Environmental Interactions during a Sudden Extreme Rain Event, following the landfall of Tropical Cyclone Bilis(2006)

Tropical Cyclone (TC) Bilis made landfall on the China coast at 0500UTC 14 July 2006. Following the landfall, sudden and unforecast torrential rain commenced some 400¬¬¬km southwest of the circulation center at around 1200UTC 14 July 2006. The rain caused catastrophic flooding with large loss of life. Prior to the rain event, a monsoon trough over the south China coast strengthened. Evidence suggests that this was influenced by an amplifying extratropical wave over central China. It is demonstrated that as the wave and the monsoon trough amplified, a large agradient wind developed over Bilis' northwest sector. The vortex response included a westward and southward expansion of the mid-tropospheric circulation, initiated by an outward surge of high potential vorticity (PV) from Bilis. The response weakened both the circulation and the rainfall near Bilis' center. A PV budget indicates that over the rainfall area, differential PV advection below 400hPawas present before, and isentropic ascent was diagnosed throughout, the torrential rainfall period. We propose that the former process locally raised the isentropes to provide conditions very favorable for isentropic ascent. To tie together the developing monsoon trough, the Bilis circulation and the rainfall, Lagrangian trajectories are used to demonstrate that air within the rain system originated from low levels in the accelerating monsoon westerly flow. The air then ascended and circulated through the Bilis circulation and exited with high PV values towards the rain area in the middle troposphere.