Impact of the Monsoonal Surge on Extreme Rainfall of Landfalling Tropical Cyclones

ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 38, MAY 2021, 771–784

• Original Paper •

Dajun ZHAO1,2, Yubin YU*1, and Lianshou CHEN1

1State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081, China 2University of Chinese Academy of Sciences, Beijing 100049, China

(Received 18 August 2020; revised 8 December 2020; accepted 6 January 2021)

ABSTRACT A comparative analysis and quantitative diagnosis has been conducted of extreme rainfall associated with landfalling tropical cyclones (ERLTC) and non-extreme rainfall (NERLTC) using the dynamic composite analysis method. Reanalysis data and the tropical cyclone precipitation dataset derived from the objective synoptic analysis technique were used. Results show that the vertically integrated water vapor transport (Qvt) during the ERLTC is significantly higher than that during the NERLTC. The Qvt reaches a peak 1−2 days before the occurrence of the ERLTC and then decreases rapidly. There is a stronger convergence for both the Qvt and the horizontal wind field during the ERLTC. The Qvt convergence and the wind field convergence are mainly confined to the lower troposphere. The water vapor budget on the four boundaries of the tropical cyclone indicates that water vapor is input through all four boundaries before the occurrence of the ERLTC, whereas water vapor is output continuously from the northern boundary before the occurrence of the NERLTC. The water vapor inflow on both the western and southern boundaries of the ERLTC exceeds that during the NERLTC, mainly as a result of the different intensities of the southwest monsoonal surge in the surrounding environmental field. Within the background of the East Asian summer monsoon, the low-level jet accompanying the southwest monsoonal surge can increase the inflow of water vapor at both the western and southern boundaries during the ERLTC and therefore could enhance the convergence of the horizontal wind field and the water vapor flux, thereby resulting in the ERLTC. On the other hand, the southwest monsoonal surge decreases the zonal mean steering flow, which leads to a slower translation speed for the tropical cyclone associated with the ERLTC. Furthermore, a dynamic monsoon surge index (DMSI) defined here can be simply linked with the ERLTC and could be used as a new predictor for future operational forecasting of ERLTC. Key words: landfalling tropical cyclones, extreme rainfall, monsoon surge, dynamic composite analysis Citation: Zhao, D. J., Y. B. YU, and L. S. Chen, 2021: Impact of the Monsoonal Surge on Extreme Rainfall of Landfalling Tropical Cyclones. Adv. Atmos. Sci., 38(5), 771−784, https://doi.org/10.1007/s00376-021-0281-1. Article Highlights: • The low-level jet accompanying the southwest monsoonal surge increases the inflow of water vapor which could enhance the convergence of the landfalling tropical cyclones, thereby resulting in the extreme rainfall. • Southwest monsoonal surge decreases the zonal mean steering flow, which leads to a slower translation speed for the tropical cyclone associated with the ERLTC. • A dynamic monsoon surge index (DMSI) defined here can be simply linked with the ERLTC and could be used as a new predictor in operational forecasting of ERLTC.

record for China (1748.5 mm on 1 August 1996) was recor- 1. Introduction ded at Ali Mountain in Taiwan Province during Typhoon Tropical cyclones are major synoptic systems and often Herb (1996). The maximum 24-h rainfall record of 1062 produce destructive rainstorms. There are many records of mm in the Chinese mainland was recorded during the “75.8 extraordinary rainstorm in Henan province ” caused by extreme rainfall related to the activity of tropical cyclones Typhoon Nina (1975) (975) (Chen et al., 2012). Rainstorms

( Tao, 1980). For example, the maximum 24-h rainfall associated with tropical cyclones have a widespread impact in terms of both their duration and range (Lei, 2020) and * Corresponding author: Yubin YU can lead to both economic and social losses, making dis-

Email: [email protected] aster prevention and mitigation challenging (Chen et al.,

772 MONSOON SURGE IMPACT ON TYPHOON EXTREME RAINFALL VOLUME 38

2012). Previous studies have shown that the highest propor- monsoon is not constant in its intensity after onset and tions of rainfall induced by tropical cyclones occur in East shows marked low-frequency oscillations. A monsoon surge Asia (Khouakhi et al., 2017), and that heavy rainfall in this occurs when the wind speed increases dramatically, fol- region is more sensitive to changes in the water vapor con- lowed by significant changes in the weather (Dictionary of tent of the atmosphere than general rainfall (Trenberth, Atmospheric Science 1994). Monsoon surge is usually 1999). The Intergovernmental Panel on Climate Change defined as the band-pass filtered zonal wind at 850 hPa (Ju Fifth Assessment Report noted that the frequency and intens- et al., 2005, 2007) or regional mean of total wind at 850 hPa ity of extreme rainfall have been increasing as a result of over a specific area (Dong et al., 2010). In addition, the mon- global warming and increasing amounts of atmospheric soon surge is always identified with southeast oriented water vapor. We therefore need to carry out systematic stud- cloud clusters in infrared satellite imagery in real time opera- ies on landfalling tropical cyclone extreme rainfall tional forecasting. When the southwest monsoonal is strong, (ERLTC). a low-level jet tends to form and approach typhoons from Extreme rainfall events have caused great losses in their south side to transport sufficient water vapor, which is recent years as a result of their high frequency and wide- conducive to the formation and maintenance of heavy rain- spread impact, but are still a great challenge in operational fall (Tao, 1980). Monsoon surges are crucial in rainstorms forecasting. Extreme rainfall is generally defined by either that cause flooding and provide the water vapor required for the relative (95th or 99th percentiles) or absolute (daily or the rainstorm (Tao and Wei, 2007). accumulated rainfall during the precipitation process) The southeastern coast of China is the most active area threshold methods. The relative method focuses on the clima- in the East Asian summer monsoon region, and landfalling tology of extreme rainfall to determine a pattern (Chris et typhoons often occur where monsoon surges and typhoons al., 2002; Knight and Davis, 2009). The absolute method is have more opportunity to interact with each other (Chen and commonly used in synoptic studies. For example, a Xu, 2017). Dong et al. (2010) reported that a southwesterly typhoon-induced extraordinary rainstorm in China with 24-h monsoonal surge can intensify the transport of water vapor rainfall ≥1000 mm was defined as an extreme rainfall by to westward-moving typhoons, and that the enhanced south- Chen and Xu (2017), although there have been seven westerly flow increases the convergence near the typhoon, extreme rainfall associated with typhoons that have met this which favors the development of ascending motion and intens- criterion since 1960. Six of these extreme rainfall occurred ifies the rainfall. Based on these studies, it is easy to asso- in Taiwan, but only one in the Chinese mainland. A max- ciate monsoon surges with LTCER and the surges may influ- imum daily rainfall ≥50 mm is also used to define an ence the occurrence of such rainfall. Wang et al. (2010) also extreme rainfall during a typhoon (hoon (Jiang et al., 2018; showed that the monsoon surge can increase the torrential Qiu et al., 2019). Using this definition, the operational fore- rains induced by the landfalling typhoon. casting terms of typhoon-induced torrential rain, heavy torren- Extreme rainfall, including ERLTC, has increased in fre- tial rain and extraordinary storm all refer to the extreme rain- quency within the current backdrop of global warming, fall associated with typhoons. hence we need to improve our understanding of the causat- As a crucial component of the Earth’s atmospheric circu- ive mechanisms for these events. The main objective of this lation, monsoons are essential for the occurrence of rainfall. study is to conduct a dynamic composite analysis of the occur- China is located in the world’s largest monsoonal climate rence of ERLTC and to focus on the impact of the mon- zone, the Asian−Australian monsoon region, and therefore soonal surge on it. Landfalling tropical cyclones with non- the Asian monsoon has a great impact on rainstorms in extreme rainfall (NERLTC) are also analyzed. China (Zhao et al., 2019). Statistical and numerical studies The rest of the paper is organized as follows. Section 2 have shown that low-level jets (LLJs) mainly consist of describes the data and methods. Section 3 details the water boundary layer jets (BLJs) and synoptic system-related vapor flux composite and comparative analysis in terms of LLJs (SLLJs) over southern China, which are the key the vertically integrated water vapor transport (Qvt), the Qvt factors in regulating heavy rainfall (Du and Chen, 2019a), convergence, lateral boundary budget and the vertical distribu- convection initiation (Du and Chen, 2019b) and the sub- tion. Section 4 discusses the impacts of the low-level jet and sequent upscale convective growth (Du et al., 2020). Chen steering flow associated with the monsoon surge. Our discus- et al. (2010) concluded that the heavy rainfall from landfall- sion and conclusions are presented in section 5. ing typhoons depends upon the transport of water vapor, the extratropical transition process, land surface processes, topo- 2. Data and methods graphy, and mesoscale convective systems. The relationship between the rainfall associated with landfalling trop- The 6-h reanalysis data from the National Centers for ical cyclones and summer monsoon jets has also been invest- Environmental Prediction−National Center for Atmo- igated. Results show that the tropical cyclones causing wide- spheric Research dataset with a spatial resolution of 2.5°× spread heavy rainfall are often consistently associated with 2.5° and the tropical cyclone best-track dataset from the a low-level jet after landfall and the water vapor flux and lat- China Meteorological Administration (Ying et al., 2014) are ent heat are significantly higher than tropical cyclones that used. In addition, the tropical cyclone precipitation dataset

only cause weak rainfall (Cheng et al., 2012). The summer identified by objective synoptic analysis (Ren et al., 2001,

MAY 2021 ZHAO ET AL. 773

2007) is also employed, which partitions the rainfall lones” refers to the landfall of either the center of the trop- induced by tropical cyclones from the total rainfall based on ical cyclone or the tropical cyclone rain belt. Thus, it cov- station observations from the Chinese mainland, Macau, ers both the landfalling tropical cyclones and side-swiping Hong Kong and Taiwan Island. The dataset also includes rain- tropical cyclones in the operational forecasting definition. fall record over land induced by side-swiping tropical cyc- The average daily rainfall during ERLTC and NERLTC is lones which did not make landfall (fall (Feng et al., 2020), 1223.0 and 165.7 mm, respectively (Table 1). We are more in addition to the rainfall from tropical cyclones that do concerned with the differences between extreme rainfall and make landfall. These data have been widely applied in stud- ordinary torrential rain accompanied with landfalling ies of typhoon-induced rainfall over China (Ren et al., 2006; typhoons, not the so-called “dry typhoon” which can only gen- Jiang et al., 2018; Qiu et al., 2019; Liu and Wang, 2020). erate precipitation over 24-h≤50 mm. Our study has taken more factors into consideration in Figure 1 shows the tracks and intensity categories of defining typhoon extreme rainfall, including the number of the tropical cyclones during the ERLTC (Fig. 1a) and NER- samples and the representativeness of the tropical cyclones LTC (Fig. 1b). These tropical cyclones occurred during the triggering the extreme rainfall, the differences among peak season of tropical cyclone activity (July−September) typhoon-induced torrential rain, heavy torrential rain and and showed a mainly northwestward movement. All these extraordinary storms, and extreme rainfall caused by tropical cyclones made landfall or affected Taiwan and the typhoons on Taiwan, Hainan Island and the Chinese main- Chinese mainland, reaching the category of a typhoon or land. We defined typhoon-induced extraordinary rain- above before landfall. To avoid the influence of terrain and storms in China as a rainfall event in which the 24-h rain- to make the tropical cyclone samples more comparable, we fall was ≥600 mm at a single rain gauge station. Statistic- selected the tropical cyclones associated with ERLTC and ally, there are 38 records of single-station typhoon extreme NERLTC in Ali Mountain, Taiwan. This paper focuses on rainfall from 1960 to 2019, caused by a total of 26 the main differences between the tropical cyclones that bring extreme rainfall and those that bring non-extreme rain- typhoons. Among these 26 typhoons, 14 resulted in extreme fall under similar backgrounds of atmospheric circulation rainfall in Taiwan, 5 resulted in extreme rainfall in Hainan

and the same terrain. and 7 resulted in extreme rainfall in the Chinese mainland. Seven ERLTC occurred at the same rain gauge station at Ali Mountain, Taiwan Island, which is more than 25% of all the 3. Comparative analysis of water vapor flux typhoons according to our definition. These typhoons are the main object of study in this paper. We carried out compar- 3.1. Vertically integrated water vapor transport ative analysis and dynamic diagnosis of the large-scale circu- The vertically integrated water vapor transport (Qvt) of lations of selected tropical cyclones using dynamic compos- an air column is defined as (Ding, 1989; Chen and Huang, ite analysis (e analysis (Li et al., 2004). The ERLTC and 2007 ): NERLTC were identified based on the similarities of the sea- ∫ ∫ son of occurrence (major season, July−September), track 1 Ps 1 Ps Q = qVdp = q(u,v)dp , (1) (northwestward), and location (Ali Mountain in Taiwan vt g g Island) of the tropical cyclones. On this basis, typhoons 300 300 9608, 0908, 6312, 1307 and 0813 were selected as associ- where q (kg kg−1) and V (m s−1) are the specific humidity ated with ERLTC, whereas typhoons 9417, 1315, 0713, and wind vector for each layer of the air column, respect- 0505 and 0605 were selected as associated with NERLTC. ively, u and v are the zonal and meridional wind compon- This research focuses on the precipitation induced by trop- ents, respectively, Ps (Pa) is the surface air pressure, g is the −1 −1 ical cyclones over land. The term “landfalling tropical cyc- gravitational acceleration and the unit of Qvt is kg m s .

Table 1. Overview of selected ERLTCs and NERLTCs.

Extremes of Time at which extreme Central Maximum Tropical cyclone daily rainfall rainfall occurred Center of minimum wind speed number (mm) (LST, LST=UTC+8) tropical cyclone pressure (hPa) (m s−1) ERLTC 9608 1748.5 7-31-02:00−8-1-02:00 (121.0°E, 24.9°N) 960 40 0908 1623.5 8-8-20:00−8-9-20:00 (119.8°E, 26.7°N) 975 30 6312 1248.0 9-10-20:00−9-11-20:00 (121.6°E, 25.6°N) 945 55 1307 757.0 7-12-20:00−7-13-20:00 (118.8°E, 26.1°N) 985 30 0813 738.0 9-13-20:00−9-14-20:00 (121.3°E, 25.4°N) 970 33 NERLTC 9417 52.7 8-31-20:00−9-1-20:00 (119.7°E, 25.3°N) 970 35 1315 96.5 8-28-20:00−8-29-20:00 (122.3°E, 26.6°N) 990 23 0713 255.0 9-17-20:00−9-18-20:00 (121.4°E, 26.2°N) 940 50 0505 350.0 7-17-20:00−7-18-20:00 (121.2°E, 24.7°N) 955 40 0605 74.5 7-24-20:00−7-25-20:00 (117.7°E, 24.4°N) 985 28

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Fig. 1. Track and intensity categories of the selected (a) ERLTC and (b) NERLTC during periods of extreme rainfall. TD, TS, STS, TY, STY and SupTY are representing the tropical depression, tropical storm, severe tropical storm, typhoon, strong typhoon and super typhoon, respectively.

We aimed to explore the evolutionary characteristics in the water vapor fluxes within the typhoon circulation and compare the differences of water vapor between before and after the occurrence of extreme rainfall. This is ERLTC and NERLTC. Figure 2 shows the evolution of the an indication of the occurrence of extreme rainfall. A compar- area-averaged Qvt during ERLTC and NERLTC, where rect- ison between ERLTC (Fig. 2a) and NERLTC (Fig. 2b) angles of different colors indicate the overall evolution of shows that the Qvt in the tropical cyclone circulation and the the water vapor flux of the typhoon and the surrounding envir- surrounding environmental field is significantly higher for onmental field as a function of the side lengths relative to the ERLTC than for the NERLTC. the eye of the typhoon center. Differences in the water vapor fields can result in very The different variation trends for the Qvt are consistent different intensities and distributions of rainfall, which with each other. The Qvt increases continuously, reaching a could, in turn, determine the spatiotemporal distribution and maximum 1.5 days before the occurrence of extreme rain- intensity of typhoon rainstorms (Ye and Li, 2011). To fur- fall and then decreases rapidly. This evolutionary trend is ther investigate the main water vapor transport channels in most pronounced within the rectangles with side lengths of the tropical cyclones that trigger extreme rainfall, Fig. 3 5° and 10°, indicating that there are significant differences shows the Qvt between 1000 and 300 hPa during ERLTC

−1 −1 Fig. 2. Variations in the regional average Qvt during (a) ERLTC and (b) NERLTC (units: kg m s ). The horizontal axis shows the time in days relative to the occurrence of rainfall, −1 (−2) means the day (two days) before the extreme rainfall

occurrence, and so on. Color curves indicate distances from TC centers in degrees latitude and longitude.

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−1 −1 Fig. 3. The Qvt (units: kg m s ) for (a, d, g, j) ERLTC and (b, e, h, k) NERLTC and their differences (c, f, i, l) (units: −1 −1 kg m s ). (a−c) and (d−f) show the Qvt two days and one day before rainfall, respectively, (g−i) shows the Qvt on the day when the rainfall occurred and (j−l) show the Qvt one day after rainfall. The center of the tropical cyclone is set as the origin of the coordinates, which are positive to the north and east direction and negative to the south and west. Horizontal and

vertical axes indicate the distance in latitude and longitude degree from the composite TC center.

776 MONSOON SURGE IMPACT ON TYPHOON EXTREME RAINFALL VOLUME 38 and NERLTC and their differences. There are two major chan- vapor was located within 10° of the center of the tropical cyc- nels for the transport of water vapor from the southwest and lone related to ERLTC, where there are large areas of water southeast regions for both types of tropical cyclone, but the vapor convergence on the southwestern and southeastern southwestern water vapor channel of a tropical cyclone associ- sides, associated with the southwestern and southeastern ated with the ERLTC is much broader and continuously water vapor fluxes, respectively (Fig. 4a). Although there is provides water vapor for the development of the cyclone. also a strong area of convergence of water vapor near the cen- However, the southeastern oriented water vapor channel inter- ter of the tropical cyclone related to NERLTC (Fig. 4b), the acts with the two types of tropical cyclones in different quad- corresponding intensity and range are significantly smaller rants—in the first quadrant (northeast relative to the than those in ERLTC. There is also an area of strong water typhoon center) for ERLTC and in the fourth quadrant (south- vapor divergence within 10° northeast of the center of trop- east relative to the typhoon center) for NERLTC. In other ical cyclone related to the NERLTC. These characteristics words, the environmental southeastern oriented water vapor are unchanged during the two days before the occurrence of channel combines with the typhoon in its northeast (south- extreme rainfall (figure not shown). The distribution of the east) direction in ERLTC (NERLTC). Tropical cyclones asso- water vapor flux divergence for both types of tropical cyc- ciated with the ERLTC have a continuous and stable inflow lone is mainly determined by the wind field convergence of water vapor from the southeastern sector. The differ- term (Figs. 4c−f), indicating that the contribution of wind con- ences in water vapor transport (Figs. 3c, 3f, 3i and 3l) show vergence to water vapor flux convergence is significantly that the southwestern channels for the transport of water greater than that of water vapor advection. Specifically, vapor is powerful and long-lasting in the environmental wind convergence is mainly within the circulation of the field for tropical cyclones associated with the ERLTC. A con- two types of tropical cyclones as well as the environmental tinuous inflow of water vapor from the southwest for trop- fields on their southeastern and southwestern sides (Figs. 4c− ical cyclones associated with the ERLTC occurs from two d). The wind convergence is much wider and stronger days before the extreme rainfall to one day after its occur- within the circulation of tropical cyclones associated with rence, intensifying the water vapor flux on the southwest- ERLTC and within the environmental field on its southwestern side of the tropical cyclones. Thus, a more intense southern side compared with tropical cyclones associated with westerly transport of water vapor in the environment is a NERLTC. For the water vapor advection term, wind conver- key feature differentiating tropical cyclones associated with gence (Figs. 4e, 4f) contributes less (relatively more) to the the ERLTC from those associated with the NERLTC. total water vapor flux convergence in ERLTC (NERLTC). However, such difference does not have a significant effect 3.2. Qvt divergence and decomposition terms on the total water vapor flux convergence. Therefore, trop- The Qvt can reflect the source of the water vapor for rain- ical cyclones associated with ERLTC are associated with fall and the relationship between the transport of water strong wind convergence to their southwestern and southeast- vapor and the weather systems in the environment. The loca- ern sides—that is, with the presence of stronger moisture tion and intensity of the rainfall are more closely related to advection in the environment—resulting in a stronger water the divergence of the Qvt. vapor flux convergence in the circulation of the tropical cyc- The Qvt divergence C of the air column is decomposed lones. However, the wind convergence in the environment as follows (Chen and Huang, 2007): to the southwest and southeast of tropical cyclones associ- ∫ ∫ ated with NERLTC is relatively weak and the northern side 1 Ps 1 Ps C = − ∇ · Q ≈ − ∇ · (qV)dp = − (q∇ · V)dp− of the tropical cyclones is dominated by dry advection, mak- vt g g ∫ 300 300 ing a negative contribution to the water vapor flux conver- 1 Ps q(V · ∇q)dp , (2) gence in NERLTC. g 300 3.3. Qvt budget on the lateral boundaries of tropical where C is the source (sink) for water vapor transport. C > cyclones 0 (C < 0) indicates the convergence (divergence) of water We used the regional average water vapor budget pro- vapor, meaning a water vapor transport sink (source). C con- posed by Ding (1989): sists of two components: the convergence of the wind [the ∫ ∫ ( ) first term on the right-hand side of Eq. (2)] and the advec- ρ hu 0 ∂ ∂ q + ∇ · + qw σ = − + , tion term for water vapor [the second term on the right-hand q V dzd m Es (3) σ hs σ ∂t ∂z side of Eq. (2)]. A positive (negative) value of the wind field convergence or the wet (dry) advection of water vapor where σ denotes the area of the selected region, hu and hs facilitate (suppresses) the convergence of water vapor. are the top and bottom heights of the integration (300 and ∂ /∂ Figure 4 shows the spatial distributions of the Qvt diver- 1000 hPa, respectively), q t is the local variation term for gence and the two components (convergence and advection water vapor. ∇q · V is the water vapor flux divergence term. of water vapor) during the ERLTC and the NERLTC on the ∂qw/∂z is the vertical water vapor transport term. m is the date when extreme rainfall occurred. On the day on which water vapor condensation term and Es is the evaporation ∇ · the extreme rainfall occurred, strong convergence of water term. q V is calculated using the linear integral equation

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Fig. 4. Distributions of (a, b) Qvt (vector) and Qvt divergence (shaded), (c, d) convergence and (e, f) water vapor advection during ERLTC and the NERLTC for the day on which extreme rainfall occurs (units: 10−5 kg m−2 hPa−1 s−1). ERLTC are shown on the left panels, NERLTC on the right panels.

778 MONSOON SURGE IMPACT ON TYPHOON EXTREME RAINFALL VOLUME 38 ∫ H hu (ρ/σ) vnqdldp, where vn is the normal component per- rainfall, respectively, and then decreases rapidly once the hs extreme rainfall begins. However, there is no significant dif- pendicular to the boundaryH and is positive outward. The spe- ference between the inflow of water vapor on the northern cific calculation for vnqdl is: and eastern boundaries before and after the occurrence of I ∑k ∑k extreme rainfall. Similarly, there are also water vapor vnqdl = −vi qi∆ls + vi qi∆ln+ inflows on the southern, western and eastern boundaries of i=1 i=1 the rectangular region for tropical cyclones associated with ∑n ∑n NERLTC. However, there is a continuous outflow of water − ∆ + ∆ . u j q j lw u j q j le (4) vapor on the northern boundary before the occurrence of rain- j=1 j=1 fall (Fig. 5b), which is consistent with previous findings (Ding and Liu, 1986). The inflows of water vapor on both The four terms on the right-hand side of Eq. (4) repres- the western and southern boundaries during ERLTC are lar- ent the amount of water vapor entering the selected region ger than those during NERLTC, but there is no obvious differ- from the southern, northern, western and eastern boundar- ence in the water vapor budget between the two types of trop- ies, respectively, where k and n are the number of grids ical cyclones on the eastern boundary. Therefore the inflow along the meridional and zonal directions in the selected of water vapor on the northern boundary is an important fea- region, and ∆ls, ∆ln, ∆lw and ∆le are the grid spacings along ture that distinguishes ERLTC from NERLTC. the four boundaries. Our study area is a square with sides of 10° longitude/latitude centered on the eye of the typhoon. 3.4. Vertical distribution of water vapor flux The variations in the water vapor flux at each lateral bound- Figure 6 shows the vertical profiles of the water vapor ary and the total water vapor flux during the ERLTC and fluxes averaged over the rectangular area with side lengths the NERLTC were calculated to determine the characterist- of 10° from the center of the typhoon for the two types of trop- ics of the water vapor budget in the typhoon area during the ical cyclones. These profiles were used to determine the distri- ERLTC. bution of water vapor fluxes in different layers. We investigated the evolution of the Qvt in ERLTC and The fluxes of water vapor in both types of tropical cyc- NERLTC along the four boundaries (east, west, south and lones decrease with height as a result of their structural charac- north) of the rectangular region with side lengths of 10° teristics—i.e., areas with high wind speeds and large from the center of the typhoon to quantitatively determine amounts of water vapor are located in the lower and middle the transport of water vapor between the environment and troposphere. However, the water vapor flux is significantly the tropical cyclones (Fig. 5). The water vapor flows enter higher in the tropical cyclones associated with ERLTC than through all four boundaries during ERLTC (Fig. 5a), with those associated with NERLTC and can be almost twice as the largest (smallest) inflow on the southern (northern) bound- large in the same layer. The regionally averaged water ary. The inflow of water vapor increases on the southern vapor flux below 850 hPa in ERLTC is at a maximum from and western boundaries before the occurrence of ERLTC, two days before the occurrence of extreme rainfall until the reaching a peak 1−2 days before the occurrence of extreme day on which the extreme rainfall occurs. By contrast, the

Fig. 5. Evolution of the Qvt budget for ERLTC and NERLTC along the four boundaries of the rectangular region −1 −1 with side lengths of 10° from the center of the typhoon (units: kg m s ).

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Fig. 6. Vertical profiles of the average water vapor flux in the rectangular area with side lengths of 10° from the center of the two types of tropical cyclone for (a) ERLTC and (b) NERLTC (units: kg m−1 s−1). regionally averaged water vapor flux in NERLTC reaches a rainstorms. However, the tropical cyclone associated with maximum three days before the occurrence of rainfall and the NERLTC does not interact with the southwestern low- then decreases from two days before the occurrence of rain- level jet. The area near the tropical cyclone with high wind fall. Combined with the results for the Qvt (Fig. 3), this speeds is always to the east, mainly as a result of the strong shows that the lower troposphere makes the highest contribu- pressure gradient between the subtropical high and the trop- tion to the Qvt, and therefore the water vapor flux in the ical cyclone. lower troposphere is significantly different between the 4.2. Monsoon surge slows the movement of tropical

ERLTC and NERLTC. cyclones Many previous studies have documented that a slower 4. Impact of the monsoonal surge translation speed of a tropical cyclone favors local extreme rainfall by extending the impact period of the typhoon, espe- 4.1. Monsoon surge and low-level jet cially on Taiwan Island (Chien and Kuo, 2011; Su et al., 2012; Wu, 2013; Chen and Xu, 2017). Figure 8 shows the Our analyses show the strong convergence of Q dur- vt translation speed of our selected examples of ERLTC (NER- ing ERLTC, which is mainly caused by the strong conver- LTC) and the average speed calculated using the latitudinal gence of wind. This section further explains the configura- and longitudinal position at 6-h intervals in the best-track data- tion of circulation that allows for such a strong conver- set. gence of the water vapor flux during ERLTC. Figure 7 Figure 8 clearly shows that the average translation shows the configuration of the wind and humidity fields at speed of the ERLTC (NERLTC) is slower (faster) during 850 hPa and the west Pacific subtropical high at 500 hPa for the period of extreme rainfall. Specifically, the average trans- the same time period. lation speed of the ERLTC (NERLTC) is 15.5 (20.5) km h−1. There is no significant difference between the humid- The average translation speed of the ERLTC is only 75% of ity fields of the two types of tropical cyclones. The tropical the average for the NERLTC. Tu and Chou (2013) sugges- cyclone itself and its western side are both areas of high ted that intense and long-lasting typhoon rainfall is mainly a humidity and these areas are slightly larger in regions with result of this slower translation speed, which may be associ- ERLTC. However, the configurations of the wind field are ated with weakening of the steering flow. Figure 9 shows markedly different for the two types of tropical cyclone. Dur- the zonal component of the steering flow of ERLTC and NER- ing ERLTC, the tropical cyclones are linked to the southwest- LTC at each level between 1000 and 200 hPa and the ern low-level jet and interact with each other for a long time whole-layer zonal mean component of the steering flow. (Figs. 7a and 7c), causing the southern monsoonal trough to Before the occurrence of extreme rainfall, the direction of advance eastward and the subtropical high to retreat east- the zonal steering flow in ERLTC and NERLTC were sim- ward and then move northward. This is consistent with the ilar at the middle and higher levels, but were different in mag- conclusions in Cheng et al. (2012) that the water vapor and nitude. The tropical cyclones associated with ERLTC had a instability energy associated with low-level jet have an import- much stronger westward zonal steering flow while the NER-

ant influence on landfalling typhoon induced extraordinary LTC had an eastward zonal steering flow in the lower tropo-

780 MONSOON SURGE IMPACT ON TYPHOON EXTREME RAINFALL VOLUME 38

Fig. 7. Specific humidity (shading; units: 10−2 kg kg−1) at 850 hPa, wind speed ≥12 m s−1 (black contours), zonal wind u=0 (red solid line; units: m s−1) and the 500-hPa geopotential height (blue solid line; units: 10 gpm) for the (a, c) ERLTC and (b, d) NERLTC (a, b) one day before and (c, d) on the day on which the extreme rainfall occurred. sphere. This directly affected the whole-layer zonal mean east to west after formation, it is often difficult to distin- steering flow and therefore the ERLTC had a relatively guish the circulation of the typhoon itself from the mon- weak eastward zonal mean steering flow, which contrib- soon surge circulation during and after the landfalling. We uted to the slower translation speed relative to the NER- therefore define a dynamic monsoon surge index (DMSI)

LTC.

as: 4.3. Dynamic monsoon surge index ∑N 1 [ ] Within the background of the East Asian summer mon- DMSI = U x − ,y − , (5) N 850 40W 10W 20N 0 soon, the activities of monsoonal surges are characterized n=1 by a clear increase in wind speed, possibly associated with the activity of low-level jets. Monsoon surges have previ- where U850 denotes the zonal wind at 850 hPa, x40W−10W indic- ously been defined as the regional average of the zonal ates the longitude range from 40° to 10° west of the center wind at 850 hPa in a given region (Dong et al., 2010; Hai et of the tropical cyclone and similarly y20N−0 indicates that

al., 2017). As a landfalling typhoon generally moves from the latitude varies within 20° south of the center of the trop-

MAY 2021 ZHAO ET AL. 781

Fig. 8. Translation speed (units: km h−1) of the five selected ERLTC (red dashed lines) and their average (red solid line) and the five NERLTC (blue dashed lines) and their average (blue solid lines) calculated from their latitudinal and longitudinal position records at 6-h intervals in the best-track dataset.

Fig. 9. Time series of the zonal component of steering flow (vectors) and its magnitude (shading; units: m s−1) at each level between 1000 and 200 hPa of (a) ERLTC and (b) NERLTC. The vectors in the lower box of each panel show the whole- layer zonal mean component of steering flow. ical cyclone (as shown in the rectangle in Fig. 10a). The ensures that, in dynamic coordinates, the tropical cyclone is DMSI is the average of N grid values in the region, which always at the center of the study area, whereas the mon- means that the location of the center of the tropical cyclone soon surge is always to the southwest of the tropical cyc-

at each time is taken as the dynamic regional center. This lone.

782 MONSOON SURGE IMPACT ON TYPHOON EXTREME RAINFALL VOLUME 38

Fig. 10. (a) Domain for the definition of the DMSI overlain by the 850 hPa wind field (units: m s−1) of the ERLTC one day before the occurrence of extreme rainfall. (b) Evolution of the DMSI for the two types of tropical cyclone (units: m s−1).

Figure 10b shows the evolution of the DMSI during the tributes the most to the environmental fields in both types of ERLTC and NERLTC. The DMSI clearly distinguishes tropical cyclones, whereas the water vapor advection term ERLTC from NERLTC, with the DMSI of ERLTC being sig- has a relatively minor role. The strong wind convergence in nificantly greater than that of NERLTC before and after rain- the ERLTC is mainly caused by the low-level jet overlain fall. The DMSI increases continuously three days before the by the circulation of the tropical cyclone, whereas there is extreme rainfall in ERLTC, peaks on the day on which the no low-level jet in the environment of cyclones associated rainfall occurs, and then decreases rapidly. The DMSI also with NERLTC. increases before the occurrence of rainfall in NERLTC, but (3) The water vapor flux budget on the boundaries indic- with a relatively small magnitude. The DMSI proposed here ates that there is inflow of water vapor from the four boundar- indicates significance for ERLTC and could be taken as a pre- ies 10° from the center of the tropical cyclone during the

dictor of ERLTC in operational forecasting. ERLTC, whereas there is a continuous outflow of water vapor on the northern boundary during the NERLTC before 5. Discussion and conclusions the rainfall occurs. Quantitatively, the inflow of water vapor is larger on both western and southern boundaries during We carried out composite and comparative analyses of the ERLTC than during the NERLTC, but there is little differ- two types of tropical cyclone (10 cases in total) based on an ence on the eastern boundary. objective definition of ERLTC from the perspective of mon- (4) The southwest monsoonal surge can decrease the soon surges. zonal mean steering flow, which leads to a slower transla- (1) The Qvt peaks 1−2 days before the occurrence of the tion speed and extends the period of influence of the ERLTC and decreases dramatically after the occurrence of ERLTC. The circulation of the landfalling typhoon and the rainfall. The Qvt in the circulation of the tropical cyclone monsoon circulation can be clearly distinguished by our and its surrounding environmental field during the ERLTC newly defined DMSI. The DMSI is significantly higher dur- is significantly larger than that during the NERLTC. In ing the ERLTC than during the NERLTC and increases dra- terms of the regionally averaged water vapor fluxes, both matically before the occurrence of ERLTC. Given its clear the circulation of the ERLTC and its environment are physical implications, the DMSI could be used as a pre- moister than that of the NERLTC. dictor for ERLTC and could be used in operational forecast- (2) There is a wider and more persistent water vapor ing. channel to the southwest of the tropical cyclone during the Our conclusions suggest that monsoon surge activity ERLTC than during the NERLTC. The low-level jet to the has a significant impact on ERLTC. Previous reports of southwest of the tropical cyclone during the ERLTC is con- ERLTC have mainly focused on a single case (Ge et al., nected to the tropical cyclone for a long time and continu- 2010; Chien and Kuo, 2011; Ding, 2015), whereas our find- ously enhances the cyclonic circulation of the tropical cyc- ings are based on several and are an extension of previous lone. The Qvt convergence is stronger during the ERLTC studies concluding that low-level jets have an important influ-

than during the NERLTC. The wind convergence term con- ence on the formation of typhoon-induced extraordinary

MAY 2021 ZHAO ET AL. 783

storms (Cheng et al., 2012). From the perspective of mon- lish abstract) soon surges, understanding the formation and development Ding, Y. H., and Y. Z. Liu, 1986: Research on water vapor of low-level jets and their interaction with tropical cyclones budget in Typhoon 7507. Acta Oceanologica Sinica, 8, can help to improve our understanding of the mechanisms 291−301. (in Chinese) of ERLTC. Nevertheless, this study is limited to synoptic- Dong, M. Y., L. S. Chen, Y. Li, and C. G. Lu, 2010: Rainfall rein- forcement associated with landfalling tropical cyclones. J. scale analyses and more detailed processes; in particular, Atmos. Sci., 67, 3541−3558, https://doi.org/10.1175/2010JAS mesoscale, convective, and microphysical processes during 3268.1. the ERLTC have not been investigated as a result of the Du, Y., and G. X. Chen, 2019a: Climatology of low-level jets and coarse resolution of the data. We are planning numerical their impact on rainfall over southern China during the experiments to explore this further in future studies. early-summer rainy season. J. Climate, 32, 8813−8833,

https://doi.org/10.1175/JCLI-D-19-0306.1. Acknowledgements. This work was supported by the Du, Y., and G. X. Chen, 2019b: Heavy rainfall associated with National Science Foundation of China (Grant Nos. 41775048, double low-level jets over southern China. Part II: Convec- 42030611), National Basic Research Program of China (Grant No. tion initiation. Mon. Wea. Rev., 147, 543−565, https://doi.org/

2015CB452804), the Open Grants of the State Key Laboratory of 10.1175/MWR-D-18-0102.1. Severe Weather (Grant No. 2020LASW-B06). The authors thank Du, Y., G. X. Chen, B. Han, C. Y. Mai, L. Q. Bai, and M. H. Li, Dr. Fumin REN for providing the TC-induced OSAT precipitation 2020: Convection Initiation and growth at the coast of South China. Part I: Effect of the marine boundary layer jet. Mon. dataset over China in this study. The best-track data is from Wea. Rev., 148, 3847−3869, https://doi.org/10.1175/MWR- http://tcdata.typhoon.org.cn and the NCEP/NCAR reanalysis data

D-20-0089.1. is from https://rda.ucar.edu/datasets/. The authors are grateful to Feng, T., F. M. Ren, D. L. Zhang, G. P. Li, W. Y. Qiu, and H. the editor and the three anonymous reviewers for providing insight- Yang, 2020: Sideswiping tropical cyclones and their associ- ful comments that significantly improved the quality of this paper. ated precipitation over China. Adv. Atmos. Sci., 37,

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