Based on self‐organizing maps (SOM), clustering analysis is performed on the lower‐level large‐scale flow fields centred at the tropical cyclone (TC) genesis points averaged over the 24‐hr TC preformation period. A total of 461 TCs over the western North Pacific (WNP) from June to November in 2000–2019 are classified into five patterns, that is, convergence (CON) pattern, monsoon trough (MT) pattern, pre‐existing cyclone (PC) pattern, subtropical high (SH) pattern and easterly wave (EW) pattern. The most frequent and southmost pattern, the CON pattern, is characterized by a confluent zone of monsoonal southwesterly flows and easterly flows. The second dominant pattern, the MT pattern, occurs the westmost and has a larger cyclonic vorticity in the strongest monsoon trough among the five patterns. In contrast, cyclogenesis in the SH pattern is featured by both the confluent zone and the cyclonic wind shear largely attributed to the subtropical high. The PC pattern features a pre‐existing cyclone to the northwest. The EW pattern with easterly flows in all four adjacent quadrants has the smallest number of TC formation and relatively northeast genesis location. Statistics also shows that the CON pattern dominates in late season, while the other four mainly appear in summertime. Large‐scale environmental conditions are quantified and comparisons with previous studies are also discussed.
The sudden transition to rapid weakening immediately after a tropical cyclone (TC) reaches its intensity peak (hereafter TP‐RW) is a challenge to operational TC intensity forecasts, but the characteristics and the controlling factors of the TP‐RW have not been investigated so far. In this study, the main characteristics of the TP‐RW events over the western North Pacific (WNP) and the corresponding key controlling factors and their relative importance are analyzed and quantified. Results show that about 2.5 TP‐RW events occurred each year, which corresponds to about 10% of total TCs over the WNP during 1982–2018. The TP‐RW events occurred in all months but peaked in fall and were the most likely to occur for TCs with intensities between 90 and 145 kt. The sea surface temperature, the TC maximum potential intensity and translational speed, the environmental vertical wind shear and relative humidity are key to the TP‐RW. Four distinct clusters of the TP‐RW events are identified based on their tracks using the K‐means clustering algorithm. The relative contribution of each of the identified key factors to each cluster is quantified using the growth rate from the simplified dynamical system for TC intensity prediction developed by DeMaria. The reversal of the growth rate from positive to negative values is shown to be a good indicator of the TP‐RW processes. Different clusters are predominantly controlled by different key factors. The findings from this study can help better understand TC intensity changes and improve TC intensity forecasts for such kind of RW.
Heavy rainfall events (HREs) have great socioeconomic impacts around the world and cause deadly and destructive disasters. China is located in the East Asia monsoon area and frequently susceptible to heavy rainfall and flooding extremes (Cao & Gao, 2007;H. Li et al., 2017). Studies on HREs have always been concerned widely and highly (Chen et al., 2012;Junker et al., 1999).Although HREs generally have a short-lived and localized nature (Jo et al., 2020), they are actually modulated by synoptic-scale systems, such as the western Pacific subtropical high (WPSH, Ding & Chan, 2005), tropical cyclone (TC, also known as typhoon, Ross & Kurihara, 1995), and southeasterly moisture transport produced by their interaction (Y. Nie & Sun, 2022). H. Wang et al. (2021) divided the persistent HREs over the Yangtze-Huaihe River Valley during 1981-2020 into three types according to pattern correlation. They emphasized that the WPSH and TC play significant roles in persistent HREs. Y. Wang et al. ( 2009) performed numerical sensitivity experiments to suggest that Typhoon Songda (2004) enhanced the transport of moisture to the heavy rainfall region through the outer circulation and was responsible for >90% of the precipitation in Japan. Many studies have revealed that when a TC approaches the WPSH, the pressure gradient between them increases rapidly, establishing or strengthening the low-level easterly jet, which is beneficial to transport water vapor toward the heavy rainfall region (Schumacher et al., 2011).
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