How Do Weak Tropical Cyclones Produce Heavy Rainfall When Making Landfall Over China

In this study, we investigate rainfall characteristics, such as rainfall intensity, rainfall coverage, and the location of heavy rain, associated with tropical cyclones (TCs) that made landfall over China during 2005-2014, using the observations of Tropical Rainfall Measuring Mission and environmental fields. Results show that before landfall, the stronger the TC itself is, the stronger the TC rainfall intensity is in both eyewall and inner rainband regions. However, there is no obvious difference in rainfall intensity in the outer core region for the TCs with different intensity. The coverage of heavy rainfall is also found to be correlated with increasing TC intensity before landfall. It is found that the weak TC with heavy rainfall is usually under strong westward vertical wind shear (VWS), resulting from the impact of enhanced upper-level easterlies associated with a significant northward-shifted South Asia high. In particular, when the TC is located under the right-hand side of the entrance of the upper-level easterly jet, the updraft over the downshear side of the TC circulation is further enhanced under the combined influence of VWS and anomalous upper-tropospheric divergence caused by the jet. The precipitation thus manifests a clear asymmetric feature with severe rainfall over the downshear sector of the weak TC. Our results suggest that the interaction between a weak TC and synoptic systems deserves as much attention as that between a strong TC and surrounding systems in landfalling TC forecasting and research.