Influences of Continental High on Inland Torrential Rain Associated with Severe Tropical Storm Bilis (0604)

The remnant vortex of severe tropical storm Bilis (0604) endures over land for a few days, results in a large range of torrential rain over South China and leads to a calamity after it lands in Fujian Province on 14 July 2006. Many studies are carried out and reveal the crucial effect of low level southwesterly monsoon jet to the extreme rain event. But the influence of the continental high on the torrential rain is also worth being discussed. The analysis of circulation shows that during the torrential rain process associated with Bilis, the continental high at upper level strengthens, moves eastwards, and stretches downwards to the low level after Bilis makes landfall, which strengthens the northeasterly stream at the southeastern side of the continental high. When Bilis moves westwards, the northeasterly stream at the low and middle levels in the northwestern quadrant of Bilis, overlapping with that southeast to the continental high, is strengthened and increases the transfer of water vapor and instability energy to the inland, which is favorable to the maintenance of the remnant low and the typhoon trough. The vorticity above the inland rainstorm area remains high during the process. The budget shows that it is the long existing remnant vortex and typhoon trough whose intense convergence at the low level produces a large amount of vorticity, triggers the successive development of MCS and results in the continuous heavy rain. The contribution of the continental high is to help the maintenance of the remnant low and the typhoon trough, and to enlarge the horizontal advection of vorticity at the upper level. And a set of numerical sensitivity experiments reveals that, based on the background of strong southwestern monsoon, the intensification and eastward stretch of the continental high not only forces Bilis to move southwestwards, but also provides strong divergence at upper level over Bilis. Besides, the continental high influences the intensity and the distribution of heavy rain. The endurance of the remnant vortex and the typhoon trough, due to the strong low level southwesterly monsoon jet and the continental high, enhances the intensification of vorticity above the inland rainstorm area and the occurrence of torrential rain. And in the sensitivity experiment of weakened continental high, the northeasterly winds in the northwestern quadrant of Bilis become weakened and the transfer of water vapor to the inland rainstorm area and the vorticity decrease accordingly, which leads to the northward movement of rain bands and the decline of rain intensity with the decrease of the 24 h rainfall maximum in the inland by 1/3, compared with the control experiment.