Tropical cyclones (TC) are the most destructive weather phenomena in the world. These storm systems are common in large parts of highly populated tropics and subtropics with favorable atmospheric conditions. Society’s vulnerability to them and the associated annual economic costs have risen steadily: mean worldwide insured losses averaged 75 billion USD per year in the 10 years between 2009 and 2019. Cinco et al. (2016) analyzed TC data and observed that in the period from 1951 to 2013, an average of 19.4 TCs entered the Philippine Area of Responsibility in the Western North Pacific, and nine TCs moved over the islands. Consequently, the Philippines have the highest number of landfalling storms and the highest rate of severe TC worldwide. In turn, the TCs that move over the islands into the South China Sea frequently affect the coast of Vietnam. Through a spatial assessment of TC vulnerability, Nguyen et al. (2019) have demonstrated a high or very high susceptibility in most parts of coastal Vietnam. The most extreme event over the WNP in the last century was Typhoon Haiyan, which caused 6,300 deaths and widespread economic and socioeconomic damage. This significant susceptibility implies the high importance of improving weather forecast models for greater predictive capability. In recent decades, the quality of forecasting tropical cyclone tracks has increased steadily at the European Center for Medium-Range Weather Forecast and other numerical weather prediction centers. Nevertheless, intensity predictions still present more significant challenges.

Text sample: Chapter 2.1.3, Regional differences within the WNP: The favorable synoptic genesis parameters, dynamic and thermodynamic environmental conditions, and thresholds depend on the ocean basin. Therefore, an adequate study of the considered basin is crucial. The SCS is the largest semi-enclosed marginal sea in theWNP, with an area of 3.5·106km2. According to Liang (1991), the East Asian monsoon strongly influences the basin. Southwesterly summer winds occur in May in the southern and central parts and in June in the entire basin. In the northern section, the winter monsoon begins in September and expands over the whole ocean basin in the SCS up to November. In between, there is an area with confluent winds: westerly in the south and easterly in the north. This confluence zone leads to powerful vertical motions. Therefore, the zone is a preferred location for TCG (Sec.2.1.2). Consequently, TCs occur frequently in winter in the southern basin and in summer in the northern part. Environmental factors also influence TC frequency. In winter, the sea surface temperatures are under the threshold of 26?C over the northern and western parts of the SCS (Fig.2.5). Additionally, the vertical wind shear values and relative humidity are unfavorable for TC genesis (Wang et al., 2007, their Fig.2.7, Fig.2.6). In contrast, a reverse distribution of vertical wind shear is seen in summer, with the highest values over 10ms-1 in the southern parts and favorable conditions in the central and northern parts. In winter, the relative vorticity is enhanced close to the coast of the largest island of the Philippines, Luzon , which indicates a preferred location for TCG. Yuan et al. (2015) investigated the frequency of precursor synoptic-scale disturbances associated with TCG over the SCS between 2000 and 2011. All 35 genesis events occurred between May and December. The authors also observed that most cyclones developed in association with a synoptic wave train at the confluence zone followed by the monsoon shear line. This is in contrast to the other parts of the WNP, where the monsoon shear line is the most frequent genesis factor. The WNP is the only ocean basin where TCs develop in all months of the year (McBride, 1995). However, similar to the SCS are the most TCs occurring between May and December (Fig.2.8). In contrast to the SCS, the highest storm frequency is visible earlier in August. This shift and the higher relative frequency in autumn are related to northerly cold surges that influence on model predicted TCI is studied in (Sec.5.1.4).

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