Research On The Secondary Circulations And The Track Turning Of Bilis(2006) Based On The Potential Vorticity (PV) Inversion

The Weather Research and Forecast (WRF) mesoscale numerical model is applied to simulate Bilis(2006) in high resolution. Based on the successful simulation of the track and sea level pressure of Bilis(2006), the PV inversion and the PV-ω system proposed by Wang and Zhang (2003) are applied to diagnose the nonlinear balanced character and the quasi-balanced character of the simulated Bilis(2006), and the relationship between the environmental vertical wind shear and the secondary circulation of Bilis(2006). The piecewise PV inversion proposed by Kieu and Zhang(2010) is applied to discuss the influence of the asymmetric mesoscale convective systems to the track turning of Bilis(2006). It is shown that:(1)The inversion indicated that the tangential wind and the vertical velocity and are well recovered. The recalculated azimuthally averaged tangential winds is about80%of the simulation. The distribution of vertical velocity is almost in phase with latent heating, the recalculated quasi-balanced vertical velocity is about70%～80%of the simulation. It is shown that the convection is strong at the southwest part of the storm, with high PV, while at the northeast part of the storm is just the opposite, which displayed strong wavenumber-1asymmetries.(2)The secondary circulations associated with the heating, friction and the dry dynamic terms respective have significant difference. The heating term contributes more to the secondary circulation, the two strong updraft area of its secondary circulation is located southwest of the storm center (down shear side) about150-200km and400-500km, which correspond the latent heating of precipitation by the eye wall and spiral rain band, while the secondary circulation at the northeast of the storm center (up shear side) is rather weak. The weak convergence upward motion of the friction term is located in the eye, which behaves like Ekman pumping. The shear related dry dynamic term will produce a secondary circulation opposed to the environmental vertical wind shear, if the vertical wind shear is quite strong. The updraft is located southwest of the storm center with convergence below and divergence aloft, and the northeast of the storm center is downdraft. The southweat-northeastly flow aloft reduce20%～25%of the destructive action of the imposed shear. The secondary circulation increase (decrease) the precipitation of up (down) shear side, and the wavenumber-1asymmetries distribution of precipitation of Bilis(2006) is thus formed. The secondary circulation associated with the dry dynamic term is relatively weak when the vertical wind shear is weak.(3)The mesoscale convective systems in the southwest part of the storm play an important part in the track turning of Bilis(2006) after its landing.The large-scale steering flow and the cyclone track of Bilis(2006) have significant difference in latitude direction,even opposite in longitude direction at several time levels after landing. The result of piecewise pv inversion indicated that the steering flow associated with the mesoscale convective systems is southwestward,while the large-scale steering flow is northwestward,there for the mesoscale convective systems play an important role in the track turning of Bilis(2006).