A Numerical Study of Windstorms in the Lee of the Taebaek Mountains, South Korea: Characteristics and Generation Mechanisms

atmosphere

Article A Numerical Study of Windstorms in the Lee of the Taebaek Mountains, South Korea: Characteristics and Generation Mechanisms

Joohyun Lee 1, Jaemyeong Mango Seo 1,2, Jong-Jin Baik 1, Seung-Bu Park 1 and Beom-Soon Han 1,* 1 School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Korea; [email protected] (J.L.); [email protected] (J.M.S.); [email protected] (J.-J.B.); [email protected] (S.-B.P.) 2 Precipitating Convection Group, Max Planck Institute for Meteorology, 20146 Hamburg, Germany * Correspondence: [email protected]

Received: 19 February 2020; Accepted: 22 April 2020; Published: 24 April 2020

Abstract: The Yeongdong region, located east of the Taebaek Mountains, South Korea, often experiences severe windstorms in spring, causing a lot of damages, especially when forest fires spread out rapidly by strong winds. Here, the characteristics and generation mechanisms of the windstorms in the Yeongdong region on 8 April 2012 are examined through a high-resolution Weather Research and Forecasting (WRF) model simulation. In the Yangyang area, the steep descent of the isentropes on the lee slope of the mountain and their recovery farther leeward are seen. Inversion layers and incoming flow in hydraulic jump regime suggest that the hydraulic jump is responsible for the downslope windstorm. In the Jangjeon area, the plume-shaped wind pattern extending seaward from the gap exit is seen when the sea-level pressure difference between the gap inside and the gap exit, being responsible for the gap winds, is large. In the Uljin area, downslope windstorms pass over the region with weak wind, low Richardson number, and deep planetary boundary layer (PBL), making banded pattern in the wind and PBL height fields. This study demonstrates that the characteristics of the windstorms in the lee of the Taebaek Mountains and their generation mechanisms differ depending on local topographic features.

Keywords: downslope windstorms; hydraulic jump; gravity waves; gap winds; planetary boundary layer

1. Introduction There are many meteorological phenomena directly associated with topography, which include mountain/valley winds, downslope windstorms, gap winds, lee waves, lee vortices, cold-air damming, and banner clouds [1]. When winds blow down on the lee slope of a mountain, they become strong, occasionally producing severe downslope windstorms. Downslope windstorms are observed in many regions of the world, and they are called with local names, for example, föhn in the Alps, bora in the Adriatic, and chinook in the Rocky Mountains. When winds blow through a gap of mountains, strong winds, called gap winds, can be produced. Gap winds are also observed in many regions of the world, such as the Strait of Juan de Fuca and the Rhine region. Downslope windstorms and gap winds have received considerable attention because of their fascinating nature as well as damages they cause. Three diﬀerent mechanisms have been proposed to explain the generation/development of downslope windstorms. Summaries of the mechanisms are given in [1,2]. The hydraulic theory, based on [3], suggests that there is a similarity between hydraulic jumps and downslope windstorms and that downslope windstorms can be generated when a transition from subcritical ﬂow to supercritical

Atmosphere 2020, 11, 431; doi:10.3390/atmos11040431 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 431 2 of 16

flow occurs. For the atmospheric flow to undergo the transition, some circumstance, such as wave breaking, capping by a mean-state critical layer, or Scorer-parameter layering, is required [2]. The other two generation mechanisms for downslope windstorms, which are related to mountain waves, are the partial reflection and the critical-level reflection. Eliassen and Palm [4] found that when upward propagating (internal) gravity waves encounter a region where the Scorer parameter changes rapidly with height, the waves are partly reflected. Klemp and Lilly [5] considered a multi-layer atmosphere and suggested that severe downslope windstorms can occur when reflected downward propagating gravity waves are optimally superposed with upward propagating gravity waves. Clark and Peltier [6] found that downslope windstorms occur after upward propagating gravity waves break. At the wave-breaking region, which is characterized by a locally reversed flow and the Richardson number smaller than 0.25, incident gravity waves are reflected rather than absorbed. The reflection of upward propagating gravity waves at the critical layer leads to amplifying resonant gravity waves, producing downslope windstorms. In addition, they found that for reflected and upward propagating gravity waves to interfere constructively and resonate, the wave-induced critical layer should be located at heights of z = (n + 3/4) λz, where n = 0, 1, 2, ... and λz is the vertical wavelength of the hydrostatic gravity wave. The generation/development mechanisms for gap winds have also been proposed. In the beginning, a funnel effect, also known as the Venturi effect, was proposed. According to [7], when air enters a gap, the speed of the air becomes high within the gap due to mass conservation. However, in reality, gap winds are in general strongest near the exit of a gap, not in the inner region of the gap. To explain this, a mechanism taking account of the pressure gradient around an exit was proposed [8,9]. When the flow escapes from a gap, the depth of the flow becomes shallow in order to conserve mass and accordingly, the pressure at an exit decreases. The resultant horizontal pressure gradient produces the strongest winds near the exit of the gap. Many numerical modeling studies have been performed to explain the generation/development of observed downslope windstorms [10–15]. Decker and Robinson [10] showed that the downslope windstorm in New Jersey is caused by hydraulic jump or trapped lee waves, using a regime diagram suggested by [16], and partial reflection is not enough to explain the observed wind speed. Tollinger et al. [15] showed that the maximum wind speed in the case of March 1972 northwest Greenland windstorm is mainly explained by hydraulic jump and wave breaking contributes to maximum wind speed. Shestakova et al. [14] also showed that Novorossiysk bora winds can be explained by combined mechanisms of hydraulic jump and wave breaking. The relative importance of these mechanisms depends on elevated inversion-layer strength and mean-state critical-level height. The Yeongdong region, which is located east of the Taebaek Mountains, South Korea, often experiences severe downslope windstorms in spring. Springtime windstorms occurring in this region have been historically called Yang-Gan Winds, Yang-Gan denoting Yangyang and Ganseung which belong to the Yeongdong region. Since the damages caused by dry and strong Yang-Gan winds are severe, many studies on Yang-Gan winds have been conducted. Lee [17] showed that downslope windstorms in the Yeongdong region on 11 February 1996 are mainly caused by the hydraulic jump mechanism rather than the partial reflection of gravity waves at the tropopause, based on a numerical simulation. Jang and Chun [18] investigated mechanisms for downslope windstorms in Gangneung, the Yeongdong region. Using surface and upper-air sounding data, they analyzed 92 cases with the observed maximum instantaneous wind speed exceeding two standard deviations of the total mean and showed that most of the cases can be explained by the hydraulic jump, partial reflection or critical-level reflection mechanism. Lee and In [19] conducted numerical sensitivity experiments of downslope windstorms to inversion layer and found that the strong windstorms are generated when the thickness of the inversion layer is thin and the altitude of the inversion layer, which causes maximum wind speeds, differs depending on the atmospheric stability. Since topographic features in the Yeongdong region are diverse, generation mechanisms for windstorms in this region can differ depending on local topographic features. In this study, we examine Atmosphere 2020, 11, 431 3 of 16 the characteristics of the 8 April 2012 windstorms in the lee of the Taebaek Mountains and their generation mechanisms according to local topographic features through a high-resolution numerical model simulation. In Section2, the synoptic analysis for the selected case is presented. In Section3, the numerical model and the simulation design are described. In Section4, the simulation results are Atmosphere 2020, 11, x FOR PEER REVIEW 3 of 16 presented and discussed. In Section5, a summary and conclusions are given. numerical model simulation. In Section 2, the synoptic analysis for the selected case is presented. In 2. SynopticSection Analysis3, the numerical model and the simulation design are described. In Section 4, the simulation Theresults 8 Aprilare presented 2012 windstorm and discussed. case In isSect analyzedion 5, a summary in this study.and conclusions Figure1 areshows given. the surface and 850-hPa weather charts at 0900 LT (local time = UTC + 9 h) and 2100 LT on this day. The locations 2. Synoptic Analysis of Yangyang, Jangjeon, Uljin, and Osan stations and topographic features are shown in Figure2b. At 0900 LT,Thewhich 8 April is 2012 a time windstorm before the case maximum is analyzed instantaneous in this study. Figure wind speed1 shows is the observed surface and at Yangyang 850- station,hPa the weather low andcharts high at 0900 pressure LT (local systems time = areUTC located + 9 h) and north 2100 and LT southon this of day. the The Korean locations Peninsula, of Yangyang, Jangjeon, Uljin, and Osan stations and topographic features are shown in Figure 2b. At respectively (Figure1a). This synoptic pressure pattern is favorable for strong westerly winds in 0900 LT, which is a time before the maximum instantaneous wind speed is observed at Yangyang spring.station, The the analysis low and of high radiosonde pressure data systems at Osan, are located an upstream north and station, south of reveals the Korean that thePeninsula, prevailing windrespectively direction in (Figure the lower 1a). This atmosphere synoptic onpressure 8 April pattern 2012 isis westerlyfavorable tofor southwesterly, strong westerlyso winds it is almostin perpendicularspring. The to analysis the ridge of radiosonde line of the data Taebaek at Osan, Mountains an upstream which station, is reveals elongated that the approximately prevailing wind in the northwest-southeastdirection in the lower direction atmosphere in the middle on 8 April eastern 2012 part is westerly of South to Korea. southwesterly, This is a goodso it is condition almost for the generationperpendicular of downslope to the ridge windstorms. line of the Taebaek The northern Mountains low which moves is elongated southeastward, approximately and the in southern the high movesnorthwest-southeast eastward. The direction pressure in the gradient middle in eastern the middle part of easternSouth Korea. part ofThis South is a good Korea condition becomes for larger at 2100the LT generation (Figure1 ofb). downslope The 850-hPa windstorms. weather The charts nort (Figurehern low1c,d) moves also southeastward, show a similar and synoptic the southern pressure high moves eastward. The pressure gradient in the middle eastern part of South Korea becomes larger pattern. In addition, the 850-hPa temperature becomes high over South Korea with time (Figure1c,d), indicating the 850-hPa level near the mountain top height becomes more stable.

FigureFigure 1. Surface 1. Surface weather weather charts charts at at (a ()a 0900) 0900 LT LT andand (b)) 2100 2100 LT LT and and 850-hPa 850-hPa weather weather charts charts at (c) at 0900 (c) 0900 LT andLT ( dand) 2100 (d) LT2100 on LT 8 Aprilon 8 April 2012. 2012. In (a ,Inb), (a the) and mean (b), sea-levelthe mean pressuresea-level (bluepressure solid (blue lines) solid is contouredlines) is in contoured in 4 hPa intervals. In (c) and (d), the geopotential height (blue solid lines) is contoured in 4 hPa intervals. In (c,d), the geopotential height (blue solid lines) is contoured in 30 gpm intervals and 30 gpm intervals and the temperature (red dashed lines) is contoured in 3 °C intervals. These weather the temperature (red dashed lines) is contoured in 3 C intervals. These weather charts are provided by charts are provided by the Korea Meteorological Administration◦ [20]. The yellow and green marks the Koreaon the Meteorological cloud coverage symbol Administration in (a) and (b [20) represent]. The yellow the fog andand greenrain, respectively. marks on the cloud coverage symbol in (a,b) represent the fog and rain, respectively. On 8 April 2012, the 10-m maximum instantaneous wind speed is 11.3 m s−1 at Osan station and 22.2 m s−1 at Yangyang station, showing a large difference in wind speed between the upstream and Atmosphere 2020, 11, 431 4 of 16

1 On 8 April 2012, the 10-m maximum instantaneous wind speed is 11.3 m s− at Osan station and Atmosphere 12020, 11, x FOR PEER REVIEW 4 of 16 22.2 m s− at Yangyang station, showing a large diﬀerence in wind speed between the upstream and downstream stations of the Taebaek Mountains. This implies that the strong winds on the lee side are downstream stations of the Taebaek Mountains. This implies that the strong winds on the lee side directly related to the mountain. are directly related to the mountain. 3. Model and Simulation Design 3. Model and Simulation Design In this study, the Weather Research and Forecasting (WRF) model version 3.8.1 [21] is used to simulateIn this the study, windstorms. the Weather The model Research domain and conﬁguration, Forecasting (WRF) the terrain model height version in the 3.8.1 innermost [21] is domain,used to andsimulate the areas the windstorms. and cross-section The model lines for domain analysis configuration, are presented the in terrain Figure 2height. Five in nested the innermost domains centereddomain, onand the the Yeongdong areas and region cross-section are considered. lines for The analysis horizontal are resolutionspresented in of domainsFigure 2. 1,Five 2, 3, nested 4, and 5domains are 27 km centered (160 on160 the grid Yeongdong points), 9 region km (331 are considered.331), 3 km (601The horizontal601), 1 km resolutions (901 901), of domains and 333 m1, × × × × (10002, 3, 4, and1000), 5 are respectively. 27 km (160 There × 160 are grid 59 points), layers in 9 thekm vertical(331 × 331), direction. 3 km (601 The vertical× 601), 1 grid km size,(901 × which 901), × isand stretched 333 m (1000 with × height, 1000), isrespectively. ~70 m in the There lowest are model 59 layers layer in andthe vertical ~410 m direction. in the highest The modelvertical layer. grid Thesize, time which steps is stretched for domains with 1, height, 2, 3, 4, andis ~70 5 are m 60,in the 20, lowest 6.66, 2.22, model and layer 0.3 s, and respectively. ~410 m in The the model highest is integratedmodel layer. over The 36 time h from steps 2100 for LT domains on 7 April 1, 2, to 3, 0900 4, and LT 5 on are 9 April60, 20, 2012, 6.66, and2.22, the and data 0.3 ons, respectively. 8 April 2012 areThe analyzed. model is integrated over 36 h from 2100 LT on 7 April to 0900 LT on 9 April 2012, and the data on 8 April 2012 are analyzed.

Figure 2. ((aa)) Five Five nested nested domains domains and and (b ()b terrain) terrain height height (m) (m) in the in theinnermost innermost domain domain (domain (domain 5). The 5). Theareas areas enclosed enclosed by the by red the redlines, lines, A1 and A1 and A2, A2,include include Jangjeon Jangjeon and and Uljin Uljin stations. stations. The The red redsolid solid lines lines C1 C1and and C2 C2are arepassing passing Yangyang Yangyang and and Osan Osan stations stations and and Uljin Uljin station, station, respectively. respectively.

For thethe initialinitial andand boundaryboundary conditions,conditions, thethe ERAERA interiminterim reanalysisreanalysis datadata withwith 0.750.75°◦ horizontalhorizontal resolutionresolution [[22]22] areare used.used. TheThe ShuttleShuttle RadarRadar TopographyTopography MissionMission (SRTM)(SRTM) datadata with with a a resolution resolution of of 3 300˝ are usedused toto representrepresent fine-scalefine-scale topographictopographic featuresfeatures [[23].23]. TheThe followingfollowing physicalphysical parameterizationparameterization schemesschemes areare selectedselected forfor thethe windstormwindstorm simulation:simulation: the Mellor-Yamada-JanjićplanetaryMellor-Yamada-Janjić planetary boundaryboundary layerlayer schemescheme [[24],24], thethe unifiedunified NoahNoah landland surfacesurface modelmodel [[25],25], thethe WRFWRF single-momentsingle-moment 6-class6-class cloudcloud microphysics schemescheme [[26],26], the Dudhia shortwave radiation scheme [[27],27], the Rapid Radiative Transfer Model (RRTM)(RRTM) longwave longwave radiation radiation scheme scheme [28 [28],], and and the the Kain–Fritsch Kain–Fritsch cumulus cumulus convection convection scheme scheme [29], which[29], which is applied is applied only toonly domains to domains 1 and 1 2. and 2.

4. Results and Discussion

4.1. Validation Using radiosonde radiosonde data data at at Osan Osan station, station, which which is located is located at the at upwind the upwind side of side the area of the of interest, area of interest,the vertical the profiles vertical of proﬁles the simulated of the simulated wind speed wind and speed air temperature and air temperature are validated are validated (Figure 3). (Figure At 09003). AtLT, 0900 although LT, although the WRF the model WRF overestimates model overestimates the wind the speed wind above speed a height above of a height~1 km, ofthe ~1 simulated km, the wind speed profile is similar to the observed profile. The model also reproduces the temperature profile well. The WRF results and observation both show the elevated inversion layer below a height of ~1 km, and the simulated inversion layer is thicker than the observed inversion layer. It seems that the difference of the inversion layer thickness is to some extent caused by the different vertical

Atmosphere 2020, 11, 431 5 of 16 simulated wind speed profile is similar to the observed profile. The model also reproduces the temperature profile well. The WRF results and observation both show the elevated inversion layer below a height of ~1 km, and the simulated inversion layer is thicker than the observed inversion layer. It seems that the difference of the inversion layer thickness is to some extent caused by the different verticalAtmosphere resolutions 2020, 11, x of FOR the PEER radiosonde REVIEW data and WRF model. Below a height of ~1 km, the5 ofvertical 16 resolution of the WRF model stretches with height from ~70 m to ~200 m, but the vertical resolution of resolutions of the radiosonde data and WRF model. Below a height of ~1 km, the vertical resolution the radiosonde data varies irregularly from ~20 m to ~400 m. At 2100 LT, the simulated wind speed of the WRF model stretches with height from ~70 m to ~200 m, but the vertical resolution of the above a height of ~1.5 km is slightly higher than the observation. The overall pattern of the simulated radiosonde data varies irregularly from ~20 m to ~400 m. At 2100 LT, the simulated wind speed above winda speed height is of similar ~1.5 km to is theslightly observation. higher than The the simulated observation. temperature The overall profilepattern isof alsothe simulated well matched wind with the observed one, except below a height of ~500 m.

FigureFigure 3. Vertical 3. Vertical proﬁles profiles of windof wind speeds speeds at at (a ()a 0900) 0900 LT LT and and ( b(b)) 21002100 LT and temperatures temperatures at at (c ()c 0900) 0900 LT and (LTd) 2100and (d LT) 2100 at Osan LT at station. Osan station. The black The black and and red red lines lin indicatees indicate the the radiosonde radiosonde observation observation data data and WRFand simulation WRF simulation data, respectively. data, respectively.

FigureFigure4 shows 4 shows the the time time series series of of the the 10-m 10-m instantaneousinstantaneous wind wind speed speed and and wind wind direction direction at at Yangyang, Uljin, and Osan stations. At Yangyang station, there are two periods in which strong Yangyang, Uljin, and Osan stations. At Yangyang station, there are two periods in which strong winds appear in both the observation and the simulation: around 0300 LT and 1800 LT on 8 April. windsThe appear maximum in both wind the speed observation in the first andperiod the is simulation:15.9 m s−1 at 0220 around LT in 0300 the observation, LT and 1800 and LT it onis 17.3 8 April. 1 The maximumm s−1 at 0400 wind LT inspeed the simulation. in the first In the period first period, is 15.9 the m smodel− at 0220well simulates LT in the the observation, maximum wind and it is 1 17.3 mspeed s− at which 0400 is LT 1.4 in m the s−1 stronger simulation. and occur In the 1first h 40 period,m later compared the model to well the observation. simulates the The maximum maximumwind 1 speedwind which speed is 1.4 in mthe s second− stronger period and is 22.2 occur m s 1−1 h at 40 1950 mlater LT in compared the observation, to the and observation. it is 20.5 m Thes−1 at maximum 1520 1 −1 1 windLT speed in the in simulation. the second The period maximum is 22.2 wind m sin− theat second 1950 LT period in the is observation,simulated to be and 1.7 itm is s 20.5 weaker m s − at 1 1520 LTand in occur the simulation. 4 h 30 m earlier The maximumcompared to wind the observation. in the second The period reason is simulatedfor the shift to in be the 1.7 maximum m s− weaker and occurwind 4speed h 30 can m earlier be attributed compared to the to fact the that observation. the upwind The flow reason in the for simulation the shift is in enhanced the maximum earlier wind speedthan can that be attributedin the observation. to the factThe simulated that the upwind 10-m wind flow direction in the is simulation well matched is enhancedwith the observed earlier than one. The wind direction at Yangyang station is mainly westerly to southwesterly for both the simulation and the observation. Atmosphere 2020, 11, x FOR PEER REVIEW 6 of 16

The observed 10-m instantaneous wind speed at Uljin station is generally weaker than that at Yangyang station. However, the strong winds at Uljin station also appear in two periods on 8 April. The strong winds in the first period appear around 1040 LT in both the observation and the simulation. The model simulates the maximum wind speed in the first period as 15.7 m s−1, which is very close to the observed value of 16.2 m s−1. However, the maximum wind speed in the second period, which is 15.1 m s−1 at 1620 LT in the observation, fails to be simulated. The reasons for this will be explained in Subsection 4.5. The wind direction at Uljin station is southerly to southwesterly Atmospherefor both2020 the, 11 ,simulation 431 and the observation. 6 of 16 Osan station, located at the upwind side, has relatively low 10-m wind speed compared to Yangyang and Uljin stations. It clearly shows that the wind speed increases in the downwind side of that inthe the mountains observation. like Yangyang The simulated and Uljin 10-m stations. wind The direction model reproduces is well matched this wind with speed the difference observed one. The windwell. Although direction the at observed Yangyang high-frequency station is mainly fluctu westerlyation of the to 10-m southwesterly wind direction for does both not the appear simulation and thein the observation. simulation, the model well simulates the overall trend of the 10-m wind direction at Osan station.

Figure 4. Time series of (a–c) the 10-m instantaneous wind speed and (d–f) the 10-m instantaneous Figure 4. Time series of (a–c) the 10-m instantaneous wind speed and (d–f) the 10-m instantaneous wind direction at (a,d) Yangyang, (b,e) Uljin, and (c,f) Osan stations. The black and red lines indicate wind direction at (a,d) Yangyang, (b,e) Uljin, and (c,f) Osan stations. The black and red lines indicate the observed data and simulation results, respectively. The blue dashed line in (b,e) indicates the time the observed data and simulation results, respectively. The blue dashed line in (b,e) indicates the time series of the simulated 10-m wind speed and the wind direction at the location 10Δx (3.33 km) west seriesof of Uljin the station. simulated 10-m wind speed and the wind direction at the location 10∆x (3.33 km) west of Uljin station. Although there are some limitations in simulating the maximum wind speeds at Yangyang and UljinThe observedstations, they 10-m are instantaneoussimulated within wind an error speed of 2 at m Uljin s−1 in stationwind speed, is generally except for weaker the maximum than that at Yangyangwind speed station. in the However, second period the strong at Uljin winds station. at The Uljin overall station trend also of the appear simulated in two 10-m periods wind speed on 8 April. The strongis also similar winds into the observed ﬁrst period one, appear although around the maximum 1040 LT wind in both speed the at observation Yangyang station and the appears simulation. 1 The modelat different simulates times the in maximumthe simulation wind and speed observ in theation. ﬁrst The period 10-m as wind 15.7 mdirection s− , which shows is verya good close to agreement between the simulation1 and the observation. Thus, it would be appropriate to examine the observed value of 16.2 m s− . However, the maximum wind speed in the second period, which is the characteristics1 of the 8 April 2012 windstorms and their generation mechanisms using the 15.1 m s− at 1620 LT in the observation, fails to be simulated. The reasons for this will be explained in simulation data. Section 4.5. The wind direction at Uljin station is southerly to southwesterly for both the simulation and the observation.

Osan station, located at the upwind side, has relatively low 10-m wind speed compared to Yangyang and Uljin stations. It clearly shows that the wind speed increases in the downwind side of the mountains like Yangyang and Uljin stations. The model reproduces this wind speed difference well. Although the observed high-frequency fluctuation of the 10-m wind direction does not appear in the simulation, the model well simulates the overall trend of the 10-m wind direction at Osan station. Although there are some limitations in simulating the maximum wind speeds at Yangyang and 1 Uljin stations, they are simulated within an error of 2 m s− in wind speed, except for the maximum wind speed in the second period at Uljin station. The overall trend of the simulated 10-m wind speed is also similar to the observed one, although the maximum wind speed at Yangyang station appears at different times in the simulation and observation. The 10-m wind direction shows a good agreement between the simulation and the observation. Thus, it would be appropriate to Atmosphere 2020, 11, 431 7 of 16

Atmosphere 2020, 11, x FOR PEER REVIEW 7 of 16 examine the characteristics of the 8 April 2012 windstorms and their generation mechanisms using the simulation4.2. Overall data. Characteristics of the Simulated Winds

4.2. OverallTo see Characteristics the overall characteristics of the Simulated of the Winds simulated winds, the 10-m wind speed and wind vector in the innermost domain are analyzed (Figure 5). At 0900 LT on 8 April, the winds are relatively strongTo see on the the overalllee slope characteristics of the Taebaek ofMountains the simulated (Figure winds, 5a). The the winds 10-m on wind the windward speed and side wind of the vector in themountain innermost range domain are relatively are analyzed weak. (Figure On the5). lee At of 0900 the LTmountain on 8 April, range, the there winds are are areas relatively of plume- strong on theshaped lee slope wind of pattern the Taebaek extending Mountains seaward, (Figure particularly5a). The noticeable winds north on the of windwardJangjeon station. side ofIn the East mountain rangeSea/Sea are relativelyof Japan, the weak. southerly On thewinds lee are of predomina the mountainnt. At range, 1200 LT, there the winds are areas on the of plume-shapedwindward side wind patternbecome extending stronger than seaward, at 0900 particularly LT and the winds noticeable blow approximately north of Jangjeon perpendicular station. Into the mountain East Sea /Sea of range (Figure 5b). This is largely because of the enhanced prevailing winds associated with the Japan, the southerly winds are predominant. At 1200 LT, the winds on the windward side become northern low and southern high (see Figure 1). Additionally, severe downslope windstorms in the strongerlee of the than Taebaek at 0900 Mountains LT and are the clearly winds evident. blow approximately At 1500 LT, just before perpendicular the simulated to the maximum mountain 10- range (Figurem wind5b). speed This appears is largely at Yangyang because station, of theenhanced the windstorm prevailing along the winds Taebaek associated Mountains with is extended the northern lowseaward and southern and intensified high (see (Figure Figure 5c).1 At). 1800 Additionally, LT, the banded severe pattern downslope of strong-wind windstorms regions and in the weak- lee of the Taebaekwind regions, Mountains which are is clearlyparallel to evident. the coastal At 1500line, appears LT, just near before Uljin the (Figure simulated 5d). maximum 10-m wind speed appearsTo examine at Yangyang these characteristics station, the in windstorm detail, we z alongoom in the on Taebaekthree areas Mountains to take advantage is extended of the seaward andhigh-resolution intensiﬁed (Figure simulation;5c). At(1) 1800Yangyang LT, the area banded for which pattern we focus of strong-windon the downslope regions windstorm and weak-wind (C1 regions,in Figure which 2b), is(2) parallel Jangjeon to area the for coastal which line, we focus appears on the near plume-shaped Uljin (Figure wind5d). pattern (A1 in Figure 2b), and (3) Uljin area for which we focus on the banded wind pattern (A2 and C2 in Figure 2b).

FigureFigure 5. 5.Simulated Simulated 10-m10-m wind speed speed and and wind wind vector vector fields ﬁelds at (a) at 0900 (a) LT, 0900 (b LT,) 1200 (b )LT, 1200 (c) LT,1500 ( cLT,) 1500 LT, and (d) 1800 LT. The red dots indicate the locations of Jangjeon, Yangyang, and Uljin stations. and (d) 1800 LT. The red dots indicate the locations of Jangjeon, Yangyang, and Uljin stations. 4.3. Downslope Windstorm (Yangyang) To examine these characteristics in detail, we zoom in on three areas to take advantage of the high-resolution simulation; (1) Yangyang area for which we focus on the downslope windstorm (C1 in Atmosphere 2020, 11, 431 8 of 16

Figure2b), (2) Jangjeon area for which we focus on the plume-shaped wind pattern (A1 in Figure2b), and (3) Uljin area for which we focus on the banded wind pattern (A2 and C2 in Figure2b).

4.3. Downslope Windstorm (Yangyang) Atmosphere 2020, 11, x FOR PEER REVIEW 8 of 16 For the analysis of the downslope windstorm in the Yangyang area, potential temperature, wind speed, windFor vector,the analysis and of vertical the downslope temperature windstorm gradient in the at Yangyang 0900 LT and area, 1500 potent LTial are temperature, plotted in the wind vertical cross-sectionspeed, wind along vector, C1 in and Figure vertical2b (Figure temperature6). Note gradie thatnt the at location0900 LT and of Osan 1500 stationLT are isplotted denoted in the by the red arrowvertical in cross-section Figure6. At 0900along LT, C1 thein Figure downslope 2b (Figure windstorm 6). Note appears that the along location/over of the Osan downslope station is of the Taebaekdenoted Mountains by the red (Figure arrow6 a).in Figure The isentropes 6. At 0900 (potentialLT, the downslope temperature windstorm isolines) appears descend along/over steeply the on the lee slopedownslope of the mountainof the Taebaek and Mountains recover farther (Figure leeward. 6a). The Thisisentropes structure (potential of isentropes temperature suggests isolines) that the hydraulicdescend jump steeply is responsible on the lee slope for the of downslopethe mountain windstorm and recover in farther the Yangyang leeward. area.This structure In addition, of the jump-likeisentropes motion suggests of the that inversion the hydr layeraulic over jump the is downslope responsible offor the the mountain downslope shows windstorm that the in hydraulic the Yangyang area. In addition, the jump-like motion of the inversion layer over the downslope of the jump is involved in the downslope windstorm (Figure6b) [ 30,31]. The downslope windstorm is mountain shows that the hydraulic jump is involved in the downslope windstorm (Figure 6b) [30,31]. enhancedThe downslope in the lee windstorm of the mountain is enhanced range in atthe 1500 lee of LT, the and mountain the structure range at of 1500 hydraulic LT, and jumpthe structure propagates leeward,of hydraulic passing jump Yangyang propagates station. leeward, These passing correspond Yangyang to the station. rapid increaseThese correspond of the 10-m to windthe rapid speed at Yangyangincrease from of the 0600 10-m LT wind to 1500 speed LT at (see Yangyang Figure 4froma). 0600 LT to 1500 LT (see Figure 4a).

FigureFigure 6. Vertical 6. Vertical cross-sections cross-sections of of the the potential potential temperature temperature (blue (bluelines in lines a,c), wind in a, cspeed), wind (a,c), speed wind (a,c), windvector vector (a (,ca),,c and), and vertical vertical temperature temperature gradient gradient (b,d) along (b,d) C1 along at (a C1,b) 0900 at (a ,LTb) 0900and (c LT,d) and1500 (LT.c,d )The 1500 LT. The referencereference point point on on the the horizontal horizontal axis axis(0 km) (0 indicates km) indicates the location the location of Yangyang of Yangyang station. The station. red arrow The red arrowon on the the horizontal horizontal axis indicates axis indicates the location the location of Osan ofstation. Osan The station. zero vert Theical zero temperature vertical gradient temperature lines are indicated by black lines. gradient lines are indicated by black lines.

TheThe hydraulic hydraulic jump jump in in the the Yangyang Yangyang areaarea is mainly caused caused by by the the elevated elevated inversion inversion near nearthe the mountain top height. At 0900 LT, the strong elevated inversions above the upslope and the ridge of mountain top height. At 0900 LT, the strong elevated inversions above the upslope and the ridge of the mountain range are clearly shown (Figure 6b). The elevated inversion can act as a free surface in the shallow water theory and help the atmospheric flow to undergo a transition from subcritical to

Atmosphere 2020, 11, 431 9 of 16 the mountain range are clearly shown (Figure6b). The elevated inversion can act as a free surface in Atmosphere 2020, 11, x FOR PEER REVIEW 9 of 16 the shallow water theory and help the atmospheric flow to undergo a transition from subcritical to supercriticalsupercritical flow flow [32]. [32]. At At 1500 1500 LT, LT, the the elevated elevated inversion inversion is is dissipated dissipated because because of diurnal of diurnal heating heating in in the daytimethe daytime (Figure (Figure6d) and6d) and the the downslope downslope windstorm windstormis is weakenedweakened after after 1500 1500 LT. LT. To quantitativelyTo quantitatively investigate investigate atmospheric atmospheric conditions conditions for for the the formation formation ofof thethe hydraulichydraulic jump, jump, the Fr-M thediagram Fr-M diagram [33] is shown [33] is shown in Figure in Figure7. The 7. Froude The Froude number numberFr Frand and the the dimensionless dimensionless height MM are givenare as given follows: as follows: U Fr== Up , (1) Fr g,H (1) gH′ 0 h M = m . (2) hH M = m . (2) Here, U is the wind speed, g0 = g∆θ/θ0 is theH reduced gravity where g is the gravitational acceleration,Here,∆ θU isis thethe potentialwind speed, temperature g′ = gΔθ/θ0 diis fftheerence reduced between gravity the where inversion g is the layer gravitational top and the inversionacceleration, layer bottom, Δθ is the and potentialθ0 is the temperature potential temperature, difference betweenH is the the inversion inversion layer layer top top height, and the and hm is theinversion mountain layer top bottom, height. and The θ inversion0 is the potential layer toptemperature, (bottom) H is is defined the inversion by the layer highest top height, (lowest) and model level of the layer in which the vertical temperature gradient is positive. U and θ are averaged from hm is the mountain top height. The inversion layer top (bottom) is defined by the0 highest (lowest) the surface to the inversion layer top height. Note that U is the wind speed in the C1 direction in model level of the layer in which the vertical temperature gradient is positive. U and θ0 are averaged Figurefrom2b. the The surface simulation to the inversion data at the layer location top height. of Osan Note stationthat U is are the used wind for speed the in calculation, the C1 direction and the data withoutin Figure the2b. The inversion simulation are data excluded. at the location The radiosonde of Osan station data are at Osanused for station the calculation, are also used and the for the calculation.data without In the theFr-M inversiondiagram, are excluded. regimes I,The II, III,radios andonde IV representdata at Osan subcritical station are flow, also hydraulicused for the jump, supercriticalcalculation. flow, In andthe Fr-M totally diagram, blocked regimes flow, I, respectively. II, III, and IVThe representFr-M subcriticaldiagram (Figureflow, hydraulic7) shows jump, that the simulatedsupercritical incoming flow, flows and totally before blocked 0400 LT, flow, when respectively. downslope The windstorm Fr-M diagram and (Figure the structure 7) shows of that hydraulic the jumpsimulated are not evident, incoming are flows mainly before in 0400 regime LT, when IV. Afterdownslope 0400 windstorm LT, the simulated and the structure incoming of hydraulic flows are in regimejump II. Theare not observed evident,incoming are mainly flows in regime are inIV. regime After 0400 II for LT, all the times. simulated These incoming results flows imply are that in the regime II. The observed incoming flows are in regime II for all times. These results imply that the incoming flows with a favorable condition for hydraulic jump play an important role in the formation incoming flows with a favorable condition for hydraulic jump play an important role in the formation of theof downslope the downslope windstorm. windstorm.

FigureFigure 7. Diagram 7. Diagram for asymptotic for asymptotic solutions solutions to shallow to shallow water wa ﬂowter flow over over an isolated an isolated obstacle obstacle as a as function a of thefunction Froude of number the Froude and number dimensionless and dimensionless height. The he redight. and The blackred and dots black (stars) dots represent(stars) represent the condition the of simulatedcondition (observed) of simulated incoming (observed) ﬂow incoming before 0400 flow LTbefore and 0400 after LT 0400 and LT, after respectively. 0400 LT, respectively. 69 samples 69 in the samples in the simulation and 4 observed samples are used for this diagram. simulation and 4 observed samples are used for this diagram.

Atmosphere 2020, 11, x FOR PEER REVIEW 10 of 16 Atmosphere 2020, 11, 431 10 of 16 Additionally, an overall feature of the topography is that the topographic height increases gradually and then decreases rapidly in the downwind direction. This means that the leeward slope Additionally, an overall feature of the topography is that the topographic height increases is much steeper than the windward slope. This topographic feature is also favorable for the gradually and then decreases rapidly in the downwind direction. This means that the leeward downslope windstorm. slope is much steeper than the windward slope. This topographic feature is also favorable for the The critical-level reflection is not relevant as the generation mechanism for the downslope downslope windstorm. windstorm in the Yangyang area. Due to the strong prevailing winds resulting from the synoptic The critical-level reflection is not relevant as the generation mechanism for the downslope pressure pattern of the northern low and the southern high, it is difficult for a critical level to be self- windstorm in the Yangyang area. Due to the strong prevailing winds resulting from the synoptic induced. The climatological statistics of springtime windstorms in Gangneung, the Yeongdong pressure pattern of the northern low and the southern high, it is difficult for a critical level to be region reveal that only 5% of the downslope windstorms are related to the critical-level reflection self-induced. The climatological statistics of springtime windstorms in Gangneung, the Yeongdong [18]. region reveal that only 5% of the downslope windstorms are related to the critical-level reflection [18]. In the Yangyang area, the maximum wind speed appears earlier in the WRF model than the In the Yangyang area, the maximum wind speed appears earlier in the WRF model than observation. However, this shift in the maximum wind speed of the WRF model (Figure 4a) non- the observation. However, this shift in the maximum wind speed of the WRF model (Figure4a) significantly impacts the favorable conditions for the downslope windstorm discussed in this section non-significantly impacts the favorable conditions for the downslope windstorm discussed in this (e.g., the elevated inversion layer near the mountain top height and the steeper leeward slope). section (e.g., the elevated inversion layer near the mountain top height and the steeper leeward slope). Therefore, the generation mechanism of the downslope windstorm conjectured from these conditions Therefore, the generation mechanism of the downslope windstorm conjectured from these conditions remains unchanged. remains unchanged. 4.4. Gap Winds (Jangjeon) 4.4. Gap Winds (Jangjeon) Figure 8 shows the simulated 10-m wind fields at 0900 LT and 1500 LT in the Jangjeon area. At Figure8 shows the simulated 10-m wind fields at 0900 LT and 1500 LT in the Jangjeon area. 0900 LT, just north of Jangjeon station, the area of the plume-shaped wind pattern extending seaward At 0900 LT, just north of Jangjeon station, the area of the plume-shaped wind pattern extending seaward from the mountain gap is apparent (Figure 8a; see Figure 9a for the gap location). As the winds from the mountain gap is apparent (Figure8a; see Figure9a for the gap location). As the winds become become strong along the coastal region with time, the plume-shaped wind pattern is less identifiable strong along the coastal region with time, the plume-shaped wind pattern is less identifiable (Figure8b). (Figure 8b). Notice that the strong windstorms appear on the lee slope of the Taebaek Mountains Notice that the strong windstorms appear on the lee slope of the Taebaek Mountains (Figure8). (Figure 8).

Figure 8. Simulated 10-m wind speed and the wind wind vector vector fields ﬁelds in in A1 A1 at at ( (aa)) 0900 0900 LT LT and and ( (bb)) 1500 1500 LT. LT. TheThe terrainterrain heightheight (blue(blue lines)lines) isis contouredcontoured upup toto 15001500 mm inin intervalsintervals ofof 100100 m.m. The red circle indicates thethe locationlocation ofof JangjeonJangjeon station.station.

The decrease in in the the pressure pressure near near the the exit exit of ofa gap a gap is a is mechanism a mechanism for gap for gapwinds winds [34–37]. [34– The37]. Thepresent present numerical numerical simulation simulation is suitable is suitable for revealing for revealing such sucha local a localpressure pressure change change because because the high- the high-resolutionresolution grid gridsystem system and andthe thehigh-resolution high-resolution terrain terrain data data are are used. used. The The local local pressure pressure difference diﬀerence between thethe gapgap insideinside andand the the gap gap exit exit is is analyzed analyzed using using sea-level sea-level pressure pressure at at 17 17 points points in in and and out out of theof the gap gap (Figure (Figure9a). 9a). The The sea-level sea-level pressure pressure and an thed the 10-m 10-m wind wind speed speed along along the the 17 points17 points are are shown shown in Figurein Figure9b,c. 9b,c. Before Before 0900 0900 LT, LT, the sea-levelthe sea-level pressure pressu direﬀ differenceserences are are large large and and the localthe local minimum minimum of the of

Atmosphere 2020, 11, 431 11 of 16

Atmosphere 2020, 11, x FOR PEER REVIEW 11 of 16 sea-level pressure and the local maximum of the 10-m wind speed at the gap exit are clearly represented. Aroundthe sea-level 1500 LT, pressure the diﬀ anderences the of local the sea-level maximum pressure of the and 10-m the 10-m wind wind speed speed at the between gap exit the gap are insideclearly (pointsr 1–12) and the gap exit (points 13–14) are smaller than around 0900 LT.

Figure 9. (a) Terrain height (m)(m) inin A1.A1. Hovmöller diagrams of the ( b) sea-level pressure and (c) 10-m wind speed and 10-m 10-m wind wind vector vector at at the the 17 17 points. points. Th Thee red red dots dots in inFigure Figure 9a9 aindicate indicate the the selected selected 17 17points points in inthe the gap. gap. Vertical Vertical cross cross sections sections of of the the te terrainrrain height height at at the the 17 17 points points are are added added in in the lower panels of Figure9 9b,c.b,c. TheThe redred triangletriangle indicatesindicates thethe locationlocation ofof JangjeonJangjeon station. station.

To examineexamine the the relationship relationship between between the sea-levelthe sea-level pressure pressure and the and 10-m the wind 10-m speed, wind the speed, difference the ofdifference the sea-level of the pressure sea-level and pressure the 10-m and wind the 10-m speed wind between speed the between gap inside the andgap theinside gap and exit the are gap plotted exit inare Figure plotted 10 in. TheFigure values 10. The of the values gap insideof the gap and insi thede gap and exit the are gap averaged exit are over averaged the points over the 1–12 points and the 1– points12 and 13–14,the points respectively. 13–14, respectively. Around 0500 Around LT, the 0500 large LT, sea-level the large pressure sea-level difference pressure ( 3.4 diff hPa)erence induces (−3.4 1 − thehPa) large induces 10-m the wind large speed 10-m diff wierencend speed (~12 mdifference s− ). It is (~12 well matchedm s−1). It withis well the matched local maximum with the of 10-mlocal windmaximum speed of at 10-m the gapwind exit speed (Figure at 9thec). gap When exit the (Figur sea-levele 9c). pressure When the di sea-levelfference decreasespressure difference with time fromdecreases 0500 with LT, the time 10-m from wind 0500 speed LT, the di ff10-merence wind also speed decreases. difference However, also decreases. the wind However, speed near the the wind gap exitspeed is similarnear the to gap the exit one is of similar early morningto the one (Figure of early9c). morning It is because (Figure the 9c). larger-scale It is because synoptic the larger-scale pressure patternsynoptic increases pressure the pattern wind increases speed at the the gap wind inside. speed Accordingly, at the gap inside. a plume-shaped Accordingly, wind a plume-shaped pattern, which iswind an indicativepattern, which of the is windan indicative speed increase of the wind at the speed gap increase exit, appears at the bettergap exit, in theappears morning better than in the in themorning afternoon. than in the afternoon. Atmosphere 2020, 11, 431 12 of 16

Atmosphere 2020, 11, x FOR PEER REVIEW 12 of 16

FigureFigure 10.10. Time series of thethe sea-levelsea-level pressurepressure (blue(blue line)line) andand thethe 10-m10-m windwind speedspeed (black(black line)line) didifferencesﬀerences betweenbetween thethe gapgap insideinside andand thethe gapgap exit.exit. TheThe valuesvalues ofof thethe gapgap insideinside andand thethe gapgap exitexit areare averagedaveraged overover pointspoints 1–121–12 andand pointspoints 13–14,13–14, respectively.respectively.

4.5.4.5. AssociationAssociation withwith PlanetaryPlanetary BoundaryBoundary LayerLayer (Uljin)(Uljin) ToTo analyze analyze the the banded banded wind wind pattern pattern in thein the Uljin Uljin area, area, the 10-mthe 10-m wind wind and planetaryand planetary boundary boundary layer (PBL)layer height(PBL) height fields arefields plotted are plotted (Figure (Figure11). In the 11). Mellor-Yamda-JanjićPBL In the Mellor-Yamda-Janji scheme,ć PBL thescheme, PBL height the PBL is determinedheight is determined by the lowest by modelthe lowest level abovemodel the level surface above at whichthe surface TKE decreasesat which to TKE a prescribed decreases lower to a bound.prescribed At 1800lower LT, bound. the well-defined-banded At 1800 LT, the well-def patternined-banded of very strong-wind pattern of very regions strong-wind and weak-wind regions regionsand Atmosphereweak-wind is evident. 2020, 11regions The, x FOR banded PEER is evident. REVIEW wind pattern The banded is parallel wind to thepattern coastal is line.parallel In the to strong-the coastal and13 weak-windline. of 16 In the regions,strong- and the westerlyweak-wind and regions, the southerly the westerly winds areand prevailing, the southerly respectively. winds are Comparison prevailing, respectively. of the wind fieldComparisonHowever, and the PBL ofthe the heightaforementioned wind field field shows and features the that PBLdo in thenot height directly banded field show wind shows the pattern, effects that theinof thethe strong-wind PBL banded on the wind regionwindstorm. pattern, coincides the withstrong-windAdditionally, the region region of it shalloweris coincidesnot clear PBL, how with whilethe the role the region of weak-wind the PBLof shallower as regionan absorber coincidesPBL, and while the with PBLthe the asweak-wind regionan effective of deeperregion mountain differently affect the windstorm. Therefore, further in-depth research about the effects of planetarycoincides boundarywith the region layer. of This deeper implies planetary that strong bounda windsry layer. can be This related implies to PBL that height. strong winds can be the PBL on windstorms is needed. related to PBL height. To investigate the interaction between the downslope windstorm and the PBL, the potential temperature, wind speed, wind vector, and the bulk Richardson number smaller than 0.25 at 1800 LT are plotted in the vertical cross-section along C2 in Figure 2b (Figure 12). In the east of Uljin station, the weak-wind regions appear in the PBL and have low bulk Richardson number due to strong wind shear. The downslope windstorm does not penetrate the weak-wind region with low bulk Richardson number, but instead passes over it. The PBL height also follows this wavy flow pattern and is well matched with the weak-wind and low bulk Richardson number regions. As with a flow over a mountain, the wind speed increases on the downwind of these regions. The banded wind pattern appears as a result of the weak-wind regions and the increased wind speed on the downwind side of these regions. The interaction between the PBL and the downslope windstorm is unclear. Some studies [38–41] show that the PBL absorbs downward reflected gravity waves, thus effectively limiting trapped mountain waves or significantly reducing the drag and momentum flux of mountain waves above. Accordingly, the strong windstorms due to wave-involved mechanisms can be weakened. However, the PBL as an effective mountain is reported [42–44]. According to these studies, mountain waves can be forced by an effective mountain, not by a real mountain only [42–44], and flow regime can also be influenced by the PBL height [43].

Considering those effects in the present study, the role of the PBL as an energy absorber is unclear.Figure FigureHowever, 11. Simulated11. Simulated the height (a )(a 10-m) 10-m wi wind thwind maximum speed speed andand windwind vector vectorspeed fields ﬁeldsis nearand and ( bthe) ( bplanetary) PBL planetary top boundary height, boundary layer not layer near the height field in A2 at 1800 LT. The red dot indicates the location of Uljin station. surface,height suggesting ﬁeld in A2 atthat 1800 the LT. ThePBL red could dot indicates act as thean location effective of Uljinmountain station. [42]. The large vertical displacement of the isentropes above the PBL in the east of Uljin station and the windstorm appearing at the downslope of the PBL imply that the PBL could force gravity waves and change flow regime.

Figure 12. Vertical cross-section of the wind speed, wind vector, potential temperature (blue lines), and bulk Richardson number smaller than 0.25 (black hatch lines) along C2 at 1800 LT. The PBL height is represented by magenta line. The reference point on the horizontal axis (0 km) indicates the location of Uljin station.

The time variation of the observed wind speed at Uljin station (Figure 4b) can be explained by the banded wind pattern coupled with the spatiotemporal variation of the PBL height (Figure 13). As the deep PBL developed in the afternoon is pushed seaward, it passes over Uljin station and the wind speed decreases. After the deep PBL passes over Uljin station, the wind speed increases, because it is located upwind of the deep PBL. However, the WRF model failed, in location, to simulate this

Atmosphere 2020, 11, x FOR PEER REVIEW 13 of 16

However, the aforementioned features do not directly show the effects of the PBL on the windstorm. Additionally, it is not clear how the role of the PBL as an absorber and the PBL as an effective mountain differently affect the windstorm. Therefore, further in-depth research about the effects of the PBL on windstorms is needed.

Atmosphere 2020, 11, 431 13 of 16

To investigate the interaction between the downslope windstorm and the PBL, the potential temperature, wind speed, wind vector, and the bulk Richardson number smaller than 0.25 at 1800 LT are plotted in the vertical cross-section along C2 in Figure2b (Figure 12). In the east of Uljin station, the weak-wind regions appear in the PBL and have low bulk Richardson number due to strong wind shear. The downslope windstorm does not penetrate the weak-wind region with low bulk Richardson number, but instead passes over it. The PBL height also follows this wavy ﬂow pattern and is well matched with the weak-wind and low bulk Richardson number regions. As with a ﬂow over a mountain, the windFigure speed 11. increases Simulated on the(a) 10-m downwind wind speed of these and regions.wind vector The fields banded and wind(b) planetary pattern boundary appears aslayer a result of theheight weak-wind field in A2 regions at 1800 and LT. the The increased red dot indicates wind speed the location on the of downwind Uljin station. side of these regions.

Figure 12.12. VerticalVertical cross-sectioncross-section of of the the wind wind speed, speed, wind wind vector, vector, potential potential temperature temperature (blue (blue lines), lines), and bulkand bulk Richardson Richardson number number smaller smalle thanr than 0.25 0.25 (black (black hatch hatch lines) lines) along along C2 C2 at 1800at 1800 LT. LT. The The PBL PBL height height is representedis represented by by magenta magenta line. line. The The reference reference point point on on the the horizontal horizontal axis axis (0 km)(0 km) indicates indicates the the location location of Uljinof Uljin station. station.

The interactiontime variation between of the the observed PBL and wind the downslopespeed at Uljin windstorm station (Figure is unclear. 4b) Somecan be studies explained [38–41 by] showthe banded that the wind PBL pattern absorbs coupled downward with the reflected spatiotemp gravityoral variation waves, thus of the eff PBLectively height limiting (Figure trapped 13). As mountainthe deep PBL waves developed or significantly in the afternoon reducing is thepushed drag se andaward, momentum it passes fluxover of Ul mountainjin station wavesand the above. wind Accordingly,speed decreases. the strongAfter the windstorms deep PBL duepasses to wave-involvedover Uljin station, mechanisms the wind speed can be increases, weakened. because However, it is thelocated PBL asupwind an effective of the mountain deep PBL. is reportedHowever, [42 the–44 ].WR AccordingF model tofailed, these studies,in location, mountain to simulate waves this can be forced by an effective mountain, not by a real mountain only [42–44], and flow regime can also be influenced by the PBL height [43]. Considering those effects in the present study, the role of the PBL as an energy absorber is unclear. However, the height with maximum wind speed is near the PBL top height, not near the surface, suggesting that the PBL could act as an effective mountain [42]. The large vertical displacement of the isentropes above the PBL in the east of Uljin station and the windstorm appearing at the downslope of the PBL imply that the PBL could force gravity waves and change flow regime. However, the aforementioned features do not directly show the effects of the PBL on the windstorm. Additionally, it is not clear how the role of the PBL as an absorber and the PBL as an effective mountain differently affect the windstorm. Therefore, further in-depth research about the effects of the PBL on windstorms is needed. The time variation of the observed wind speed at Uljin station (Figure4b) can be explained by the banded wind pattern coupled with the spatiotemporal variation of the PBL height (Figure 13). As the deep PBL developed in the afternoon is pushed seaward, it passes over Uljin station and the wind speed decreases. After the deep PBL passes over Uljin station, the wind speed increases, because it is located upwind of the deep PBL. However, the WRF model failed, in location, to simulate this Atmosphere 2020, 11, 431 14 of 16 Atmosphere 2020, 11, x FOR PEER REVIEW 14 of 16 increaseincrease inin windwind speedspeed well.well. The time variation ofof the simulated maximummaximum windwind speedspeed atat thethe locationlocation 3.333.33 kmkm westwest ofof UljinUljin station,station, asas illustratedillustrated byby thethe blueblue dotteddotted lineline inin Figure4 4b,b, reasonably reasonably simulates simulates thethe observedobserved strongstrong winds winds at at 1620 1620 LT. LT. After After the the strong strong winds winds at 1620 at 1620 LT, theLT, deep the deep PBL returnsPBL returns to Uljin to station,Uljin station, decreasing decreasing the wind the speedwind speed there. there.

FigureFigure 13.13. Hovmöller diagram for the simulated planetary boundaryboundary layerlayer heightheight alongalong C2.C2. The black solidsolid lineline indicatesindicates thethe locationlocation ofof UljinUljin station.station. 5. Summary and Conclusions 5. Summary and Conclusions In this study, the characteristics and generation mechanisms of the windstorms occurring in In this study, the characteristics and generation mechanisms of the windstorms occurring in the the lee of the Taebaek mountain on 8 April 2012 were investigated using the high-resolution WRF lee of the Taebaek mountain on 8 April 2012 were investigated using the high-resolution WRF model model simulation. Three different areas (Yangyang, Jangjeon, and Uljin) were selected for the analysis. simulation. Three different areas (Yangyang, Jangjeon, and Uljin) were selected for the analysis. In In the Yangyang area, the severe downslope windstorm in the lee of the Taebaek Mountains appears. the Yangyang area, the severe downslope windstorm in the lee of the Taebaek Mountains appears. The steep descent of the isentropes on the lee slope of the mountain and their recovery farther leeward The steep descent of the isentropes on the lee slope of the mountain and their recovery farther are typical features of hydraulic jump. Inversion layers near the highest mountain top height and leeward are typical features of hydraulic jump. Inversion layers near the highest mountain top height incoming flow in hydraulic jump regime support the hydraulic jump mechanism for the generation of and incoming flow in hydraulic jump regime support the hydraulic jump mechanism for the the downslope windstorm. In the Jangjeon area, the plume-shaped wind pattern extending seaward generation of the downslope windstorm. In the Jangjeon area, the plume-shaped wind pattern from the exit of the gap is clearly seen. The gap winds are induced by the large pressure gradient extending seaward from the exit of the gap is clearly seen. The gap winds are induced by the large between the inside and exit of the gap. In the Uljin area, the banded pattern of strong-wind regions and pressure gradient between the inside and exit of the gap. In the Uljin area, the banded pattern of weak-wind regions is distinct. The banded wind pattern is parallel to the coastal line. Interestingly, strong-wind regions and weak-wind regions is distinct. The banded wind pattern is parallel to the we found that the strong-wind (weak-wind) region coincides with the region of shallower (deeper) coastal line. Interestingly, we found that the strong-wind (weak-wind) region coincides with the planetary boundary layer and that in the weak-wind region, the southerly winds are prevailing. This is region of shallower (deeper) planetary boundary layer and that in the weak-wind region, the because the downslope windstorm does not penetrate the weak-wind region with low bulk Richardson southerly winds are prevailing. This is because the downslope windstorm does not penetrate the number and passes over it. The windstorms and their features are well simulated in our simulation weak-wind region with low bulk Richardson number and passes over it. The windstorms and their with the gray-zone resolution (∆x = 333 m). However, we need a scale-aware PBL parameterization or features are well simulated in our simulation with the gray-zone resolution (Δx = 333 m). However, a large-eddy simulation to better simulate the fine structure and evolution of the windstorms. we need a scale-aware PBL parameterization or a large-eddy simulation to better simulate the fine In this study, we showed that even if windstorms directly related to topography occur on the structure and evolution of the windstorms. same day, their generation mechanisms can differ depending on the local topographic features. This In this study, we showed that even if windstorms directly related to topography occur on the suggests that even under the same synoptic conditions, local topographic features should be taken into same day, their generation mechanisms can differ depending on the local topographic features. This consideration in forecasting windstorms in mountainous regions. suggests that even under the same synoptic conditions, local topographic features should be taken Authorinto consideration Contributions: in forecastingJ.L. conducted windstorms the numerical in mountainous simulation and regions. prepared the figures. J.L., J.M.S., J.-J.B., S.-B.P., and B.-S.H. contributed to the analysis and interpretation of the numerical simulation results. J.L., J.-J.B., andAuthor B.-S.H. Contributions: wrote the manuscript. J.L. conducted All authors the numerical have read simulation and agreed and to pr theepared published the figures. version J.L., of J.M.S., the manuscript. J.-J.B., S.- Funding:B.P., and B.-S.H.This work contributed was supported to the byanal theysis Research and interpretation Institute of Basicof the Sciences numerica fundedl simulation by the Nationalresults. J.L., Research J.-J.B., Foundationand B.-S.H. ofwrote Korea the (NRF-2019R1A6A1A10073437). manuscript

Funding: This work was supported by the Research Institute of Basic Sciences funded by the National Research Foundation of Korea (NRF-2019R1A6A1A10073437).

Atmosphere 2020, 11, 431 15 of 16

Acknowledgments: The authors are grateful to two anonymous reviewers for providing valuable comments on this study. The authors thank the supercomputer management division of the Korea Meteorological Administration for providing us with the supercomputer resource. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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