Numerical Prediction of the Heavy Rainfall Vortex Over Eastern Asia Monsoon Region

Transcription

1 730 Journal of the Meteorological Society of Japan Vol. 62, No. 5 Numerical Prediction of the Heavy Rainfall Vortex Over Eastern Asia Monsoon Region By Shou-Jun Chen* and Lorenzo Dell'Osso European Centre for Medium Range Weather Forecasts, Shinfield Park, Reading, UK (Manuscript received.13 March 1984, in revised form 18 June 1984) Abstract A series of numerical forecasts were carried out to investigate a heavy rainfall event over the monsoon region of Eastern Asia which was caused by a vortex formed over the Tibet Plateau during Summer. The ECMWF global grid point model was capable of predicting the vortex four days in advance. The importance of the moist processes in the development of the vortex was investigated by comparing the results from a moist and a dry (without latent heat) versions of the model. It was found that the low level jet associated with the vortex and the upper level easterlies are more active when the latent heat is included in the model. The ECMWF limited area model with resolution 0.5* of latitude and longitude (*37km at 45*N) was used in order to obtain a more accurate and detailed forecast. Some intermediate scale disturbances were predicted over the Tibet Plateau and verified with satellite pictures and observations. This result is significant for the problem of numerical weather prediction over the Asian Continent. 1. Introduction It is well known that during the summer monsoon season some intermediate scale or *- mesoscale (Orlansky, 1975) disturbances (such as vortices, shear lines etc. with a horizontal scale of 103km and a life time of 3-4 days) frequently develop over the East Asian continent, and they often cause widespread heavy precipitation. The total precipitation from these disturbances usually exceeds 100mm of water in 24 hours. How to forecast such disturbances is an important problem for the Chinese meteorologists. There have been many synoptic investigations of these disturbances (e. g. Tao, 1979). Some numerical prediction experiments have also been performed with a limited area model (Zhang, 1981) ; but the model was too simple (5 levels) and only a 24 hour forecast was * On leave from Department of Geophysics, Peking University, China made. Recently Anthes et al. (1982) studied the development of intense mesoscale features which occur in North America. A 10 layer model was used with a horizontal resolution of about 100km, simple physics and initial condition derived from synoptic data. They found that their model was capable of forecasting the development and maintenance of many mesoscale phenomena for up to 24 hours. Their results show that mesoscale features develop in a favourable synoptic scale environment, and that they can be simulated if a suitable synoptic pattern is well predicted. et al. (1982) showed Bengtsson that hurricane type vortices are simulated in the ECMWF global model without any disturbance being apparent in the initial data. This result suggests that it may be possible to forecast intermediate scale disturbances over the Asian summer monsoon more than 48 hours in advance. During July 1981, very heavy rainfall

2 October 1984 S. J. Chen and L. Dell'Osso 731 Fig. 1 (a) Accumulated precipitation (mm) for the 48 hour period, 00GMT 12-00GMT 14 July (b) 700mb subjective analysis for 12GMT 12July was recorded on the eastern periphery of the Tibet Plateau, in the Si-Chuan province of South West China. The accumulated rainfall from 00GMT 12July to 00GMT 14July 1981 was more than 300mm (Fig. la). The area with total rainfall over 100mm was about one hundred thousand kilometers square and that with over 200mm was sixty thousand kilometers square. Most precipitation (about 70-80%) occurred during the 24 hour period starting 12GMT 12July, and during this period more than forty counties recorded rainfall exceeding 100mm. This caused a severe flood and heavy damages in south west China. The disturbance that caused this event was a vortex (with horizontal scale about 1000km) which originated on the eastern periphery of the Tibet Plateau at 700mb (Fig. lb). Such a vortex is called the "south west vortex" by Chinese meteorologists (SW vortex hereafter). Forecasting the SW vortex is a difficult problem because the influence of orography must be taken into consideration. During the summer the Tibet Plateau is not in the midlatitude westerlies, so the SW vortex is not a typical lee cyclone. It has a warm structure and can only be identified in the lower troposphere. The mechanism for the genesis of such a vortex is not yet clear. Since the original disturbance is formed over the Tibet Plateau, the interaction between the westerlis, south west monsoon and orography as well as the release of latent heat, may be important in the development of the vortex. The ECMWF global grid point model (Burridge and Haseler, 1977; Tiedtke et al., 1979) -resolution 1.875* of latitude and longitudeis used to show that it is possible to predict a SW vortex in a medium range forecast. Then some experiments are performed with a limited area version of the ECMWF model in order to study the importance of latent heat in the development of this vortex. The use of this model, in all respects similar to the global model, is simply due to economical reasons. Finally the limited area model with an improved resolution of 0.5* is used to investigate the effect of increased horizontal resolution and additional orographic detail upon the forecast over the Tibet Plateau. 2. Forecasting the change of large scale pattern In Fig. 2 the map of the five day mean 500mb geopoential height for 11-15July 1981 shows the main features of the large scale pattern during the period of heavy precipitation. Two points are worth noting : (i) There was a relatively low index pattern in high latitudes over East Asia. A pronounced long wave trough was located on 105*E which is about 5* further west than normal. The axis of the trough extended south of 30*N and this caused the cold air behind the trough to flow towards the Tibet Fig. 2 Five day mean 500mb map for 12GMT 11-12GMT 15July Units: 10gpm.

3 732 Journal of the Meteorological Society of Japan Vol. 62, No. 5 Fig. 3 Initial analysis of 500mb geopotential height, valid 12GMT 8July 1981 (top left). Right column: forecasts for +2, +4 days respectively. Left column: verifying analyses for the corresponding times.

4 October 1984 S. J. Chen and L. Dell'Osso 733 Plateau. (ii) There was an abnormal development of the Indian low and Western Pacific subtropical high. The Western Pacific high extended westward to 105*E (identified by the 5880m contour of the geopotential height), 10* further west than normal. At the same time the Indian monsoon low (located at 30*N, 80*E) was about 4* further north than normal so that it had the same latitude as the western Pacific high. Consequently, due to the increased east-west pressure gradient, the SW monsoon was strengthened. Therefore a large amount of warm, moist air from the Bay of Bengal travelled along and over the Plateau into south west China and interacted with the The character of the large scale pattern described above was established through a large scale change in the planetary wave patterns (Fig. 3 a, b, c). At 12GMT 8July 1981 (Fig. 3a) there was a ridge over Bakel Lake (105*E). The trough along the east coast of North America deepened, then the long wave ridge over Europe gradually intensified, moved slowly eastwards and developed to a blocking high by 12GMT 10July (Fig. 3b). Subsequently a ridge developed rapidly over south Siberia, merged with the high over Iran, and caused a strong meridional motion. The cold trough ahead of this ridge moved to 102*E at 12GMT of 12July (Fig. 3c) and became quasistationary. At that time the SW vortex was formed and heavy precipitation began. A 96 hour global forecast was performed starting from 12GMT 8July Figs. 3d and e show the 2 and 4 day forecasts. Comparison with the verifying analyses (Figs. 3b and c) demonstrates that the SW vortex which formed on the eastern periphery of Tibet was well predicted ; note also the accurate forecasts of the block over Europe, the ridge over Siberia and, in particular the trough along 102*E. However the forecast was rather poor over the tropical region. For example, the monsoon low (90*E, 20*N) in Fig. 3c was under predicted by about 30gpm and no closed low was formed ; also the west Pacific high over the east coast of Asia at 30*N was overpredicted by about 40gpm, but a strong SW monsoon flow over south west China could be identified. The model predicted the variation of the large scale circulation over Asia and North Pacific and it gave a rough indication Fig. 4 Analyses of 700mb and 200mb wind and geoptential height for 00GMT 13July 1981 (left). Forecasts for +4.5 days of the 700mb and 200mb wind and geopotential height (right). Thin lines are isotachs (ms-1), thick lines are geopotential height in 10m.

5 734 Journal of the Meteorological Society of Japan Vol. 62, No. 5 of the development of the SW vortex and the consequent severe weather. 3. The SW vortex It is appropriate to review the genesis of the SW vortex from a synoptic view before discussing the model simulations. On 00GMT 11July there was a weak shear line over the Tibet Plateau at 500mb. When the cold trough moved across the northern part of the Plateau, it merged with the shear line and, at the same time, the monsoon developed from the south and interacted with the shear line. A weak eddy developed over the eastern Plateau on 12GMT, 12July and a loose cloud cluster could be seen from satellite imagery (Wang 1982). The cloud cluster grew into a large scale white and bright feature about 1030km in length and 540km in width by 00GMT 13July. During this period, the eddy intensified rapidly and became a typical SW vortex. Basically the SW vortex was initiated over the Tibet Plateau and developed on its eastern boundary. Due to the sparseness of observation over the Plateau, it is difficult to give a detailed synoptic analysis of the disturbance in the development stage. Thus the results of the model are compared with the analysis only when the vortex moves to the eastern edge of the Plateau. Fig. 4a shows the wind field analysis at 700mb for 00GMT 13July ; note there is a well developed vortex at 30*N, 100*E. A pronounced south west low level Fig. 5 (a) Analysed vertical cross-section along 105*E for 00GMT 13July (b) 4.5 days forecast cross section along 105*E. Thin lines are isentropes (isolines every 2K), thick lines are isotachs of the component of the wind perpendicular to the plane of cross section (in ms-1, positive into the plane), arrows are proportional to wind speed parallel to the plane of cross section.

6 October 1984 S. J. Chen and L. Dell'Osso 735 jet (LLJ hereafter) with a maximum wind of more than 15 ms-1 was formed on the eastern side of the vortex, and heavy rainfall occurred on the left side of this jet. At 200mb (Fig. 4b) the subtropical jetstream was located near 40*N, with winds in excess of 45 ms-1 at 45*N, 110*E. The 700m trough was on the right side of the subtropical jet and this location is favourable for the development of ascent (Uccellini and Johnson, 1979). Figs. 4c and d show the corresponding 700mb and 200mb wind field from the 108 hour forecast starting at 12GMT 8July. At 700mb the vortex was formed but the position was about 200km further north than observed ; also the LLJ developed but it was about 5 ms-1 stronger than that in the analysis. The predicted scale of the vortex is too large and the vortex is also too intense-this may be due to the coarse grid of the model. At 200mb the position and intensity of the subtropical jetstream are well predicted and the relative positions of the vortex and the upper westerlies are quite good. The structure of the SW vortex can be seen in the cross-section of potential temperature and wind speed perpendicular to the section along 105*E (Fig. 5a). The axis of the vortex, as defined by the zero isotach, was slightly tilted to the north and it cannot be identified above 500mb. This vortex was asymmetric with rather weak easterlies (5ms-1) to the north and a strong LLJ on the south. From the distribution of potential temperature a warm belt to the south of the LLJ could be identified. Upward motion with a maximum up to -1.5Pa s-1 (this vertical velocity was diagnosed by Huo (1982) using the real wind data and equation of continuity) occurred near the core of the vortex. The subtropical jet stream was located north of the heavy rainfall area and the relationship between the LLJ, upper level jet and heavy rain area was very similar to the patterns discussed by Chen (1961). In Fig. 5b a cross-section of the results from the 108 hour forecast is shown for comparison with the analysis. At the initial time there was no pronounced LLJ so it was generated with the development of the SW vortex. The LLJ is well predicted and the structure of the vortex in the model is similar to that of the analysis. Two mechanisms can cause the enhancement of the LLJ :convective momentum transport (Ninomiya 1971) and the release of latent heat. The former requires the presence of a westerly flow in the upper troposphere. However, in this case there is, as usual in the Mei-yu season (Ninomiya 1980), a tropical easterly flow above the LLJ (Fig. 5). Therefore, it appears that the latent heat may be considered the major cause of the enhancement of LLJ. At 850mb a moist tongue (mixing ratio 18g/kg) accompanied the LLJ as it spread Fig. 6 (a) 4.5 day prediction of mixing ratio (gkg-1) at 850mb for 00GMT 13July (b) Same as (a) but for *e; isolines every 5K. (c) 4.5 days predicted distribution of *e at 700mb and 500mb along 30*N; shaded area represents potential instability (upper). 4.5 day predicted accumulated precipitation along 30*N for 00GMT 12 to 00GMT 13July 1981 (lower).

7 736 Journal of the Meteorological Society of Japan Vol. 62, No. 5 Fig. 7 (a) 4 day forecast accumulated precipitation (mm) for 00GMT 13July 1981 (lower). (b) Observed precipitation for 12GMT 12July to 12GMT 13July from southern to northern China (Fig. 6a). A value as high as 360K (Fig. 6c) indicated that this is a typical tropical monsoon air mass (Fig. 6b). The moist tongue had two dry zones on its eastern and western flanks, and the heavy rainfall area on its western side. The monsoon tropical air was potentially unstable when the heavy precipitation began. It is interesting to compare the predicted rainfall with the observed. Fig. 7 shows the forecast and observed accumulated precipitation between 12GMT 12July and 12GMT 13 July. The predicted rain area was about 250km west of the observed ; this means that the vortex moved more slowly in the model than in reality. The total predicted rainfall was about one third of the observed. The deficiency in rainfall in the model during the heavy precipitation is a common problem in numerical weather prediction. 4. The effect of latent heat release Previous studies about the development of heavy rainstorms show that the latent heat plays an important role in the "moist baroclinic process" (Xie, 1979) ; thus we want to investigate its effects on the development of a SW vortex. To do this, use is made of a limited area version of the ECMWF grid point model which covers the region 5*-60*N, *E; the gridlength is the same as in the global model. The forecast was started from Fig. 8 Difference fields-experiment M (with latent heat) minus experiment D (without latent heat) -from the 48h forecast verifying at 00GMT 13July (a) Geopotential height (solid lines in m) and temperature (dashed lines in *) field at 700mb. (b) Same as (a) but at 200mb. (c) Stream line and isotach (dashed lines in ms-1) at 700mb. (d) Same as (c) but for 200mb.

8 October 1984 S. J. Chen and L. Dell'Osso GMT 11July (when the SW vortex began to develop) and the boundary values were updated with analysed data. Both a moist and dry version of the model was used, the latter being the version without latent heat. Experiments with these models will be referred to as M and D respectively. The difference fields of the geopotential height, temperature and wind between M and D after 48hr forecast, Fig. 8, indicate that the effect of latent heat was by no means negligible. In the lower troposphere at 700mb (Fig. 8a) the latent heat induced a negative height anomaly with maximum height decrease of nearly 60gpm, accomanied by a positive temperature anomaly up to 3* in the region around 30*N, 105*E. The area of temperature anomaly was larger than the rainfall area because the release of latent heat in cumulus clouds causes increased ascent-the compensating subsidence then produced heating in the environment. In the upper troposphere at 200mb (Fig. 8b), a positive height anomaly was created and also a warming took place due to the vertical transport of sensible heat by cumulus convection. However, the effect of the latent heat was more pronounced on the wind field than on the temperature field at 700mb (Figs. 8c and d). There it generated a convergent cyclonic flow over the rain area with a maximum wind speed increase of 7ms-1, while a divergent anti-cyclonic flow appeared at 200mb with a wind speed increase of over 10ms-1 compared to those in the D-case respectively. The results are consistent with observations and other previous modelling studies (Ninomiya 1971 and Anthes et al., 1982). Fig. 9 shows the vertical cross-section of =dp/dt (a, b), potential temperature and * winds (c, d) along 105*E from the 48 hours forecast from experiments M and D. In M a large upward motion through the whole troposphere (-0.6Pa s-1 at 400 mb) was predicted between 28*-37*N; that coincided with the area of heavy precipitation. Its magnitude was one half of the diagnostic results from Huo (1982), while in experiment D the vertical motion was rather weak, only Pa s-1. The latent heat enhanced the Fig. 9 (a) 48h forecast vertical velocity on the cross section along 105*E in the experiment M. Units: Pa s-1. (b) 48h forecast vertical velocity on the cross section along 105* in the experiment D. (c) Same as (a) but for the isentropes and wind field. (d) Same as (b) but for the and wind field. Units as in Fig. 5. isentropes

9 738 Journal of the Meteorological Society of Japan Vol. 62, No. 5 vertical velocity by a factor of about six in the heavy rainfall area. At the same time, due to mass continuity, strong subsidence (0.25Pa s-1) occurred to the south of the LLJ causing a warming; consequently there was an increase in the temperature gradient across the LLJ and the speed of the LLJ increased due to the thermal wind relationship. Enhanced ascent induced a vertical circulation that in turn affects the horizontal flow, as can be seen in Fig. 9. In experiment M there was an indirect circulation around the LLJ, with southerly winds below 400mb and a northerly wind aloft. These flows are so this circulation strengthened ageostrophic and maintained both the LLJ in the lower troposphere and the easterlies in the upper troposphere. A direct circulation below the subtropical jet in experiment M (Fig. 9) was more pronounced than that in D ; the upper branch of the meridional circulation was also along the pressure gradient thus generating the kinetic energy necessary to maintain the subtropical jet stream. Fig. 10 shows the difference of the wind fields between experiments M and D for the cross-section along 105*E. Positive (negative) values indicate that the westerlies (easterlies) are stronger in experiment M than in D. There were two positive centres near the LLJ and the subtropical jet, and one negative centre (-15ms-1) near the easterly jet. This means that LLJ, subtropic jet and easterly jet were all enhanced by the latent heat. Fig. 10 The difference of the 48h forecast wind field between experiment M and experiment D on the cross section along 105*E. Fig. 11 Schematic illustration of the enhance ment of LLJ, subtropical jet and easterlies jet due to the vertical circulation induced by the release of convective latent heat. Although the condensation takes place over a small area, it can influence a quite large region because a larger area of subsidence is induced by the mass compensation. This process is illustrated schematically in Fig. 11. Due to the convective latent heating, the vertical velocity increases, as does the convergence and cyclonic circulation in the lower troposphere ; an indirect circulation is induced to the south and a direct circulation to the north. The wind speed of the LLJ to the south of the heavy rainfall area is enhanced due to the cross-isobaric flow and the subsidence warming south of the LLJ. In the upper troposphere, convective warming increases the pressure gradient to the north of the heavy rainfall area and sets up a cross flow along the pressure gradient isobaric causing an increase in speed of both the subtropical jet and the tropical iasterlies. This process is similar to that for the Mei-yu quasistationary front discussed by Ninomiya (1980), but in Ninomiya's case there is a shift of the subtropical jet to the north instead of the enhancement of easterlies. The previous discussion can be supported by the diagnostic analysis of the kinetic energy (KE) budget. The KE equation in the *- coordinate system can be written as :

10 October 1984 S. J. Chen and L. Dell'Osso 739 troposphere the KE also decreased, but in M the local change of KE was increased more than in D, especially in the lower and upper troposphere where the maximum difference Here sigma *= p/ps, ps is the surface pressure, reached 6 W m-2. The maximum value of Rk V is the horizontal wind components, *= at 150mb is consistent with the presence of d*/dt is the vertical wind component, * the the jet stream at that level. and *-1/2(u2+*2) the kinetic geopotential In the lower troposphere, the boundary flux energy of horizontal flow. The terms on the of KE was quite small for both forecasts indicating that the external sources of KE are left hand side are the local change, horizontal flux convergence and vertical flux convergence relatively unimportant for the SW vortex of the KE while on the right hand side there development. are the generation and dissipation of the KE The KE generation in the lower troposphere respectively. was due to isallobaric forcing as well as frictional forcing; the generation substantially The calculation was carried out on surfaces using the method presented by Savijarvi (1983). increased in experiment M. This increase The KE budget is computed for the period was accompanied by the deepening of the T+24 to T+48 of the forecast; this corresponds to when the vortex developed. The Another marked increase in the KE genera- vortex and the strengthening of the LLJ. domain is 95*-115*E and 20*-40*N, and it tion in M appeared in the upper troposphere contained both the SW vortex and the LLJ. with a maximum near 150mb, while in D Figs. 12 a and b show the KE budget for there was a KE sink at the same level. On the dry experiment (D) and the difference between experiments M and D. In D, the local amount of KE generation in M arose from the the whole, during this period a substantial change of KE was very small below 500mb. flow induced by release of latent ageostrophic This means that there was not a pronounced heat (i. e. the ageostrophic wind from high to development of a SW vortex. In the upper low pressure caused a positive generation). Fig. 12 (a) Kinetic energy budget in experiment D, 24-48h forecast, 75*- 95*E 20 40*N area mean. (b) Difference of kinetic energy budget between M and D in the same area and period. Thick solid lines are local change of KE, thick dashed lines are Rk, thin lines are horizontal flux divergence and thin dashed lines are generation of KE.

11 740 Journal of the Meteorological Society of Japan Vol. 62, No Experiments with a high resolution model Grammeltvedt (1969) examined the problem of resolution in the context of a simple primitive equation model and noted that a wave needs to be described by at least 15 grid points in order to produce a realistic two to three day prediction. A model with a grid length of 1.875* latitude/longitude is not able to describe in detail the SW vortex that has a horizontal scale of about 10* in latitude and longitude. For this reason the ECMWF limited area fine mesh model was used with a higher resolution. The grid length was a quarter of that used in the coarse mesh version-*0.47* latitude/longitude which is equivalent to 37km at 45*N. The physical processes were identical to those in the ECMWF global grid point model and the orographic height was the mean, over the grid square, of the U S Navy orography (resolution 10' in latitude and longitude). With this refined orography the maximum slope at the southern Fig. 13 (a) Initial analysis 500mb wind field, valid 00GMT 11July (b) 24h forecast 500mb wind field in the fine mesh model.

12 October 1984 S. J. Chen and L. Dell'Osso 741 edge of the Tibet Plateau reached 2.4/100, about twice that found in the coarse mesh model. Despite this increase, there appeared to be no noise generated during the integration due to the steepness of the mountains. Initial data for the fine mesh was interpolated from the coarse mesh analysis in which the coarse orography was used. An adjustment was carried out by changing the orography steadily during the first 12 hours of the integration, Dell'Osso (1983). Fig. 13a shows that only large scale features were apparent in the initial data ; for example the cold air behind the trough north of the Tibet Plateau at 40*N, 80*-90*E and the south west monsoon at 29*N, *E south of it. There was an absence of disturbances over the Plateau. After 24 hours of integration, some intermediate scale disturbances developed ; such disturbances were Fig. 13 (c) 48h forecast 500mb wind field in the fine mesh model. (d) Observed 500mb wind field at 00GMT 13July 1981; verification for (c). Dashed line is the contour of 3000m orograhic height, thin lines are isotachs in ms-, thick arrow line in the main stream line show the vortex, double dashed line is the shear line over Tibetan Plateau.

13 742 Journal of the Meteorological Society of Japan Vol. 62, No. 5 mainly due to the interaction of the large scale flow and the Tibet Plateau. The most pronounced features were an eddy with a scale of about 300km over the eastern part of the Plateau (32*N, 98*E) and a shear line (33*N, 80-90*E) over the south central Plateau (Fig. 13b). This eddy was the initial stage of the SW vortex, and 24 hours later (Fig. 13c) it moved eastwards and its scale increased to 600km. The wind on the southern part of the eddy strengthened, causing the SW monsoon to turn east and flow into southwest China. Fig. 13d shows the observed wind field at 500mb on 00GMT 13July note the SW vortex at 32*N, 102*E and the shear line located at 31*N, 89*-98*E. Comparison with Fig. 13c, shows that the vortex and shear line were well captured although the movement of the SW vortex in the model appeared a little slow. The shear line over the Plateau is an important feature. The observations show that the eastern part of this shear line was already present at 00GMT 11July (Wang 1982) but it was not clearly defined in the corresponding coarse grid analysis which formed the initial conditions for the model (Fig. 13a). After 24 hours of integration the shear line developed -this shows that a shear line can be simulated by using a fine mesh model. During the next 24 hours it moved southwards and became stationary along the valley of the Yalutsanpo river. Because there are no upper air observations over the Plateau west of 90*E, we cannot verify the predictions exactly from the analyses ; but east of 90*E the data is more abundant and the eddy and shear line are clearly defined (Fig. 13d). These two disturbances can also be identified in satellite photographs (Fig. 14). A bright white cloud cluster with its core near 32*N, 103*E (marked A) was associated with the SW vortex, and this was about 100km east of the predicted position of the vortex. Also the monsoon clouds from the Bay of Bengal flow over the southern slope of the Plateau and into the central region-the shape of the boundary of the clouds was similar to the shape of the shear line found in Fig. 13c. The vortex like Fig. 14 Satellite photograph for 18GMT 12July structure near 31*N, 92*E (marked B) was associated with a disturbance on the shear line over the Plateau. From the experience of meteorologists working near the Plateau, it is known that some quasi-stationary disturbances can form over the Plateau under favourable large-scale conditions. Due to the scale of the Plateau, such disturbances are limited to the *-

14 October 1984 S. J. Chen and L. Dell'Osso 743 Note at that time, the initial eddy was already formed at 500mb over the Plateau (Fig. 13b) ; 12 hours later, just when a part of the eddy moved out of the Plateau, a rapid development took place at 700mb (Fig. 15c)-the LLJ with a maximum speed of 20ms-1 suddenly appeared. The isotachs are very similar to those of the subjective analysis (Fig. 15d). The low level development was caused by positive vorticity in the middle troposphere becoming superimposed on low level potentially unstable air. The subsequent assen caused the release of latent heat which enhanced the development. The forecast rainfall on day 2 is displayed in Fig. 16a and the observed in Fig. 16b. The location of the rain area was about 100 km west of that observed. The maximum accumulated precipitation was 74mm; it did not increase with respect to the coarse mesh forecast, but it is worth noting that the proportion contributed by the large scale processes increased markedly in the fine mesh model. In the coarse mesh version 90% of Fig. 15 (a) Initial 700mb wind field, valid 00GMT 11July (b) 24h forecast 700mb wind in the fine mesh model.

15 744 Journal of the Meteorological Society of Japan Vol. 62, No. 5 Fig. 15 (c) 36h forecast 700mb wind field in the fine mesh model. (d) Observed 700mb wind field at 12GMT 12July 1981; verification for (c). Units as in Fig. 13. precipitation was due to the convection, but in the fine mesh the large scale condensation increased to 54% of total predicted precipitation. This is due to the decrease of grtidlength causing some larger scale convective systems to be directly described by the fine mesh without need for parameterisation. In the investigation of intense convective systems in North America, Ninomiya (1971) and Maddox (1980) found that *-mesoscale convective cloud clusters can develop in the upper troposphere ; they were called mesoscale convective complex (MCC) by Maddox. Whilst over North America a large out flow takes place on the north west side of the MCC (Maddox 1980), over the Monsoon region of East Asia, in summer, heavy rainfall occurs to the south of the subtropical jet stream and under the Tibet subtropical high ; the influence of easterlies is also pronounced. Maddox and Fritsch (1981) were able to simulate a MCC in a 20km grid length primitive model. Ninomiya and Tatsumi (1981) using a 77km fine mesh model, obtained a

16 October 1984 S. J. Chen and L. Dell'Osso 745 about 1000km in size, is similar to a MCC. From Fig. 17 that shows the 48 hour forecast wind field at 200mb along with the 12 hour accumulated rainfall, it can be seen that above the heavy precipitation area there was a 200 km narrow belt of high winds up to 20ms-1 which blew southward and mixed with the easterlies. The maximum divergence was 4.16x10-5 s-1 similar to that obtained by Fridrish and Maddox (1981). This result shows that the fine mesh experiment, besides a more detailed description of the overall phenomenon, is capable of forecasting the MCC. 6. Summary and conclusions Fig h forecast wind field at 200mb. The heavy solid lines are the last 12h accumulated precipitation (contours at 10mm, 20mm and 40mm), thin lines are isotachs in ms-1, thick dashed line is the 3000m orographic height. diffluent flow with anticyclonic vorticity and divergence over the area of maximum precipitation of the Baiu-font at 300mb after a 24 hour integration. These features are like the features of upper outflow over intense convections (MCC). In our case, the observed cloud cluster, In this study numerical models are used to study a heavy rainfall vortex that originated over the eastern part of Tibetan Plateau. For the global model, the large scale circulation pattern which is favourable for the genesis of a SW vortex was well predicted four days in advance. The vortex appeared in the model forecast and the structure was similar to that observed, although its scale was too large. To understand the role of the latent heat in the formation of the SW vortex, "moist" and "dry" experiment were carried out. In the dry case, latent heat, condensation and processes were absent. The convective experiments showed that at 700mb the latent heat induced a positive temperature anomaly of up to 3*C over a region larger than the rainfall area-the release of latent heat in cumulus clouds induced increased upward motion with compensating downward motion ; the net effect was the heating of the environment. The difference of the wind speed fields showed a region of convergent cyclonic flows over the rain area with a maximum of 7ms-1 at 700mb whilst at 200mb there is a divergent flow with a maximum wind anticyclonic speed difference of 10ms-1. Compared to the dry case the release of latent heat enhanced the vertical velocity by a factor of about six over the rainfall area in the moist case. An indirect circulation with southerly winds below 400mb and northerly wind above intensified, in turn, both the LLJ in the lower troposphere

17 746 Journal of the Meteorological Society of Japan Vol. 62, No. 5 and the easterlies in the upper troposphere. The LLJ, the subtropical jet and the easterly jet were all enhanced by latent heat. So long as the kinetic energy budget is concerned, it appeared that the local change of kinetic energy was very small below 500 mb in the dry case and that the vortex developed less than in the moist case. The generation of kinetic energy due to isollabaric forcing, as well as frictional effects, was substantially larger in the moist case in the lower troposphere. In the upper troposphere the kinetic energy generation was positive in the moist case and negative in the dry one. A large amount of KE arose from the ageostrophic flow induced by the heat latent release. Since the vortex was formed over the Tibetan Plateau, the effect of surface sensible heat flux should be taken into consideration. Some numerical experiments have been done by Dell'Osso and Chen (1984). They found that the sensible heating is not uniform over the Plateau in the model ; the relative maximum sensible heating is on the northern and southern flanks of the Plateau. Thus the sensible heating damps rather than enhances the vortex development. Early synoptic and diagnostic studies also found that such a vortex is not a thermal low (Yeh and Gao, 1979). disturbances (an eddy with Mesoscale a scale of 300km over the eastern part of the Plateau and a shear line over the south central part) were predicted 24 hours ahead using the ECMWF limited area model (resolution 37km at 45*N). In the 48 hour forecast the eddy created over the Tibetan Plateau moved eastwards and a LLJ with maximum 20ms-1 appeared. The wind field was very similar to that observed. The location of the eddy and shear line were verified using satellite pictures. The use of a numerical model with a fine resolution and a detailed description of orography appears to be capable of increasing the possibility of forecasting disturbances that have devastating consequences over Eastern Asia. References Anthes, R. A., Y. H. Kuo, S. G. Benjamin and Y. F. Li, 1982: The evolution of the mesoscale environment of severe local storms : Preliminary modeling results. Mon. Wee. Rev., 110,

18 Octobar 1984 S. J. Chen and L. Dell'Osso 747 Tellus, 35A, Tao, S. Y. et al., 1979: The heavy rainstorm in China. Sinica press. Beijing, 225pp. Tiedtke, M., Geleyn, J.-F., Hollingsworth, A. and Louis, J.-F., 1979: ECMWF model parameterization of sub-grid scale processes. ECMWF Tech, Rep. No. 10, 46pp. Uccellini, L.W. and D. R. Johnson, 1979: The coupling of upper and lower tropospheric jet streaks and implications for the development of severe connective storms. Mon. Wee. Rev., 107, Wang, Z. Y., 1982: Analyses of heavy precipitation in Si-Chuan province during July, Met. Institute of the Si-Chuan province, Tech. Report, Xie, Y. P., 1979: Dynamics of the moist baroclinic atmosphere. Collect papers on heavy rainstorm. Science Press Jilin Yeh, T. C. and Y. X. Gao, 1979: The meteorology of the Qinghai-Tibetan Plateau. Science Press. Beijing, 278pp. Zhang, Y. L., 1981: Heavy rainfall and formation of sub-synoptic systems. Acta Met. Sinica,139, Shou-Jun Chen and Lorenzo Dell'Osso European Center for Medium Range Weather Forecasts