Hindawi Advances in Meteorology Volume 2019, Article ID 1853797, 15 pages https://doi.org/10.1155/2019/1853797

Research Article Analysis of the Gale in the Bohai Sea Caused by “Yagi”

Tiantian Hu,1 Binxian Liu ,1 Di Wu,2 and Xiaoyuan Yi3

1Tianjin Central Observatory for Oceanic Meteorology, Tianjin Meteorological Bureau, Tianjin 300074, 2Department of Aviation Meteorology, College of Air Traffic Management, Civil Aviation University of China, Tianjin 300300, China 3Tianjin Meteorological Observatory, Tianjin Meteorological Bureau, Tianjin 300074, China

Correspondence should be addressed to Binxian Liu; [email protected]

Received 13 April 2019; Revised 17 July 2019; Accepted 21 July 2019; Published 28 August 2019

Academic Editor: Alastair Williams

Copyright © 2019 Tiantian Hu et al. ,is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In this study, we analyzed the meteorological processes associated with 2018 tropical cyclone No. 14, “Yagi.” TC Yagi continued moving northeastward after losing its numerical designation from the National Meteorological Center of the China Meteo- rological Administration (CMA) because of weakening and then restrengthened when it moved over the Bohai Sea, inducing an ocean gale on 14-15 August 2018. ,e results of our investigation revealed that the continued northeastward movement of Yagi on 14 August was related to the divergence of the upper-level westerly jet stream, the northward shift of the subtropical high in the midtroposphere, as well as the steering flow and asymmetrical air flow around the disturbance itself in the lower troposphere. ,e enhancement of Yagi over the Bohai Sea on the night of 14 August was related to the decrease of friction over the ocean and the increase of diabatic heating from the sea surface flux. ,e wind speed increased to a maximum when the depression moved over the Bohai Sea, an occurrence that was not only due to the enhancement of the cyclone itself but also due to the flow of cold air from high latitudes along the north side of the Bohai Sea. ,e behavior of the cold air was related to the shift of the convergence zone in the upper-level westerly jet at 200 hPa, long-wave troughs and ridges at 500 hPa, and terrain effects. ,us, the gale development in the Bohai Sea was due to both the enhancement of tropical cyclone Yagi after it moved over the ocean and the flow of cold air from high latitudes.

1. Introduction of China [4, 5]. TC intensity is classified into six levels by the China Meteorological Administration (CMA), as listed in Tropical cyclones (TCs), including , tropical Table 1. storms, and tropical depressions, are strong, warm-core Research has revealed that there are many factors which atmospheric vortices that develop over the tropical ocean affect the development and movement of TCs, including sea [1]. TCs are always accompanied by strong gales, heavy surface temperature [6–8], ambient airflow, the β effect [9], rainfall, and [2]. China is one of the countries the internal TC structure, terrain, and the interaction be- that are most seriously impacted by TCs, with an average of tween multiscale weather systems [10–13]. ,ere are three 7-8 TCs making landfall there each year [3]. TCs are born in dynamic factors involved in TC movement: external forces, the Northwest Pacific Ocean and are classified into two internal processes, and the interaction between them [14– categories based on their track: one type moves west- 16]. ,e tracks of TCs are primarily determined by the northwestward into the South China Sea or the environmental air flow [17, 18]. Large-scale environmental Sea, making landfall in either the , , or steering flow is the domination external influence on a TC, China, and the other type moves northwest along a parabolic accounting for 70–90% of a TC movement [19, 20]. ,e track, suddenly turning northward towards the north coast structure of TCs and their atmospheric flow field determine 2 Advances in Meteorology

Table 1: TC intensity levels as defined by the CMA. 2. Data and Methods Maximum average wind speed near the TC type ,e best-track dataset of TC Yagi from the China Meteo- surface of the storm center (m/s) rological Administration (CMA) website (http://data.cma. Tropical depression 10.8–17.1 cn) was used in this study. In order to analyze the gale in the Tropical storm 17.2–24.4 Severe tropical storm 24.5–32.6 Bohai Sea which was caused by Yagi on 14-15 August 2018, 32.7–41.4 the hourly observed data from two automated weather Severe typhoon 41.5–50.9 stations on two platforms in the Bohai Sea were used, in- Super typhoon ≥51.0 cluding wind speed and wind direction data. Visible cloud imagery from the FY2 satellite was also utilized to determine the intensity of tropical depression Yagi once it had moved that TCs born in the Northwest Pacific Ocean generally move over the Bohai Sea. northwestward [21, 22]. ,e western Pacific subtropical high In order to analyze the characteristics of the atmospheric (WPSH), the westerly trough, and the internal force of the circulation of Yagi, 6-hourly daily reanalysis data from the TCs themselves are the main influence factors of the tracks of European Centre for Medium-Range Weather Forecasts TCs that are born in the western Pacific Ocean. Wang et al. Interim (ERA-Interim) product were used. ,e horizontal [23] considered that the anticyclonic flow field of the WPSH resolution of this dataset is 0.25° × 0.25° [32]. directly affects seven possible TC paths, which include Tropical depression Yagi reversed its track after weak- moving northward along the coast of East Asia, making ening on 14 August. ,e northeastward curving of Yagi took landfall in East China, making landfall in South China, it over the Bohai Sea, where it strengthened. ,us, the moving westward, turning out to sea after landfall, veering in steering flow was a key factor in the track of Yagi [18, 20, 33]. the offshore, and veering in the open sea. Furthermore, In this study, we computed the steering flow by vertically westerly flow, especially the distribution and variation of the integrating the pressure-weighted flow from 850 to 200 hPa westerlies in the upper levels of midlatitudes and high lati- [20]. tudes, exerts significant influence on the characteristics of To further illustrate the effects of the sea surface flux on various TC tracks. In addition, the interactions between the development of tropical depression Yagi, the apparent synoptic-scale vortices and TCs [24], as well as interactions of heat source was computed [34]. ,e apparent heat source a TC with other TCs, also have important effects on TC tracks (Q1) can be calculated as k [25]. ,e Bohai Sea and the Yellow Sea constitute the zT → zθ p northern ocean of China. ,e gales and storm surges caused Q � C ⎡⎣ + V · ∇T + ω � � ⎤⎦, (1) 1 p zt zp p by northward-moving TCs are the most serious summertime 0 → meteorological disasters in that region. For this reason, many where T is the temperature; V is the horizontal velocity meteorologists have investigated the mechanisms of north- vector; p is the air pressure; ω is the vertical pressure velocity ward-moving TCs [26–29]. ,e structure of a TC can be − 1 in units of Pa·s ; θ is the potential temperature; p0 is the described by a number of parameters associated with the TC reference air pressure of 1000 hPa; and k is a constant, with a wind [30]. Intensity (i.e., the maximum sustained surface value of 0.286. wind) is critically important because the wind-related damage ,e spatial intensity variation of Q1 can reflect the of a TC is proportional to at least the square of the maximum diabatic heating change [34]. surface wind. ,e gale processes are characterized by intense In order to examine the effects of friction and surface pressure gradients and strong winds such as northeasters, flux change on tropical depression Yagi when it moved from thunderstorms, and frontal systems that bring similar types of land to sea, we used the Weather Research and Forecasting weather patterns [31]. (WRF) mesoscale model version 3.9.1 to simulate this ,is study analyzed the meteorological processes asso- process. ,e two-layer nesting scheme was used for simu- ciated with the gale in the Bohai Sea that was caused by lation (Figure 1(a)). ,e outer layer included mideastern tropical depression Yagi on 14-15 August 2018. In addition, China and the Northwest Pacific, while the inner layer in- the effects of the WPSH, upper-level trough, jet stream cluded North China as well as the Bohai Sea and Yellow Sea. variability, and sea surface flux on the track and intensity ,e parameter settings used in the simulation, as found in changes of Yagi were investigated. ,is manuscript is or- Peng et al.’s study [35], are listed in Table 2. ganized as follows: Section 2 describes the data and methods. ,e WRF model was run from 0600 UTC 14 August to Section 3 describes the observed features of the track of TC 0000 UTC 16 August 2018. Since tropical depression Yagi Yagi and the induced gale process in the Bohai Sea. Section 4 entered the south Bohai Sea around 1200 UTC 14 August investigates the development mechanism of tropical de- and made landfall again around 1200 UTC 15 August, the pression Yagi in terms of atmospheric circulation. Section 5 first 6 hours (from 0600 UTC to 1200 UTC 14 August) were includes the model simulation and two sensitivity experi- used as the spin-up time, with the analysis beginning at 1200 ments used to explain the effects of sea surface change on the UTC 14 August. ,e driving wind data were the 6-hour data development of Yagi. Our conclusions and discussion are of the National Centers for Environmental Prediction presented in Section 6. (NCEP), and the driving sea surface temperature data were Advances in Meteorology 3

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Figure 1: (a) Simulation domain; (b) land use categories in the control experiment (CE); (c) land use categories in sensitivity experiment 1 (SE_1). ,e numbers on the legend bar represent the 20 MODIS land use categories modified by the IGBP (1: evergreen needleleaf forest; 2: evergreen broadleaf forest; 3: deciduous needleleaf forest; 4: deciduous broadleaf forest; 5: mixed forest; 6: closed shrubland; 7: open shrubland; 8: woody savanna; 9: savanna; 10: grassland; 11: permanent wetland; 12: cropland; 13: urban and built-up land; 14: cropland/ natural vegetation mosaic; 15: snow and ice; 16: barren or sparsely vegetated; 17: water; 18: wooded tundra; 19: mixed tundra; 20: barren tundra).

Table 2: Simulation scheme. changes. In sensitivity experiment 1 (SE_1), we changed the land use categories in the Bohai Sea and Yellow Sea from Term Scheme water to cropland in domain 2 (Figures 1(b) and 1(c)). ,e Central position 39°N, 122°E sea surface was changed to land in SE_1. In sensitivity Horizontal resolution 27 km, 9 km experiment 2 (SE_2), we set isfflx � 0 in namelist.input so Time step 90 s that the sea surface heat and moisture fluxes were turned off Boundary layer scheme YSU scheme Land surface scheme Standard Noah land surface model during the simulation. ,e intensity and track of Yagi were compared in the CE, SE_1, and SE_2. the daily data of the National Oceanic and Atmospheric 3. Observational Characteristics Administration (NOAA). In order to analyze the effects of the underlying surface TC Yagi was born in the Northwest Pacific Ocean at 0600 changes on the development of tropical depression Yagi, in UTC 7 August 2018 and made landfall on the East China the control experiment (CE), we ran the model without any coast at 1600 UTC 12 August (Figure 2(a)). Yagi had the 4 Advances in Meteorology

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Figure 2: (a) Track of TC Yagi. ,e different-colored points represent different intensities. Green points represent Yagi after it lost its numerical designation; yellow points represent tropical depression strength; orange points represent tropical storm strength; red-orange points represent severe tropical storm strength. (b) Visible cloud image of Yagi from the FY2 satellite at 0400 UTC 15 August. (c) Terrain around the Bohai Sea. Points A and B represent the location of the 2 automated observation platforms.

intensity of a severe tropical storm when it made landfall. satellite imagery (Figure 2(b)). ,e terrain around the Bohai As Yagi continued moving northwestward, its intensity Sea can be seen in Figure 2(c). During the daylight hours of weakened to tropical storm and then to tropical de- 15 August, the maximum average hourly wind speed in the pression. Tropical depression Yagi lost its TC designation Bohai Sea reached as high as 33.1 m/s, almost matching the number from the National Meteorological Center of the intensity of a severe tropical storm. Figure 3 shows the wind China Meteorological Administration (CMA) at 0000 changes observed by the two platforms in the Bohai Sea. UTC 14 August at 35°N, 116.1°E. Tropical depression Yagi Platform A is in the center of the Bohai Sea, while platform B then turned northeastward and continued moving. ,e is in the northern portion of the Bohai Sea (the locations of center of the depression strengthened after it moved into the platforms are illustrated in Figure 2(c)). It can be seen the southwest side of the Bohai Sea at 1800 UTC 14 that the wind speed strengthened between 0200 and 0600 August. At approximately 1200 UTC 15 August, tropical UTC 15 August. ,e maximum wind speed observed by depression Yagi again made landfall on the south coast of platform A was 31.7 m/s at 0400 UTC 15 August, while the the Bohai Sea, where it then weakened and dissipated maximum wind speed observed by platform B was 22.8 m/s (Figure 2(a)). at 0400 UTC 15 August. ,e wind direction at platform A During the daylight hours of 15 August, tropical de- was northeast, and the wind direction at platform B was pression Yagi strengthened after moving into the Bohai Sea, northwest on the evening of the 14th, veering to the north where it remained for an extended period of time and and northeast during the night of the 14th and during the presented a distinct typhoon circulation structure in the entire day of the 15th. Advances in Meteorology 5

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Figure 3: Wind change from 0800 UTC 14 August to 0000 UTC 16 August 2018. ,e hourly data were observed at (a) platform A and (b) platform B.

4. The Development Mechanism of Tropical surface pressure field around Yagi. ,e circulation center Depression Yagi was located over land, to the south of the Bohai Sea. ,e gale which was caused by the periphery of the circulation center To analyze the development mechanism of tropical de- of Yagi began to affect the central and western Bohai Sea. At pression Yagi after its track turned northeastward, as well as the same time, as the high-pressure system at high latitudes the factors leading to the gale in the Bohai Sea, the ERA- moved southeast, the terrain effect of Baekdu Mountain Interim reanalysis data were utilized to determine the at- caused the cold air to flow from the northeast over the north mospheric circulation pattern. TC Yagi weakened to a side of the Bohai Sea. During the interaction of Yagi with the tropical depression and lost its TC number at 0000 UTC 14 cold air, the average wind speed strengthened sharply over August 2018. At that time, the circulation center of the the ocean, a common phenomenon with tropical cyclones. depression was located at 34.7°N, 116.1°E, over North China. At 0000 UTC 15 August (Figure 4(c)), the center of tropical Based on the mean sea level pressure and 10 m winds at 0000 depression Yagi entered the Bohai Sea from the south, with UTC 14 August (Figure 4(a)), there was a high-pressure the cyclone strengthening after its center moved over the sea center at high latitudes (50°N, 110°E). ,e 1000–500 hPa surface and forming an apparent clear, warm circulation thickness was also calculated in order to analyze the ther- center, which is characteristic of TCs. ,e cold air from high modynamic characteristics. It could clearly be seen that the latitudes expanded farther southward. ,e southern pe- high-pressure area had a cold air structure. With the east- riphery of the high-pressure center and the northwest pe- ward movement of the system, the high-pressure center riphery of Yagi formed a tight pressure gradient zone, gradually moved southeastward, thereby inducing the cold strengthening the wind speed in the zone, which induced a air to also move southeastward. At 1200 UTC 14 August heavy gale in the Bohai Sea. At approximately noon (0400 (Figure 4(b)), the center of tropical depression Yagi had UTC 15 August), the average wind speed reached its weakened. ,ere were no closed isobar contours in the maximum. At 1200 UTC 15 August (Figure 4(d)), with the 6 Advances in Meteorology

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Figure 4: Surface circulation patterns: (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. ,e contours represent the mean sea level pressure (hPa), the vectors represent the 10 m wind speed (m/s), the shading represents the 1000–500 hPa thickness (m), and the red dots represent the location of “Yagi.” further expansion of the high-pressure system from high August (Figure 5(a)), obvious positive divergence existed latitudes, the southward cold air pushed Yagi back over land above the Bohai Sea. ,is indicated that the upper-level jet on the south side of the Bohai Sea. ,is effectively destroyed stream was divergent in this region, which induced an the warm center structure of tropical depression Yagi, and its updraft in the midlevels and low levels and convergence in intensity weakened. At this point, there were no closed the lower troposphere. ,is structure benefited tropical contours around Yagi in the surface pressure field. ,e depression Yagi as it moved northeast into the Bohai Sea. pressure gradient was tight on the northwest side of Yagi and Meanwhile, the convergence area of the upper westerly jet the southeast side of the high-pressure periphery, i.e., in the located at higher latitudes, which favored the formation of a Bohai Sea area. ,e wind direction was northeast, and the downdraft, corresponded to the formation of a cold high- average wind speed had slightly weakened over the Bohai pressure system at the surface. At 1200 UTC 14 August Sea. With the further expansion of the cold air from high (Figure 5(b)), the eastward movement of the convergence latitudes, tropical depression Yagi was pushed back over area of the upper westerly jet induced the cold high-pressure land, where it gradually dissipated. ,e meteorological system at the surface to move east. ,e divergence area of the processes leading to the gale over the Bohai Sea caused by upper-level westerly jet at 200 hPa was still located above the Yagi came to an end. Bohai Sea, which induced tropical depression Yagi to From the geopotential height (GPH) and wind field at continue moving northeastward into the Bohai Sea. At 0000 200 hPa on 14-15 August 2018 (Figure 5), it can be seen that UTC 15 August (Figure 5(c)), the convergence zone of the the westerly jet at high latitudes exhibited an obvious wave upper-level westerly jet still moved southeastward, and its pattern. ,e meridional wind was distinct. At 0000 UTC 14 intensity had increased, thereby inducing a stronger Advances in Meteorology 7

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Figure 5: 200 hPa circulation patterns: (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. ,e contours represent the GPH (gpm), the vectors represent winds at 200 hPa, the shading represents the divergence at 200 hPa (×10− 5·s− 1), and the red dots represent the location of “Yagi.” southeastward expansion of cold air at the surface. At 1200 of the effects of “Bebinca” on its southern side. ,is aided the UTC 15 August (Figure 5(d)), the long wave troughs and continued northeastward movement of tropical depression ridges of the upper-level westerlies continued to progress Yagi along the western edge of this high. By 1200 UTC 14 eastward, and the convergence area of the upper-level August (Figure 6(b)), the system at high latitudes had moved westerly jet now controlled the entire area above the Bohai eastward. With the progression of the trough, the cold air at Sea. ,us, the cold air had become the predominant factor the surface continued moving southeastward. ,e ridge of on the surface of the Bohai Sea at that time, forcing tropical the subtropical high continued moving northward, exhib- depression Yagi back over land. iting a zonal distribution that extended inland. ,is helped From the circulation patterns at 500 hPa on 14-15 Au- to steer Yagi northward into the Bohai Sea. At 0000 UTC 15 gust (Figure 6), it can be seen that there were two troughs August (Figure 6(c)), the “two troughs and one ridge” and one ridge at high latitudes. ,e meridional wind was configuration at high latitudes continued to move eastward. obvious, and the location of the periphery of the subtropical ,rough the effects of the trough at 500 hPa to the east, the high (588 gpm contour) was farther to the north. An obvious cold air at low levels flowed into the Bohai Sea from the cyclone was present in the South China Sea, which was the north along Baekdu Mountain. At this time, tropical de- severe tropical storm “Bebinca.” At 0000 UTC 14 August pression Yagi strengthened after it moved over the sea. ,e (Figure 6(a)), there was a trough east of eastern China at high location of the subtropical high was also farther north than latitudes. ,e trough was conducive to the pooling of cold air usual, and the 588 line extended inland. ,e regions con- at the surface. ,e location of the periphery of the sub- trolled by the subtropical high in the midlatitudes expanded. tropical high was farther north than usual probably because ,is induced tropical depression Yagi to remain over the sea 8 Advances in Meteorology

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Figure 6: 500 hPa circulation patterns: (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. ,e blue contours represent the GPH (gpm), the red contours represent the temperature (K), and the red dots represent the location of “Yagi.” for an extended period. At 1200 UTC 15 August period. At 1200 UTC 15 August (Figure 7(d)), the steering (Figure 6(d)), with the eastward movement of the trough at flow around Yagi had turned to the southwest, pushing the 500 hPa, the southward expansion of the cold air, and the depression back over land. generation of TC No. 18 “Rumbia” in the East China Sea, the ,e steering flow was the vertical integration of the subtropical high was split, with its eastern portion soon pressure-weighted flow from 850 to 200 hPa [20]. In order to appearing over the ocean. ,e southward expansion of cold further explore the steering flow pattern, we generated the air and the weakening of the subtropical high led to Yagi 700 hPa circulation patterns to reveal the effects of the low- making landfall again, after which it soon weakened and level jet. From the circulation patterns at 700 hPa (Figure 8), dissipated. at 0000 UTC 14 August (Figure 8(a)), a pronounced ,e curving of tropical depression Yagi to the northeast asymmetric structure existed on the east and west sides of had a significant impact on its redevelopment once it tropical depression Yagi. ,e southerly airflow on the east emerged over the ocean, resulting in a terrible gale in the side of the depression was noticeably stronger than the Bohai Sea. ,e upper-level westerly jet as well as the sub- northerly airflow on its west side. ,e stronger southerly tropical high impacted the northeastward turning of Yagi. In airflow induced the depression to move northward. ,e addition, the steering flow was also an important mechanism southerly airflow on the east side of the depression then in the curving of its track [20] (Figure 7). At 0000 UTC 14 shifted to southwesterly and was now stronger than the August (Figure 7(a)), the steering flow was northward in the northeasterly airflow on the west side of the depression, environment surrounding Yagi, inducing the depression to thereby inducing the depression to move farther north- continue moving northward toward the Bohai Sea. At 1200 eastward [9]. ,is was due to the fact that Yagi moved along UTC 14 August (Figure 7(b)), Yagi was close to the south the edge of the subtropical high. An obvious low-level side of the Bohai Sea, embedded in a decidedly northeast southerly jet stream existed on the edge of the subtropical steering current. ,is forced tropical depression Yagi into high, thus making the wind speed on the east side of tropical the Bohai Sea from its southwest coast. At 0000 UTC 15 depression Yagi stronger than on its west side. ,is asym- August (Figure 7(c)), Yagi was in the Bohai Sea, and the metric structure was the most significant factor inducing steering flow around it had weakened, allowing the de- Yagi to abruptly turn to the northeast. From 1200 UTC 14 pression to remain over the Bohai Sea for an extended August (Figure 8(b)) to 0000 UTC 15 August (Figure 8(c)), Advances in Meteorology 9

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Figure 7: Steering flow (×102 m/s): (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. ,e black dots represent the location of “Yagi.” as the depression moved north, the asymmetric structure at 850 hPa, weak cold advection occurred, indicating that the weakened, allowing Yagi to remain over the Bohai Sea for an cold air extended to the southeast. At 1200 UTC 14 August extended period. At 1200 UTC 15 August (Figure 8(d)), the (Figure 9(b)), the vorticity advection at 700 hPa along the northeasterly airflow on the northwest side of Yagi track of tropical depression Yagi was not apparent, but the strengthened, while the southerly airflow on its east side cold advection at 850 hPa above the Bohai Sea had weakened, compelling the depression to return back to land, strengthened. ,is indicated that the intensity change of where it weakened and dissipated. Comparing Figure 8 to Yagi was not obvious, but the cold air advection at 850 hPa Figure 7, we can see that the low-level jet steam pattern was had increased. At 0000 UTC 15 August (Figure 9(c)), consistent with the steering flow, indicating that the low- positive vorticity advection occurred over the southern level jet stream played an important role in the steering flow portion of the Bohai Sea, with negative vorticity advection around tropical depression Yagi. over its northern portion. ,is configuration implied that In order to analyze the diagnostic features of Yagi’s tropical depression Yagi would strengthen in the southern development, we generated the vorticity advection at part of the Bohai Sea and remain over the Bohai Sea for an 700 hPa and the temperature advection at 850 hPa (Fig- extended period. Cold advection at 850 hPa above the Bohai ure 9). At 0000 UTC 14 August (Figure 9(a)), positive Sea also occurred. At 1200 UTC 15 August (Figure 9(d)), vorticity advection existed on the south side of the Bohai Sea positive vorticity advection at 700 hPa was present from the along the track of Yagi at 700 hPa, along a southwest- south Bohai Sea to the land, indicating that Yagi had northeast axis. ,is indicated that the depression would returned back to land. Otherwise, at 850 hPa above the Bohai strengthen when it move along that axis. Over the Bohai Sea Sea, the cold advection was strong, indicating that the gale in 10 Advances in Meteorology

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12 312 12 35°N 35°N 10 10 8 8 6 6 20 20 312 30°N Reference vector 30°N Reference vector 110°E 115°E 120°E 125°E 110°E 115°E 120°E 125°E

(c) (d)

Figure 8: 700 hPa circulation patterns: (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. ,e contours represent the GPH (gpm), the vectors represent the winds at 700 hPa, and the shading represents the wind speed (m/s). the Bohai Sea at that time was primarily caused by the cold Yagi was over land, adjacent to the south side of the Bohai air. Sea. ,e north side of Yagi had already generated the ocean gale. At 0000 UTC 15 August (Figure 10(d)), the center of 5. Model Simulation and Sensitivity Test Yagi was located over the central Bohai Sea, and the intensity of the depression had strengthened significantly. ,e ,e effects of the atmospheric circulation on the develop- maximum surface winds over the ocean at this time ment and movement of tropical depression Yagi have been exceeded 30 m/s. At 1200 UTC 15 August (Figure 10(g)), the discussed previously. ,e depression strengthened appre- center of tropical depression Yagi had returned to land from ciably after it moved over the Bohai Sea not only because the south coast of the Bohai Sea, and the maximum winds friction was reduced but also because of the combined in- over the ocean had weakened compared to the values ob- fluence of sea surface heating and moisture flux. To analyze served at the previous time (Figure 10(d)). For the 10 m wind the effects of the surface change from land to sea on the field of the CE, the results of the simulation were approx- development of Yagi, we designed one control experiment imately the same as the observations. In SE_1, in which the (CE) and two sensitivity experiments (SE_1 and SE_2). ,e sea surface was changed to cropland, the 10 m wind fields of introduction of the CE, SE_1, and SE_2 can be found in SE_1 and the CE were almost the same at 1200 UTC 14 Section 2. August (Figure 10(b)). However, at 0000 UTC 15 August ,e 10 m wind field comparison between the CE, SE_1, (Figure 10(e)), the SE_1 wind speed was lower than the and SE_2 from 1200 UTC 14 August to 1200 UTC 15 August corresponding value in the CE (Figures 10(e) vs. 10(d)), and is shown in Figure 10. In the CE, at 1200 UTC 14 August the SE_1 center of the circulation was loosely structured. At (Figure 10(a)), the circulation center of tropical depression 1200 UTC 15 August (Figure 10(h)), the strong wind zone Advances in Meteorology 11

45°N 45°N 4 16 16 14 14 –2 12 12 10 10 0 8 4 –2 8 40°N 4 0 0 6 40°N 6 4 –2 –2 4 4 2 –6 2 –2 –2 –2 –4 –2 –4 35°N –2 0 –6 35°N –6 –8 4 –8 –10 –10 0 –12 –12 –14 –14

4 –16 4 –16 30°N 30°N 110°E 115°E 120°E 125°E 130°E 110°E 115°E 120°E 125°E 130°E

(a) (b) 45°N 45°N 0 16 12 16 14 0 4 14 12 4 0 0 12 8 10 0 10 8 16 –2 8 40°N 0 40°N 4 6 4 6

8 –2 4 12 4 4 2 0 2 –2 0 –2 8 –4 –4 0 35°N –6 35°N 4 –6 0 –8 8 –8 –10 –10 –12 0 –12

0 –14 –14 0

–16 4 –16 30°N 30°N 110°E 115°E 120°E 125°E 130°E 110°E 115°E 120°E 125°E 130°E

(c) (d)

Figure 9: Vorticity advection at 700 hPa (contour: ×109/s2) and temperature advection at 850 hPa (shading: ×105 K/s): (a) 0000 UTC 14 August; (b) 1200 UTC 14 August; (c) 0000 UTC 15 August; (d) 1200 UTC 15 August. ,e red dots represent the location of “Yagi.” seemed larger in SE_1 than in the CE, and the cyclone center cropland surface did, however, significantly impact the shifted more to the east. In SE_2, the sea surface heat and breakdown of the structure of Yagi. Additionally, changes to moisture flux were turned off in the model. At 1200 UTC 14 the surface friction and surface flux also affected the track of August (Figure 10(c)), the 10 m wind fields of SE_2 and the Yagi, a finding that warrants further investigation. ,e CE approached that of SE_1, and the location of the center of sensitivity experiments revealed that the surface change tropical depression Yagi as well as its intensity was nearly the from land to sea played an important role in the develop- same in SE_2, SE_1, and the CE. At 0000 UTC 15 August ment and movement of tropical depression Yagi. (Figure 10(f)), compared with SE_1 and the CE, the intensity Along the track of tropical depression Yagi (the red line of the center of Yagi was noticeably weaker in SE_2, with the in Figure 10(a)), the vertical cross section was generated in mean wind over the sea ranging from approximately 17.2 to order to analyze the vertical spatial structure of the potential 24.4 m/s. At 1200 UTC 15 August (Figure 10(i)), the center vorticity (the contours, which represent the intensity of of tropical depression Yagi was still located on the south side Yagi) and the apparent heat source (the shading, which of Bohai Sea in SE_2, while it had already moved back over represents diabatic heating) (Figure 11). land in the CE and SE_1. In addition, the wind speed in- From this figure, it can be seen that, at 1200 UTC 14 tensity in SE_2 was weaker than the CE and SE_1 values, August (Figures 11(a)–11(c)), the intensity of Q1 and the ranging from approximately 13.9 to 20.7 m/s. From the potential vorticity on the vertical cross section in SE_1 comparison of SE_1, SE_2, and the CE, it can be deduced (Figure 11(b)) and the CE (Figure 11(a)) are similar, while that the intensity of tropical cyclone Yagi and the wind speed the intensity of Q1 in SE_2 is weaker (Figure 11(c)). At this intensity in the Bohai Sea weakened a small amount when time, the center of tropical depression Yagi was still located the sea surface was changed to cropland and changed no- over land, along the coast. At 0000 UTC 15 August ticeably after the sea surface heat and moisture flux were (Figures 11(d)–11(f)), the center of Yagi had moved over the turned off. ,ese results indicate that the heat and moisture Bohai Sea (the center was located at approximately 200 km in flux from the sea surface played a more important role in the the profile). In the CE (Figure 11(d)), the values of Q1 below development of Yagi than the reduction of friction when the 500 hPa were strongly negative, indicating the convergence surface changed from land to sea. ,e friction of the of diabatic heating at low levels and the absorbance of 12 Advances in Meteorology

CE SE_1 SE_2

42°N 42°N 42°N

40°N 40°N 40°N

38°N 38°N 38°N

36°N 36°N 36°N 1200UTC14 1200UTC14 1200UTC14 1200UTC14 1200UTC14 1200UTC14

34°N 34°N 34°N

20 20 20 32°N Reference vector 32°N Reference vector 32°N Reference vector 112°E 114°E 116°E 118°E 120°E 122°E 124°E 126°E 128°E 130°E 112°E 114°E 116°E 118°E 120°E 122°E 124°E 126°E 128°E 130°E 112°E 114°E 116°E 118°E 120°E 122°E 124°E 126°E 128°E 130°E Wind speed Wind speed Wind speed

3.4 6.4 9.4 12.4 15.4 18.4 21.4 24.4 27.4 28.5 3.4 6.4 9.4 12.4 15.4 18.4 21.4 24.4 27.4 28.5 3.4 6.4 9.4 12.4 15.4 18.4 21.4 24.4 27.4 28.5 (a) (b) (c) CE SE_1 SE_2

42°N 42°N 42°N

40°N 40°N 40°N

38°N 38°N 38°N 0000UTC15 36°N 36°N 0000UTC15 36°N 0000UTC15

34°N 34°N 34°N

20 20 20 32°N Reference vector 32°N Reference vector 32°N Reference vector 112°E 114°E 116°E 118°E 120°E 122°E 124°E 126°E 128°E 130°E 112°E 114°E 116°E 118°E 120°E 122°E 124°E 126°E 128°E 130°E 112°E 114°E 116°E 118°E 120°E 122°E 124°E 126°E 128°E 130°E Wind speed Wind speed Wind speed

3.4 6.4 9.4 12.4 15.4 18.4 21.4 24.4 27.4 28.5 3.4 6.4 9.4 12.4 15.4 18.4 21.4 24.4 27.4 28.5 3.4 6.4 9.4 12.4 15.4 18.4 21.4 24.4 27.4 28.5

(d) (e) (f) CE SE_1 SE_2

42°N 42°N 42°N

40°N 40°N 40°N

38°N 38°N 38°N

36°N 1200UTC15 1200UTC15 36°N 1200UTC15 36°N

34°N 34°N 34°N

20 20 20 32°N Reference vector 32°N Reference vector 32°N Reference vector 112°E 114°E 116°E 118°E 120°E 122°E 124°E 126°E 128°E 130°E 112°E 114°E 116°E 118°E 120°E 122°E 124°E 126°E 128°E 130°E 112°E 114°E 116°E 118°E 120°E 122°E 124°E 126°E 128°E 130°E Wind speed Wind speed Wind speed

3.4 6.4 9.4 12.4 15.4 18.4 21.4 24.4 27.4 28.5 3.4 6.4 9.4 12.4 15.4 18.4 21.4 24.4 27.4 28.5 3.4 6.4 9.4 12.4 15.4 18.4 21.4 24.4 27.4 28.5 (g) (h) (i)

Figure 10: Comparison of 10 m winds in the CE, SE_1, and SE_2. ,e red line represents the track of Yagi (from 37°N, 117°E, to 39°N, 122°E), the vectors represent the 10 m wind, and the shading represents the wind speed (m/s). (a–c) At 1200 UTC 14 August; (d–f) at 0000 UTC 15 August; (g–i) at 1200 UTC 15 August. (a), (d), and (g) show the CE; (b), (e), and (f) show SE_1; (c), (f), and (i) show SE_2. diabatic heat from the sea surface by Yagi. However, at the heights to the right of the vortex center, revealing that the midlevels of the troposphere, Q1 became strongly positive, vortex absorbed diabatic heating from the surface ahead of it indicating that the midlevel of the troposphere was the and released diabatic heating behind it, a configuration that source of the diabatic heating. In other words, tropical would induce a vertically tilted vortex structure. ,e ab- depression Yagi released a great deal of heat through the solute values of Q1 in the midlevels and low levels of the latent heat of condensation at this height. At 0000 UTC 15 troposphere in SE_2 at this time (Figure 11(f)) were smaller August in the CE, the potential vorticity center was strong, than those in the CE and SE_1, and the SE_2 intensity of the indicating that the intensity of the low-pressure center was potential vorticity was weaker. ,ese results indicate that, strong. At the same time in SE_1 (Figure 11(e)), the negative after the sea surface heat and moisture flux were turned off, region of Q1 tilted a greater distance, appearing at lower tropical depression Yagi absorbed less diabatic heating from Advances in Meteorology 13

CE SE_1 SE_2 200 200 200 0.75

250 250 250 1 0.75 1.75 2 1 1 1 1 0.75

300 1 300 300 1 1.75

1 1.75 0.75 1 400 2 1 400 400 2 1 1.75 0.75 500 500 500 0.75 1 600 600 600 1

2 0012UTC14 0012UTC14 0012UTC14 Pressure (hPa) Pressure (hPa) Pressure (hPa) 700 700 1.75 700 0.75 2

850 850 850 1 1 0.75 1000 1000 0.75 1000 0 100 200 300 400 0 100 200 300 400 0 100 200 300 400 Distance (km) Distance (km) Distance (km) Apparent heat source (K/day) Apparent heat source (K/day) Apparent heat source (K/day)

–400 –300 –200 –100 50 150 250 350 –400 –300 –200 –100 50 150 250 350 –400 –300 –200 –100 50 150 250 350

(a) (b) (c) CE SE_1 SE_2 200 200 200 5 2 5 4 2 3 4 3 2 3 1 2 1 2 250 2 2 250 250 0 1 1 1 300 1 300 1 300 1 2 1 2 1 2 1 2 3 2 1 1 2 3 4 3 400 3 400 400 3 4 4 1 4 1 5 3

1 4 3

4 1 500 500 500 1 2 4 3 5 2 2 4 5 4 5 1 5 1 3 2 3 3 1 2 1 600 6 600 600 0000UTC15 0000UTC15

0000UTC15 5

Pressure (hPa) 2 Pressure (hPa) 3 Pressure (hPa) 700 1 700 1 700 1 4

5 3

850 1 850 850 4 2 5 4 3 1 3 1 2 0 1 2 1 1000 0 1 1000 1000 0 0 0 100 200 300 400 0 100 200 300 400 0 100 200 300 400 Distance (km) Distance (km) Distance (km) Apparent heat source (K/day) Apparent heat source (K/day) Apparent heat source (K/day)

–400 –300 –200 –100 50 150 250 350 –400 –300 –200 –100 50 150 250 350 –400 –300 –200 –100 50 150 250 350

(d) (e) (f) CE SE_1 SE_2 200 200 200 4 1 3 2 2 1

3 2 250 250 2 1 250 1 2 1 3 1

2 1 2 1 1 1 300 300

300 2 4 3 1 3 1 2 0 2 3 2 3 2 2 400 400 1 1 400 1 2 1 3 4 2 1 2 3 2 3 0 3 –1 2 2 3 500 4 500 4 500 2 1 2 3 4 2 1 1 2 0 4 2 5 4 1 1 2 1 3 600 2 600 600 4

2 0012UTC15 3 0012UTC15 0012UTC15 –1 Pressure (hPa) Pressure (hPa) Pressure (hPa) 3 1 2 1 3 700 1 1 700 700 5 4 850 0 850 2 850 1 3 1 1 3 0 2 1 1 2 0 1 1 1000 0 1000 1000 0 0 100 200 300 400 0 100 200 300 400 0 100 200 300 400 Distance (km) Distance (km) Distance (km) Apparent heat source (K/day) Apparent heat source (K/day) Apparent heat source (K/day)

–400 –300 –200 –100 50 150 250 350 –400 –300 –200 –100 50 150 250 350 –400 –300 –200 –100 50 150 250 350

(g) (h) (i) Figure 11: Vertical cross section of the apparent heat source Q1 (shading: K/day) and potential vorticity (contour: PVU) along the track of Yagi (the red line in Figure 9(a)) in the CE and SEs. (a–c) At 1200 UTC 14 August; (d–f) at 0000 UTC 15 August; (g–i) at 1200 UTC 15 August. (a), (d), and (g) show the CE; (b), (e), and (h) show SE_1; (c), (f), and (i) show SE_2. the lower troposphere, leading to a weakening of its intensity induced an ocean gale on 14-15 August 2018. At 0000 UTC and the subsequent release of less diabatic heat in the 14 August, tropical cyclone No. 14, “Yagi,” weakened and midtroposphere. By 1200 UTC 15 August (Figures 11(g)– lost its numerical designation from the CMA and continued 11(i)), the center of Yagi had moved back over land in the CE moving northeastward. During the night of 14 August, Yagi and SE_1, and the released diabatic heating from this de- entered the Bohai Sea from the south coast. Because of the pression in the CE and SE_1 was stronger than it was in SE_2 decreased friction over the ocean, as well as the increased because of the previous development intensity of Yagi. diabatic heating from the sea surface and moisture flux, the intensity of the cyclone strengthened noticeably, and the 6. Conclusions and Discussion wind speed along its periphery increased sharply. Mean- while, during the daylight hours of 15 August, there existed a ,is study analyzed the meteorological processes that oc- cold high-pressure system at high latitudes at the surface; curred as tropical depression Yagi entered the Bohai Sea and cold air from this system flowed over the Bohai Sea along the 14 Advances in Meteorology north coast, influenced by the local terrain. A concentrated changed to cropland in SE_1, the intensity of Yagi decreased pressure gradient zone was present on the southern pe- and its structure became looser. After turning off the sea riphery of the high-pressure center, which, in combination surface heat and moisture flux in SE_2, the intensity of Yagi with the northwest side of TC Yagi, formed a tight pressure and the wind speed along its periphery decreased signifi- gradient region, which induced the ocean gale that cantly, indicating that the sea surface friction and diabatic strengthened during the daylight hours of 15 August. ,e heating from the sea surface heat and moisture flux played a combined effect of the rejuvenated tropical cyclone Yagi and key role in the intensity and structural changes of Yagi after the flow of cold air induced the gale in the Bohai Sea. During it moved out over the ocean. the evening of 15 August, as the cold air continued to push southeastward, tropical depression Yagi moved back over Data Availability land, where it rapidly weakened and dissipated. ,us, the sharp increase of wind speed in the Bohai Sea when the ,e platform automatic station hourly wind data and the center of Yagi moved over the water, the return of the tropical cyclone track data are provided from China Na- depression to land, and the dissipation of the cyclone were tional Meteorological Data Service Center (http://data.cma. all connected with the surface change from land to sea and cn). ,e ERA-Interim dataset is obtained from https://www. the cold air from high latitudes. ecmwrf.int/en/forecasts/datasets/archive-datasets/ In addition, in the upper levels of the troposphere, the reanalysis-dataset/era-interim. ,e WRF model version long wave adjustment of the 200 hPa westerly jet at high 3.9.1 is freely available at https://www.mmm.ucar.edu/ latitudes provided favorable conditions for Yagi to continue weather-researchand-forecasting-model. ,e NCEP FNL moving northeastward and for the development of cold air data are obtained from https://rda.ucar.edu/datasets/. And in the lower troposphere via the divergence and convergence the NOAA sea surface temperature data are obtained from of the upper-level westerly jet. As the upper trough and ridge ftp://polar.ncep.noaa.gov/pub/history/sst. progressed eastward, the convergence region of the westerly jet also shifted to the east, which induced the surface cold air to move southeastward, as the divergence area weakened Conflicts of Interest and disappeared. At the middle tropospheric height of ,e authors declare that there are no conflicts of interest 500 hPa, it can be seen that the northeastward track of regarding the publication of this paper. tropical depression Yagi was along the periphery of the subtropical high on 14 August. ,e more northward location of the subtropical high was the primary factor behind the Acknowledgments continued northeastward movement of tropical depression ,is work was supported by the National Natural Science Yagi, a configuration that was also influenced by TC Foundation of China (No. 41575049), Forecaster Special “Bebinca” in the South China Sea. During the daylight hours Projects of the China Meteorological Administration (CMA) of 15 August, the effects of the more northward subtropical (CMAYBY2019-009), and Science and Technology Collab- high and the cold air from high latitudes combined to induce orative Innovation Fund Projects of Circum-Bohai-Sea tropical depression Yagi to remain over the Bohai Sea for an Region (QYXM201712 and QYXM201808). ,e authors extended period of time. During the evening of 15 August, thank LetPub (http://www.letpub.com) for its linguistic the subtropical high split, with a portion of it moving back assistance during the preparation of this manuscript. over the ocean. Under the influence of the cold air, Yagi returned to land and dissipated. ,us, the track of Yagi was not only related to the location of the subtropical high but References also connected with the formation of TC “Bebinca.” ,e steering flow explained the reason that the track of Yagi first [1] K. A. 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