Potential Vorticity Diagnosis On The Formation,Development And Eastward Movement Of A Tibetan Plateau Vortex And It's Influence On The Downstream Precipitation

Tibetan Plateau Vortex(TPV)plays a very important role in the summer rainfall in east China.TPV is not only a precipitation weather system for plateau area,but under some special circulation pattern,can also move out of Tibetan Plateau(TP)and bring disastrous weather such as rainstorm to the downstream region.The dynamic and thermodynamic characteristics of the atmosphere can be represented by potential vorticity(PV),which is widely used in diagnosing weather system.During the period of 28 June to 1 July,2016,a Tibetan Plateau Vortex was generated,which then developed and moved eastward to subtropical region of China,resulting in precipitation in the lower reaches of the Yangtze River area.In the potential vorticity(PV)framework,the process is diagnosed by using the second Modern-Era Retrospective analysis for Research and Applications(MERRA-2)data and the Tropical Rainfall Measuring Mission(TRMM)precipitation data.The formation and development mechanism of the Tibetan Plateau Vortex and it's influence to the large-scale dynamic process to the downstream region have been explored.The main conclusions are summarized as follows.(1)The surface heating over the TP processes obvious diurnal variation.It changes from heat source in the daytime to cold source in the night and directly influences the vertical gradient of diabatic heating.Negative PV is generated near the surface in daytime and positive PV is generated in night,demonstrating a prominent diurnal cycle.When the night-time the release of latent heat in cumulus convection exist over TP,positive PV generation can become very strong and cannot be compensated for by the daytime negative PV generation,the diurnal cycle of usual PV generation could be destroyed and the TP low vortex is formed.(2)Diabatic term in PV equation is the main contribution for the positive changing PV at the fast developing stage of Tibetan Plateau Vortex.At this stage,strong diabatic heating associated with heavy precipitation can reinforce the positive PV generation at low level and the low vortex developed.(3)As the low vortex system continues to propagate eastward,the high PV forcing moving out of Tibetan Plateau can form positive zonal PV advection in the middle troposphere and negative meridional PV advection in the lower troposphere in the middle-lower Yangtze area.The PV advection increasing with height acts as the large circulation background over the middle and lower reaches of the Yangtze River is in fever of the development of air ascent.