Interaction Between Landfalling Tropical Cyclone and Summer Monsoon with Influences on Torrential Rain

Based on Tropical Cyclone Yearbooks, NCEP/NCAR reanalysis data and dynamic composite analysis, the relationship between the torrential rain associated with landfalling tropical cyclones and summer monsoon jets is studied. Several sets of numerical experiments are carried out to analyze influences on heavy rain of interaction between the landfalling tropical cyclone Bilis (2006) and the summer monsoon. Composite analysis show that tropical cyclones resulting in a large range of torrential rain always link with a low-level jet for a long time even after landfall, which makes them obtain sufficient water vapor flux. While for the weak rainfall tropical cyclones, the linking might break before landfall, which cuts off the water vapor transport from the low level jet. Numerical experiments reveal that water vapor transport from the low-level jet of summer monsoon is favorable to the maintenance of the tropical cyclone structure of warm core. Cutoff of the water vapor transport will lead to the stretch of a dry tongue into the cyclonic circulation from the boundary, destruct the tropical cyclone structure, and make the mesoscale and microscale synoptic systems weakened, which results in the obvious weakness of the rainfall intensity and the reduction of heavy rain coverage. Water vapor transport from the southern boundary is much more important than any other boundary. In the sensibility numerical experiment, the wave spectrum method is applied to alter the mesoscale and microscale winds to strengthen and weaken the monsoon jet speed. Results show that moderate change of wind speed in low level jet only change the distribution and coverage of heavy rain a little, but the grid number of extremely strong rainfall (more than 200 mm) varies notably. Under the background of summer monsoon, a tropical cyclone moving into the monsoon channel will strengthen wind speed of the low level jet, which strengths the transport of water vapor at low level. Furthermore, the movement of tropical cyclone over land changes the distribution of water vapor, instability energy and the relevant heavy rain, and the unique dynamic structure of tropical cyclone, and strong convergence at low level and strong divergence at upper level, could strengthen the rain intensity.