Yihui Ding and Er Lu
National Climate Center, Beijing 100081, China

1. Meiyu Research in China and the Purpose of the Case Study

The climatic moving process of rain belt in East China during rainy season is that it moves northwards with main precipitation region appearing in South China, Changjiang-Huaihe River basin (hereafter referred to as Jianghuai basin), and North China in early, middle and late stage of rainy season respectively. Jianghuai basin, as the middle region rain belt stays at during its moving process, is a sensitive area where flood or drought forms frequently. The Meiyu phenomena over Jianghuai basin, as a short-range synoptic problem, has been studied comprehensively and many results about its mechanism have been achieved. However, as a long-range synoptic or short-range climatic problem, flood/drought over Jianghuai basin still needs more investigation as to its physical essence.

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. Results of the Case Study and Comments on Meiyu Forecast

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.

2.2 Summer monsoon behavior

The first episode of heavy rain occurs before the onset of summer monsoon and is mainly induced by synoptic systems in subtropical westerlies. While the activity of southwest monsoon becomes most important during the second and third episode of heavy rain when subtropical high in west Pacific and southeast monsoon keep stationary. The maintenance and break of Meiyu are of the results of the activity of southwest monsoon. A significant effect of southwest monsoon is to transport convective air from tropical area (Arabian Sea and Bangladesh Gulf) to Jianghuai basin, thus the rainfall much more convective than in normal case forms in mid-late stage of the Meiyu, especially in the third episode.

2.3 Precipitation nature and cumulus convection

Calculation of apparent heat source/apparent moisture sink and analysis of TBB data all show that the nature of precipitation has distinct seasonal variation, from frontal precipitation in the first episode to extremely strong convective precipitation in the third episode.

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.

2.4 Low frequency activity and seasonal variation feature

The activity of low frequency oscillation widely exists in East Asia during this Meiyu season. The precipitation process over Jianghuai basin has the period of 10-20d, and this period can also be found in the wind over the wide area from subtropical region to mid-high latitudes. Particularly, the process of the onset and southward invasion of cold air in north clearly shows such low frequency activity.

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.

2.5 Comments on the three rain episodes and Meiyu forecast

The three episodes of heavy rain of this Meiyu event are of most representative. They result from unique atmospheric circulation patterns at different stages of its transition season. The first episode occurs at the early stage of the transition period from winter monsoon to summer monsoon, and it falls into the type of frontal precipitation with characteristics of vigorous onset and southward invasion of cold air. The third episode appears in midsummer when southwest monsoon becomes very active with a significant effect of transporting convective atmosphere from tropical ocean, while in the north there is still cold air involved although it is not so vigorous as in the early stage. This is an episode of typical strong convective precipitation. The nature of the second episode is situated between the above two episodes and is a transition period from frontal precipitation to strong convective precipitation.

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.


Ding, Y., 1993: Study on the 1991 excessively heavy rain over Changjiang-Huaihe River basin. China Meteorological Press, Beijing, pp. 255 (in Chinese).

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).

Drs. Yihui Ding and Er Lu
National Climate Center
Beijing 100081, China
E-mail: yhding@public.bta.net.cn