| The Tibetan Plateau vortex(TPV)is one of the most important rain-generating systems over the Tibetan Plateau(TP)in boreal summer.Its generation and activity not only bring more than half of the precipitation in the main body of TP in the warm season,but also have an inseparable relationship with the flood disaster in southwestern and eastern China.Since the launch of the Qinghai-Tibet Plateau Meteorology Experiment(QXPMEX),numerous studies on the mechanism of the eastward movement,synoptic analysis,and activity characteristics of TPVs have been conducted.Nevertheless,few studies focus on its genesis mechanism.It makes scientific significance to investigate it systematically.To reveal the dynamic and thermodynamic process related to TPV generation in boreal summer,a flood-causing TPV event occurred in late June 2016 was selected for a detailed analysis from the potential vorticity(PV)perspective.Besides,the temporal and spatial climatic characteristics of TPVs are also investigated seriously.Based on the 24-hour increment of equivalent potential temperature( ?24h?e)index,the June TPVs are divided into two groups and their genesis causes are explored respectively.Main conclusions are summarized as follows.(1)Against a favorable circulation background,TPV genesis is affected synergistically by both adiabatic and diabatic PV processes.At midnight on 27–28 June2016,a TPV event generated over the western TP that subsequently caused a downstream record-breaking rainstorm and extremely severe natural disaster.A remarkable circulation anomaly occurred over the TP and its peripheral area,with abnormal easterly flow developing over its southern rim in the middle and lower troposphere in the subtropical zone,replacing the normal westerly flow in late June 2016.Its forefront merged with the southwesterly flow from the west and penetrated and converged over the western TP where the surface was warmer than normal,forming a low-level jet and downward slantwise isentropic surfaces in situ.When the air parcel slid down the slantwise isentropic surface,its vertical relative vorticity developed owing to slantwise vorticity development associated with PV restructuring.At the same time,the penetrating southwesterly flow brought abundant water vapor to the western TP and induced increasing sub-cloud entropy and air ascent there.Low-layer cloud formed and the cloud liquid water content increased.The strong latent heat release in association with the formation of cloud produced strong diabatic heating near 400 h Pa at night and strong PV generation below.(2)The nocturnal genesis of the TPV case is related to the diurnal variation between the upper and lower layer of diabatic heat.The nonuniformity of diabatic heat served as the source and sink of PV.Meanwhile,the vertical stretch process was the bridge between the local changes of standard static stability and vorticity.Attributed to the combined effect of solar radiation and the moist process,diabatic heating at 350 h Pa and 500 h Pa showed a distinct diurnal circle.At the top level,the diabatic heat was weak during daytime and strengthened significantly at night.At 500 h Pa,the sensible heat was obvious during the daytime while it turned negative at night owing to the reduction of sensible heat and radiative cooling.It induced a decreased static stability during daytime and an increased static stability with denser isentropic surfaces at night.In addition,the ascending motion at high level was significantly stronger due to the enhancement of the high-level heating,while the 500 h Pa vertical ascending motion weakened,resulting in the vertical stretch.Although the vertical stretch of air column cannot offset the enhancement effect of diabatic heating on the variability of static stability,it inhibited the development of static stability to a certain extent and was crucial to the growth of vorticity on the other hand.(3)In climatology,the TPV occurrence number has a maximum in the warm season and at night over the western TP,with typical interannual variability.Based on the TPV complete dataset and MERRA-2 reanalysis dataset,the temporal and spatial characteristics of TPVs are revealed.The annual average number is 63,with a slightly increasing inter-decadal trend.An overwhelming majority occur in the warm season and at night.TPVs are more likely to generate in the mountains over the western TP.Kunlun Mountain is constantly a high-incidence area of TPVs in boreal summer.And the proportion of TPVs number originating near Gangdise Mountain rises significantly in August.(4)The TPV events can be classified into two categories from the perspective of genesis mechanism.Based on ?24h?e index during TPVs generation,TPV events can be divided into two types.For an event that generates in an increasing equivalent potential temperature environment within 24 hours,the index is assigned positive,and vice versa.It determines the difference in genesis mechanism.TPV events with positive ?24h?e correspond to the dipole pattern of geopotential height increment,with positive over the eastern TP and negative over its west.This corresponds to the easterly wind increment at 700 h Pa,which turns to southwesterly flow over the southwestern TP and penetrates the western TP.It is conducive to the 500 h Pa water vapor flux convergence in the high-incidence area near Kunlun Mountain.The strongest convergence at 500 h Pa,divergence at 250 h Pa as well as ascending motion benefit a prominent diabatic heat incremental center near 300 h Pa,favorable for PV generation.When ?24h?e is negative,an upstream abnormal trough develops to the north TP at 250 h Pa.The stronger South Asian High is located to the western TP.The induced anomalous westerly over the western TP transports abnormal positive PV,reinforcing the positive zonal vorticity advection.Similarly,at 500 h Pa,the strong zonal vorticity advection is kept by enhanced westerly speed over the southern TP,leading to the TPV genesis.In short,the unusual large-scale circulation has a dynamic influence on TPV genesis. |
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