Vortex Structure And Development Mechanism. Meiyu

Mei-yu front and low along Yangtze River is an important and difficult event for meteorologists of China. A durative heavy rainfall process occurred on the middle and lower reaches of Yangtze River in lat Junl999.lt was just during the period of the operation of HUBEX experiment which provided a great deal of intensive rainfall data and upper-air sounding data. So it is a good chance for us to do research on the Mei-yu front and low. Using the daily rainfall data of 740 stations over China and NCEP daily up-ai data from 1971 to 2000, we analyzed the average Mei-yu climate field. Based on the climate field and Mei-yu intensity index, we confirm that the year of 1999 is a typical Mei-yu. Besides, the intensity of Mei-yu rainfall in 1999 is the highest since we had record, so the analysis of Mei-yu in 1999 can present the common characteristics of Mei-yu. We have done intensive research on the characteristics of large scale background, rainfall field, the characteristic of Mei-yu front and low in 1999, also we confirm the close relationship between the Mei-yu low and heavy rainfall, and the possible developing mechanism. 1) THE ANALYSE OF MEI-YU FRONT STRUCTUREThere is stable Mei-yu front along the middle and lower reaches of Yangtze River during the Mei-yu period in 1999.The temperature contrast in the low level of Mei-yu front almost disappeared, which existed only in the upper level. So the Mei-yu front is "footless" from the temperature field. It was a typical Mei-yu front on the middle and upper level of the troposphere which became wider with the height increased. The temperature field of Mei-yu front area looked like "sandwich" with the warmer air along the each side of the colder air of Mei-yu area. Below 600hPa, the front of east Asia was Mei-yu front; Mei-yu equatorial front and polar front from the south to the north; while above 600hPa, there was only Mei-yu front. Upon these conclusions, we put forward a conception model of east-Asia front. High pseudo-potential temperature along Mei-yu front area mainly presented the deep convection of Mei-yu area, so there would be a front along each side of the high pseudo-potential temperature, then the pseudo front would appear. On the other hand, there will be on pseudo front appears if we use the conventional definition of front. Only above SOOhPa, the Mei-yu front is similar when we use the two ways to analysis.2) THE ANALYSE OF THE STRUCTURE OF MEI-YU LOW AND THE ELATIONSHIP BETWEEN THE LOW AND RAINFALLThree lows moved along the Mei-yu front towards the northeast during Mei-yu period in 1999, which produced three intensive rainfall periods along the Yangtze River:illthe station of Huangshan, the most heavy rainfall center, totally had 850mm rain during 11 days. The reason for the heavy rainfall was the sustaining moisture supply from the south, and the deep moist layer and the high relative humidity in the south-east quadrant. The influence of the low to the rainfall of different part of Yangtze River had special characteristic: the first low mostly influenced upper reaches, while the second and the third influenced the middle and the lower reaches of Yangtze River. When the lows moved to the sea and Japan, the rainfall presented the characteristic of the high-level cold low rain produce.The temperature structure of the second and the third low was different with the different developing phases. Before moving into the ocean, the low was colder on the low stratosphere, while into the ocean, the low was colder on the whole level below 200hPa. On the beginning , the first low was warmer from the low level to the high level. Only after developing, it also became colder on the low level and wanner on the upper level, which was the result of the release of coagulation heat with rainfall.The low-level jet stream has three vibration processes, which corresponded to the three lows' beginning and developing. The low-level jet stream is the result of the spread of the momentum from the upper-level jet stream. The beginning of t...