Numerical Study of the Mesoscale Systems in the Spiral Rainband of 0509 Typhoon Matsa

Advanced Research WRF is used to simulate the local heavy rainstorm process caused by typhoon Matsa in northeastern coast of Zhejiang province in 2005. The results show that the rainstorm was mainly caused by the secondary spiral rainband of Stationary Band Complex (SBC) structure. Within the secondary spiral rainband there was strong meso--scale convergence line generated in the boundary layer, corresponding very well to the Doppler radar echo band. The convergence line was consisted of several smaller convergence centers, and all of these convergence columns inclined outward. Along the convergence line there was precipitation larger than 20 mm occurring in the next 1 hour. During the heavy rainstorm process, the Doppler radar echo band, convergence line and the next 1-h precipitation moved and evolved synchronously. Further study reveals that the vertical shear of radial wind and the low-level jet of tangential wind contributed to the genesis and development of convergence columns. The collaboration of ascending leg of the clockwise secondary circulation of radial wind and the favorable environment of the entrance region of the low-level jet of tangential wind further strengthened the convergence. The warm-moist inflow in the low level was brought in by the inflows of the clockwise secondary circulation and uplifted intensely at the effect of convergence. In the convective instable environment, strong convection was triggered to produce heavy rainstorm.