Essential Role of Synoptic Environment on Rainfall Distribution of Landfalling Tropical Cyclones Over China

摘要

How the rainfall characteristics of landfalling tropical cyclones (TCs) over China change with the dry-air intrusion is explored through analyzing Tropical Rainfall Measuring Mission (TRMM) and environmental fields. It is found that the rainfall area of landfalling TC is positively correlated to the midlevel environmental relative humidity: the larger the surrounding relative humidity before TC landfall, especially in the southern quadrants of the TC, the larger the rainfall coverage at landfall. Even when situated in a dry environment, the TC may produce severe concentrated rainfall with stronger intensity than in a moist environment. Results show that interaction between synoptic environment and TC is essential for influencing rainfall distribution for landfalling TCs over China. As a TC moves with northward component under two subtropical highs and westerly trough, it is under the influence of significant dry-air intrusion, which results in limited rainfall area. The increasing northwesterly vertical wind shear that is nearly opposite to the TC movement, on the one hand enhances the upshear-side subsidence, which offsets the friction-induced ascent ahead of TC. On the other hand, it strengthens the downshear-side updraft with the corporation of increasing synoptic convergence and results in severe asymmetric rainfall there. When a TC moves under the influences of enhanced subtropical high and monsoonal southwesterlies, it is under the moist environment that causes a larger rainfall area. Influenced by the weaker vertical wind shear with a similar direction as the TC movement, the rainfall distributes relatively symmetrically with heavy rain over the downshear-friction convergence overlapping region at landfall.