A DIAGNOSTIC ANALYSIS OF THE WEAKENING OF WEST PACIFIC SUBTROPICAL ANTICYCLONE DURING THE PERIOD OF SOUTH CHINA SEA SUMMER MONSOON ONSET IN 1998

The onset of South China Sea summer monsoon in 1998 occurred on May 21st. Using the U.S. National Centers for Environmental Prediction reanalysis data, this paper examines the physical process of the weakening of a subtropical anticyclone in West Pacific during the onset period using the Zwack-Okossi vorticity equation. Results show that during the pre-onset period, the positive vorticity advection in front of an upper tropospheric trough was the most dominant physical mechanism for the increase of the cyclonic vorticity on the 850-hPa layer over the South China Sea and its nearby region. The secondary contribution to the increase of the cyclonic vorticity was the warm-air advection. After the onset, the magnitude of the latent-heat warming term rapidly increased and its effect on the increase of the cyclonic vorticity was about the same as the positive-vorticity advection. The adiabatic term and divergence term contributed negatively to the increase of the cyclonic vorticity most of the time. Thus, the positive vorticity advection is the most important physical mechanism for the weakening of the West Pacific subtropical anticyclone over the South China Sea during the onset period.

THE ESTABLISHMENT OF THE SOUTH CHINA SEA SUMMER MONSOON IN 1998:THE NORTHWARD JUMP OF SOUTH ASIA ANTICYCLONE AND ITS EFFECT ON THE ONSET OF SOUTH CHINA SEA SUMMER MONSOON

By using the NCEP/NCAR reanalysis data,the activities of the 200 hPa South Asia Anticyclone (SAA) and its effect on the onset of South China Sea (SCS) summer monsoon in May 1998 are studied.The results show that:(1)During the pre-onset period the reversal of the mid- upper troposphere meridional horizontal temperature gradient over South Asia to the east of 90°E was caused mainly by warm horizonal thermal advections over this area.About 36 h after the reversal,the SCS summer monsoon bursts.(2)The variation of thermal wind circulation caused by the reversal of temperature gradient gave rise to the 200 hPa center of the SAA to jump from 15°N to 21°N within 12 hours.(3)The northward jump of the SAA and the intrusion eastward of the SAA under the effect of the warm horizonal thermal advections resulted in the rapid development of a trough over the South China coast.This induced the West Pacific Subtropical Anticyclone ridge,which originally controlled the South China Sea,to weaken rapidly and retreat eastward.Subsequently,the SCS summer monsoon built.(4)The superimposition in phase of the anticyclone ridge moving eastward over the middle-latitudes of China with the SAA has a rather important influence on the development of the trough over the South China coast.

ADVANCE AND RETREAT OF THE SUMMER MONSOON IN CHINA

By using the daily-14 year(1983—1996)NCEP/NCAR 2.5°×2.5° reanalysis data,we carefully study in each pentad the advance and retreat of the summer monsoon in China and given to it a new definition.This definition considered the intensity of southwesterly winds at 850 hPa together with its degree in temperature and moisture.The result revealed that: (1)The advance of the summer monsoon in China shows three abrupt northward shifts and four relatively stationary stays.The four stable stages correspond to the peak of the pro-summer rainy period in South China,the“Meiyu”season in the Changjiang(Yangtze)-Huaihe River Valleys.the rainy season in the downstream of the Huanghe(Yellow)Riyer Valleys and the rainy season in northern China.The retreat of the summer monsoon is so fast that it totally retreats from the mainland at about the mid-August. (2)The northward advance of summer monsoon in China is basically controlled by the seasonal variation latitudinally of the upper level planetary westerlies.It is in roughly accord with the temporal variation in the position of 15 m s^(-1) isotaeh at 200 hPa.The fast retreat of the summer monsoon is mainly due to the blocking effect of the Tibetan Plateau. (3)The advance of 500 hPa subtropical high of the western Pacific is also in aecordanee with the advance of the summer monsoon in China.During the advancement of the summer monsoon, the eastward movement of the subtropical high shows great meaning that it creates the essential condition for the convergence of southward intrusion cold airs with the warm and humid southwesterly winds,which result in precipitation.There are three manifest eastward movements of the subtropical high during its northward advancement.They coincide correspondingly to the beginning of the peak of the pre-summer rainy period in South China,the“Meiyu”season in the Changjiang(Yangtze)-Huaihe River Valleys and finally the rainy season in northern China.The western part of the subtropical high moves eastward to the region of Japan in late July and the beginning of August.It then stays there for quite a long time which results in the straight movement of cold airs intruding from the north to the east of Tibetan Plateau,i.e.the eastern region of China.This provides good condition for the fast southward retreat of the summer monsoon. (4)The intensifieation and development of the Tibetan high at 200 hPa are closely related to the eastward movement of the subtropical high,they often occur simultaneously.

NUMERICAL SIMULATION OF A MESOSCALE CONVECTIVE SYSTEM(MCS)DURING THE FIRST RAINY SEASON OVER SOUTH CHINA

Based on the NCEP/NCAR reanalysis data and observations collected during the SCSMEX,a mesoscale convective system (MCS) occurring over South China during 23-24 May 1998 has been studied with a numerical simulation using the Fifth Generation Penn-State/NCAR Mesoseale Modeling System (MMS).The successful simulations present us some interesting findings.The simulated MCS was a kind of meso-β scale system with a life cycle of about 11 hours.It generated within a small vortex along a cold front shear line.The MCS was characterized by severe convection.The simulated maximum vertical velocity was greater than 90 cm s^(-1),and the maximum divergence at about 400 hPa.The rainfall rate of MCS exceeded 20 mm h^(-1).To the right of the simulated MCS,a mesoscale low-level jet (mLLJ) was found.A strong southwesterly current could also be seen to the right of MCS above the mLLJ.This strong southwesterly current might extend up to 400 hPa.A column of cyclonic vorticity extended through most part of the MCS in the vertical direction.Additionally,the simulated MCS was compared favorably with the observational data in terms of location,precipitation intensity and evolution.

NUMERICAL PREDICTION EXPERIMENT OF AN ADVECTION FOG IN NANLING MOUNTAIN AREA

Now more comprehensive cloud microphysical processes have been included in advanced three- dimensional mesoscale meteorological model such as PSU/NCAR MM5 model,so the model can be used in the prediction of fog.In this paper,MM5 was utilized to simulate an advection fog occurring in Nanling Mountain area.The simulated results were compared with the facts obtained by detailed observation experiment.The results showed that the simulation was successful in the following aspects:(1)the formation and development of the fog;(2)the temporal variation of the maximum liquid water content;(3)the diffusion of the cold air,especially the temporal variation of the ground temperature;and (4)the uplift of the air and the formation and development of the low-lever inversion.Besides,we did some sensitivity numerical experiments and discussed the effects of the radiation,the release of condensation latent heat and the change of soil moisture and temperature on the formation and development of fog.The success of numerical simulation experiment of fog has proved that the numerical forecasting of fog is promising.