Effects of Vertical Wind Shear on Intensity and Rainfall Asymmetries of Strong Tropical Storm Bilis (2006)

The relationships between environmental vertical wind shear (VWS) and the intensity and rainfall structure of tropical storm (TS) Bilis (2006) have been investigated using the surface rainfall estimated from the TRMM/TMI data, the winds at 850hpa and 200hpa of the NCEP/NCAR reanalysis data, the observed precipitation at 5-minute interval from automatic weather stations over China, and the best track tropical cyclone data. The results show that the contemporaneous and 6h-lagged correlation coefficients between VWS and storm intensity (the minimum surface pressure at the storm centre ) are 0.59145 and 0.57438 (P<0.01), respectively. The average VWS value is about 10.9 ms-1, a slightly greater than the moderate value, which suppresses the intensification of Bilis.

There is a distinctive asymmetric structure of precipitation of Bilis. The azimuthally averaged rainfall rate was smaller near the eyewall than that in the outer spiral rainband. As the storm intensified, the band of the maximum rainfall rate moved outwards. The VWS contributed greatly to the asymmetric distribution of precipitation and the heavy precipitation band near and outside the eyewall with the maximum precipitation in the downshear-left of the VWS. Results from sensitivity numerical experiments show that the environmental VWS is the dominant dynamical factor responsible for the wavenumber-one asymmetric rainfall structure of Bilis and the effects of terrain and underlying surface play a secondary role.