NUMERICAL STUDY ON THE DYNAMIC AND THERMODYNAMIC EFFECTS OF THE QINGHAI-XIZANG PLATEAU ON THE SEASONAL TRANSITION IN THE EARLY SUMMER IN EAST ASIA

By employing a five-layer seasonal numerical weather prediction model and utilizing the National MeteorologicalCenter (NMC) objective analysis data on May 10,1991 as the initial field,three numerical experiments——diabatic withorography,adiabatic with orography and adiabatic without orography——have been carried out to investigate the dy-namic and thermodynamic effects of the Qinghai-Xizang Plateau (hereafter the Plateau) on the seasonal transition inthe early summer in East Asia.The results show that the typical seasonal transition features such as the northward shiftof the subtropical westerly jet stream,the adjustment of the long-wave in middle and high latitudes and the changes ofthe circulation in the lower troposphere can be well simulated,if the dynamic and thermodynamic effects of the Plateauare both considered in the model as in the experiment adiabatic with orography.In other two experiments,it is failed tosimulate out the seasonal transition features.The results also show that the thermodynamic effect of the Plateau acts on the atmosphere as a gradually enhancedheating source in the early summer,which makes the air temperature in 500 hPa over the Plateau increase by nearly 10℃in a month and accelerates the northward shift of the subtropical westerly jet stream about seven latitudes more north-ward in twenty days and helps the shifted jet maintain at the northern periphery of the Plateau.It is also helpful to theformation and maintenance of the Lake Balchas trough and the northward and westward extension of the Pacificsubtropical high.On the other hand,the dynamic effect of the Plateau weakens the northward shift of the subtropicalwesterly jet stream and barricades the westward extension of the Pacific subtropical high.If only the dynamic effect ofthe Plateau is considered in the model without consideration of the thermodynamic effect of the Plateau,not only theshifted jet withdraws southward but also the simulated Lake Balchas trough is 10 longitudes more eastward than that inthe diabatic experiment,and the simulated Pacific subtropical high is more eastward and northward.However,the dy-namic effect of the Plateau strengthens the vertical movements in the west and east of the Plateau and makes thelow-level jet maintain in the southeast of the Plateau.The path of the cold air and the tunnel of the water vapor in thelower troposphere during the seasonal transition period in the early summer in East Asia are determined by the couplingof the dynamic and thermodynamic effects of the Plateau.