Diagnosis of the Secondary Circulation of Tropical Storm Bilis (2006) and the Effects of Convective Systems on Its Track

We diagnose characteristics of the quasi-balanced flow and secondary circulation (SC) of tropical storm Bilis (2006) using the potential vorticity (PV)- inversion method. We further analyze how secondary steering flows associated with mesoscale convective systems affected the track of tropical storm Bilis after it made landfall. The quasi-balanced asymmetric and axisymmetric circulation structures of tropical storm Bilis are represented well by the PV- inversion. The magnitude of the nonlinear quasi-balanced vertical velocity is approximately 75% of the magnitude simulated using the Weather Research and Forecasting (WRF) model. The SC of Bilis (2006) contained two strong regions of ascending motion, both of which were located in the southwest quadrant of the storm. The first (150-200 km southwest of the storm center) corresponded to the eyewall region, while the second (approximately 400 km southwest of the storm center) corresponded to latent heat release associated with strong precipitation in major spiral rainbands. The SC was very weak in the northeast quadrant (the upshear direction). Dynamical processes related to the environmental vertical wind shear produced an SC that partially offset the destructive effects of the environmental vertical wind shear (by 20%-25%). This SC consisted of upward motion in the southwest quadrant and subsidence in the northeast quadrant, with airflow oriented from southwest to northeast at high altitudes and from northeast to southwest at lower levels. The inverted secondary zonal and meridional steering flows associated with continuous asymmetric mesoscale convective systems were about -2.14 and -0.7 m s-1, respectively. These steering flows contributed substantially to the zonal (66.15%) and meridional (33.98%) motion of the storm at 0000 UTC 15 July 2006. The secondary steering flow had a significant influence on changing the track of Bilis from southward to northward. The direction of the large-scale meridional steering flow (3.02 m s -1) was opposite to the actual meridional motion (-2.06 m s-1).