Distribution Of The Surface Potential Vorticity Density Over Tibetan Plateau And Its Impact On The Downstream Extreme Weather Events

The dynamic and thermodynamic characteristics of the atmosphere can be represented by potential vorticity,which is widely used in diagnosing weather system.Surface potential vorticity can transport to atmosphere and act as its source and sink.And the distribution of its source and sink can also help in analyzing some phenomenon from dynamic perspective.In this paper,potential vorticity equation is used in deriving surface potential vorticity density equation.Potential vorticity density(PVD)equals potential vorticity multiply by density,which equals to absolute vorticity in isentropic coordinates.The surface air convergence on the eastern flank of the Tibetan Plateeau(TP)can increase the in situ surface potential vorticity density.Since the elevated TP intersects with the isentropic surfaces in the lower troposphere,the increased PVD on its eastern flank thus forms a PVD forcing source of the intersected isentropic surface at the boundary.Then reanalysis data and model result are used in analyzing the distribution of PVD and its forcing terms over TP region,in su mmer and winter,respectively.And the influence of surface PVD forcing over the TP region on the extreme winter storm over southeast China in 2008 is investigated by using numerical experiments based on the atmospheric circulation model FAMIL.The mainly result shows that:(1)Potential vorticity equation is used in deriving surface potential vorticity density equation and the algorithm of its forcing terms.Atmosphere divergence,diabatic and friction function are three forcing term for potential vorticity density.Potential vorticity is conserved,but the PVD not,the convergence of atmosphere can increase PVD and influence the source and sink of PVD.(2)TP plays an important role in changing of PVD because of its interacting with the ground.By analyzing the three forcing terms,the result shows that atmosphere divergence contributes most.It has a large value in the east boundary of TP during winter,and a large value in the central during su mmer.The contribution of diabatic term is weaker and friction term can almost be neglected.(3)The extreme weather in southeast China during 1.24-1.27 in 2008 is investigated by using numerical simulation and sensitivity experiment,which is designed by reducing the surface PVD over the TP region.It is demonstrated that in the lower troposphere,the increase of surface PVD over the TP region can generate anomalous cyclonic circulation over southern China which plays an important role in increasing southerly winds and the water vapor transport and also increases the northward negative absolute vorticity advection.In the upper layers,the surface PVD which is generated on the eastern TP region can propagate on isentropic surface along westerly wind and results in positive absolute vorticity advection in the downstream places.Consequently,due to the development of both ascending motion and water vapor transport downstream of the TP,the extreme precipitation event occurred over southern China.