br> (2) The synoptic field of enlarged type<br> At the surface, when the enlarged type appears over South China, there is a stationary front with a zone of large temperature gradient off the southern coast of China, which roughly corre-sponds to the southern end of the stratiform clouds. The whole area covered with clouds is swept by northeast winds from the Siberian High (Fig. 1-a). On the other hand, the South China Sea is cloudless because an anticyclone exists there.<br> At 850 mb, the advection of warm southwest winds from the anticyclone glides upon the under-lying northeasterlies toward the zone of large temperature gradient at this level, which lies to the north of that of the surface (Fig. 1-b). I think this advection results in the formation of stratiform clouds at the southern part of the cloud area under the stable stratification because of the ascending air current of quasi-geostrophic wind over the front.<br> At 700mb, the large temperature gradient zone shifts more to the north than at the 850mb level. And the westerlies which glide over the stratiform cloud layer are far stronger because of the ad-vection of warm air at the 850mb (Fig. 1-c). I think these westerlies result in the formation of clouds at the northern hart of the stratiform cloud area. The Szechwan Basin experiences only very weak and irregular winds because of the shelter effect of the Tibetan Plateau.<br> The strong westerlies which are the downward extension of a subtropical jet stream exist at 500 mb (Fig. 1-d).<br> The model of synoptic fields for the enlarged type is shown in Fig. 13-a.<br> (3) The synoptic field of diminished type<br> This type is also under the influence of the winter monsoon from the Siberian High. Surface winds are generally weak and variable on the continent. Relatively strong northeast winds are reported on the South China Sea. The Siberian air-mass undoubtedly reaches the Philippines, but there is no front between the Siberian air-mass and the subtropical air-mass (Fig. 2-a).<br> The northeast winds are predominant even at the 850mb level, indicating a thick Siberian air-mass over South China.