A Study of Qinghai-Xizang Plateau Lee wave Rainstorm

                Lin BiYuan   Sunhong      Xiaolu-Lin

      (Meteorological service of ) (Planning Commission of )  

                ,,410007 

  In some terrential rains course of the middle and upper reaches of the Changjiang River, at 12 hours before the terrential rain proceses happened, there is not a trough above Sichuan basin in 500hpa.When the terrential rain is going to happen,the small trough appeared suddenly above Sichuan basin in 500hpa. This type of  rainstorm  is not only difficult to predict,  but also difficult to find the real reason which produce the torrential rain in analyse afterwards°£July 3-6 1983ChangjiangRiver  

   1. The type of 500hpa lee wave

   The torrential rain course  in the middle and upper reaches area of the

  for , was consist of two torrentials rains. The first

 torrential rain is from  from 17 o'clock area of the Changjiang River, has no low value system°£ But we can see from fig 1 and fig 2 , from to Qinghai-Tibet plateau

 southern side, There are large stretch of southwest air current areas. After 12 hours,

 a small-scale low troughs has appeared in . Obviously, the torrential rain was caused by this low trough .

  1.1. An environmental condition helping the small trough to produce

   (1)Topography. We know, in the east of Qinghai-Tibet Plateau, the Hengduan  mountain lies across west of the basin , Form a natural protective barrier. The  highest of mountain is higher than 5000m, with great drop between . Above-mentioned low troughs were just produced under the topographical condition

   (2)The air current. From fig1 and fig2 We can see at 20 o'clock third day and 20 o'clock forth day, vast area of middle-south of Qinghai-Tibet Plateau (35˚N) exist an area of W-SW air stream. At 8 o'clock third day, the centre of largest wind speed is in Lijiang, the largest wind speed reaches 18 m/s; at 20 o'clock third day, wind speed center moves to Xichang ( the mountain ridge), its value is 20 m/s; At 8 o'clock forth day , the largest wind speed centre moves to Enshi ( behind the mountain), its value slightly reduces, it is 16 m/s. At 08 o'clockHengduan mountain, but also from ground to 500 hPa, before little trough appeared ( 20 o'clocks third day and 20 o'clock forth day ), It is W-SW air current between the higher and lower level. This phenomenon and little troughs didn't appear at the other time ( see 1 form).

    (3) stability of stratification. During the 2^nd July to 5^th July, the atmospheres of  1000-500hpa, appeared stable stratification at   and at 20 o'clockChongqingSichuanBasin  It can see that the

 temperature-rise is earlier in 500hpa than in 700hpa from fig4(c),and the air temperature on the ground reduced is earlier than in 850hpa. Just this time, the small trough appeared in 500hpa above . These environmental conditions are just helpful of forming lee wave.

   1.2 Fundamental feature of small trough in 500hpa above

   (1) Time scale less than 12hours;

   (2)The space scale is about 500 km, equal of north and south distance belong  meso-¦Á   (3)This trough is moved slowly, present the quasi-stationary state;

   (4)These smalls troughs appear when the centre of wind speed of

 W-SW air current moved behind mountain, and exist or die with the size and existance of w-sw air current in front of the mountains;

   (5)These troughs is warm.

 1.3  The analysis of precipitation course

Fig4  7,1-6,1983  24our temperature-difference       Tenchueng(a)  Xichang(b)  Chongqin(c)

   (1)The precipitation centre. There are two strong precipitation centres. One is Wufeng, the other is in Luotian, (see fig5). 

 These two centres are in correspondence with the two wave troughs of lee;

   (2) Precipitations of two centres exists seesaw structure.

     2 Genesis and development of  lower eddy in lower layer

 Becurse the lee trough in 500hpa appeared, field of pressure of lower atmosphere layer is influenced obviously. At ¦Áb 

   3.Prediction of lee interrential rain 

   following is inportant:

   (1) topograph. abruptness of mountain is large;

   (2) wind speed more than 12m/s;

   (3)air temperature in 500hpa rises suddenly; 

   (4)stratification is stability. ¦Ã¦Ã¦Ã ¦Ã 

   4. summary.  main result in this paper is in the following:

   (1) some terrential rain in middle and upper reachs of Changjiang river are caused by the lee trough in Qinghai-Tibet Pleteau.

   (2) This lee trough appeaerance is sudden. It can be a lee wave.

   (3). The lee wave can cause meso-b   (4).Precipitation of two centre in lee rainstorm exists seesaw phenomenon.