atmosphere

Article Numerical Simulation of a Heavy Rainstorm in Northeast Caused by the Residual Vortex of Typhoon 1909 (Lekima)

Yiping Wang * and Tong Wang

School of Atmospheric Sciences, University, Nanjing 210023, China; [email protected] * Correspondence: [email protected]

Abstract: From 14 to 17 August 2019, a heavy rainstorm occurred in Northeast China due to the combined influence of the residual vortex of typhoon 1909 (Lekima) and cold air intrusion. Based on the precipitation data of China Meteorological observation stations, surface and upper charts, HMW-8 satellite images, NCEP/NCAR 0.25◦ × 0.25◦ reanalysis data and WRF4.0 numerical prediction model are used to carry out numerical simulations. According to the weather situation and numerical simulation results, the cause of 1 h severe precipitation is thoroughly studied. Results show that: (1) According to the weather situation, the precipitation process can be divided into two stages. The first stage is from 1412 to 1612 August 2019, which is caused by the interaction between the residual vortex, the inverted trough of typhoon 1909 (Lekima) and the upper trough. The rain belt lies from northeast to southwest, and the rainfall center has typical meso-β-scale characteristics. At the second stage from 1612 to 1712 August 2019, the residual vortex of typhoon reaches Province, at the same time, 500 hPa cold vortex falls to the south; (2) Based on the 1 h rainfall of automatic weather stations, it can be seen that there are three rainfall peaks from 00 UTC 14 to 12 UTC 17, which are 53.2 mm in the Middle East of Province, 38.2 mm in the south of 1610 Liaoning Province, and 21.3 mm in the east of 1707 Heilongjiang Province respectively. (3) Before the occurrence of 1 h heavy

 rainfall, the water vapor is concentrated in the middle and lower troposphere. The residual vortex  trough of typhoon 1909 extends northward, converges with the southwest airflow at the edge of

Citation: Wang, Y.; Wang, T. the subtropical high, and transports water vapor and energy to the northeast. The convective cloud Numerical Simulation of a Heavy clusters generated ahead of the trough move southeast, then merge into the mesoscale convective Rainstorm in Northeast China system in the inverted trough; (4) In the Bohai Bay and North , there is a vortex-like zone Caused by the Residual Vortex of composed of several convergence centers, and the convergence zone in typhoon-inverted trough Typhoon 1909 (Lekima). Atmosphere meets with the trough in Central Jilin. There exist a rising area and a positive vorticity belt in the 2021, 12, 120. https://doi.org/ typhoon-inverted trough, and the center of heavy rain lies in front of the positive vorticity center. At 10.3390/atmos12010120 the west of the inverted trough, there is a large center of positive vertical wind shear, and a small center in the east. The center of heavy rainfall is located on the line between the maximum and Received: 4 November 2020 minimum centers, which is close to the right of the maximum center; (5) The high energy tongue is Accepted: 12 January 2021 transported from the center of the typhoon to the northeast along the inverted trough of the typhoon, Published: 16 January 2021 and the southwest airflow at the edge of the subtropical high. There is a zone titled downward from northwest to southeast that contains dry and cold air, where there is convective instability; (6) Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- The strong precipitation area is located on the lee in the northwest of Changbai Mountain. There ms in published maps and institutio- is a convergence area in the middle of the troposphere, and a strong divergence area in the upper nal affiliations. troposphere, with remarkable topographic effect, and the west divergence column inclines on the east convergence column.

Keywords: northeast rainstorm; typhoon residual vortex; typhoon-inverted trough; convergence Copyright: © 2021 by the authors. Li- and divergence columns; convective instability; vertical wind shear; terrain uplift censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- 1. Introduction tribution (CC BY) license (https:// creativecommons.org/licenses/by/ Northeast China is located in the northernmost part of China. In summer, it is affected 4.0/). by the interaction of westerly zone weather systems, high latitude, subtropical and tropical

Atmosphere 2021, 12, 120. https://doi.org/10.3390/atmos12010120 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, 120 2 of 19

weather systems, and affected by the influence of Northeast special terrain, which makes the summer rainstorm in Northeast China have the characteristics of suddenness, strong intensity, short duration and locality [1]. Since typhoons usually occur in tropical low latitude areas, few typhoons can directly affect Northeast China, so the previous case studies mainly focus on the places of middle and low latitudes. Typhoons and typhoon- inverted troughs are important weather systems causing rainstorms in Northeast China in the summer. Due to the limitation of observation data, numerical simulation is an important tool to study rainstorms in Northeast China. Many meteorologists have performed a lot of beneficial research on rainstorm related to typhoon and typhoon-inverted trough. Li et al. used the numerical study on impacts of the upper westerly trough on the extratropical transition process of (9714), they found that the extratropical transition process was sensitive to the intensity of the upper trough, which affected the process in terms of strong cold advection, positive vorticity advection and strong upper divergence ahead of the trough, which was conducive to the maintenance and development of tropical cyclones [2]. Shen et al. studied torrential rain caused by a residual vortex of (1513), they found that the strong precipitation area mainly located on the left side of the shear vector between 200–850 hPa due to the interaction between landing typhoon and westerly belt system in middle and high latitudes [3]. By studying this case, Ye et al. found that the main mechanism of mesoscale convective activity is the combination of the convergence of low-level airflow strengthened by the uplift of terrain, and the middle-level divergent flow. The uplift of terrain strengthens the convergence of low-level airflow [4]. By the numerical simulation of torrential rain in the northeast of Huaihe basin, Wang et al. found that the persistent vertical wind shear provides kinetic energy for the development of the MCSs (mesoscale convective system), hence promoting the baroclinic development of convective systems, and the concentration of heavy rain at the specific location [5]. At the east side of the deep convective system, there was a downshear circulation, which accelerated the upper-level westerly and the low-level easterly, thus intensified the vertical wind shear of the upper and lower layers, the low-level convergence and upper-level divergence were intensified too, so the pumping effect from the upper levels to the lower levels was intensified correspondently [6]. Sun et al. found that two mesoscale convective cloud clusters developed in the typhoon-inverted trough were related directly to the severe heavy rainfall in Hebei province. The moisture transportation of southerly low-level jet (LLJ) was very important to the development of convection [7]. When analyzing the effects of vertical wind shear and storm motion on the asymmetric structure of precipitation, Chen et al. found that when the vertical wind shear (200–850 hPa) was greater than 7.5 m/s, it played a decisive role in the asymmetric distribution of precipitation on the left side of the downshear in the Northern Hemisphere [8]. From a numerical study of the severe heavy rainfall associated with typhoon 0505(Haitang), Yu et al. found that the topography of southern and northern enhanced the landing typhoon rainstorm and led to the uneven distribution of precipitation [9]. Wu and Lin thought that the terrain effect has a very important influence on the precipitation of tropical cyclones [10,11]. From the numerical simulation of rainfall and precipitation mechanism associated with typhoon Sinlaku(0216), Niu et al. found that the rainfall on the upwind slopes of the mountains increased and decreased on the downwind slopes due to the topographic effect [12]. In this paper, a heavy rainstorm that occurred in Northeast China between 14 and 17 August 2019 is studied, and during this period, the place experienced the joint influence of typhoon 1909’s residual vortex and cold air intrusion. The synoptic situation of this rainstorm is studied by using the surface and upper weather charts. The distribution characteristics of the rainstorm are analyzed by using the total rainfall, 1 h precipitation intensive observation data and satellite images, then the NCEP reanalysis data and WRF version 4.0 model are used for numerical simulation. Using the results, the dynamic conditions, water vapor conditions, vertical shear and other physical parameters were diagnosed and analyzed, and the influence of topography on the rainstorm was analyzed too. Atmosphere 2021, 12, 120 3 of 19

2. Data and Numerical Simulation Description 2.1. Observational DataSets Upper and surface weather charts, 1-h encrypted precipitation data of automatic meteorological stations in August 2019 were obtained from China Meteorological Data Network.

2.2. Satellite Cloud Image 1-h infrared HMW-8 satellite images were downloaded from the University of Kochi, , the scope is: 20◦ S–70◦ N, 70–160◦ E, 0.05◦ × 0.05◦ horizontal resolution.

2.3. NCEP 0.25◦ × 0.25◦ Reanalysis Data These NCEP FNL (Final) operational global analysis and forecast data are on 0.25-degree by 0.25-degree grids prepared operationally every 6 h. This product is from the Global Data Assimilation System (GDAS), which continuously collects observational data from the Global Telecommunications System (GTS), and other sources, for many analyses. The FNLs are made with the same model which NCEP uses in the Global Forecast System (GFS), but the FNLs are prepared about an hour or so after the GFS is initialized. The FNLs are delayed so that more observational data can be used. The GFS is run earlier in support of time-critical forecast needs, and uses the FNL from the previous 6 h cycle as part of its initialization. The analyses are available on the surface, at 26 mandatory (and other pressure) levels from 1000 millibars to 10 millibars, in the surface boundary layer and at some sigma layers, the tropopause and a few others. Parameters include surface pressure, sea level pressure, geopotential height, temperature, sea surface temperature, soil values, ice cover, relative humidity, u- and v- winds, vertical motion, vorticity and ozone.

2.4. WRF Simulations The Advanced Research WRF (ARW) modeling system has been in development for the past eighteen years. The current release is Version 4.0, available since June 2018. The ARW is designed to be a flexible, state-of-the-art atmospheric simulation system that is portable and efficient on available parallel computing platforms. The ARW is suitable for use in a broad range of applications across scales ranging from meters to thousands of kilometers. Used in this study is performed by NCAR, which is publicly available at 0.25◦ × 0.25◦ reanalysis data. One-way nested-grids are employed (Figure1). The centers of Domain1, Domain2 are at (45.0◦ N, 130.0◦ E), with their grid spacings of 9 and 3 km respectively. The grid sizes are 480 × 480 and 721 × 661 correspondingly. There are 33 layers in the vertical direction. The simple ice phase scheme and Kain–Fritsch cumulus parameterization scheme are used [13]. The integrations are performed for 84 h from 0000 UTC 14 to 1200 UTC 17 August 2019. Atmosphere 2021, 12, x FOR PEER REVIEW 4 of 20

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Figure 1. Two nested model domains (D01,D02) and their horizontal resolutions (9 km and 3 km respectively). FigureFigure 1.1. Two nested model domains (D01,D02) (D01,D02) and and their their horizontal horizontal resolutions resolutions (9 (9 km km and and 3 3km km 3. Resultsrespectively).respectively). 3.1. Typhoon Track and Surface Precipitation 3.3. ResultsResults According3.1. Typhoon to the Track typhoon and Surface information Precipitation provided by the data center 3.1. Typhoon Track and Surface Precipitation of the China MeteorologicalAccording to the Administration typhoon information (http://www.typhoon.org.cn), provided by the tropical cyclone and databased center on According to the typhoon information provided by the tropical cyclone data center the movingofthe path China of typhoon Meteorological 1909 (Lekima) Administration (Figure (http://www.typhoon.org.cn 2a), which occurred in ),the and Pacific based of the China Meteorological Administration (http://www.typhoon.org.cn), and based on on the moving path of typhoon 1909 (Lekima) (Figure2a), which occurred in the Pacific Ocean eastthe of movingthe path of typhoon on 4 August 1909 2019 (Lekima), at 1745 (Figure UTC 2a), 9, it which landed occu onrr edWenling in the City, Pacific Zhejiang OceanProvince, east and of the moved Philippines northward on 4 Augustgradually, 2019, then at 1745 passing UTC through 9, it landed Zhejiang on Wenling and City,Ocean Zhejiang east of the Province, Philippines and moved on 4 August northward 2019, gradually,at 1745 UTC then 9, it passing landed through on Wenling Zhejiang City, Provinces. At about 0400 UTC 11, “Lekima” set sail from City, andZhejiang Jiangsu Province, Provinces. and Atmove aboutd northward 0400 UTC gradually, 11, “Lekima” then set pass sailing from through Lianyungang Zhejiang City, and Jiangsu Province.JiangsuJiangsu Province.P Atrovinces. about At At1250 about about UTC 1250 0400 11, UTC UTCit landed 11, 11, it landed “ inLekima in Qingdao” set City, sail City, from Shandong Lianyungang Province, Province, City, and at thatandJiangsu time, at that theProvince. time, maximum the At maximum about wind 1250 force wind UTC near force 11, theit near land center theed center in of Qingdao the of thetyphoon typhoonCity, Shandong 1909 1909 was was Province,23 23 m/s. m/s. It movedIt andinto moved at the that intoBohai time, the theSea Bohai maximum at 2100 Sea at UTC 2100wind 11, UTC force and 11, near weakened and the weakened center to of the the to the tropicaltyphoon tropical 1909depression depression was 23 m/s.at at 0000 UTC0000It 13, move UTCthend into 13,stopped then the Bohai stopped numbering Sea numbering at 2100 at 0600 UTC at UTC 060011, and UTCthat weaken thatday. day. ed to the tropical depression at 0000 UTC 13, then stopped numbering at 0600 UTC that day.

Figure 2. Cont.

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Figure 2. (a) Schematic diagram of Typhoon 1909 (Lekima) moving path; (b) Accumulated precipita- Figure 2. (a) Schematic diagram of Typhoon 1909 (Lekima) moving path; (b) Accumulated precipi- tion from 12 UTC 14 August to 12 UTC 16 August 2019; (c) Accumulated precipitation from 12 UTC tation from 12 UTC 14 August to 12 UTC 16 August 2019; (c) Accumulated precipitation from 12 16 August to 12 UTC 17 August 2019. UTC 16 August to 12 UTC 17 August 2019. From 14 to 17 August 2019, affected by the northward movement of the residual Fromcirculation 14 to of17 typhoon August 1909 2019, (Lekima) affected and by the th colde northward air intrusion, movement a large range of the of rainstormsresidual cir- culationwas of suffered typhoon in Northeast1909 (Lekim China.a) and the cold air intrusion, a large range of rainstorms was sufferedAccording in Northeast to the synoptic China. situation, the precipitation process can be divided into two Accordingstages. The to first the stage synoptic is from situation, 12 UTC 14 the to precipitation 12 UTC 16, which process is caused can be by divided the interaction into two stages.between The first the stage residual is from vortex 12 andUTC the 14 inverted to 12 UTC trough 16, ofwhich typhoon is caused 1909, together by the interaction with the cold air intrusion, and the rain belt extends from southwest to northeast (Figure2b). The between the residual vortex and the inverted trough of typhoon 1909, together with the horizontal width of the rainband composed by the rainfall larger than 50 mm is about cold air100 intrusion, km, and the and length the israin about belt 600 extends km. The from rainfall southwest centers are to embeddednortheast in(Figure many blocks,2b). The horizontalwhich width are typical of the meso- rainbandβ-scale composed characteristics. by the There rainfall are eleven larger stations than whose50 mm rainfalls is about are 100 km, andgreater the thanlength 100 is mm, about six stations600 km. in The Heilongjiang rainfall centers Province, are four embedded stations in in Jilin many Province blocks, whichand are one typical station meso- in Liaoningβ-scale Province characteristics. separately. There The rainstormare eleven center stations 154.4 whose mm is located rainfalls are greaterin Huadian, than 100 Jilin mm, Province. six stations The maximum in Heilongjiang 1 h rainfall Province, is 53.2 mm,four stations which occurs in Jilin at theProv- ince andperiod one of station 19–20 UTC in Liaoning on 14 August. Province At the separately. second stage, The from rainstorm 12 UTC 16 center to 12 UTC 154.4 17, mm the is locatedresidual in Huadian, vortex centerJilin Province. of typhoon The 1909 maximum entered Heilongjiang 1 h rainfall Province.is 53.2 mm, The which precipitation occurs at was caused by the addition of residual circulation of typhoon 1910(Krosa) and the southeast the period of 19–20 UTC on 14 August. At the second stage, from 12 UTC 16 to 12 UTC 17, movement of the upper cold vortex into Liaoning Province (Figure2c). There are two the residualrainfall vortex centers, center one located of typhoon at the central 1909 junctionentered ofHeilongjiang Liaoning and Province. Jilin Provinces, The precipita- and the tion wasother caused at the centralby the junction addition of Jilinof residual and Heilongjiang circulation Provinces, of typhoon distributed 1910(Krosa) in blocks. and The the southeastrainstorm movement center 71.0of the mm uppe is locatedr cold at vortex , into Jilin Liaoning Province. Province (Figure 2c). There are two rainfall centers, one located at the central junction of Liaoning and Jilin Provinces, and the other at the central junction of Jilin and Heilongjiang Provinces, distributed in blocks. The rainstorm center 71.0 mm is located at Liaoyuan, Jilin Province. From the 1 h intensive precipitation of the automatic weather stations, it can be seen that there are three rainfall peaks during the period from 00 UTC 14 to 12 UTC 17, the first time is at 20 UTC 14, the precipitation is 53.2 mm, at the place of central and eastern Jilin

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From the 1 h intensive precipitation of the automatic weather stations, it can be seen that there are three rainfall peaks during the period from 00 UTC 14 to 12 UTC 17, the first time is at 20 UTC 14, the precipitation is 53.2 mm, at the place of central and eastern Jilin Province (42.6◦ N, 126.5◦ E); the second time is at 10 UTC 16 the precipitation is 38.2 mm, at the place of southern Liaoning Province (40.3◦ N, 122.2◦ E); the third time is at 07 UTC 17, the precipitation is 21.3 mm, at the place of eastern Heilongjiang Province (47.00◦ N, ◦ Atmosphere 2021, 12, x FOR PEER REVIEW 130.72 E). 6 of 20

3.2. Synoptic Situation Analysis Figure3a,b show the synoptic situation at 500 hPa at 1900 UTC on 14 August 2019, Province (42.6° N, 126.5° E); the second time is at 10 UTC 16 the precipitation is 38.2 mm, there are two cold vortices and one warm ridge locate in the middle and high latitudes of at the place of southern Liaoning Province (40.3° N, 122.2° E); the third time is at 07 UTC 110–150◦ E, the east (west) cold vortex is 5640 gpm, −18 ◦C (5600 gpm, −20 ◦C), which 17, the precipitation is 21.3 mm, at the place of eastern Heilongjiang Province (47.00° N, is in the Bering Sea (the Lake Baikal). The west cold vortex was split into two 5640 gpm 130.72° E). centers, another small 5640 gpm center is separated to the southwest. As the transverse 3.2. Synoptictrough Situation moves to Analysis the south and merges into the North China trough, the cold air behind the shallow trough continuously diffuses and moves toward the southeast. For the strong Figure 3a,b show the synoptic situation at 500 hPa at 1900 UTC on 14 August 2019, subtropical high, its 5880 gpm isoline reaches the sea of Japan and Sakhalin, which is there are two cold vortices and one warm ridge locate in the middle and high latitudes of a huge warm zone. Figure3c shows, at 850 hPa, the position where the small trough 110–150° E, the east (west) cold vortex is 5640 gpm, −18 °C (5600 gpm, −20 °C), which is in in Heilongjiang and Jilin Provinces, intersect with the inverted trough of typhoon 1909 the Bering Sea (the Lake Baikal). The west cold vortex was split into two 5640 gpm centers, residual vortex is at the eastern junction of Liaoning and Jilin Provinces, where, at the sea another small 5640 gpm center is separated to the southwest. As the transverse trough level pressure field (Figure3d), the residual vortex of typhoon 1909 (Lekima) is located, moves to the south and merges into the North China trough, the cold air behind the shal- since the intrusion of cold air exists with the conflict of cold and warm air, so it is easy low trough continuously diffuses and moves toward the southeast. For the strong sub- to trigger strong convective systems there. There are three times 20, 21 and 22 UTC 14 tropical high, its 5880 gpm isoline reaches the sea of Japan and Sakhalin, which is a huge successively 1 h rainfall centers appeared at Huadian (42.59◦ N, 126.45◦ E), Jilin Province, warm zone. Figure 3c shows, at 850 hPa, the position where the small trough in Hei- they were 53.2 mm, 40.7 mm and 24.2 mm respectively, this indicated that the residual longjiang and Jilin Provinces, intersect with the inverted trough of typhoon 1909 residual vortex of typhoon 1909 played a role in organizing and strengthening convective systems. vortex is at the eastern junction of Liaoning and Jilin Provinces, where, at the sea level From Figure4, the HMW-8 satellite image at 2000 UTC on 14 August 2019 shows pressure field (Figure 3d), the residual vortex of typhoon 1909 (Lekima) is located, since that double arc-shaped rain belts appeared in Northeast China, and the south branch was the intrusion of cold air exists with the conflict of cold and warm air, so it is easy to trigger over Changbai mountain, and the north one was between Wanda mountain and Sanjiang strong convective systems there. There are three times 20, 21 and 22 UTC 14 successively plain, separately. From the location of the weather systems, the south one is related to the 1 h rainfall centers appeared at Huadian (42.59° N, 126.45° E), Jilin Province, they were 53.2 mm,residual 40.7 vortexmm and of typhoon24.2 mm 1909,respectively, and the this north indicated one is related that the to residual the airflow vortex at the of edge of typhoonthe subtropical1909 played high.a role in organizing and strengthening convective systems.

Figure 3. Cont.

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Figure 3. (a) Potential height (shaded areas, gpm) and horizontal winds (barbs, kts) at 500 hPa; (b) Figure 3. (a) Potential height (shaded areas, gpm) and horizontal winds (barbs, kts) at 500 hPa; (b) Temperature (shaded areas, ◦C) and horizontal winds (barbs, kts) at 500 hPa; (c) Potential height Temperature (shaded areas, °C) and horizontal winds (barbs, kts) at 500 hPa; (c) Potential height (shaded(shaded areas, areas, gpm)gpm) andand horizontalhorizontal winds winds (barbs, (barbs, kts) kts) at at 850 850 hPa; hPa; (d ()d Pressure) Pressure (shaded (shaded areas, areas, hPa) hPa) andand horizontal horizontal windswinds (barbs,(barbs, kts)kts) at sea level at 19 UTC on on 14 14 August August 2019. 2019.

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From Figure 4, the HMW-8 satellite image at 2000 UTC on 14 August 2019 shows that double arc-shaped rain belts appeared in Northeast China, and the south branch was over Changbai mountain, and the north one was between Wanda mountain and Sanjiang plain, Atmosphere 2021, 12, 120 separately. From the location of the weather systems, the south one is related8 ofto 19 the resid- ual vortex of typhoon 1909, and the north one is related to the airflow at the edge of the subtropical high.

FigureFigure 4. 4.The The HMW-8 HMW satellite-8 satellite image image (TBB, (TBB, unit: ◦unit:C) at °C 20)UTC at 20 on UTC 14 August on 14 August 2019. 2019.

FromFrom 1512 1512 on, on, at 500at 500 hPa, hPa the, coldthe cold vortex vortex in Lake in BaikalLake Baikal continues continues to descend to descend south- south- wardward and and the the transverse transverse trough trough turns turns vertical. vertical. The 5640 The gpm5640 cold gpm vortex cold wasvortex split was into split into two parts and moved southward. The south one fell to the place of Hebei Province and two parts and moved southward. The south one fell to the place of Hebei Province and the west part of Liaoning Province, meanwhile, the north one reached the junction of Mongoliathe west andpart Inner of Liaoning Mongolia, Province, and the coldmeanwhile, vortex in the the north Bering one Sea reache was stable,d the while junction the of Mon- subtropicalgolia and highInner weakened Mongolia, and and retreated the cold eastward, vortex within the a bodyBering decrease. Sea wa Meanwhile,s stable, while the the sub- contourtropical line high of 5800weaken gpmed shrinks and toretreat the islanded eastward, of Japan, thewith northeast a body of decrease China and. Meanwhile the sea , the ofcontour Japan is line a huge of 5800 warm gpm ridge. shrinks At 850 to hPa, the typhoonisland of 1910 Japan, moved the northwardnortheast of and China entered and the sea theof seaJapan of Japan, is a huge guided warm by the ridge. residual At vortex850 hPa, of typhoontyphoon 1909, 1910 its move invertedd northward trough moving and entered throughthe sea theof Japan, border guided of Liaoning by the and residual Jilin Provinces, vortex andof typhoon entering 1909 into, the its centralinverted part trough of mov- Heilongjianging through and the Jilin border Provinces. of Liaoning At 1600, and at 850 Jilin hPa, Provinces, the residual and vortex ente typhoonring into 1909 the was central part located in Jilin Province, and due to the incorporation of typhoon 1910 circulation, the wind of Heilongjiang and Jilin Provinces. At 1600, at 850 hPa, the residual vortex typhoon 1909 speed on both sides of the inverted trough of the residual vortex is strengthened, Nenjiang, ,was located , in and Jilin Province, are and at thedue west to ofthe the incorporation inverted trough, of whiletyphoon , 1910 Yichun, circulation, andthe Linjiangwind speed are aton the both East. sides The of precipitation the inverted centers trough of of 6 the h appear residual in Suifenhevortex is Riverstrengthened, (44.2Nenjiang,◦ N, 131.1 Qiqihar,◦ E, Heilongjiang Harbin, and Province), Changchun 41 mm, are at (45.2the west◦ N, 130.6of the◦ E, inverted Heilongjiang trough, while Province),Yanji, Yichu 41 mm.n, and They Linj alliang appear are at at the the west East of. theThe surface precipitation inverted centers trough. of At 6 1610, h appear in theSuifenhe second River rainfall (44.2 peak° 38.2N, 131.1 mm occurred° E, Heilongjiang in Gaizhou P (40.3rovince),◦ N, 122.2 41 m◦ m,E), atJixi the (45.2 south° N, of 130.6° E, LiaoningHeilongjiang Province, Province), near the 41 Bohai mm. Sea. They Because all appear the residual at the vortex west ofof typhoonthe surface 1909 inverted reaches trough. LiaoningAt 1610, Province,the second as doesrainfall the peak 5640 gpm38.2 coldmm vortex,occurred they in canGaizhou interact (40. with3° N, each 122.2° other E), at the sosouth as to of trigger Liaoning strong Province, convective near systems. the Bohai At 850 Sea. hPa, Because the inverted the residual trough ofvortex typhoon of typhoon 1910 residual vortex is located in the eastern part of Heilongjiang Province. At 1700, at 1909 reaches Liaoning Province, as does the 5640 gpm cold vortex, they can interact with 500 hPa the 5640 gpm cold vortex merged with the residual vortex of typhoon 1910, at theeach junction other ofso Heilongjiangas to trigger and strong Jilin convective Provinces. Atsystems. 850 hPa, At the 85 typhoon0 hPa, the 1910 inverted residual trough of vortextyphoon reaches 1910 the residual east of the vortex sea of is Japan, located where in the it merges eastern with part the of denatured Heilongjiang cold vortex Province. At and1700, typhoon at 500 1909 hPa residualthe 5640 vortex gpm to cold form vortex into a vortexmerged of with 1320 gpm.the residual The cold vortex air enters of typhoon in1910, Heilongjiang at the junction and Jilin of Provinces Heilongjiang from the and west, Jilin and Provinces. the third 1At h rain850 peakhPa, appearsthe typhoon in 1910 ◦ ◦ 1707,residual which vortex is 21.3 reaches mm in Huachuan(47.00 the east of the N,sea 130.43 of Japan,E),Heilongjiang where it merges Province. with At the 1712, denatured thecold upper vortex trough and at typhoon 500 hPa moved1909 residual southeast, vortex and then to form the residual into a vortex vortex weakenedof 1320 gpm. and The cold disappeared.air enters in Heilongjiang and Jilin Provinces from the west, and the third 1 h rain peak 3.3.appears Analysis in of1707, Water which Vapor is Condition 21.3 mm in Huachuan(47.00° N, 130.43° E), Heilongjiang Prov- ince.At At 1412, 1712, the the residual upper vortex trough of at typhoon 500 hPa 1909 moved reaches southeast, the Yellow and Sea then about the 150residual km vortex southeastweakened of Liaodongand disappeared. Peninsula, and its inverted trough extends northward, and converges with the southwest airflow at the edge of the subtropical high, which transports water

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3.3. Analysis of Water Vapor Condition Atmosphere 2021, 12, 120 9 of 19 At 1412, the residual vortex of typhoon 1909 reaches the Yellow Sea about 150 km southeast of Liaodong Peninsula, and its inverted trough extends northward, and con- verges with the southwest airflow at the edge of the subtropical high, which transports watervapor vapor and energy and energy to the northeast,to the northeast, and the and center the ofcenter the severeof the tropicalsevere tropical storm 1910 storm (Krosa) 1910 (Krosa)is close is to close the Kyushu to the Kyushu Island of Island Japan, of its Japan, outer its cloud outer system cloud issystem transported is transported from Kyushu, from Kyushu,South Korea South and Korea the Northand the Yellow North Sea Yellow to the Sea southeast to the ofsoutheast Liaoning of Province Liaoning (where Province the (whereresidual the vortex residual located), vortex and located), water vapor and water is conveyed vapor to is theconveyed northeast. to the After no 1412,rtheast. there After are 1412,many there convergence are many centers convergence along the centers inverted along trough, the inverted and it is trough located, and in central it is located Liaoning, in centralJilin and Liaoning Heilongjiang, Jilin and Provinces. Heilongjiang At 1419, Provinces. at 850 At hPa, 1419, Figure at 855a0 showshPa, Figure that the 5a shows water thatvapor the starts water from vapor the centerstarts offrom typhoon the center 1910, of and typhoon transports 1910, northward and transports along the northward southerly alongairflow the on southerly the east airflow side of on the the inverted east side trough, of the inverted as does the trough, convergence as does the center convergence of water −8 −1 −2 −1 centervapor of (Figure water5 b).vapor At 1419,(Figure the 5b center). At 1419, was –21the center× 10 wasg·hPa –21 × 10·cm−8 g∙hPa·s −1,∙cm at the−2∙s junction−1, at the −8 −1 −2 −1 junctionof Liaoning of Liaoning and Jilin and Provinces, Jilin Provi atn 1420,ces, at the 1420, central the central part − part18 × −1108 × 10g·−8hPa g∙hPa·cm−1∙cm−2·s∙s−1 −8 −1 −2 −1 changedchanged from from massive toto banded.banded. AtAt 1421,1421, thethe− −1199 ×× 1010−8 g∙hPag·hPa−1∙cm·cm−2∙s−1,· swhich, which is at isthe at centerthe center of Jilin of JilinProvince, Province, is located is located at the at thecenter center of the of therainstorm. rainstorm. Therefore, Therefore, the thecenter center of typhoonof typhoon 1910 1910 transports transports water water vapor vapor to the to thenorthwest northwest along along the inverted the inverted trough, trough, and andun- derunder the theorganization organization of ofthe the residual residual vortex vortex of of typhoon typhoon 1909, 1909, it it provides provides abundant abundant water water vapor conditions for the generation of heavy rainfall. Consequently, the first time 1 h vapor conditions for the generation of heavy rainfall. Consequently, the first time 1 h in- intensive precipitation peak of 53.2 mm occurred in Huachuan, central and Eastern Jilin tensive precipitation peak of 53.2 mm occurred in Huachuan, central and Eastern Jilin Province. Province.

Figure 5. Horizontal winds (barbs;kts) and (a) vapor flux(shaded;g/(cm.hPa.s)); (b) water vapor flux divergence (shaded; Figure 5. Horizontal winds (barbs;kts) and (a) vapor flux(shaded;g/(cm.hPa.s)); (b) water vapor flux divergence (shaded; 10−8 g/(cm2.hPa.s)) at 19 UTC on 14 August 2019. 10−8 g/(cm2.hPa.s)) at 19 UTC on 14 August 2019. At 1600, at 500 hPa, the split cold vortex (5640 gpm) reaches upper Hebei and Liaoning Provinces.At 1600, At at 1610, 500 hPa, at 850 the hPa, split the cold residual vortex vortex (5640 ofgpm) typhoon reaches 1909 upper arrives Hebei at the and Middle Liao- ningEast ofProvinces. Jilin Province, At 1610, the 500at 850 hPa hPa, cold the vortex(5640 residual vortex gpm) moves of typhoon toward 1909 southeast, arrives convec- at the Middletive systems East of transport Jilin Province, to the eastthe 500 of LiaoninghPa cold Province,vortex(5640 the gpm transportation) moves toward of water southeast, vapor convectiveis not sufficient, systems the transport west dry to air the intrudes east of Liaoning to eastward, Province, and convergesthe transpor withtation the of wet water air vapordelivered is not from sufficient the east., the west dry air intrudes to eastward, and converges with the wet air deliverFigureed6 afrom shows thethat east. on the vertical section of 42.5 ◦ N latitudinal relative humidity, there is an 80% vertical quasi-saturated water vapor column around 124◦ E that reaches 400 hPa. Meanwhile, because there is a dry area above 700 hPa to the west of 121◦ E, and a long and narrow relative dry column around 125◦ E is above 500 hPa, the humidity

gradient on both sides of the vertical saturated water column is very large. As the relative Atmosphere 2021, 12, x FOR PEER REVIEW 10 of 20

Figure 6a shows that on the vertical section of 42.5° N latitudinal relative humidity, Atmosphere 2021, 12, 120 there is an 80% vertical quasi-saturated water vapor column around 124° E that reaches10 of 19 400 hPa. Meanwhile, because there is a dry area above 700 hPa to the west of 121° E, and a long and narrow relative dry column around 125° E is above 500 hPa, the humidity gradient on both sides of the vertical saturated water column is very large. As the relative ◦ dry columncolumn isis placed placed on on the the lower lower saturated saturated water water vapor vapor block block around around 125 125E,° itE, is it favorable is favor- forable the for formation the formation of convective of convective instability. instability. The 70%The water70% water vapor vapor column column between between 126– ◦ 129126–129E reaches° E reaches 300 300 hPa. hPa. Additionally, Additionally there, there is is a longa long and and narrow narrow east-west east-west drydry zonezone ◦ between 700–850700–850 hPa to the easteast ofof 130130° E.E. Therefore,Therefore, thethe convectionconvection onon thethe westwest sideside ofof thethe typhoon typhoon-inverted-inverted trough trough is is more more vigorous vigorous than than that that on onthe the east east side side (i.e., (i.e., near nearthe west the westedge edgeof theof subtropical the subtropical high). high).As a result, As a the result, precipitation the precipitation on the west on theside west of the side typhoon of the- typhoon-invertedinverted trough is troughlarger than is larger that than on the that east on side. the east side.

Figure 6. Vertical cross section of relative humidity (shaded;%) along 42.5◦ N(a), and 126.5◦ E(b) at 19 UTC on 14 August 2019.Figure 6. Vertical cross section of relative humidity (shaded;%) along 42.5° N (a), and 126.5° E (b) at 19 UTC on 14 August 2019. Figure6b shows that, on the vertical profile of the meridional relative humidity along 126.5Figure◦ E, the 6 70%b shows water that vapor, on the columns vertical near profile 39–42 of the◦ N meridional and 46–47◦ relativeN both humidity reach 300 along hPa, the126.5 dry° E, region the 70% of 300–850water vapor hPa tocolumns the north near around 39–42° 46 N◦ Nand aloft 46– tilted47° N fromboth northreach to300 south. hPa, Thethe dry 90% region saturated of 30 water0–850 vaporhPa to columnthe north between around 39 46◦°N N andaloft 42 tilt◦ Ned reachesfrom north 520 to hPa, south. the wetThe center90% saturated relative water between vapor 42◦ columnN and 45between◦ N with 39° its N humidityand 42° N morereaches than 520 90% hPa, is the below wet 700center hPa, relative and there between is a dry 42 column° N and less 45° than N wit 50%h its that humidity reaches 500more hPa. than Therefore, 90% is below the upper 700 layerhPa, and is dry there and is the a dry lower column layer isless wet, than which 50% isthat conducive reaches to500 the hPa. formation Therefore, of convective the upper instabilitylayer is dry in and the the lower lower layer layer there. is wet, As awhich result, is it conducive is beneficial to the to increase formation precipitation of convective in thisinstability area. in the lower layer there. As a result, it is beneficial to increase precipitation in this area.Figure 7a shows that, along the 42.5 ◦ N zonal vertical profile of water vapor flux, waterFigure vapor 7a is shows mainly that concentrated, along thein 42.5 the° lowerN zonal troposphere, vertical profile below of 850water hPa, vapor and flux, it is transportedwater vapor from is mainly east to concentrated west. Figure 7inb showsthe lower that, troposphere, on the vertical below profile 850 of hPa, water and vapor it is ◦ fluxtransported along the from meridional east to west. direction Figure of 126.5 7b showsE, the that water, on vaporthe vertical is transported profile of from water south vapor to ◦ ◦ north,flux along concentrated the meridional in the southdirection of 45 of 126.5N, 35–40° E, theN iswater larger, vapor and is it transported reaches 500 hPafrom among south ◦ ◦ 40–45to north,N. concentrated Figure7c shows in that,the south along of 42.5 45° NN, the 35– zonal40° N vertical is large profiler, and ofit waterreaches vapor 500 fluxhPa divergence,among 40–45 water° N. Figure vapor is7c concentrated shows that, along in the 42.5 lower° N troposphere the zonal vertical below 600profile hPa of around water ◦ ◦ 124vaporE, flux and divergence, below 700 hPawater around vapor 127is concentratedE. Figure7d in shows the lower that, troposphere along the meridional below 600 ◦ directionhPa around of 126.5 124° E,E, and several below water 700 vapor hPa convergencearound 127° centersE. Figu werere 7d transported shows that from, along south the ◦ ◦ tomeridional north, there direction are two of convergence 126.5° E, several centers ofwater water vapor vapor convergence at 39–40 N andcenters 42–43 wereN totrans- the ◦ southported of from 45 Nsouth below to north, 700 hPa there respectively. are two convergence centers of water vapor at 39–40° N and 42–43° N to the south of 45° N below 700 hPa respectively.

Atmosphere 2021, 12, 120 11 of 19 Atmosphere 2021, 12, x FOR PEER REVIEW 11 of 20

Figure 7. Vertical cross section of vapor flux (shaded; g/(cm.hPa.s)) along 42.5◦ N(a), and 126.5◦ E(b) at 19 UTC on Figure 7. Vertical cross section of vapor flux (shaded; g/(cm.hPa.s)) along 42.5° N (a), and 126.5° E (b) at 19 UTC on 14 14 August 2019; Vertical cross section of water vapor flux divergence (shaded; 10−8 g/(cm2.hPa.s)) along 42.5◦ N(c), and August 2019; Vertical cross section of water vapor flux divergence (shaded; 10−8 g/(cm2.hPa.s)) along 42.5° N (c), and 126.5° 126.5◦ E(d) at 19 UTC on 14 August 2019. E (d) at 19 UTC on 14 August 2019.

3.4.3.4. Dynamic Dynamic Mechanism Mechanism AfterAfter 1415, 1415, there there is is a a belt, belt, consist consistinging of of serval serval vortex vortex convergence convergence centers centers from from Bohai Bohai BayBay to to North North Korea, Korea, which which extends extends northeast northeast along along the the southwest southwest jet jet at at the the east east of of the the invertedinverted trough trough on on the the west west edge edge of the subtropical high. The typhoon typhoon-inverted-inverted trough trough andand the the upper upper trough trough converge converge in in the the middle middle of of Jilin Jilin Province, Province, which which is is the the place place of of the the rainstormrainstorm center. center. The The convergence convergence centers centers and and the the divergence divergence centers centers match match with with each each other, which is favorable for the formation of the vertical secondary circulations. other, which is favorable for the formation of the vertical secondary circulations. Figure8a shows the maximum divergence center at 850 hPa is −6 × 10−5 s−1, it is Figure 8a shows the maximum divergence center at 850 hPa is −6 × 10−5 s−1, it is in the in the middle of Liaoning and Jilin Province. At 1610, typhoon 1910 reaches the north of middle of Liaoning and Jilin Province. At 1610, typhoon 1910 reaches the north of Japan, Japan, the cold vortex is west of Heilongjiang and Jilin Provinces. There is a convergence the cold vortex is west of Heilongjiang and Jilin Provinces. There is a convergence center center in the middle of the saddle field and near the circulation of the upper cold vortex, in the middle of the saddle field and near the circulation of the upper cold vortex, the

Atmosphere 2021, 12, 120 12 of 19

the center is in the east of Jilin Province. There is a convergence center at the bottom of the cold vortex, which is −6 × 10−5 s−1, with a westerly flow, at the south of Liaoning and the Bohai Sea. From 1706 to 1708, the 500 hPa cold vortex moves to the Heilongjiang and Jilin, the southwest jet in front of the trough begins to strengthen, and there appears to be a strong convergence center −6 × 10−5 s−1 in Jilin and the east of Heilongjiang Province. Figure8b shows, at 1419, at 850 hPa, that there is a positive vorticity belt about 1000 km long, 300 km wide located in the inverted trough, ranging from the central part of Liaoning Province to the eastern part of Heilongjiang Province. This comes across typhoon 1909’s residual vortex, the positive vorticity is northward transported from typhoon 1910, the positive vorticity center is 50 × 10−5 s−1, located at the middle of Liaoning Province. In front of it, the heavy rainstorm center is located, and then it moves to Jilin Province only after 1423. At 1610, there is a positive vorticity center 8 × 10−5 s−1 at the bottom of the cold vortex, which is at the westerly flow, at the junction of southern Liaoning and Bohai Sea. The north and south branches meet in the south of Liaoning Province. At 1707, at the east of Heilongjiang Province, there is a shear and positive vorticity belt with a center value of 8 × 10−5 s−1. Figure8c shows, at 1419, at 850 hPa, there is a rising area in the inverted trough, and there are several rising centers in Liaoning and Jilin, the center 0.9 m/s is in Liaoning, at the junction of Jilin and central Liaoning, there is a positive and a negative in the inverted trough, 0.4 m/s, −0.4 m/s, conducive to form the secondary circulation, and enhanced to 1423. At 1610, the vertical velocity is not strong. At 1706, there appears a positive rising speed center 0.4 m/s is located at Jilin and east of Heilongjiang. It shows that the convection is not very strong in this precipitation process. Figure8d shows, at 1419, between the height of 1500–6000 m, there is a positive vertical wind shear belt located at the west side of the inverted trough, at the junction of eastern Hebei and western Liaoning. The high-value center 28 m/s is located at the junction of , Liaoning and Jilin Provinces, and a small center of vertical wind shear 2 m/s at the north edge of the subtropical high is located at the south of Sihote mountain, and the rainstorm center is located on the line between the maximum center and the minimum center of vertical wind shear, which is close to the right side of the large value center. The results show that the right side of the downshear center is favorable for the occurrence of heavy rainfall. Figure9a shows at 126.5 ◦ E, there is a rising column with the center value of 0.1 m/s at the height of 450–600 hPa, and at the west side, there is an ascending column with the center value of 0.15 m/s at the height of 400–500 hPa, it shows that there is an eastward-moving convection cell at the west of the rainstorm center, and the intensity is strengthened. The results show that the convective system in front of the westerly trough moves eastward and replenishes new energy and water vapor. Figure9b shows, at 300–600 hPa, 39–43 ◦ N, there is a mass updraft center with the center value of 0.15 m/s, whose horizontal width is around 400 km and vertical height is around 4 km. The middle-level vortex may be the function of the typhoon residual vortex. Figure9c shows there is a zonal westerly belt with several gale cores embedded in the upper 100–300 hPa, a 20 m/s gale core to the west of 125◦ E is caused by the northward cold vortex descending southward, and the gale core between 130–135◦ E is 25 m/s, which is caused by the southwest airflow in front of the westerly trough under the blocking of the subtropical high, indicating that there is a westerly jet eastward continuously in the latitudinal direction. Near the ground, east of 126◦ E, due to the east side of the typhoon-inverted trough, there is a maximum easterly airflow core between 131–135◦ E. The vertical wind shear between the upper and lower layers is between 132–134◦ E, which is bigger than 40 m/s. From 120 to 126◦ E, the easterly flow gradually decreases, and from 300 hPa to the ground, the westerly flow sinks. Figure9d shows the jet core with a southerly wind of 30 m/s at 100–350 hPa is caused by the southwest airflow in front of the upper trough. From 39–43◦ N, the upper trough is deep, and the surface in front of the westerly trough is a southerly wind. From 43◦ N to the north, the lower layer is located at the west of the typhoon-inverted trough, which is the northerly wind. The upper-level in front of the upper trough is the southerly wind. There Atmosphere 2021, 12, 120 13 of 19

is a vertical wind shear in the upper and lower layers, and it inclines northward with the height.

Figure 8. Horizontal winds (barbs;kts) and (a) divergence(10−5 s−1); (b) vorticity (10−5 s−1); (c) ver- tical velocity (m/s); (d) vertical wind shear between 1500–6000 m(m/s) at 19 UTC on 14 August 2019.

3.5. Thermal and Instability Characteristics At 1419, at 850 hPa, the high energy tongue is transported from the center of the typhoon 1910 along the southwest airflow of the typhoon-inverted trough and the edge of the subtropical high to the northeast. Figure 10a shows, at 1419, along 42.5◦ N the zonal vertical profile of pseudo equivalent potential temperature and specific humidity, between ◦ 120–124 E, at the height of 400–850 hPa θse is the low-value area, less than 320 K, which indicates that dry and cold air intrudes from the west. The specific humidity above 700 hPa is less than 1 g/kg. From the west to the east of 120◦ E, the dry and cold air is obliquely intruded into the high humidity center between 124–135◦ E from the middle troposphere. The maximum value of θse at the center is greater than 352 k, and the specific humidity at 850 hPa is more than 14 g/kg. At 1421, the maximum vertical velocity between 42–44◦ N ◦ 124–126 E is more than 1 m/s. The center area of the low value of θse is located at 700 hPa,

1

Atmosphere 2021, 12, 120 14 of 19

which indicates that the area below is a convective instability area. It can be seen that this area is in the convective instability area before and when the rainstorm occurs. As long as the upward movement exists, the convective systems will be triggered and the convective upward motion will be maintained and strengthened. Figure 10b shows, at 1419, Atmosphere 2021, 12, x FOR PEER REVIEW the meridional vertical profile of pseudo equivalent potential temperature14 of 20 and specific ◦ ◦ humidity along 126.5 E, and it can be seen that to the south of 44 N under 700 hPa, θse is greater than 348 K and the warm moist air with specific humidity greater than 16 g/kg ◦ ◦ flow graduallyextends decreases, 47 Nand northward from 300 withhPa to the the southerly ground, wind.the wester Therely isflow a high-value sinks. Figure area at 41–43 N. The low value of θ is between the height of 500–700 hPa to the north of 45◦ N, the θ 9d shows the jet core with a southerlyse wind of 30 m/s at 100–350 hPa is caused by the se is 316 K at and the altitude is about 650 hPa. The intensified dry and cold air intrudes southwest airflow in front of the upper trough. From 39–43° N, the upper trough is deep, obliquely from the middle and upper levels. The front area at the junction of warm and and the surface in front of the westerly trough is a southerly wind. From 43° N to the humid air is stable at 41–47◦ N. The center of the low value of θ is located at 3–4 km north, the lower layer is located at the west of the typhoon-inverted trough, whichse is the height. The convective instability is strengthened in the middle and low troposphere, northerly wind. The upper-level in front of the upper trough is the southerly wind. There which leads to more intense precipitation. As the unstable state continues, there is still is a vertical wind shear in the upper and lower layers, and it inclines northward with the strong precipitation during 1420–1422. height.

Figure 9. Vertical cross section of vertical velocity (shaded; m/s)) along 42.5◦ N(a), and 126.5◦ E(b) vertical cross section of Figure 9. Vertical cross section of vertical velocity (shaded; m/s)) along 42.5° N (a), and 126.5° E (b) vertical cross section ◦ c ◦ d of u (shaded; m/s)u (shaded; along 42.5 m/s)° N along (c), and 42.5 v(shaded;N( ), and m/s) v(shaded; along 126.5 m/s)° E along (d) at 126.5 19 UTCE( on) 14 at 19August UTC on2019. 14 August 2019.

3.5. Thermal and Instability Characteristics At 1419, at 850 hPa, the high energy tongue is transported from the center of the ty- phoon 1910 along the southwest airflow of the typhoon-inverted trough and the edge of the subtropical high to the northeast. Figure 10a shows, at 1419, along 42.5° N the zonal vertical profile of pseudo equivalent potential temperature and specific humidity, be- tween 120–124° E, at the height of 400–850 hPa θse is the low-value area, less than 320 K, which indicates that dry and cold air intrudes from the west. The specific humidity above 700 hPa is less than 1 g/kg. From the west to the east of 120° E, the dry and cold air is obliquely intruded into the high humidity center between 124–135° E from the middle troposphere. The maximum value of θse at the center is greater than 352 k, and the specific humidity at 850 hPa is more than 14 g/kg. At 1421, the maximum vertical velocity between 42–44° N 124–126° E is more than 1 m/s. The center area of the low value of θse is located at 700 hPa, which indicates that the area below is a convective instability area. It can be

Atmosphere 2021, 12, x FOR PEER REVIEW 15 of 20

seen that this area is in the convective instability area before and when the rainstorm oc- curs. As long as the upward movement exists, the convective systems will be triggered and the convective upward motion will be maintained and strengthened. Figure 10b shows, at 1419, the meridional vertical profile of pseudo equivalent potential temperature and specific humidity along 126.5° E, and it can be seen that to the south of 44° N under 700 hPa, θse is greater than 348 K and the warm moist air with specific humidity greater than 16 g/kg extends 47° N northward with the southerly wind. There is a high-value area at 41–43° N. The low value of θse is between the height of 500–700 hPa to the north of 45° N, the θse is 316 K at and the altitude is about 650 hPa. The intensified dry and cold air intrudes obliquely from the middle and upper levels. The front area at the junction of warm and humid air is stable at 41–47° N. The center of the low value of θse is located at 3–4 km height. The convective instability is strengthened in the middle and low tropo- sphere, which leads to more intense precipitation. As the unstable state continues, there is still strong precipitation during 1420–1422. Atmosphere 2021, 12, 120 15 of 19

Figure 10. Vertical cross section of pseudo equivalent potential temperature (solid line; K) and specific humidity (shaded; Figure 10. Vertical cross section of pseudo◦ equivalent potential◦ temperature (solid line; K) and specific humidity (shaded; g/kg) and along 42.5°g/kg) N and (a),along and 126.5° 42.5 N(E (ba)), at and 19 126.5UTC onE( 14b) August at 19 UTC 2019. on 14 August 2019.

3.6. Topographic3.6. Effect Topographic Effect Figure 11 showsFigure the topographic 11 shows the dist topographicribution map distribution of Northeast map China of Northeast, the first China,stage the first stage precipitation ofprecipitation the residual ofvortex the residualof typhoon vortex 1909 of (Lekima) typhoon occur 1909 (Lekima)red in Huadian occurred of Jilin in Huadian of Jilin Province, whichProvince, is located which at the is leeward located at slope the leeward of the northwest slope of the part northwest of Changbai part ofMoun- Changbai Mountain, tain, and the topographicand the topographic effect is significant. effect is significant. However, However, the second the second and third and stages third stages of of precipita- precipitation occurredtion occurred in Gaizhou in Gaizhou of Liaoning of Liaoning Province Province and Huachuan and Huachuan of Heilongjiang of Heilongjiang Province Atmosphere 2021, 12, x FOR PEER REVIEWrespectively, which are plain areas with very flat terrain, and the role of topography16 of 20 is Province respectively, which are plain areas with very flat terrain, and the role of topog- raphy is relativelyrelatively small [12] small. [12].

FigureFigure 11. 11.TopographicTopographic height height map map of ofNortheast Northeast China. China.

In Inorder order to further to further study study the role the roleof the of topography the topography of Changbai of Changbai Mountain Mountain at the first at the ◦ heavyfirst rainfall heavy rainfallstage, a stage,meridional a meridional section along section 126.5° along E was 126.5 madeE was (Figure made 12) (Figure [14]. Figure 12)[14 ]. 12a shows that the divergence distribution at 1419 the whole layer above the high terrain is almost a southerly flow before the typhoon landing, and the updraft is not obvious. There is a small convergence area −10 × 10−5 s−1 in the lower layer of the windward slope on the south side of the high terrain. On the leeward slope of Changbai Mountain, there is a convergence area −10 × 10−5 s−1 in the middle troposphere, and a large strong diver- gence area 15 × 10−5 s−1 in the upper troposphere. Figure 12b shows, at 1420, as typhoon 1909 moves closer to the land, affected by the terrain uplift, the convective systems were uplifted continuously on the windward slope of the south side of the high terrain. The range of convergence area in the middle troposphere of the leeward slope is enlarged. Figure 12c shows, at 1421, the convective systems are still excited continuously on the windward slope of the south side of the high terrain. However, driven by the northward movement of the upper southerly weather system, the upper troposphere divergence col- umn moves northward, and the height of the convergence column below it decreases. As a result, the convective intensity in the rainstorm area is weakened.

Atmosphere 2021, 12, 120 16 of 19

Figure 12a shows that the divergence distribution at 1419 the whole layer above the high terrain is almost a southerly flow before the typhoon landing, and the updraft is not obvious. There is a small convergence area −10 × 10−5 s−1 in the lower layer of the windward slope on the south side of the high terrain. On the leeward slope of Changbai Mountain, there is a convergence area −10 × 10−5 s−1 in the middle troposphere, and a large strong divergence area 15 × 10−5 s−1 in the upper troposphere. Figure 12b shows, at 1420, as typhoon 1909 moves closer to the land, affected by the terrain uplift, the convective systems were uplifted continuously on the windward slope of the south side of the high terrain. The range of convergence area in the middle troposphere of the leeward slope is enlarged. Figure 12c shows, at 1421, the convective systems are still excited continuously on the windward slope of the south side of the high terrain. However, driven by the northward movement of the upper southerly weather system, the upper troposphere divergence column moves northward, and the height of the convergence column below it decreases. As a result, the convective intensity in the rainstorm area is weakened.

Figure 12. vertical profile of the resultant wind (Plume) along longitude 126.5◦ E divergence (shaded, unit: 10−5 s−1), v and w * 20 (barbs; m/s) at (a) 19 (b) 20 (c) 21 UTC on 14 August 2019.

Figure 13a shows, at 1419, that there are two convection systems along latitudi- nal 42.5◦ N in the vertical direction between 120–130◦ E. The convective system in the 123–125◦ E is caused by the westerly trough, with the convergence column in the west and the divergence column in the east, the divergence column tilts above the convergence column to form an inclined convective system. The convective systems are excited by the

2 Atmosphere 2021, 12, 120 17 of 19

typhoon-inverted trough in the east of 126–128◦ E, and the convective system delivers from the west, when they pass through the windward slope, because of the terrain effect and the vertical upward motion of the western edge of the subtropical high is very strong, the vertical motion is enhanced due to the strengthened convection and the pumping effect of the upper troposphere divergence center. Figure 13b shows, at 1420, with the westerly upper trough moving closer to the mountainous area, due to the topographic uplift, the convective system is uplifted on the windward slope of the west side of the highland. Atmosphere 2021, 12, x FOR PEER REVIEW Figure 13c shows, at 1421, due to the topographic uplift, that the convective18 of 20 system is

uplifted in the windward slope on the west side of the highland. Due to the weakening of the convective convergence column in the west, and the vertical upward motion of the subtropical highsubtropical edge decreases, high edge which decreases, leads to which the weakening leads to the of weakeningconvective ofactivity convective and activity and the decrease in precipitation in the rainstorm area. the decrease in precipitation in the rainstorm area.

−5 −1 Figure 13. VerticalFigure profile 13. Verticalof the resultant profile ofwind the (Plume) resultant along wind latitudinal (Plume) along 42.5° latitudinalN divergence 42.5 (shaded,◦ N divergence unit: 10 (shaded,s ), u and unit: 10−5 s−1), u w * 20 (barbs; m/s)and at w ( *a) 20 19:00 (barbs; (b) m/s)20:00 at(c) ( a21:00) 19:00 UTC (b) on 20:00 14 August (c) 21:00 2019. UTC on 14 August 2019.

4. Conclusions 4. Conclusions A heavy rainstorm occurred in Northeast China from 14 to 17 August 2019. In this A heavy rainstorm occurred in Northeast China from 14 to 17 August 2019. In paper, a synoptic method is used to study the weather situation, and the numerical simu- this paper, a synoptic method is used to study the weather situation, and the numerical lation results are studied. Main conclusions are drawn as follows: simulation results are studied. Main conclusions are drawn as follows: 1. The precipitation in the first stage is caused by the interaction between the residual 1. The precipitation in the first stage is caused by the interaction between the residual vortex of typhoon 1909 (Lekima) and its inverted trough and the upper trough. The vortex of typhoon 1909 (Lekima) and its inverted trough and the upper trough. The rain belt extends from southwest to northeast, and the rainfall center has typical rain belt extends from southwest to northeast, and the rainfall center has typical meso-β-scale characteristics. At the second stage, the upper cold vortex falls to the meso-β-scale characteristics. At the second stage, the upper cold vortex falls to the south, the precipitation is caused by the interaction between the cold vortex and the residual circulation of typhoon 1909 and 1910. 2. Before the occurrence of the first 1 h precipitation peak, the upper cold vortex was split into two parts, then the cold air flows southward along with the transverse trough, thus forming an extrusion between the subtropical high. Due to the organi- zation of the residual vorticity of typhoon 1909, a vortex-like belt composed of con- vergence centers is formed near the rainstorm center. There is a positive vorticity belt in the typhoon-inverted trough. The center of heavy rain is located in front of the positive vorticity center. The secondary circulation forms between the ascending area of the inverted trough, and the sinking center at the inner edge of the subtropical high and the west side of the inverted trough. The water vapor is concentrated in the

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south, the precipitation is caused by the interaction between the cold vortex and the residual circulation of typhoon 1909 and 1910. 2. Before the occurrence of the first 1 h precipitation peak, the upper cold vortex was split into two parts, then the cold air flows southward along with the transverse trough, thus forming an extrusion between the subtropical high. Due to the organization of the residual vorticity of typhoon 1909, a vortex-like belt composed of convergence centers is formed near the rainstorm center. There is a positive vorticity belt in the typhoon-inverted trough. The center of heavy rain is located in front of the positive vorticity center. The secondary circulation forms between the ascending area of the inverted trough, and the sinking center at the inner edge of the subtropical high and the west side of the inverted trough. The water vapor is concentrated in the middle and low troposphere. Starting from the center of typhoon 1910, the water vapor and energy are transported to the northeast along the southwest airflow of the typhoon-inverted trough and the edge of the subtropical high. The upper trough intersects with the residual vortex trough of typhoon 1909, and convective systems are triggered, thus resulting in three times consecutive 1 h heavy rainfalls in the central and eastern part of Jilin Province. 3. There is a zone tilted downward from northwest to southeast that contains dry and cold air, and it is in a convectively unstable state. Between the vertical direction of 1500–6000 m, the heavy rainfall center is located on the line between the maximum and minimum centers of vertical wind shear, which is close to the right of the max- imum center. Upper westerly momentum propagates downward, the center of the rainstorm is located at the west of the downshear, and the upper southerly momen- tum propagates downward. The system tilts northward with the height, and the rainstorm area is located at the south side of the downshear. The divergent column tilts above the convergence column thus form the inclined convective systems. 4. The heavy rainfall area is located in the leeward slope of the northwest of Changbai Mountain. The strong pumping effect is formed between the upper strong divergence area and the middle convergence area, which strengthens the vertical velocity, and the terrain effect is significant. The upper cold vortex fell to the south, and the circulation of typhoon 1910 merges into the residual vortex of typhoon 1909, which results in the second 1 h precipitation peak. The fusion of typhoon 1909, 1910 residual vortices and the cold vortices at the lower layer, which results in the third 1 h precipitation peak.

Author Contributions: Y.W. initiated the study and did the analysis of meteorological conditions, numerical simulation, dynamic diagnosis and analysis; wrote the original draft. T.W. contributes to full text discussion, checking and proofreading. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the National key R & D Program Fund of the Ministry of science and technology. (Grant No. 2018YFC1507301). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Due to the amount of data is very large, the data are not publicly available. Acknowledgments: We thank to China Meteorological Administration for providing surface and sounding observation data, hourly precipitation observation data, NCAR for providing NCEP reanal- ysis data, WRF version 4.0 mesoscale numerical prediction model and NCL mapping software, Kochi University of Japan for providing hmw-8 cloud images. We are grateful to the High Performance Computing Center (HPCC) of Nanjing University for doing the numerical calculations in this paper on its blade cluster system. Conflicts of Interest: The authors declare no conflict of interest. Atmosphere 2021, 12, 120 19 of 19

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