Actual rain belt moving process and precipitation distribution in particular years show distinct annual variability. Extremely abnormal precipitation situation may be summarized as two opposite patterns as illustrated in the following two cases: (1) In 1991, rain belt locates persistently in Jianghuai basin that leads to a rarely-seen flood damage, while drought occurs in both South China and North China; (2) In 1994, drought appears in Jianghuai basin, but two rain belts exist in the south and north. These two patterns may surely have different circulation processes and characteristics.
It is a widely accepted viewpoint that the characteristics and physical causes of the long-range synoptic process which induces serious flood or drought still need better understanding, and this is an important procedure for the final purpose of enhancing the forecasting ability of serious flood/drought. So, in the future research regarding Meiyu and flood/drought in China, more attention should be paid to the analysis of those distinguished disaster cases, and the achievements may be used in the diagnostic study of the output of some climate models which are suitable for flood/drought forecast.
In the recent years we have carefully studied the 1991 excessively heavy rain and flood over Jianghuai basin. This Meiyu begins in 18 May and ends in 14 July, with three episodes of heavy rain in the period: 18-26 May (1st episode), 2-20 June (2nd episode), and 30 June - 13 July (3rd episode). Our study emphasizes the aspects of cold air activity, summer monsoon, cumulus convection and the nature of precipitation. Based on the results, a physical picture about the situation of the Meiyu is obtained and further comments on Meiyu forecast is presented.
2.1 Blocking situation and cold air activity20 Blocking situation maintains persistently over Eurasia continent in the whole Meiyu season, although there is a transition from double blocking high pattern in the 1st episode to bipolar blocking high pattern in the 3rd episode. The maintenance of blocking high is benefit to latitudinal exchanges of cold and warm airs.
Isentropic Potential Vorticity (IPV) analysis reveals that cold air activity is an important condition for the formation of the three episodes of heavy rain. The picture of latitude-time section of IPV (Lu and Ding, 1994) clearly indicates that there are three periods, each corresponding to an episode of heavy rain, when cold air with high IPV appear over Jianghuai basin (30N). Cold air invades Jianghuai basin in lower layer of atmosphere and mainly along the track of northeast-southwest. During the invasion process, cold air is frequently cut off by southwest (and sometimes southeast) warm airflow, and finally an isolated body of cold air with maximum IPV forms persistently over Jianghuai basin. The gradual weakening of the cold air activity may well reflect the characteristics of the transition from winter monsoon to summer monsoon. For the three times of southward invasion of cold air, the south limit cold air reaches is always Jianghuai basin. This is an interesting feature of this Meiyu case. That cold and warm airs meet and interact in Jianghuai basin for such a long period results in the big flood in the Meiyu season. It seems occasional that the three episodes of heavy rain all drop in the same place, but in fact such a kind of big flood is truly rare in history.
In TBB field, cumulus convection zone appears as a narrow belt which is mainly affected by southwest airflow and may basically represents the low level jetstream in East Asia. Whether cumulus convection belt stays along or shifts from Jianghuai basin determines the maintenance or break of the Meiyu.
Seasonal variation is an essential feature of this Meiyu case. Cold air activity has the experience from vigorous onset and invasion in the first episode to relatively weak activity in the third episode. With the strengthening of southwest monsoon, the cumulus convection becomes most active, thus the nature of precipitation in Jianghuai basin adjusts from frontal precipitation in the first episode to strong convective precipitation in the third episode.
Having different synoptic backgrounds and precipitation natures, the three episodes of heavy rain play quite different roles in the formation of the big flood. With such understanding, we put forward three general conceptual models of Meiyu (early, typical, and convective Meiyu). The forecast of Meiyu should be considered in time frame of a long period from spring to summer, and such long period may be divided into several subperiods, thus Meiyu prediction being made stage by stage. Meanwhile, in order to determine the location of major seasonal rain belt, we should emphasize not only the activity of subtropical high in west Pacific and southeast monsoon which generally control the northward and southward movement of rain belt, but also the activity of southwest monsoon which may lead to the zonal movement of rain belt.
Lu E., and Y. Ding, 1994: Potential vorticity analysis and cold air activity during the 1991 excessively heavy rain over Changjiang-Huaihe River basin. J. Appli. Meteor., 5 (3), 266-274 (in Chinese with English abstract).
Lu E., and Y. Ding, 1997: Low frequency oscillation in East Asia during the 1991 excessively heavy rain over Changjiang-Huaihe River basin. Acta Meteorologica Sinica, 11 (1), 12-22.
Lu E., and Y. Ding, 1997: Nature of precipitation and activity of cumulus convection during the 1991 Meiyu season of Changjiang-Huaihe River basin. Acta Meteorologica Sinica (in printing).
Lu E., and Y. Ding, 1997: Analysis of summer monsoon activity during the 1991 excessively heavy rain over Changjiang-Huaihe River basin. J. Appli. Meteor. (in Chinese with English abstract, in printing).
Ding, Y., and E. Lu, 1996: An essay of Meiyu prediction in the light of the syno-dynamical study of summer monsoon behavior during 1991 flood in China. J. Environment and Climate Research. (in Chinese with English abstract).