Rainfall Reinforcement Associated with Landfalling Tropical Cyclones

摘要

Landfalling tropical cyclones (TCs) often bring about heavy rainfall, which typically decreases with the weakening of the TCs. However, some TCs may suddenly be reinvigorated after they become remnants over land. Such TCs may produce even stronger rains than those at the time of their landfall. This reinvigorating phenomenon is known as "rainfall reinforcement associated with landfalling tropical cyclones" (RRLTC). The TCs triggering rainfall reinforcement account for 9.7% of the total number of TCs that make landfall on mainland China and often cause problems and surprises for forecasters. The TCs with rainfall reinforcement mostly make landfall in the area of the southeastern coast of China and move primarily along two tracks, spreading northward or westward. RRLTC often occurs in the remnant of a tropical depression that has already been downgraded from typhoon intensity, particularly in a period when the remnant has slowed down or even stagnated. The highest frequency of RRLTC occurrence is during the third day after landfall and in the northeast quadrant of a TC moving northward and the southwest quadrants of a TC moving westward. Diagnostic analysis shows that an RRLTC with a northward track can be mainly attributed to the in-teraction between westerly troughs and the tropical cyclone. In this way, a remnant gains baroclinic energy from the midlatitude trough. Such an interaction does not appear for northward track TCs without rainfall reinforcement. Rainfall reinforcement for TCs with a westward track is mainly due to the interaction between monsoon surge cloud clusters and tropical cyclones, which is favorable for moisture and latent heat gain. Analyses show that the westward TCs would not have rainfall reinforcement without such an interaction. RRLTC requires new energy transport into TCs. The results of the present study indicate that baroclinic potential energy and latent heat are the two major energy sources that will trigger the remnant revival and rainfall reinforcement. Land surface topography also plays an important role in increasing the rainfall of TCs.