Impacts of cyclogenesis and moisture transport by the enhanced southwesterly monsoon flow on heavy rainfall over southern Taiwan during SCSTIMX (2018)

Impacts of cyclogenesis and moisture transport by the enhanced southwesterly monsoon flow on heavy rainfall over southern Taiwan during SCSTIMX (2018) | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Impacts of cyclogenesis and moisture transport by the enhanced southwesterly monsoon flow on heavy rainfall over southern Taiwan during SCSTIMX (2018) Chuan-Kai Wang, Yi-Leng Chen, Chuan-Chi Tu, Pay-Liam Lin This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7241626/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract During the 13–20 June 2018 South China Sea Two-Island Monsoon Experiment (SCSTIMX), two tropical disturbances formed along western end of a Mei-Yu front over the northern South China Sea (SCS) between the cold northeasterlies (> 15 m s − 1 ) coming from the Taiwan Strait and warm, moist southwesterlies. Both disturbances exhibit mixed baroclinic characteristics and an asymmetric tropical cyclone (TC) structure. The first disturbance formed around 1800 UTC 13 June and moved northeastward along the Mei-Yu front, deepened, and became tropical storm Gaemi before making landfall over southern Taiwan in the early morning on 15 June. With the deepening of Gaemi, the southwesterly flow on the southern/southeastern flank of the storm strengthened (> 20 m s − 1 ). During 14–15 June, heavy orographic rainfall occurred over southern Taiwan with a maximum (> 550 mm) on the windward side of the Central Mountain Range (CMR). The second tropical disturbance over the northern SCS made landfall over southern China around 0600 UTC 18 June. During 19–20 June, the southwesterly monsoon flow strengthened (> 20 m s − 1 ) due to relatively large pressure gradients between the remnants of the second tropical disturbance and the West Pacific Subtropical High (WPSH). During 19–20 June, the horizontal moisture fluxes associated with the enhanced southwesterly monsoon flow mainly occurred below the 850-hPa level. On 19 June, the maximum rainfall axis (~ 300 mm) occurred on the southwestern windward slopes due to orographic blocking and lifting of the warm, moist south/southwesterly flow under favorable large-scale settings. On 20 June, with the development of the cold pool (~ 25°C) and offshore flow over southwestern Taiwan due to rain evaporative cooling, the maximum rainfall (~ 300 mm) occurred along the southwestern coast. Mei-Yu frontal cyclones Moisture transport Heavy Rainfall Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 1. INTRODUCTION Tu et al. (2020) identified two main heavy rainfall periods (1–4 June and 14–18 June 2017) over the Taiwan area during the South China Sea Two Island Monsoon Experiment (SCSTIMX-2017) (Sui et al., 2020 ). They found that these two periods are closely related to the large-scale moisture transport within the marine boundary layer (MBL) from the northern South China Sea (SCS) to the Taiwan area. The Marine Boundary Layer Jet (MBLJ) develops and intensifies in response to sub-synoptic pressure gradients when the Mei-Yu trough over southern China deepens and/or the west Pacific subtropical high (WPSH) strengthens and extends westward as found in Tu et al. (2019). They suggest that moisture transport within the MBL from the northern SCS to the Taiwan area is a favorable condition for the development of heavy rainfall during the early summer rainy season in Taiwan (e.g., mid-May to mid-June) (Kuo and Chen, 1990; Chen and Tu, 2024 ). Chen et al. ( 2007 ) identified five torrential rain (> 350 mm day − 1 ) events over southwestern Taiwan during a 10-year period (1997–2006). These events are characterized by large (> 220 g kg − 1 m s − 1 ) upstream low-level moisture fluxes averaged among 1000-, 925-, and 850-hPa levels, the presence of the 850-hPa high θe axis, and 700-hPa sub-synoptic upward motion. Chen et al. ( 2018 ) studied the widespread heavy rainfall events over Taiwan during 10–12 June 2012 and showed that the maximum low-level horizontal moisture fluxes (> 330 g kg − 1 m s − 1 ) associated with the southwesterly monsoon flow occurred in the planetary boundary layer with a maximum at the 950-hPa level. In addition to the localized heavy rainfall during the passage of a Mei-Yu jet/front system over northern Taiwan, heavy rainfall occurred on the windward slopes of the Snow Mountains (> 1,000 mm) and Central Mountain Range (CMR) (> 1,500 mm) as the low-level southwesterly monsoon flow impinged on these mountains (Chen et al., 2018 ). Tu et al. (2019) used a five-year (2008–2012) reanalysis data to study the characteristics of MBLJs over the northern SCS during the early summer rainy season over Taiwan. The MBLJ at the 950-hPa level (Tu et al., 2019, 2020, 2022 ) over the northern SCS and the sub-synoptic low-level jet (SLLJ) at the 850-700-hPa level associated with the Mei-Yu front (Chen and Yu, 1988; Chen et al., 2006 ; Chen et al., 1994 , 1997 ) are two distinct features that could occur and, possibly, co-exist during the early summer rainy season over Taiwan (Chen et al., 2018 ; Tu et al., 2019; and Ke et al., 2025 ). The SLLJ in the frontal zone is an integral part of the secondary circulation across the Mei-Yu jet/front system (Chen et al., 1994 , 1997 ; Chen and Chen, 1995 ; Chen and Chen, 2002). The MBLJ is a synoptic feature related large-scale pressure gradients between the Mei-Yu trough over southern China and the WPSH. At the jet core of the MBLJ over the northern SCS, the vertical wind profile resembles an Ekman spiral with a wind speed maximum near the top of the MBL (~ 925 hPa) (Tu et al., 2019). When both MBLJ and SLLJ are present the MBLJ is important in horizontal moisture transport from the northern SCS to the Taiwan area (Tu et al., 2019), whereas rainfall production is mainly related to secondary circulation associated with the jet/front system in the frontal zone (Chen et al., 1994 , 1997 ) or localized lifting related to orographic effects (Chen et al., 2022 ). During the early summer rainy season, the MBLJ from the southern China coast is also favorable for the occurrences of heavy rainfall over the southern China coastal areas (Du and Chen, 2018, 2019 ). During 13–20 June 2018, we show that two tropical depressions developed over the northern SCS along western end of a Mei-Yu front with appreciable temperature gradients in low levels. Although many studies show that the Convective Instability of the Second Kind (CISK) (Charney and Eliassen, 1964 ; Ooyama, 1969 , 1982 and others) is important for the development of tropical cyclones, we show that the initial disturbances could possibly develop along a Mei-Yu front over the subtropical ocean. This Mei-Yu front over the northern SCS exhibits baroclinic characteristics, as found in previous studies over southern China (Chen et al., 1989 ; Trier et al., 1990 ; Chen, 1993; Hsiao and Chen, 2014 ; Chen et al., 2022 ). We also show that the southwesterly monsoon flow could possibly strengthen on the southeasterly flank of a tropical cyclone, due to increased pressure gradients between a tropical disturbance and the WPSH bringing in moisture at low levels, which is favorable for the development of heavy precipitation over the Taiwan area. In addition, Xu et al. ( 2012 ) studied the evolution and mechanism of the mesoscale convection system during Terrain-influenced Monsoon Rainfall Experiment (TiMREX). They identified the effect of rain evaporative cooling and the cold pool that formed in the lowest 500 m due to persistent rainfall. Tu et al. (2017) used the weather research and forecasting (WRF) model to further investigate the impact of rain evaporative cooling on coastal rainfall for the same case. We confirm the findings by Xu et al. ( 2012 ) and Tu et al. (2017) that in addition to orographic effects, rain evaporation affects the location of heavy precipitation. The data and methodology used in this study are given in Section 2 . The results of cyclogenesis, horizontal moisture fluxes within the MBL, and the relationship between moisture transport from the northern SCS to the Taiwan area and heavy rainfall occurrence are presented in Section 3 . Finally, the major findings are summarized in Section 4 . 2. DATA AND METHODOLOGY Tu et al. (2020) identified the southwesterly MBLJ over the northern SCS based on two criteria: 1) maximum wind speed of more than 10 m s − 1 below the 850-hPa level; and 2) vertical shear of horizontal wind > 3 m s − 1 . Similar to Tu et al. (2020), we use the same cross section line (Fig. 1 ) to construct time series to identify periods when strong southwesterly monsoon flow is present and the relationship between the horizontal moisture fluxes by strong southwesterly flow and heavy rainfall events over southwestern Taiwan. The National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSv2) data (Saha et al., 2014) with 0.5° × 0.5° grids at 6-h intervals and 37 pressure levels are used to delineate the development, evolution, and structure of the tropical depressions in the Mei-Yu baroclinic zone over the northern SCS, and moisture transport in the MBL from the northern SCS to the Taiwan area. The rainfall data from surface weather stations and the Automatic Rainfall and Meteorological Telemetry System (ARMTS) over Taiwan (Kerns et al., 2010 ) are used to delineate the heavy rainfall events. The vertically integrated vapor transport (IVT) (Zhu and Newell, 1998 ; Lavers et al., 2012 ; Tu et al., 2019) is defined as $$\:IVT=\:\sqrt{{{Q}_{\lambda\:}}^{2}+{{Q}_{\varphi\:}}^{2}}$$ 1a , and the vertical integrals of the moisture transport components in the zonal ( λ ) and meridional ( ϕ ) directions are given as follows $$\:{Q}_{\lambda\:}=\:\frac{1}{g}{\int\:}_{{P}_{0}}^{{P}_{1}}qudp$$ 1b , $$\:{Q}_{\varphi\:}=\:\frac{1}{g}{\int\:}_{{P}_{0}}^{{P}_{1}}qvdp$$ 1c , where q is the specific humidity in kg kg − 1 , u is the zonal wind in m s − 1 , υ is the meridional wind in m s − 1 , g is the acceleration due to gravity in m s − 2 , P 0 is the surface pressure, and P 1 is the 850 hPa level. The two-dimensional frontogenesis function (Chen et al., 1994 ) is defined as $$\:F=\frac{1}{\left|\nabla\:\theta\:\right|}\left\{-\frac{\partial\:\theta\:}{\partial\:x}\left[\frac{\partial\:u}{\partial\:x}\frac{\partial\:\theta\:}{\partial\:x}+\frac{\partial\:v}{\partial\:x}\frac{\partial\:\theta\:}{\partial\:y}\right]-\frac{\partial\:\theta\:}{\partial\:y}\left[\frac{\partial\:u}{\partial\:y}\frac{\partial\:\theta\:}{\partial\:x}+\frac{\partial\:v}{\partial\:y}\frac{\partial\:\theta\:}{\partial\:y}\right]\right\}$$ 2 , where u and v are the horizontal winds and θ is the potential temperature. 3. RESULTS 3.1 Cyclogenesis along the Mei-Yu front over the northern SCS during SCSTIMX (2018) The first tropical depression (TD) formed around 1200 UTC 13 June at the western end of the NE-SW oriented Mei-Yu front over the northern SCS and moved toward Taiwan (Figs. 2 a, b, c). The TD formed between the strong (> 15 m s − 1 ) cold northeasterlies at the 925-hPa level coming from the Taiwan Strait and the warm, moist pre-frontal southwesterly flow with ~ 6°K differences in potential temperature across the frontal boundary in the subtropics (Figs. 3 a, b). At 0000 UTC 15 June, the TD intensified and was named TS Gaemi by the Japan Meteorological Agency (JMA) (Fig. 2 d). Gaemi then moved northeastward along the Mei-Yu front and became a Mei-Yu frontal cyclone (Figs. 2 e, f). At 0000 UTC on 15 June, the trough axis at the 500-hPa level extended from mid-latitudes to the subtropics around 16°N (Fig. 4 a). The horizontal distribution of vertical motion at the 500-hPa level shows rising motion (Fig. 4 a) associated with the Mei-Yu trough (Figs. 3 a, b). The 2D frontogenesis at the 850-hPa level (Fig. 4 b) shows that the axis of maximum frontogenesis occurs where the northwesterly flow behind the trough converges with the southwesterly monsoon flow. Figure 5 a shows the vertical cross-section of winds and the potential temperature along 120°E at 1200 UTC 14 June. The low-level cold ( θ 15 m s − 1 and warm ( θ > 303 K) southwesterly flow with wind speeds > 30 m s − 1 were located at 20° and 25°N, respectively. Both the southwesterly flow and northeasterly flow have maximum wind speeds around the 850-hPa level with relatively weak winds (< 5 m s − 1 ) in the storm center between 20–25°N. The equivalent potential temperature exhibits a maximum at the storm center around 22°N extending vertically upward (Fig. 5 b). It is apparent that the TD formed along the Mei-Yu frontal boundary over the northern SCS with large (> 6 K) potential temperature differences across the frontal zone. Along the southern flank of the storm, the horizontal moisture fluxes from the southwest have a maximum (> 500 g kg − 1 m s − 1 ) at the 900-hPa level (Fig. 5 c). The second TD formed around 0600 UTC on 17 June at the western end of the Mei-Yu front over the northern SCS (Figs. 2 e, f), made landfall over southern China (Fig. 2 g), and was then downgraded as a depression on 18 June (Fig. 2 h). The horizontal distribution of vertical motion at the 500-hPa level (Fig. 6 a) shows rising motion ahead of the 925-hPa wind shift line (Fig. 3 c). The TD develops under baroclinic conditions but with much weaker horizontal temperature gradients than TC Gaemi (Fig. 3 ). The axis of maximum horizontal θ gradients and the maximum 2D frontogenesis at the 850-hPa level occurred along the frontal zone (Fig. 6 b). The vertical cross section along 114°E (Fig. 7 a) shows a moderate temperature gradient (about 3°K) in the lowest levels. With weak vertical motion from 800–500 hPa around 20°N (Fig. 7 c), the high equivalent potential temperature air was brought upward (Fig. 7 b). A schematic diagram showing cyclogenesis along the Mei-Yu front over the subtropical ocean is given in Fig. 8 . 3.2 The horizontal moisture fluxes by the strengthened southwesterly monsoon flow The first and second heavy rainfall periods over southwestern Taiwan for the 2018 Mei-Yu season occurred during 14–15 June and 19–20 June, respectively (Figs. 9 and 10 ). For both periods, large horizontal moisture fluxes from the northern SCS to the Taiwan area at the 925-hPa level (Fig. 11 b) are evident due to the presence of the strengthened southwesterly flow by the presence of tropical storms (Fig. 11 ). For both periods, the total precipitable water (TPW) (Fig. 12 ) was greater than 65 mm over most of the northern SCS and the Taiwan area. The time series of the number of stations with rainfall > 80 mm day − 1 shows two separate peaks during two sub-periods (71 stations on 15 June and 199 stations on 19 June) (Fig. 10 b), with large southwesterly IVT between the surface and the 850-hPa level (> 400 kg m − 1 s − 1 ) extending from northern SCS to southwestern Taiwan (Fig. 13 ). The horizontal moisture fluxes at the 925-hPa level (> 300 g kg − 1 m s − 1 ) associated with strengthened southwesterly monsoon flow (Fig. 11 c) are closely related to heavy rainfall occurrences over Taiwan during 14–15 June and 19–20 June (Figs. 10 a, b). During 14–15 June, heavy rainfall (~ 300 mm) occurred over southwestern Taiwan (Figs. 9 b, c) due to the passage of TD Gaemi in a warm moist environment (Figs. 3 b, 13 a, b). The strengthened southwesterly flow associated with TD Gaemi, along with a large IVT between the surface and 850-hPa level (> 500 kg m − 1 s − 1 ) (Figs. 13 a, b), interacted with the Taiwan terrain and brought in widespread heavy rainfall over southwestern Taiwan (Figs. 9 b, c). After the second TD made landfall over the southern China coast around 0600 UTC 18 June (Figs. 2 g, h), a strong southwesterly monsoon flow occurred (Figs. 11 a, b). The warm, moist southwestern flow (Figs. 12 c, d) impinged on the Taiwan area and brought in heavy precipitation (> 600 mm) (Figs. 9 f, g) over southwestern Taiwan during 19–20 June. The second peak of horizontal moisture fluxes within the MBL from the northern SCS to the Taiwan area correlated well with the strength of the southwesterly monsoon flow with a peak on 19 June (Fig. 11 ). The moisture-laden southwesterly flow over the northern SCS (Figs. 12 c, d) impinged on southwestern Taiwan, which created favorable conditions for the development of heavy rainfall there (Figs. 9 and 10 ). For both strong (> 20 m s − 1 ) southwesterly flow events, they are different from the MBLJ found by Tu et al. (2019). The MBLJ is a synoptic feature associated with the deepening of the Mei-Yu trough over southern China and westward extension of the NPSH. The wind profile in the MBLJ resembles an Ekman spiral with a maximum wind speed at the top of the MBL (~ 1 km) (Tu et al., 2019). For the strengthened southwesterly flow event during 14–15 June, the strong winds at 18°N (Fig. 14 a) are embedded in a broad southwesterly monsoon flow on the south/southeastern flank of Gaemi with horizontal moisture fluxes enhanced by the TC circulations (Figs. 13 a, b). The strong winds (> 20 m s − 1 ) at 18°N extend up to the 700-hPa level with low-level northeasterly winds around 23°N (Fig. 14 a). Nevertheless, the horizontal moisture fluxes within the southwesterly monsoon flow south of the TC circulations are still mainly at low levels (Figs. 11 and 14 b). For the 19–20 June event, the broad southwesterly monsoon flow (Fig. 14 c) is affected by the outer circulation of TC after it made landfall over China with strong winds (> 20 m s − 1 ) extending up to the 700-hPa level. The relatively weak winds at 22°N (Fig. 14 c) are related to upstream flow deceleration due to orographic blocking by the terrain of Taiwan (Fig. 13 c). A vertical cross section taken at 21°N with the broad southwesterly monsoon flow over the northern SCS (Fig. 15 a) shows a jet core around 119°E at the 850-hPa level (Fig. 15 b). The relatively weak winds around 123°E in low levels (Fig. 15 b) are related to orographic effects of Philippines (Fig. 15 a). Again, the horizontal moisture fluxes within the southwesterly monsoon flow are still mainly at low levels (Fig. 15 c). A schematic diagram showing the presence of the strong southwesterly monsoon flow due to the enhanced pressure gradients between the dissipating tropical depression over southern China and the semi-permanent WPSH is given in Fig. 16 . 3.3 Localized heavy rainfall over southwestern Taiwan’s coast on 20 June In this section, we show the impact of rain evaporative cooling on the horizontal distributions of heavy rainfall on 20 June. On 19 June, the maximum rainfall axis (> 300 mm) (Fig. 9 g) occurred on the windward slopes of the CMR due to orographic blocking and lifting. The cold pool (~ 25°C) developed on 20 June over southwestern Taiwan, compared with warmer temperature over land on 18 June (Figs. 17 a, b), generated offshore flow during the daytime (Fig. 18 a). The time series analysis of temperature observations at southwestern Taiwan during 1140 LST 19 June–1140 LST 20 June reveals a significant drop (~ 2°C) due to rain evaporative cooling. Therefore, because of the convergence of the offshore flow and the southwesterly flow (Fig. 18 ), the maximum rainfall (~ 300 mm) shifted from the windward slopes of the CMR to the southwestern coast (Fig. 9 h). This heavy rainfall event ended in the late afternoon of 20 June after the weakening of the southwesterly monsoon flow (Fig. 11 b). 4. SUMMARY AND DISCUSSION During the 13–20 June 2018 South China Sea Two-Island Monsoon Experiment (SCSTIMX-2018), two tropical depressions formed at the western end of the NE-SW orientated Mei-Yu front south of Taiwan between the strong (> 15 m s − 1 ) cold northeasterlies coming from the Taiwan Strait and the warm, moist prefrontal southwesterly flow. These mesoscale disturbances developed in the open ocean in the subtropics under baroclinic settings along a Mei-Yu front. During 14–16 June, heavy rainfall (> 550 mm) occurred over southern Taiwan due to the passage of TD Gaemi in a warm moist environment. The tropical disturbance first formed around 1800 UTC 13 June on the western end of the Mei-Yu front over the northern South China Sea with mixed baroclinic Mei-Yu frontal signatures and asymmetric tropical cyclone structure. Gaemi became a named tropical storm before making landfall over southern Taiwan on 0000 UTC 15 June. With the deepening of Gaemi, the southwesterly flow on the southern/southeastern flank of the storm strengthened. During 14–15 June, heavy rainfall (> 600 mm) occurred over southern Taiwan with a maximum on the windward side of the CMR. After passing southern Taiwan, TC Gaemi continued to move northeastward along the Mei-Yu front and developed into a Mei-Yu frontal cycle. The second TD formed over the western end of the Mei-Yu front, made landfall over the southern China coast around 0600 UTC 18 June, and brought a strong southwesterly monsoon flow over the Taiwan area. On 18 June, the southwesterly monsoon flow strengthened and extended northeastward, due to the enhanced pressure gradients between the TD and the semi-permanent WPSH. It is apparent that the southwesterly flow is also affected by the outer circulation of the TD with strong winds (> 20 m s − 1 ) extending up to the 700-hPa level. During 19–20 June, the horizontal moisture fluxes associated with the strong (> 20 m s − 1 ) southwesterly monsoon flow occurred primarily within the MBL with maximum TPW > 65 mm. The warm, moist southwesterly flow impinged on southwestern Taiwan under favorable large-scale settings, including warm advection within the southwesterly monsoon flow and the passage of a 500-hPa trough to the north. On 19 June, the maximum rainfall axis (> 300 mm) occurred on the windward slopes of the CMR due to orographic lifting. On 20 June, with the development of cold pool (~ 25°C), offshore flow was generated over southwestern Taiwan during the daytime due to rain evaporative cooling associated with the heavy rains on 19 June. As a result, the maximum rainfall (> 300 mm) occurred along the southwestern coast due to the convergence between the offshore flow and the incoming southwesterly flow. This heavy rainfall event over southern Taiwan ended in the late afternoon of 20 June after the weakening of the southwesterly monsoon flow. Declarations Contributions: C.-K. Wang performed all the computations and prepared all the figures. Y.-L. Chen designed the work and wrote the manuscript. All authors reviewed the manuscript. Competing interests The authors declare no competing interests. Funding: This work is jointly funded by the National Science Foundation under Grants AGS-1142558, and AGS-1854443 to the University of Hawai‘i at Mānoa; the National Science and Technology Center (NSTC) under Grants 114-2811-M-008-016, 112-2111-M-008-023, 113-2111-M-008-027, and the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) to the National Central University, Taiwan. Author Contribution C.-K. Wang performed all the computations and prepared all the figures. Y.-L. Chen designed the work and wrote the manuscript. All authors reviewed the manuscript. Data availability The CFSv2 data is available from https://www.ncei.noaa.gov/access/metadata/landing-page/bin/iso?id=gov.noaa.ncdc:C0087 . The rainfall and surface data are available from the Central Weather Association, Taipei, Taiwan. References Charney JG, Eliassen A (1964) On the growth of the hurricane depression. 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Mon Wea Rev 140(8):2555–2574. 10.1175/MWR-D-11-00208.1 Zhu Y, Newell RE (1998) A proposed algorithm for moisture fluxes from atmospheric rivers. Mon Wea Rev 126:725–735. 10.1175/1520-0493(1998)1262.0.CO;2 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7241626","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":492489860,"identity":"14c673d6-77b6-4e29-89a7-ca0a9d8a4def","order_by":0,"name":"Chuan-Kai Wang","email":"","orcid":"","institution":"University of Hawai‘i at Mānoa","correspondingAuthor":false,"prefix":"","firstName":"Chuan-Kai","middleName":"","lastName":"Wang","suffix":""},{"id":492489862,"identity":"243c93f9-b172-488a-9690-189538a7f587","order_by":1,"name":"Yi-Leng Chen","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzElEQVRIiWNgGAWjYDACZuYGIGmTAGIfAJKMDYS1gNWkkaIFouZwAgoXLzA4ztj4uODX+Tz+GckPDzxgsJHdcICQlsOMzcYz+24XS9xIMwA6LM2YGC1t0rw9txMbbieAtBxOJEZL+2/ennOJ82+nfwBq+U+UljZmnh8HEjfczgHZcoCwFkmgX6R5G5ITN95/U3AgwSDZeCYhLXznDx/8zPPHLnHemeObP/6osJPtI6RFAaSAsQ3uTgLKQUC+AUT+IULlKBgFo2AUjFwAANUtUI3p76wyAAAAAElFTkSuQmCC","orcid":"","institution":"University of Hawai‘i at Mānoa","correspondingAuthor":true,"prefix":"","firstName":"Yi-Leng","middleName":"","lastName":"Chen","suffix":""},{"id":492489863,"identity":"d3eb4a7f-230a-43c8-bde7-686263021d38","order_by":2,"name":"Chuan-Chi Tu","email":"","orcid":"","institution":"National Central University","correspondingAuthor":false,"prefix":"","firstName":"Chuan-Chi","middleName":"","lastName":"Tu","suffix":""},{"id":492489866,"identity":"f45fa97f-169b-42bd-8747-b75417d9476d","order_by":3,"name":"Pay-Liam Lin","email":"","orcid":"","institution":"National Central University","correspondingAuthor":false,"prefix":"","firstName":"Pay-Liam","middleName":"","lastName":"Lin","suffix":""}],"badges":[],"createdAt":"2025-07-29 09:38:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7241626/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7241626/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":87888931,"identity":"989c2c45-1e66-4e1a-8662-e0186c819dc1","added_by":"auto","created_at":"2025-07-30 06:02:48","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":320322,"visible":true,"origin":"","legend":"\u003cp\u003eTerrain height (m). The black line is the NW-SE cross-section line used for time series plot in Fig. 11.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/19f84c24b307f63f02e59ba0.png"},{"id":87889872,"identity":"08120f24-8ba8-40d9-aac1-4e5a05b4b55b","added_by":"auto","created_at":"2025-07-30 06:10:48","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":844541,"visible":true,"origin":"","legend":"\u003cp\u003eThe Central Weather Administration (CWA) surface analysis at (a) 1200 UTC 13 June, (b) 1800 UTC 13 June, (c) 1200 UTC 14 June, (d) 0000 UTC 15 June, (e) 0000 UTC 17 June, (f) 0600 UTC 17 June, (g) 0600 UTC 18 June, and (h) 1200 UTC 18 June 2018.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/7237f6382fab4476eafd365e.png"},{"id":87888932,"identity":"591a843b-4f57-4e33-afd1-d57c42f108e9","added_by":"auto","created_at":"2025-07-30 06:02:48","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":996141,"visible":true,"origin":"","legend":"\u003cp\u003eThe 925 hPa geopotential height (gpm, contoured every 20 gpm), potential temperature (K, shaded), and winds (m s\u003csup\u003e−1\u003c/sup\u003e) (a full barb represents 10 m s\u003csup\u003e-1\u003c/sup\u003e) at (a) 1800 UTC 13 June, (b) 0000 UTC 15 June, (c) 0000 UTC 17 June, and (d) 0600 UTC 17 June\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/61dcea683f08d02c6d125999.png"},{"id":87889874,"identity":"8e8c8796-1cda-4d7d-9b00-ff6a47730022","added_by":"auto","created_at":"2025-07-30 06:10:48","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":536570,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The CFSv2 850 hPa frontogenesis (10\u003csup\u003e-9\u003c/sup\u003e K m\u003csup\u003e-1\u003c/sup\u003e s\u003csup\u003e-1\u003c/sup\u003e, shaded) and potential temperature (K, contoured every 2 K), and winds (m s\u003csup\u003e−1\u003c/sup\u003e), (b) The CFSv2 500 hPa vertical motion (Pa s\u003csup\u003e-1\u003c/sup\u003e, shaded), geopotential height (gpm, contoured every 30 gpm), and wind (m s\u003csup\u003e−1\u003c/sup\u003e) (full barb and half barb represent 10 and 5 m s\u003csup\u003e-1\u003c/sup\u003e, respectively) at 0000 UTC 15 June 2018.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/f42ad5d21b4871d2fcaea01e.png"},{"id":87888938,"identity":"5122f802-9101-419f-a758-0796336b7a0f","added_by":"auto","created_at":"2025-07-30 06:02:48","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":585910,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The CFSv2 vertical cross-section of winds (m s\u003csup\u003e−1\u003c/sup\u003e) (full barb, and half barb represent 10 and 5 m s\u003csup\u003e-1\u003c/sup\u003e, respectively) and potential temperature (K, contoured every 3 K) along 120°E at 1200 UTC 14 June. (b) As in (a) but for winds (m s\u003csup\u003e−1\u003c/sup\u003e) (full barb and half barb represent 10 and 5 m s\u003csup\u003e-1\u003c/sup\u003e, respectively) and equivalent potential temperature (K, shaded every 5 K). (c) As in (a) but for horizontal moisture flux vector (\u003cem\u003eqV\u003c/em\u003e)(g kg\u003csup\u003e-1\u003c/sup\u003e m s\u003csup\u003e-1\u003c/sup\u003e, shaded every 50 g kg\u003csup\u003e-1\u003c/sup\u003e m s\u003csup\u003e-1\u003c/sup\u003e) and vertical velocity (Pa s\u003csup\u003e-1\u003c/sup\u003e, contoured every 1 Pa s\u003csup\u003e-1\u003c/sup\u003e).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/39b510564eb0558e055683e1.png"},{"id":87889877,"identity":"8044dc83-7c81-48da-94b7-95a5bcb903ae","added_by":"auto","created_at":"2025-07-30 06:10:48","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":434877,"visible":true,"origin":"","legend":"\u003cp\u003eSame as Fig. 4 but for 0000 UTC 16 June 2018.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/ca9541bc4371646c26afc141.png"},{"id":87889879,"identity":"1349caa4-f241-4130-aa88-4c5cf0286f10","added_by":"auto","created_at":"2025-07-30 06:10:48","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":544379,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The vertical cross-section of winds (m s\u003csup\u003e−1\u003c/sup\u003e) (full barb, and half barb represent 10 and 5 m s\u003csup\u003e-1\u003c/sup\u003e, respectively) and potential temperature (K, contoured every 3 K) along 114°E at 0600 UTC 17 June. (b) As in (a) but for winds (m s\u003csup\u003e−1\u003c/sup\u003e) (full barb and half barb represent 10 and 5 m s\u003csup\u003e-1\u003c/sup\u003e, respectively) and equivalent potential temperature (K, shaded every 5 K). (c) As in (a) but for horizontal moisture flux vector (\u003cem\u003eqV\u003c/em\u003e)(g kg\u003csup\u003e-1\u003c/sup\u003e m s\u003csup\u003e-1\u003c/sup\u003e).\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/bdd92464b68a909f54bf5e20.png"},{"id":87889892,"identity":"85812b6f-b2d1-4d97-bc69-dd2a08b655ea","added_by":"auto","created_at":"2025-07-30 06:10:50","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":389689,"visible":true,"origin":"","legend":"\u003cp\u003eA schematic diagram showing a tropical depression/tropical storm (TD/TS) formed under baroclinic setting. The red dashed line represents the trough at 500 hPa. The red (blue) arrow represents warm, moist southwesterlies (cold, dry northeasterlies).\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/94cdbb56d21e01ecacbe1b27.png"},{"id":87888940,"identity":"aaccdcc3-3c1e-41a0-b6aa-0ee3b6a5a695","added_by":"auto","created_at":"2025-07-30 06:02:48","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":987198,"visible":true,"origin":"","legend":"\u003cp\u003eTotal daily (00-24 LT) rainfall accumulation (mm) during (a) 13 June, (b) 14 June, (c) 15 June, (d) 16 June, (e) 17 June, (f) 18 June, (g) 19 June, and (h) 20 June 2018 from rain gauge observations (Courtesy: Central Weather Association, Taipei, Taiwan).\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/f9f9816b003ae64a16c15435.png"},{"id":87888949,"identity":"7131201a-ba49-42e7-8b0f-d052485c4832","added_by":"auto","created_at":"2025-07-30 06:02:48","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":57991,"visible":true,"origin":"","legend":"\u003cp\u003eTime series of (a) averaged daily rainfall over Taiwan from rain gauges (mm), (b) number of stations recording heavy rain (\u0026gt; 80 mm day\u003csup\u003e-1 \u003c/sup\u003eor \u0026gt;40 mm hour\u003csup\u003e-1\u003c/sup\u003e) from 1 June to 30 June 2018. The Roman numbers in (a) represent the first and second heavy rainfall period.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/09796245b6216de9a222d0a9.png"},{"id":87888955,"identity":"67806695-a625-4bbe-a69c-d388d6763fea","added_by":"auto","created_at":"2025-07-30 06:02:48","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":617091,"visible":true,"origin":"","legend":"\u003cp\u003eThe Hovmöller diagrams along the NW-SE line (black line in Fig. 1) from 0000 UTC 1 June to 1800 UTC 30 June 2018: (a) 925 hPa winds (m s\u003csup\u003e-1\u003c/sup\u003e) (full barb, and half barb represent 10 and 5 m s\u003csup\u003e-1\u003c/sup\u003e, respectively) ; (b) 925 hPa geopotential height (gpm); (c) 925 hPa horizontal moisture flux vector (\u003cem\u003eqV\u003c/em\u003e) (g kg\u003csup\u003e-1\u003c/sup\u003e m s\u003csup\u003e-1\u003c/sup\u003e); and (d) 700 hPa horizontal moisture flux vector (\u003cem\u003eqV\u003c/em\u003e) (g kg\u003csup\u003e-1\u003c/sup\u003e m s\u003csup\u003e-1\u003c/sup\u003e).\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/ac77175de5881d43896b70ed.png"},{"id":87888967,"identity":"d70935ef-0f49-4d07-be10-40720a7674bb","added_by":"auto","created_at":"2025-07-30 06:02:49","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":693630,"visible":true,"origin":"","legend":"\u003cp\u003eThe total precipitable water (kg m\u003csup\u003e-2\u003c/sup\u003e) at (a) 0000 UTC 14 June, (b) 0000 UTC 15 June, (c) 0000 UTC 19 June, and (d) 0000 UTC 20 June 2018.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/6174ecbeb42825a4c2645378.png"},{"id":87888942,"identity":"fd6c67ba-625f-4472-943c-4869e1d8e5b3","added_by":"auto","created_at":"2025-07-30 06:02:48","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":772036,"visible":true,"origin":"","legend":"\u003cp\u003eThe integrated vapor transport (IVT; kg m\u003csup\u003e-1\u003c/sup\u003e s\u003csup\u003e-1\u003c/sup\u003e) between surface and 850 hPa at (a) 0000 UTC 14 June, (b) 0000 UTC 15 June, (c) 0000 UTC 19 June, and (d) 0000 UTC 20 June 2018.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/ae9f7c7931feaeac3e108e1e.png"},{"id":87888984,"identity":"6050a204-1966-4ff8-8c95-865741cdc2ef","added_by":"auto","created_at":"2025-07-30 06:02:50","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":630360,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The vertical cross-section of winds (m s\u003csup\u003e−1\u003c/sup\u003e) (full barb and half barb represent 10 and 5 m s\u003csup\u003e-1\u003c/sup\u003e, respectively) along 118°E at 0000 UTC 15 June 2018. (b) As in (a) but for horizontal moisture flux vector (\u003cem\u003eqV\u003c/em\u003e) (g kg\u003csup\u003e-1\u003c/sup\u003e m s\u003csup\u003e-1\u003c/sup\u003e). (c) and (d) As in (a) and (b) but for 0000 UTC 19 June 2018.\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/fdd2b95d96f31003132d7d64.png"},{"id":87889882,"identity":"be883727-eb34-497e-8a59-827bc86f8ef8","added_by":"auto","created_at":"2025-07-30 06:10:48","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":482592,"visible":true,"origin":"","legend":"\u003cp\u003eLegend not included with this version.\u003c/p\u003e","description":"","filename":"15.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/5e7b33e530a00376106bc6c0.png"},{"id":87889881,"identity":"f6af9d39-ddf2-4e8f-a948-82bcba1bf64f","added_by":"auto","created_at":"2025-07-30 06:10:48","extension":"png","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":375875,"visible":true,"origin":"","legend":"\u003cp\u003eA schematic diagram showing strengthening of the southwesterly monsoon flow due to enhanced pressure gradients between the West Pacific Subtropical High and the TD embedded in the Mei-Yu frontal system.\u003c/p\u003e","description":"","filename":"16.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/18ccb4b058da4c9cc1d2539e.png"},{"id":87888958,"identity":"b5c7a89a-4785-497d-a5fc-8a27dcf343c8","added_by":"auto","created_at":"2025-07-30 06:02:49","extension":"png","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":337596,"visible":true,"origin":"","legend":"\u003cp\u003eThe time series of observed surface temperature during 1140 LST (0340 UTC) 19 June to 1140 LST (0340 UTC) 20 June at (a) Kaohsiung station (120.31°W, 22.73°N), and (b) Tainan station (120.20\u003csup\u003e0\u003c/sup\u003eW, 22.99\u003csup\u003e0\u003c/sup\u003eN) (Courtesy: Central Weather Administration, Taipei, Taiwan). Locations of both stations and given in the right panel.\u003c/p\u003e","description":"","filename":"17.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/691639e1bc1f87d901ff865d.png"},{"id":87888951,"identity":"d9fe9ed5-65df-4c91-ad60-674e53f297e8","added_by":"auto","created_at":"2025-07-30 06:02:48","extension":"png","order_by":18,"title":"Figure 18","display":"","copyAsset":false,"role":"figure","size":469259,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The observed surface winds (m s\u003csup\u003e−1\u003c/sup\u003e) (full barb represent 1 m s\u003csup\u003e-1\u003c/sup\u003e) at 0500 UTC (1300 LT) 20 June (Courtesy: National Center for Disaster Reduction, Taipei, Taiwan). (b) The CFSv2 1000 hPa pressure (gpm), and winds (m s\u003csup\u003e−1\u003c/sup\u003e) (a full barb represents 10 m s-1) at 0600 UTC (1400 LT) 20 June,\u003c/p\u003e","description":"","filename":"18.png","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/55a9920dcdef5d60bf24ab08.png"},{"id":92394650,"identity":"84e941e5-e6d5-427d-9878-9653f28eb018","added_by":"auto","created_at":"2025-09-29 09:14:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":9922032,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7241626/v1/6161e78e-e687-48e4-a487-3cd21b460ed0.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Impacts of cyclogenesis and moisture transport by the enhanced southwesterly monsoon flow on heavy rainfall over southern Taiwan during SCSTIMX (2018)","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eTu et al. (2020) identified two main heavy rainfall periods (1\u0026ndash;4 June and 14\u0026ndash;18 June 2017) over the Taiwan area during the South China Sea Two Island Monsoon Experiment (SCSTIMX-2017) (Sui et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). They found that these two periods are closely related to the large-scale moisture transport within the marine boundary layer (MBL) from the northern South China Sea (SCS) to the Taiwan area. The Marine Boundary Layer Jet (MBLJ) develops and intensifies in response to sub-synoptic pressure gradients when the Mei-Yu trough over southern China deepens and/or the west Pacific subtropical high (WPSH) strengthens and extends westward as found in Tu et al. (2019). They suggest that moisture transport within the MBL from the northern SCS to the Taiwan area is a favorable condition for the development of heavy rainfall during the early summer rainy season in Taiwan (e.g., mid-May to mid-June) (Kuo and Chen, 1990; Chen and Tu, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eChen et al. (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) identified five torrential rain (\u0026gt;\u0026thinsp;350 mm day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) events over southwestern Taiwan during a 10-year period (1997\u0026ndash;2006). These events are characterized by large (\u0026gt;\u0026thinsp;220 g kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) upstream low-level moisture fluxes averaged among 1000-, 925-, and 850-hPa levels, the presence of the 850-hPa high \u003cem\u003eθe\u003c/em\u003e axis, and 700-hPa sub-synoptic upward motion. Chen et al. (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) studied the widespread heavy rainfall events over Taiwan during 10\u0026ndash;12 June 2012 and showed that the maximum low-level horizontal moisture fluxes (\u0026gt;\u0026thinsp;330 g kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) associated with the southwesterly monsoon flow occurred in the planetary boundary layer with a maximum at the 950-hPa level. In addition to the localized heavy rainfall during the passage of a Mei-Yu jet/front system over northern Taiwan, heavy rainfall occurred on the windward slopes of the Snow Mountains (\u0026gt;\u0026thinsp;1,000 mm) and Central Mountain Range (CMR) (\u0026gt;\u0026thinsp;1,500 mm) as the low-level southwesterly monsoon flow impinged on these mountains (Chen et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eTu et al. (2019) used a five-year (2008\u0026ndash;2012) reanalysis data to study the characteristics of MBLJs over the northern SCS during the early summer rainy season over Taiwan. The MBLJ at the 950-hPa level (Tu et al., 2019, 2020, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) over the northern SCS and the sub-synoptic low-level jet (SLLJ) at the 850-700-hPa level associated with the Mei-Yu front (Chen and Yu, 1988; Chen et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Chen et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1994\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) are two distinct features that could occur and, possibly, co-exist during the early summer rainy season over Taiwan (Chen et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Tu et al., 2019; and Ke et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The SLLJ in the frontal zone is an integral part of the secondary circulation across the Mei-Yu jet/front system (Chen et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1994\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Chen and Chen, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Chen and Chen, 2002). The MBLJ is a synoptic feature related large-scale pressure gradients between the Mei-Yu trough over southern China and the WPSH. At the jet core of the MBLJ over the northern SCS, the vertical wind profile resembles an Ekman spiral with a wind speed maximum near the top of the MBL (~\u0026thinsp;925 hPa) (Tu et al., 2019). When both MBLJ and SLLJ are present the MBLJ is important in horizontal moisture transport from the northern SCS to the Taiwan area (Tu et al., 2019), whereas rainfall production is mainly related to secondary circulation associated with the jet/front system in the frontal zone (Chen et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1994\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) or localized lifting related to orographic effects (Chen et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). During the early summer rainy season, the MBLJ from the southern China coast is also favorable for the occurrences of heavy rainfall over the southern China coastal areas (Du and Chen, 2018, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eDuring 13\u0026ndash;20 June 2018, we show that two tropical depressions developed over the northern SCS along western end of a Mei-Yu front with appreciable temperature gradients in low levels. Although many studies show that the Convective Instability of the Second Kind (CISK) (Charney and Eliassen, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1964\u003c/span\u003e; Ooyama, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1969\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e1982\u003c/span\u003e and others) is important for the development of tropical cyclones, we show that the initial disturbances could possibly develop along a Mei-Yu front over the subtropical ocean. This Mei-Yu front over the northern SCS exhibits baroclinic characteristics, as found in previous studies over southern China (Chen et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Trier et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Chen, 1993; Hsiao and Chen, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Chen et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). We also show that the southwesterly monsoon flow could possibly strengthen on the southeasterly flank of a tropical cyclone, due to increased pressure gradients between a tropical disturbance and the WPSH bringing in moisture at low levels, which is favorable for the development of heavy precipitation over the Taiwan area. In addition, Xu et al. (\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) studied the evolution and mechanism of the mesoscale convection system during Terrain-influenced Monsoon Rainfall Experiment (TiMREX). They identified the effect of rain evaporative cooling and the cold pool that formed in the lowest 500 m due to persistent rainfall. Tu et al. (2017) used the weather research and forecasting (WRF) model to further investigate the impact of rain evaporative cooling on coastal rainfall for the same case. We confirm the findings by Xu et al. (\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) and Tu et al. (2017) that in addition to orographic effects, rain evaporation affects the location of heavy precipitation.\u003c/p\u003e\u003cp\u003eThe data and methodology used in this study are given in Section \u003cspan refid=\"Sec2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The results of cyclogenesis, horizontal moisture fluxes within the MBL, and the relationship between moisture transport from the northern SCS to the Taiwan area and heavy rainfall occurrence are presented in Section \u003cspan refid=\"Sec3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Finally, the major findings are summarized in Section \u003cspan refid=\"Sec7\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e"},{"header":"2. DATA AND METHODOLOGY","content":"\u003cp\u003eTu et al. (2020) identified the southwesterly MBLJ over the northern SCS based on two criteria: 1) maximum wind speed of more than 10 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e below the 850-hPa level; and 2) vertical shear of horizontal wind\u0026thinsp;\u0026gt;\u0026thinsp;3 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Similar to Tu et al. (2020), we use the same cross section line (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e1\u003c/span\u003e) to construct time series to identify periods when strong southwesterly monsoon flow is present and the relationship between the horizontal moisture fluxes by strong southwesterly flow and heavy rainfall events over southwestern Taiwan.\u003c/p\u003e\u003cp\u003eThe National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSv2) data (Saha et al., 2014) with 0.5\u0026deg; \u0026times; 0.5\u0026deg; grids at 6-h intervals and 37 pressure levels are used to delineate the development, evolution, and structure of the tropical depressions in the Mei-Yu baroclinic zone over the northern SCS, and moisture transport in the MBL from the northern SCS to the Taiwan area. The rainfall data from surface weather stations and the Automatic Rainfall and Meteorological Telemetry System (ARMTS) over Taiwan (Kerns et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) are used to delineate the heavy rainfall events.\u003c/p\u003e\u003cp\u003eThe vertically integrated vapor transport (IVT) (Zhu and Newell, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Lavers et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Tu et al., 2019) is defined as\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:IVT=\\:\\sqrt{{{Q}_{\\lambda\\:}}^{2}+{{Q}_{\\varphi\\:}}^{2}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1a\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e\u003cp\u003eand the vertical integrals of the moisture transport components in the zonal (\u003cem\u003eλ\u003c/em\u003e) and meridional (\u003cem\u003eϕ\u003c/em\u003e) directions are given as follows\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{Q}_{\\lambda\\:}=\\:\\frac{1}{g}{\\int\\:}_{{P}_{0}}^{{P}_{1}}qudp$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1b\u003c/div\u003e\u003c/div\u003e,\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:{Q}_{\\varphi\\:}=\\:\\frac{1}{g}{\\int\\:}_{{P}_{0}}^{{P}_{1}}qvdp$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1c\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003eq\u003c/em\u003e is the specific humidity in kg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, \u003cem\u003eu\u003c/em\u003e is the zonal wind in m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, \u003cem\u003eυ\u003c/em\u003e is the meridional wind in m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, \u003cem\u003eg\u003c/em\u003e is the acceleration due to gravity in m s\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, \u003cem\u003eP\u003c/em\u003e\u003csub\u003e0\u003c/sub\u003e is the surface pressure, and \u003cem\u003eP\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e is the 850 hPa level.\u003c/p\u003e\u003cp\u003eThe two-dimensional frontogenesis function (Chen et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1994\u003c/span\u003e) is defined as\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:F=\\frac{1}{\\left|\\nabla\\:\\theta\\:\\right|}\\left\\{-\\frac{\\partial\\:\\theta\\:}{\\partial\\:x}\\left[\\frac{\\partial\\:u}{\\partial\\:x}\\frac{\\partial\\:\\theta\\:}{\\partial\\:x}+\\frac{\\partial\\:v}{\\partial\\:x}\\frac{\\partial\\:\\theta\\:}{\\partial\\:y}\\right]-\\frac{\\partial\\:\\theta\\:}{\\partial\\:y}\\left[\\frac{\\partial\\:u}{\\partial\\:y}\\frac{\\partial\\:\\theta\\:}{\\partial\\:x}+\\frac{\\partial\\:v}{\\partial\\:y}\\frac{\\partial\\:\\theta\\:}{\\partial\\:y}\\right]\\right\\}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e,\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003eu\u003c/em\u003e and \u003cem\u003ev\u003c/em\u003e are the horizontal winds and \u003cem\u003eθ\u003c/em\u003e is the potential temperature.\u003c/p\u003e"},{"header":"3. RESULTS","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Cyclogenesis along the Mei-Yu front over the northern SCS during SCSTIMX (2018)\u003c/h2\u003e\u003cp\u003eThe first tropical depression (TD) formed around 1200 UTC 13 June at the western end of the NE-SW oriented Mei-Yu front over the northern SCS and moved toward Taiwan (Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, b, c). The TD formed between the strong (\u0026gt;\u0026thinsp;15 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) cold northeasterlies at the 925-hPa level coming from the Taiwan Strait and the warm, moist pre-frontal southwesterly flow with ~\u0026thinsp;6\u0026deg;K differences in potential temperature across the frontal boundary in the subtropics (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, b). At 0000 UTC 15 June, the TD intensified and was named TS Gaemi by the Japan Meteorological Agency (JMA) (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). Gaemi then moved northeastward along the Mei-Yu front and became a Mei-Yu frontal cyclone (Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e2\u003c/span\u003ee, f).\u003c/p\u003e\u003cp\u003eAt 0000 UTC on 15 June, the trough axis at the 500-hPa level extended from mid-latitudes to the subtropics around 16\u0026deg;N (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). The horizontal distribution of vertical motion at the 500-hPa level shows rising motion (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e4\u003c/span\u003ea) associated with the Mei-Yu trough (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, b). The 2D frontogenesis at the 850-hPa level (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) shows that the axis of maximum frontogenesis occurs where the northwesterly flow behind the trough converges with the southwesterly monsoon flow.\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e5\u003c/span\u003ea shows the vertical cross-section of winds and the potential temperature along 120\u0026deg;E at 1200 UTC 14 June. The low-level cold (\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;297 K) northeasterly flow with speeds\u0026thinsp;\u0026gt;\u0026thinsp;15 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and warm (\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;303 K) southwesterly flow with wind speeds\u0026thinsp;\u0026gt;\u0026thinsp;30 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were located at 20\u0026deg; and 25\u0026deg;N, respectively. Both the southwesterly flow and northeasterly flow have maximum wind speeds around the 850-hPa level with relatively weak winds (\u0026lt;\u0026thinsp;5 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in the storm center between 20\u0026ndash;25\u0026deg;N. The equivalent potential temperature exhibits a maximum at the storm center around 22\u0026deg;N extending vertically upward (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). It is apparent that the TD formed along the Mei-Yu frontal boundary over the northern SCS with large (\u0026gt;\u0026thinsp;6 K) potential temperature differences across the frontal zone. Along the southern flank of the storm, the horizontal moisture fluxes from the southwest have a maximum (\u0026gt;\u0026thinsp;500 g kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) at the 900-hPa level (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e5\u003c/span\u003ec).\u003c/p\u003e\u003cp\u003eThe second TD formed around 0600 UTC on 17 June at the western end of the Mei-Yu front over the northern SCS (Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e2\u003c/span\u003ee, f), made landfall over southern China (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e2\u003c/span\u003eg), and was then downgraded as a depression on 18 June (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e2\u003c/span\u003eh). The horizontal distribution of vertical motion at the 500-hPa level (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e6\u003c/span\u003ea) shows rising motion ahead of the 925-hPa wind shift line (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). The TD develops under baroclinic conditions but with much weaker horizontal temperature gradients than TC Gaemi (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The axis of maximum horizontal θ gradients and the maximum 2D frontogenesis at the 850-hPa level occurred along the frontal zone (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). The vertical cross section along 114\u0026deg;E (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e7\u003c/span\u003ea) shows a moderate temperature gradient (about 3\u0026deg;K) in the lowest levels. With weak vertical motion from 800\u0026ndash;500 hPa around 20\u0026deg;N (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e7\u003c/span\u003ec), the high equivalent potential temperature air was brought upward (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e7\u003c/span\u003eb). A schematic diagram showing cyclogenesis along the Mei-Yu front over the subtropical ocean is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e3.2 The horizontal moisture fluxes by the strengthened southwesterly monsoon flow\u003c/h2\u003e\u003cp\u003eThe first and second heavy rainfall periods over southwestern Taiwan for the 2018 Mei-Yu season occurred during 14\u0026ndash;15 June and 19\u0026ndash;20 June, respectively (Figs.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e9\u003c/span\u003e and \u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e10\u003c/span\u003e). For both periods, large horizontal moisture fluxes from the northern SCS to the Taiwan area at the 925-hPa level (Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e11\u003c/span\u003eb) are evident due to the presence of the strengthened southwesterly flow by the presence of tropical storms (Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e11\u003c/span\u003e). For both periods, the total precipitable water (TPW) (Fig.\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e12\u003c/span\u003e) was greater than 65 mm over most of the northern SCS and the Taiwan area. The time series of the number of stations with rainfall\u0026thinsp;\u0026gt;\u0026thinsp;80 mm day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e shows two separate peaks during two sub-periods (71 stations on 15 June and 199 stations on 19 June) (Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e10\u003c/span\u003eb), with large southwesterly IVT between the surface and the 850-hPa level (\u0026gt;\u0026thinsp;400 kg m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) extending from northern SCS to southwestern Taiwan (Fig.\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e13\u003c/span\u003e). The horizontal moisture fluxes at the 925-hPa level (\u0026gt;\u0026thinsp;300 g kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) associated with strengthened southwesterly monsoon flow (Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e11\u003c/span\u003ec) are closely related to heavy rainfall occurrences over Taiwan during 14\u0026ndash;15 June and 19\u0026ndash;20 June (Figs.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e10\u003c/span\u003ea, b).\u003c/p\u003e\u003cp\u003eDuring 14\u0026ndash;15 June, heavy rainfall (~\u0026thinsp;300 mm) occurred over southwestern Taiwan (Figs.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e9\u003c/span\u003eb, c) due to the passage of TD Gaemi in a warm moist environment (Figs.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, \u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e13\u003c/span\u003ea, b). The strengthened southwesterly flow associated with TD Gaemi, along with a large IVT between the surface and 850-hPa level (\u0026gt;\u0026thinsp;500 kg m\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) (Figs.\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e13\u003c/span\u003ea, b), interacted with the Taiwan terrain and brought in widespread heavy rainfall over southwestern Taiwan (Figs.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e9\u003c/span\u003eb, c).\u003c/p\u003e\u003cp\u003eAfter the second TD made landfall over the southern China coast around 0600 UTC 18 June (Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e2\u003c/span\u003eg, h), a strong southwesterly monsoon flow occurred (Figs.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e11\u003c/span\u003ea, b). The warm, moist southwestern flow (Figs.\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e12\u003c/span\u003ec, d) impinged on the Taiwan area and brought in heavy precipitation (\u0026gt;\u0026thinsp;600 mm) (Figs.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e9\u003c/span\u003ef, g) over southwestern Taiwan during 19\u0026ndash;20 June. The second peak of horizontal moisture fluxes within the MBL from the northern SCS to the Taiwan area correlated well with the strength of the southwesterly monsoon flow with a peak on 19 June (Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e11\u003c/span\u003e). The moisture-laden southwesterly flow over the northern SCS (Figs.\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e12\u003c/span\u003ec, d) impinged on southwestern Taiwan, which created favorable conditions for the development of heavy rainfall there (Figs.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e9\u003c/span\u003e and \u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eFor both strong (\u0026gt;\u0026thinsp;20 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) southwesterly flow events, they are different from the MBLJ found by Tu et al. (2019). The MBLJ is a synoptic feature associated with the deepening of the Mei-Yu trough over southern China and westward extension of the NPSH. The wind profile in the MBLJ resembles an Ekman spiral with a maximum wind speed at the top of the MBL (~\u0026thinsp;1 km) (Tu et al., 2019). For the strengthened southwesterly flow event during 14\u0026ndash;15 June, the strong winds at 18\u0026deg;N (Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e14\u003c/span\u003ea) are embedded in a broad southwesterly monsoon flow on the south/southeastern flank of Gaemi with horizontal moisture fluxes enhanced by the TC circulations (Figs.\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e13\u003c/span\u003ea, b). The strong winds (\u0026gt;\u0026thinsp;20 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) at 18\u0026deg;N extend up to the 700-hPa level with low-level northeasterly winds around 23\u0026deg;N (Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e14\u003c/span\u003ea). Nevertheless, the horizontal moisture fluxes within the southwesterly monsoon flow south of the TC circulations are still mainly at low levels (Figs.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e11\u003c/span\u003e and \u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e14\u003c/span\u003eb).\u003c/p\u003e\u003cp\u003eFor the 19\u0026ndash;20 June event, the broad southwesterly monsoon flow (Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e14\u003c/span\u003ec) is affected by the outer circulation of TC after it made landfall over China with strong winds (\u0026gt;\u0026thinsp;20 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) extending up to the 700-hPa level. The relatively weak winds at 22\u0026deg;N (Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e14\u003c/span\u003ec) are related to upstream flow deceleration due to orographic blocking by the terrain of Taiwan (Fig.\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e13\u003c/span\u003ec). A vertical cross section taken at 21\u0026deg;N with the broad southwesterly monsoon flow over the northern SCS (Fig.\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e15\u003c/span\u003ea) shows a jet core around 119\u0026deg;E at the 850-hPa level (Fig.\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e15\u003c/span\u003eb). The relatively weak winds around 123\u0026deg;E in low levels (Fig.\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e15\u003c/span\u003eb) are related to orographic effects of Philippines (Fig.\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e15\u003c/span\u003ea). Again, the horizontal moisture fluxes within the southwesterly monsoon flow are still mainly at low levels (Fig.\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e15\u003c/span\u003ec). A schematic diagram showing the presence of the strong southwesterly monsoon flow due to the enhanced pressure gradients between the dissipating tropical depression over southern China and the semi-permanent WPSH is given in Fig.\u0026nbsp;\u003cspan refid=\"Fig22\" class=\"InternalRef\"\u003e16\u003c/span\u003e.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Localized heavy rainfall over southwestern Taiwan\u0026rsquo;s coast on 20 June\u003c/h2\u003e\u003cp\u003eIn this section, we show the impact of rain evaporative cooling on the horizontal distributions of heavy rainfall on 20 June.\u003c/p\u003e\u003cp\u003eOn 19 June, the maximum rainfall axis (\u0026gt;\u0026thinsp;300 mm) (Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e9\u003c/span\u003eg) occurred on the windward slopes of the CMR due to orographic blocking and lifting. The cold pool (~\u0026thinsp;25\u0026deg;C) developed on 20 June over southwestern Taiwan, compared with warmer temperature over land on 18 June (Figs.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e17\u003c/span\u003ea, b), generated offshore flow during the daytime (Fig.\u0026nbsp;\u003cspan refid=\"Fig24\" class=\"InternalRef\"\u003e18\u003c/span\u003ea). The time series analysis of temperature observations at southwestern Taiwan during 1140 LST 19 June\u0026ndash;1140 LST 20 June reveals a significant drop (~\u0026thinsp;2\u0026deg;C) due to rain evaporative cooling. Therefore, because of the convergence of the offshore flow and the southwesterly flow (Fig.\u0026nbsp;\u003cspan refid=\"Fig24\" class=\"InternalRef\"\u003e18\u003c/span\u003e), the maximum rainfall (~\u0026thinsp;300 mm) shifted from the windward slopes of the CMR to the southwestern coast (Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e9\u003c/span\u003eh). This heavy rainfall event ended in the late afternoon of 20 June after the weakening of the southwesterly monsoon flow (Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e11\u003c/span\u003eb).\u003c/p\u003e\u003c/div\u003e"},{"header":"4. SUMMARY AND DISCUSSION","content":"\u003cp\u003eDuring the 13\u0026ndash;20 June 2018 South China Sea Two-Island Monsoon Experiment (SCSTIMX-2018), two tropical depressions formed at the western end of the NE-SW orientated Mei-Yu front south of Taiwan between the strong (\u0026gt;\u0026thinsp;15 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) cold northeasterlies coming from the Taiwan Strait and the warm, moist prefrontal southwesterly flow. These mesoscale disturbances developed in the open ocean in the subtropics under baroclinic settings along a Mei-Yu front.\u003c/p\u003e\u003cp\u003eDuring 14\u0026ndash;16 June, heavy rainfall (\u0026gt;\u0026thinsp;550 mm) occurred over southern Taiwan due to the passage of TD Gaemi in a warm moist environment. The tropical disturbance first formed around 1800 UTC 13 June on the western end of the Mei-Yu front over the northern South China Sea with mixed baroclinic Mei-Yu frontal signatures and asymmetric tropical cyclone structure. Gaemi became a named tropical storm before making landfall over southern Taiwan on 0000 UTC 15 June. With the deepening of Gaemi, the southwesterly flow on the southern/southeastern flank of the storm strengthened. During 14\u0026ndash;15 June, heavy rainfall (\u0026gt;\u0026thinsp;600 mm) occurred over southern Taiwan with a maximum on the windward side of the CMR. After passing southern Taiwan, TC Gaemi continued to move northeastward along the Mei-Yu front and developed into a Mei-Yu frontal cycle. The second TD formed over the western end of the Mei-Yu front, made landfall over the southern China coast around 0600 UTC 18 June, and brought a strong southwesterly monsoon flow over the Taiwan area. On 18 June, the southwesterly monsoon flow strengthened and extended northeastward, due to the enhanced pressure gradients between the TD and the semi-permanent WPSH. It is apparent that the southwesterly flow is also affected by the outer circulation of the TD with strong winds (\u0026gt;\u0026thinsp;20 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) extending up to the 700-hPa level.\u003c/p\u003e\u003cp\u003eDuring 19\u0026ndash;20 June, the horizontal moisture fluxes associated with the strong (\u0026gt;\u0026thinsp;20 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) southwesterly monsoon flow occurred primarily within the MBL with maximum TPW\u0026thinsp;\u0026gt;\u0026thinsp;65 mm. The warm, moist southwesterly flow impinged on southwestern Taiwan under favorable large-scale settings, including warm advection within the southwesterly monsoon flow and the passage of a 500-hPa trough to the north. On 19 June, the maximum rainfall axis (\u0026gt;\u0026thinsp;300 mm) occurred on the windward slopes of the CMR due to orographic lifting. On 20 June, with the development of cold pool (~\u0026thinsp;25\u0026deg;C), offshore flow was generated over southwestern Taiwan during the daytime due to rain evaporative cooling associated with the heavy rains on 19 June. As a result, the maximum rainfall (\u0026gt;\u0026thinsp;300 mm) occurred along the southwestern coast due to the convergence between the offshore flow and the incoming southwesterly flow. This heavy rainfall event over southern Taiwan ended in the late afternoon of 20 June after the weakening of the southwesterly monsoon flow.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eContributions:\u003c/h2\u003e\u003cp\u003eC.-K. Wang performed all the computations and prepared all the figures. Y.-L. Chen designed the work and wrote the manuscript. All authors reviewed the manuscript.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003ch2\u003eCompeting interests\u003c/h2\u003e\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFunding:\u003c/h2\u003e\u003cp\u003eThis work is jointly funded by the National Science Foundation under Grants AGS-1142558, and AGS-1854443 to the University of Hawai\u0026lsquo;i at Mānoa; the National Science and Technology Center (NSTC) under Grants 114-2811-M-008-016, 112-2111-M-008-023, 113-2111-M-008-027, and the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) to the National Central University, Taiwan.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eC.-K. Wang performed all the computations and prepared all the figures. Y.-L. Chen designed the work and wrote the manuscript. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e\u003cp\u003eThe CFSv2 data is available from \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.ncei.noaa.gov/access/metadata/landing-page/bin/iso?id=gov.noaa.ncdc:C0087\u003c/span\u003e\u003cspan address=\"https://www.ncei.noaa.gov/access/metadata/landing-page/bin/iso?id=gov.noaa.ncdc:C0087\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. The rainfall and surface data are available from the Central Weather Association, Taipei, Taiwan.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eCharney JG, Eliassen A (1964) On the growth of the hurricane depression. 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Mon Wea Rev 126:725\u0026ndash;735. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1175/1520-0493(1998)126\u0026lt;0725:APAFMF\u0026gt;2.0.CO;2\u003c/span\u003e\u003cspan address=\"10.1175/1520-0493(1998)126%3C0725:APAFMF%3E2.0.CO;2\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Mei-Yu frontal cyclones, Moisture transport, Heavy Rainfall","lastPublishedDoi":"10.21203/rs.3.rs-7241626/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7241626/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDuring the 13\u0026ndash;20 June 2018 South China Sea Two-Island Monsoon Experiment (SCSTIMX), two tropical disturbances formed along western end of a Mei-Yu front over the northern South China Sea (SCS) between the cold northeasterlies (\u0026gt;\u0026thinsp;15 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) coming from the Taiwan Strait and warm, moist southwesterlies. Both disturbances exhibit mixed baroclinic characteristics and an asymmetric tropical cyclone (TC) structure.\u003c/p\u003e\u003cp\u003eThe first disturbance formed around 1800 UTC 13 June and moved northeastward along the Mei-Yu front, deepened, and became tropical storm Gaemi before making landfall over southern Taiwan in the early morning on 15 June. With the deepening of Gaemi, the southwesterly flow on the southern/southeastern flank of the storm strengthened (\u0026gt;\u0026thinsp;20 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). During 14\u0026ndash;15 June, heavy orographic rainfall occurred over southern Taiwan with a maximum (\u0026gt;\u0026thinsp;550 mm) on the windward side of the Central Mountain Range (CMR).\u003c/p\u003e\u003cp\u003eThe second tropical disturbance over the northern SCS made landfall over southern China around 0600 UTC 18 June. During 19\u0026ndash;20 June, the southwesterly monsoon flow strengthened (\u0026gt;\u0026thinsp;20 m s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) due to relatively large pressure gradients between the remnants of the second tropical disturbance and the West Pacific Subtropical High (WPSH). During 19\u0026ndash;20 June, the horizontal moisture fluxes associated with the enhanced southwesterly monsoon flow mainly occurred below the 850-hPa level. On 19 June, the maximum rainfall axis (~\u0026thinsp;300 mm) occurred on the southwestern windward slopes due to orographic blocking and lifting of the warm, moist south/southwesterly flow under favorable large-scale settings. On 20 June, with the development of the cold pool (~\u0026thinsp;25\u0026deg;C) and offshore flow over southwestern Taiwan due to rain evaporative cooling, the maximum rainfall (~\u0026thinsp;300 mm) occurred along the southwestern coast.\u003c/p\u003e","manuscriptTitle":"Impacts of cyclogenesis and moisture transport by the enhanced southwesterly monsoon flow on heavy rainfall over southern Taiwan during SCSTIMX (2018)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-30 06:02:43","doi":"10.21203/rs.3.rs-7241626/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6199f8ed-2c01-4cef-9c01-cdcc39663ac8","owner":[],"postedDate":"July 30th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-09-29T09:12:31+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-30 06:02:43","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7241626","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7241626","identity":"rs-7241626","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}