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The heavy rainfall caused by Super Typhoon Rammasun (2014), the strongest typhoon that has landed in China since meteorological records began, has caused huge economic and social losses to China. Preliminary analysis showed that the intensity and area of the rainfall were closely related to Wuzhi Mountain, China. However, the physical mechanisms need to be analyzed in depth. Using the WRF V3.9 model, a high-resolution numerical simulation of topographic influence on the heavy rainfall of Typhoon Rammasun was carried out. The results show that the topographic rise significantly weakened the rainfall intensity and changed the location of the heavy rainfall center of Typhoon Rammasun. If the topography of Hainan Island increased to twice the original altitude, the maximum daily rainfall caused by Rammasun decreased by 35.2%, and the heavy rainfall center shifted from western Hainan Island (WHI) to southwestern Hainan Island. On one hand, the topographic rise increased the friction of the topography and weakened the intensity of Rammasun, resulting in a weaker rainfall in WHI (the observed heavy rainfall center). On the other hand, the topographic rise changed the structure of Rammasun. The control experiment showed that the heavy rainfall in WHI was mainly caused by meso-micro scale systems on an outer spiral cloud belt of Rammasun. If the altitude rose, the low-level convergence and the mid-low level ascending movement in WHI were weakened, making it difficult to form long-lasting, quasi-stationary meso-micro scale systems, eventually leading to the rainfall center transferring to southwestern Hainan Island. [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) formed in the western North Pacific and South China Sea can undergo rapid intensification (RI) shortly before making landfall in China. Forecasting such offshore RI is a great challenge in operations. In this study the offshore RI events in a sheared environment are examined for TCs that made landfall in China during 1979–2017. It is found that there were only three offshore RI events in a sheared environment, all of which occurred to the south of Hainan Island within the monsoon trough in early to mid-July, coinciding with the termination of the mei-yu season. The specific geographic location and timing of the occurrence of the offshore RI in the sheared environment is associated with the adjustment of the East Asia summer monsoon system when the mei-yu season terminates in the Yangtze River valley. In addition to the adjustment favorable for TC intensification by enhancing the TC–trough interaction in the upper troposphere, this study suggests that two environmental factors also contribute to the offshore RI over the South China Sea in a sheared environment. One is the intrusion of dry air associated with the western North Pacific subtropical high (WNPSH) and the other is the penetration of the water vapor flux associated with the monsoon surge. The adjustment of the East Asia summer monsoon system allows the water vapor flux of the monsoon surge to penetrate the TC circulation and prevents the dry air of the WNPSH from intruding into the TC circulation. [ABSTRACT FROM AUTHOR]