Enhancement of Orographic Precipitation in Jeju Island During the Passage MARK of Typhoon Khanun (2012)

Atmospheric Research 201 (2018) 58–71

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Atmospheric Research

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Enhancement of orographic precipitation in Jeju Island during the passage MARK of Typhoon Khanun (2012)

⁎ Jung-Tae Leea, Kyeong-Yeon Kob, Dong-In Leec, , Cheol-Hwan Youd, Yu-Chieng Lioue a Division of Earth Environmental System Science, Pukyong National University, Busan, Republic of Korea b Numerical Modeling Center, Korea Meteorological Administration, Seoul, Republic of Korea c Department of Environmental Atmospheric Sciences, Pukyong National University, Busan, Republic of Korea d National Research Center for Disaster-free and Safe Ocean City, Dong-A University, Busan, Republic of Korea e Department of Atmospheric Sciences, National Central University, Jhongli City, Taiwan

ARTICLE INFO ABSTRACT

Keywords: Typhoon Khanun caused over 226 mm of accumulated rainfall for 6 h (0700 to 1300 UTC), localized around the Jeju Island summit of Mt. Halla (height 1950 m), with a slanted rainfall pattern to the northeast. In this study, we in- Khanun vestigated the enhancement mechanism for precipitation near the mountains as the typhoon passed over Jeju Orographic precipitation Island via dual-Doppler radar analysis and simple trajectory of passive tracers using a retrieved wind field. The Seeder-feeder analysis of vertical profiles of the mountain region show marked features matching the geophysical conditions. Hydraulic jump − In the central mountain region, a strong wind (≥7ms 1) helps to lift low-level air up the mountain. The time taken for lifting is longer than the theoretical time required for raindrop growth via condensation. The falling particles (seeder) from the upper cloud were also one of the reasons for an increase in rainfall via the accretion process from uplifted cloud water (feeder). The lifted air and falling particles both contributed to the heavy rainfall in the central region. In contrast, on the leeward side, the seeder-feeder mechanism was important in the formation of strong radar reflectivity. The snow particles (above 5 km) were accelerated by strong downward − winds (≤−6ms 1). Meanwhile, the nonlinear jumping flow (hydraulic jump) raised feeders (shifted from the windward side) to the upper level where particles fall. To support these development processes, a numerical simulation using cloud-resolving model theoretically carried out. The accreting of hydrometeors may be one of the key reasons why the lee side has strong radar reflectivity, and a lee side weighted rainfall pattern even though lee side includes no strong upward air motion.

1. Introduction and Durran, 2004; Smith et al., 2009a; Yu and Cheng, 2013) and many observations and simulations have shown that mechanical air lifting by Many isolated islands comprise a single mountain peak due to vol- the obstacles can produce heavy rainfall (Hobbs et al., 1973; Geerts canic activity. These bell-shaped islands can be considered to have an et al., 2000; Wu et al., 2002; Jiang, 2003; Colle, 2004; Kirshbaum et al., upslope and a downslope, which lead to orographically induced con- 2007). vection and precipitation. The variation in geographical shape of each However, if the slope serves an environment which can produce island causes different airflow patterns, especially when there is a suf- cloud and rainfall despite not long enough (small hills), the rainfall can ficient forcing to lift air over the upslope. When the background wind is be described in seeder-feeder process (Bergeron, 1965). Hill et al. strong, the surface air rises over the mountain or immediately descends (1981) indicated that Bergeron's seeder-feeder mechanism is strongly downstream after passing the summit. This geophysical airflow com- affected by low-level wind speed, with precipitation enhanced most just monly generates clouds or enhances precipitation on both the upslope above the hilltops. Smith et al. (2009a) explained that almost all rainfall and downslope regions. Most studies have focused on the windward in front of a small barrier (such as Dominica) is caused by accretion region that can contribute to the production or enhancement of oro- growth of raindrops and their fallout. This is because on a short upslope graphic rainfall by rising airflows and through the condensation process there is insufficient time for a substantial amount of advection to occur, (Hamuro et al., 1969; Pandey et al., 1999; Lin et al., 2001). The con- thereby preventing the growth of a convective cell. The observational densation process depends upon the length of the upslope (Kirshbaum results reveal the effects of the seeder-feeder mechanism on the

⁎ Corresponding author at: Pukyong National University, (48513) B13-4324 45, Yongso-ro, Nam-gu, Busan, Republic of Korea. E-mail address: [email protected] (D.-I. Lee). http://dx.doi.org/10.1016/j.atmosres.2017.10.013 Received 13 April 2017; Received in revised form 13 October 2017; Accepted 13 October 2017 Available online 16 October 2017 0169-8095/ © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). J.-T. Lee et al. Atmospheric Research 201 (2018) 58–71 windward side (Sibley, 2005; Fernández-González et al., 2016), but the features represented in the radar reflectivity, as this height is un- process can also enhance precipitation on the leeward side. The oro- obstructed by orography. graphic lifting mechanism can cause snow particles to shift and fall, The surface weather elements (rainfall, wind, pressure, temperature influencing the leeward precipitation. Misumi (1996) showed that snow and humidity) were recorded by Automatic Weather Systems (AWS) at particles and cloud water transported by the downdraft can lead to 25 sites. A minimum curvature method without tension (T = 0) was enhanced rainfall. Tang et al. (2012) also investigated the interaction used (Smith and Wessel, 1990) to show a spatial distribution of accu- between leeward mountain airflow associated with local instability and mulated rainfall, considering a potential field. The surface conditions as displaced hydrometeors from the windward side. geographical features are presented by three AWS stations. These three There were some previous studies about Jeju Island's orographic stations have different geographical positions: A1 is on the windward rainfall were conducted at mesoscale convective systems (MCSs) with side, A2 in the mountain peak region and A3 on the leeward side. low-level convergence and low Froude number (Fr) < 0.5 (Lee et al., Additionally, we used upper air sounding data from the Gosan sta- 2010; Lee et al., 2012; K. O. Lee et al., 2014). At Jeju Island, most MCS tion (47185) at 1200 UTC on 18 July 2012 to calculate the environ- cases included a type of lee side enhancement by deflected around the mental stability and Fr, i.e. the kinematic energy of rising air. The terrain. Houze (1993) introduced this enhancement type, in which the parameter is defined as follows: precipitation is enhanced by low-level convergence on the lee side. Fr/= UNho (1) In the high Fr (> 1.0) condition, however, the wind and precipitation patterns can be different from a low Fr condition since the air where Uo is the average of wind speed below the height of the moun- is likely to flow over the terrain. In windy and humid environments that tain, N is the Brunt-Väisälä frequency, and h is the height of the are typical of typhoon weather, differences can be more prominent mountain (1950 m). Uo is recalculated based radar retrieved wind in- because forced lifting and nonlinear behavior (hydraulic jump) of air stead of upper air sounding. The high Fr (> 1.0) air is tend to flow over affect microphysical processes. Typhoon Khanun (2012) is a prime the mountain, while the low Fr (< 1.0) air is likely to be deflected example of the occurrence of different precipitation patterns compared around mountain. with the MCS that occurs around Jeju Island. The maximum rainfall from Typhoon Khanun occurred in the vicinity of the mountain, with a 2.2. Wind synthesis and trajectory weighted rainfall present toward the northeast region. Nonlinear behavior of the air was evident, resulting in an increase in precipitation on Three-dimensional wind fields are commonly produced from the the lee side. In particular, because Khanun was the closest typhoon to radial velocity of two or more Doppler radars using a variational Jeju Island, it is possible to analyze the enhancement mechanism of method suggested by past studies (Scialom and Lemaître, 1990; Protat precipitation from strong winds. These singularities indicate that the and Zawadzki, 1999; Gao et al., 1999, 2004; Mewes and Shapiro, study of this case is of sufficient value. 2002). The variational-based wind analysis addresses problems in- Although many studies have reported the effects of a mountainous volved in the specification of the boundary condition between the top enhancement mechanism on precipitation in a typhoon environment, and bottom levels. This has advantages over previous approaches to the rainfall enhancement varies greatly with geographical and en- vertical velocity (Liou and Chang, 2009). However, Jeju Island is the vironmental conditions (Carruthers and Choularton, 1983; Chen et al., bell-shaped mountain region (Mt. Halla). This makes wind retrievals of 2008). Therefore, in this study, we investigated the vertical profile and orographic forcing difficult and noisy. An advanced radar wind synth- retrieved wind fields in the typhoon environment at Jeju Island using esis method to produce three-dimensional wind fields over non-flat two ground-based Doppler radars to understand the specific precipita- surfaces was suggested by Liou et al. (2012), and was implemented tion enhancement occurring due to a single isolated bell-shaped island without conversion to a terrain-following coordinate system by using an (width 78 km, height 35 km). Furthermore, we attempted to overcome immersed boundary method (Tseng and Ferziger, 2003). the limitations of data acquired from observation by analyzing the re- Following this, we created three-dimensional wind field data by the sults of a cloud-resolving model for the distribution of hydrometeors Wind Synthesis System using Doppler Measurements (WISSDOM) al- near terrain that could not be marked by a single-polarization radar. gorithm designed by Liou et al. (2012). This algorithm uses the resolving anelastic continuity equation and simplified vertical vorticity 2. Data and methods equation to retrieve reasonable wind patterns in regions with terrain forcing. Therefore it is suitable for determining a baseline wind field. 2.1. Radar, surface and sounding data Jeju Island has a mountain region along the baseline between the two radars; therefore, WISSDOM is the only current method that can re- Two ground-based S-band Doppler radars were used to analyze the trieve observational wind fields for this topography. enhancement mechanism of Typhoon Khanun on Jeju Island. These two Furthermore, the wind fields were utilized for the trajectory of S-band Doppler radars were located at Gosan and Seongsanpo me- passive tracers (Cheong and Han, 1997). The tracking experiments are teorological stations in Jeju Island (Fig. 1), operated by the Korea implemented to prove the conjectured microphysical processes based Meteorological Administration (KMA). These radars were both able to on the observation results conclusively. We conceived the experiments observe the typhoon track. KMA operated at 15 elevations volume scans for both droplet (gravitationally neutral) and drop (gravitating) of the (0.5°, 0.6°, 0.8°, 1.0°, 1.5°, 2.0°, 2.5°, 3.5°, 4.5°, 6.0°, 7.8°, 10.5°, 13.8°, cloud. Although each hydrometeor has a different fall velocity as their 18.1°, 24.0°) which proved sufficient to observe the typhoon's structure. size, the simple relationship between reflectivity and fall velocity have To avoid topographical observation errors such as ground clutter and been widely used for ease of calculation. The fall velocity of the drop is beam blockage, we used managed data which were eliminated in- estimated as Shapiro et al. (1995). stances near mountain regions. Furthermore, non-meteorological tar- w = 3.088Z 0.0957 (2) gets (birds, sea clutter, and other unreasonable values) were removed t using the texture and vertical gradient of reflectivity, as proposed by All of the trajectories in the experiment follows the atmospheric Zhang et al. (2004). Following these procedures, the preprocessed data motion and the equation of the trajectory is then: were interpolated onto a Cartesian coordinate system (Cressman, dX 1959). A 1 km horizontal resolution and 0.25 km vertical resolution = V dt (3) were used, with effective radii of 1.5 and 1.0 km, respectively. Com- posed constant altitude plan position indicator (CAPPI) images were where X is the position vector of the objects during a time step. In this used at 2.5 km above sea level (ASL) to reveal the typhoon's structural experiment, the backward scheme is used to trace next position of the

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Fig. 1. The accumulated rainfall amount (mm) over the Jeju Island during 6 h (0700–1300 UTC). The observation sites with radar (blue star) and automatic weather system (red circles mean 21 surface sites and green squares are to show time variation) are presented. The contour shows the topography of Jeju Island with 200 m interval. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

− objects on the spatiotemporally discretized fields. The densely dis- Okinawa Island. It moved northwesterly at 8 m s 1 and the center of cretized time-space (60 s) was achieved by linear interpolation of the the typhoon passed the western region of Jeju Island at 1000 UTC on 18 radar-based retrieved wind fields. To allow for sufficient movement of July 2012. While it passed the island, environmental wind fields in the the tracers, retrieved wind for 40 min were used for each tracing ex- vicinity of Jeju Island were dominated by the typhoon, and conse- periment. quently, surface observations recorded a maximum instantaneous wind − speed of 24.7 m s 1 at Seogwipo station (A1; depicted in Fig. 1). Fur- 2.3. Cloud-resolving model simulation thermore, the directional wind pattern gradually changed from southeasterly to southwesterly, while moving northward of the typhoon due To support the cloud development mechanism revealed through to cyclonic air flow. A subsequent track of the typhoon was predicted to observation results and distribution of hydrometeors near the terrain, make landfall in South Korea by KMA as well as other institutions, numerical simulation was implemented using cloud-resolving storm where it rapidly dissipated. Although Typhoon Khanun was not as simulator (CReSS). The CReSS developed by Nagoya University strong as other tropical cyclones when considering damage to human (Tsuboki and Sakakibara, 2002) is a non-hydrostatic model with fully activities, it is suitable for the study of topographical influences on compressible equations and solves it using horizontal explicit and ver- enhanced precipitation because it was closely passed through the Jeju tical implicit scheme. The grid system is composed of Arakawa C grid in Island. the horizontal and Lorenz grid in the vertical. The mode-splitting time integration technique (Klemp and Wilhelmson, 1978) is adapted to 3.2. Spatial distribution of precipitation on Jeju Island integrate the acoustic mode terms. The predicted variables are the wind component (u, v, and w), potential temperature and pressure pertur- The isolated volcanic topography of Jeju Island can cause cloud bations from the initial state. To simulate the energy transfer process in formation and enhancement of precipitation, via rising air over Mt. the cloud, one- and a half-order prognostic turbulent kinematic energy Halla in the center of the island. According to recent studies (Lee et al., closure, and bulk cold-rain parameterization (Lin et al., 1983; Cotton 2010; Lee et al., 2012), rainfall distributions caused by MCSs and et al., 1986; Murakami, 1990; Ikawa and Saito, 1991; Murakami et al., Changma fronts (low-Fr regime) at Jeju Island were mainly character- 1994) are used. ized as localized enhancements of precipitation on the windward side The output of the mesoscale model (MSM) produced by Japan caused by the topographic gradient, and on the leeward side by con- Meteorological Agency (JMA) is used as initial (1500 UTC 17 July vergent flow. However, for Typhoon Khanun, a lot of rainfall occurred 2012) and boundary conditions. For the simulation with at the peak of Mt. Halla (Fig. 1), which was a different distribution Δx=Δy=1 km, the large time step is 3.75 s. The total integration time compared with common rainfall cases (MCSs and Changma fronts). is 24 h (86,400 s). Over a 6 h period, the maximum rainfall was recorded at Witse-oreum station (242 mm, Fig. 3b) A2, the highest site among the observation 3. Typhoon Khanun overview stations (at 1673 m), located in the center of the island. The most nearshore stations show less accumulated rainfall (< 40 mm) than the 3.1. Typhoon track and features mountain region. Furthermore, increased rainfall also occurred on the northeast side of the Island. Rainfall varied between 40 and 60 mm with Fig. 2 shows the central pressure and best track of Typhoon Khanun a slightly one-sided distribution toward the northeast, and we presume as it moved northward. The lowest pressure and the highest surface that this biased distribution might be affected by other factors not − wind speed were 985 hPa and 26 m s 1, respectively, when it was near shown in MCSs case. Further discussions regarding this are given in

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Fig. 2. Track of Typhoon Khanun (2012) by observation (blue) and simulation (pink). Dotted points indicate typhoon center every 6 h (observation) and 3 h (simulation). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Sections 4 and 5. and humidity observed at three sites can be seen as a result of the condensation and evaporation processes (Smith et al., 2009a). 3.3. Surface observation The surface wind direction changed from the southeast to the southwest around 1000 UTC when the center of the typhoon was close We used three surface monitoring sites that represent the windward to Mt. Halla, and then it subsequently moved northward. This direc- side, mountain summit and leeward side of Jeju Island (sites A1, A2, tional change happened because of the cyclonic circulation of the ty- and A3) to reveal differences in meteorological parameters with regards phoon. These factors were important in deciding the position of the to geographical conditions. Fig. 3 shows a time series of meteorological forming cloud on the leeward side. Detailed information of this is de- parameters (temperature, humidity, pressure and rainfall) observed by scribed with radar analysis in Section 4. these three surface monitors as the center of Typhoon Khanun passed over the western part of Jeju Island. As the typhoon approached Jeju Island, the windward side monitoring station (A1; Fig. 3a) measured 3.4. Sounding observation increased relative humidity and a drop in pressure (dotted line), with the pressure decreasing down to a minimum of 992 hPa and the wind We considered environmental conditions based on the upper-air − speed increasing to 12 m s 1 as the typhoon passed overhead (at 1000 sounding observation. Fig. 4 shows a Skew T-log p diagram under Ty- UTC). However, at the leeward side monitoring station (A3 in Fig. 3c), phoon Khanun. The height of LCL and LFC (Level of Free Convection) relative humidity decreased as the typhoon passed over, thereby drying were about 949.9 hPa (400 m ASL) and 903.5 hPa (930 m ASL), re- air in the descent regions. Additionally, the temperature did not de- spectively. The precipitable water (PW) for entire sounding was crease at the leeward side, and there was more rainfall than at the 69.8 mm. The low height of the levels (LCL and LFC) and large PW windward side, which continued until after sunset (1040 UTC). indicate that the environment was favorable for developing precipita- fi The mountain summit monitoring site (A2) recorded > 270 mm of tion. The observed pro le clearly presents a humid and nearly saturated fi accumulated rainfall over a 10 h period. During this time, the humidity environment up to melting height (about 5 km ASL). The pro le was did not change. Due to the altitude, the ambient temperature at A2 was used in analyzing condensation process around the terrain and calcu- lower than at the other stations (A1 and A3) and did not vary much lation of Brunt-Väisälä frequency. over the 10 h period. This suggests that the mountain summit was nearly completely saturated by cloud and latent heat was released over the lifted condensation level (LCL), therefore only causing a slight temperature variation near the summit. The trends of the temperature

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− Fig. 3. Time series of temperature (°C, red line), wind (m s 1, barb), rainfall amount (blue bar graph), pressure (black dotted line), and humidity (dashed green line) observed at − Seogwipo, Witse-oreum, and Jeju station on 18 July 2012. Half and full barbs are 5 and 2.5 m s 1, respectively. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

4. Radar analysis results rainbands. In this study, developed rainfall near the top of the mountain can be defined as having a radar reflectivity above 38 dBZ at 2.5 km 4.1. Orographic rainfall enhancement over the mountain region ASL. Before the landfall, the reflectivity of rainbands was about 35 dBZ. Therefore, a reflectivity of 38 dBZ near the mountain is considered to The landfall and passing images of Typhoon Khanun over Jeju represent orographic rainfall enhancement. Forced rising air resulting Island are presented in Fig. 5. During the entire analysis period (0800 to from terrain is capable of forming many droplets, and its increase is 1330 UTC) the center of the typhoon was close to the island (within 1°). indicated by enhanced radar reflectivity. Orographic precipitation Fig. 5a shows higher radar reflectivity (> 35 dBZ) at the center of the usually occurs above 38 dBZ at 2.5 km ASL (Steiner et al., 2003; Medina island than in the surrounding area. The first rainband had an enhanced and Houze, 2003; Rotunno and Houze, 2007). In particular, rainfall precipitation near the central region, when it was clearly at the top of enhancement clearly appears near the mountain summit on Jeju Island, Mt. Halla. The enhanced rainfall was circularly distributed in the vici- as shown in Fig. 1. nity of the mountain peak, and this pattern continued for all other A monopole distribution of rainfall is one of the features of isolated

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Fig. 4. Skew T-log p plot of Gosan sounding station taken at 1200 UTC 18 July 2011. Blue and red lines indicate temperature and dew point temperature, respectively. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

bell-shaped mountains, and is especially noticeable for high wind speed underwent a sudden change, with air moving upwards. This sudden environments (Smith et al., 2009a, 2009b). Air flows over mountain upward motion was increased until it reached 6 km ASL, where the − regions when there are cross-barrier winds or high background wind wind speed achieved a maximum of > 4 m s 1 in the vertical column speed such as those of a typhoon. If the air is not stacked in front of a (Fig. 7b). The upward motion of the lee side was shown from < 2 km mountain (Grossman and Durran, 1984), strong winds induced by a ASL. It can be seen that low-level convergence caused by deflected air typhoon can force air to flow over a mountain region. In this case, the around the terrain made a lot of contribution to the convection because high wind speeds could play an important role for air passing a the rise is started from the low-level convergent region (20 km of A-AA mountain, whereby air lifted over the mountain could cause con- line). This upward motion provides a reasonableness for the develop- densation to occur. This process produced more cloud water and drop ment of the upper layer cloud. This development process in Jeju Island that can be observed by enhanced radar reflectivity. has already been revealed from previous studies (Lee et al., 2010; Lee These were indirectly validated by surface observations near the et al., 2012). In typhoon environment, however, the upward motion at mountain summit. As mentioned before (Section 3), the relatively even lee side should be considered as the results of lee side convergence but temperature is evidence that latent heat is released by a forced con- also hydraulic jump suggested by Long (1954). The low-level condensation process. Fundamentally, since typhoons are always accom- vergence shown at 0940 UTC did not occur at 1030 UTC (Fig. 7d; panied by a large amount of low-level humid air, there was abundant southerly wind) and 1140 UTC (Fig. 7f; southwesterly wind). Accord- source of latent heat released above 400 m ASL. Therefore, pre-existing ingly, the intensity and position of the ascending were different from rainbands with additional condensation processes were enhanced at the the 0940 UTC. The ascending flow at those time (1030 and 1140 UTC) − summit, as shown by radar reflectivity observations over 38 dBZ. occurred at a higher altitude than 0940 UTC and was < 3 m s 1. It was The analysis of structural feature (the w-component of the wind) is noticeable when the descending flow at the downslope was strong and illustrated in Fig. 6 showing the horizontal distribution of vertical ve- not associated with the low-level convergence (no or weak convergence locity, and Fig. 7 shows the vertical cross-sections of the measured below the ascending). Furthermore, the upward motion occurred at the parameters along the wind direction. The rainband was enhanced by lee side, but it is clearly distinguished with downward flow over the lifted air on the southeastern upslope, causing an increase in rainfall slope. This behavior closely agrees with a hydraulic jump flow depicted − near the summit. The maximum vertical velocity was over 7 m s 1 and in the several studies (Smith, 1980; Durran, 1986, 1990; Saito, 1992, the ascending flow is well defined in cross-section (Fig. 7b). The vertical 1993; Tang et al., 2012). Durran (1990) stated that if the air is ac- air motion at the upslope gradually increased from coast to summit and celerated as the variation of isentrope depth over the summit, a sudden peaked before the slope decreases. Where the vertical motion begins to rise of downward flow occurs at the lee side. This rise can be explained be estimated by the island, is coincident with the coast of the island, so as a kind of energy conversion in which the potential energy is con- that it is considered that the discontinuity of surface roughness (J. T. verted into kinematic energy along the downslope and is called a hy- Lee et al., 2014) causes convergent flow at a low level. The blue con- draulic jump. Tang et al. (2012) showed that in a typhoon environment, tours (convergence) shown in Fig. 7b prove the upward motion beyond a stable mountain wave can be broken and undergo a hydraulic jump by doubt. locally unstable condition. Such studies of high Fr regimes have re- − On the lee side, vertical motions were negative less than −6ms 1 vealed that accelerated descending along the slope creates an upward (shown in Fig. 6a) as air flowed downhill, but this downward air lee side motions. In this case, the wind over the summit accelerated to

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Fig. 5. Horizontal distribution of reﬂectivity at 2.5 km ASL for (a) 0910 UTC, (b) 0940 UTC, (c) 1010 UTC, (d) 1040 UTC, (e) 1110 UTC, (f) 1140 UTC, (g) 1210 UTC, (h) 1240 UTC, (j) 1310 UTC on 18 July 2012. The star symbol indicates radar site and rectangular domains (D1, D2, and D3) are analyzed in Fig. 8-9.

− about 5 m s 1 at 4–5 km ASL and latent heat released near the summit developed as the terrain (Houze, 2012). Furthermore, the distribution was enough to cause locally unstable conditions. Consequently, the has not been reported in previous studies that have analyzed MCS's at a upper-side cloud, shown in Fig. 7, can be described as a result of ver- low Fr (Lee et al., 2010; Lee et al., 2012). K. O. Lee et al. (2014) re- tical air motion formed by a convergent flow and hydraulic jump. The ported that a deflected flow around the terrain can cause developing effect of hydraulic jump is remarkable along the region where the precipitation in a humid environment; however, this mechanism is in- downward flow (spatially different) is strong (Fig. 7d and f). sufficient to explain the stationary precipitation enhancement since the The upward motion on the lee side is deeply concerned with the upward motion (hydraulic jump) was weak to make an enhancement. development of clouds and precipitation. The radar reflectivity at lee This indicates that other physical behaviors play an important role in side shown in Fig. 7 seems to be like overshooting of convective cloud. causing high reflectivity at lee side. The reflectivity (about 20 dBZ) is relatively higher than other regions It can be explained by the superposition of hydrometeors that were (6–10 km ASL) and is quite anomalous (0940 and 1030 UTC; Fig. 7a shifted and fell from the windward side. A lot of cloud water formed at and c) compared with typical reflectivity pattern. The feature of the lee the upslope region and snow particles that fell from the upper cloud side is most noticeable at 1140 UTC (Fig. 7e). The reflectivity above 35 were moved to the leeward region, where the hydrometeors acted to dBZ distributed between 2 and 6 km ASL and the reflectivity was form the non-active high reflectivity region. Consequently, the super- strongest at 3 km ASL, although there was no topographical upslope position of hydrometeors associated with vertical air motion leads to and the existence of a weak updraft. The high reflectivity was almost accretion and coalescence processes (Zawadzki et al., 2005; Fernández- stationary on the lee side (until 1300 UTC), and the reflectivity be- González et al., 2014), which resulted in growth of raindrop. The weak came > 41 dBZ, higher than the recorded observations at the summit. upward motion would be sufficient to transport cloud water even This reflectivity distribution does not belong to the general pattern though it cannot make the precipitation enhancement. If this suggestion

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Fig. 6. Horizontal distribution of the wind and vertical velocity at 0940, 1030, and 1140 UTC on 18 July 2012. Black star symbols indicate radar sites. The black lines (transect) of (b), (d), and (f) panels are shown in Fig. 7. is reasonable, it is not surprising that these processes are represented by 4.2. Temporal distribution for each analysis domain high radar reﬂectivity. To prove the opinions referred to in this section, structural features and pathway of the air (particle) are discussed in the In this section, time-height variation is analyzed with the regions following sections. where have diﬀerent topographical characteristics to show the enhanced precipitation clearly. Fig. 5i shows a distribution analysis box of Typhoon Khanun passing Jeju Island. This is separated into three phases: before landfall (D1), the windward side (D2), and the leeward

Fig. 7. The vertical cross-section of the radar reﬂectivity and vertical velocity as the lines depicted in Fig. 6. The blue and red contours of (b), (d), and (f) are the positive and negative − − value of divergence (10 3 s 1, every 0.5). The black shading region shows topography on Jeju Island. (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

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level divergence. During the entire analysis period, box D1 had a low- level convergence. This is a major mechanism that maintained this low- level convergence structure below 1 km ASL. The plot of Fig. 8c indicates recalculated Fr number using retrieved radar wind fields. The Fr > 1.0 is maintained at all times, even though it decreases as the typhoon passed. It means that the low-level background wind could overcome the mountain height and affect vertical air motion of the lee side. Fig. 9 shows the vertical distribution in box D2 (the windward region) and D3 (the leeward region) when Typhoon Khanun made landfall. The two vertical structures (D2 and D3; Fig. 9a and c) of maximum reflectivity are quite similar, but there is a difference in the cause of high reflectivity. The distinctive feature was found between 0.5 and 2 km ASL, where the reflectivity was above 45 dBZ. The radar reflectivity continuously enhanced between 0800 and 1300 UTC at both regions. High reflectivity values of D2 (Fig. 9a) are associated with enhanced precipitation based on forced condensation, which can occur as high as mountains. However, the high reflectivity values of D3 (Fig. 9c) are attributed to the shift of the hydrometeors grown in D2, rather than due to the condensation process. For this reason, the high reflectivity at D3 (leeward) is distributed much thinner than D2 (windward). This difference is one of the proofs of condensation and shifting processes mentioned in Section 4.1. In addition, D3 shows another distinctive feature after center of typhoon passed the region. An anomalous area of high reflectivity (> 39 dBZ) was found in D3 at 1300 UTC. The high reflectivity took place between 3 and 7 km ASL. The vertically thick reflectivity de- monstrates that the high reflectivity mentioned before (in Section 4.1) is different from general reflectivity profile at lee side. The upslope region (D2) shows a strong horizontal negative diver- − − gence (< −2.0 × 10 3 s 1) at low-level (Fig. 9b). This negative divergence was calculated by a sudden change in horizontal wind patterns caused by the topography and surface roughness changed (K. O. Lee et al., 2014). Thus, the low-level convergence (negative divergence) contributes to the upward vertical velocity on the windward side. In contrast to the upslope, downslope shows a different divergence pattern (Fig. 9d). Below 2 km ASL, most divergence values were positive − − (> 1.5 × 10 3 s 1). This is a common pattern caused by downward winds that are scattered onto the surface of the lee side.

5. Passive tracking experiments

5.1. Tracking airﬂow

Experiments that trace air parcels flowing along the wind field were conducted to clarify the process of lifting (descending) air and cloud Fig. 8. Temporal variation of vertical profile in D1 domain depicted in Fig. 5. The re- fl flectivity profile is presented by the maximum value. The wind component, relative enhancements. In Section 4, the path of air ow has been indicated as a vorticity (2 km ASL), divergence, and Froude number (D1) are mean values in the do- key factor to describe the enhanced precipitation near Jeju Island. Be- mains. cause air parcels inside clouds contain cloud water or droplets floating in the air, the trace to the air parcel means the path of the cloud's hy- side (D3). Fig. 8 shows the vertical structure of radar reflectivity, cal- drometeors (no appreciable fall speed). Therefore, the tracer can ex- fl culated wind speed, and horizontal divergence against time for D1 plain not only the ow of the air around Jeju Island but also the general fi (ocean). microphysics processes. A retrieved 3-D wind eld was utilized to move fi As the typhoon's center approached D1, the maximum reflectivity air parcels to the next position. The position is de ned by a backward fi was weak compared to the other analysis domains, being below 42 dBZ nite scheme. In order to observe the ascending process of low-level air, at 2 to 3 km ASL. There are no orographic characteristics, less surface the initial height of the parcels was set to 10 m ASL (parcel 1, 3, and 5). fl friction than both the D2 and D3 regions. The typhoon's location can be Fig. 10 shows the moving path of each air parcel owing with the fi identified by the maximum in relative vorticity (Fig. 8b). This is ac- wind eld. Parcel number 1 started from the surface (10 m ASL) and companied by the maximum v-component of the wind, which is above moved northwest, undergoing the lifting process along the southern − 30 m s 1 at 1 to 2 km ASL. The strong meridional wind causes a low- upslope. When the parcel reached the summit, it rose about 2 km in level jet in the principal rainbands (Didlake and Houze, 2009), and its height and subsequently crossed the mountain ridge. The time spent to maximum indicates that the typhoon's center is located to the west of reach the summit region from the Lifted Condensation Level (LCL; box D1. 400 m) was about 600 s, which is theoretically long enough to grow a A horizontal divergence distribution shows the general structural raindrop. In the case of environments wherein the upslope is very short feature of typhoon, with both a low-level convergence and an upper- such as on Dominica Island, the ascending time of air due to variables related to the terrain is not long enough, restricting the formation of

66 J.-T. Lee et al. Atmospheric Research 201 (2018) 58–71

Fig. 9. Temporal variation of vertical profile in D2 and D3 depicted in Fig. 5. Same as Fig. 8 else mean wind speed and relative vorticity. convective cells through condensation processes (Smith et al., 2009a), 1000 m ASL, respectively, but then could not overcome the summit although shallow convection in the model can occur within 300 s region even though Fr was sufficiently large value to pass the terrain, (Kirshbaum and Smith, 2009). However, the development of a con- and therefore this air moved around the side of the mountain. The vective cell is possible because the upslope on Jeju Island is of a suf- sharp, elliptical topography of the western upslope divides the airflow ficient width for the development of precipitation. The time taken from into two directions around the mountain and as such, the slope does not LCL to the top of the mountain (600 s) supports this argument. form a blocking obstacle to southwesterly flows, whereas the upslope in The paths of parcel numbers 3 and 5 conducted with a southwestern the other direction acts as an obstacle for air to pass over the terrain. airflow and revealed that the airflow was affected by the geographical This results in a situation wherein air can flow around the mountain shape of the western upslope. These parcels ascended to 600 m and despite a large Fr that is > 1.1. However, condensation or raindrop

Fig. 10. Track of air parcel as the retrieved wind ﬁeld. Moving track of air parcels are traced for an hour. No. 1–2 are from 0900 to 1000 UTC and No. 3–7 are from 1030 to 1130 UTC. Starting point of parcel falls into three diﬀerent places as the height: the surface (No. 1 and 3), 3 km ASL, and 5 km ASL.

67 J.-T. Lee et al. Atmospheric Research 201 (2018) 58–71 growth can still occur because a strong background wind still helps the shifted due to vertical velocity on the windward side. air to lift upslope above LCL. The path of tracers provides crucial evidence for the seeder–feeder Additionally, upward motions were found at the leeward side. process on the windward and leeward sides. The particles that fell to Parcel number 7 was tracked from 3 km ASL and reached a higher level the windward side (tracer 1, 2, and 4) were assumed to be rain water (6 km) after ascending and descending near the summit. Parcel num- and started at a height of < 5 km ASL. From a previous study on the bers 2 and 4, which started at 5 km ASL, also showed upward motion at small bell-shaped island (Smith et al., 2009a, 2012), it can be inferred the lee side after descending. The accelerated air around the summit (as that particles underwent an accretion process with lifted cloud water described in Section 4) was sufficient to cause a sudden energy con- (by terrain). The process in which precipitation increases from falling version process called a hydraulic jump at the downstream region. In particles explains the precipitation that occurs in geomorphic condi- addition, the releasing latent heat by condensation process helps create tions wherein the width of the island is too short to generate convective the unstable condition and broken mountain wave (Tang et al., 2012). cells. Since the width of Jeju Island is longer than small Island (Do- Broken waves are an ascending air from the middle layer after descent minica), this dynamic provides an explanation for the increase in pre- but not repetitive up and down patterns. In brief, the topography of Jeju cipitation. Island allows the formation of a convective cell in a typhoon environ- Tracers that fell on the lee side are assumed to be snow particles ment and can cause a hydraulic jump. The path of these tracers fully (tracer 3 and 6). Theoretically, based on the particle paths, the accre- reflects the movements of cloud water. tion and riming processes exist on the lee side. The clues to understanding the processes can be found in the path of the air indicated by 5.2. Tracking particle the parcel tracking experiment. Parcel number 1 and 3 rise to the middle level at lee side due to the upward motion. During this period, fl The experiments conducted on the tracking of airflow (Fig. 10) fo- cloud water included in each parcel shifted and owed to the lee side cused only on tracing air (without gravitational acceleration). The use and overlapped with falling particles (snow and ice). This is analogous ff of a tracer particle differs from the tracking of air parcels because the to the collection of cloud water on the windward side but di ers in that fl particles representing rain water in clouds have a gravitational accel- it was in uenced by the development processes (of the windward side), fl eration. Thus, the additional experiment considered the falling speed of through the growth of ice particles. The strong re ectivity at lee side particles (Fig. 11). A simple calculation (Shapiro et al., 1995) is used mentioned above (Fig. 7e) can be interpreted as a result of the super- that analyses the falling speed of rain water particles of about 25 dBZ position of hydrometeors. − (4.2 m s 1). This calculates the average fall velocity from radar reflectivity when the temperature is above 0 °C. Above the melting layer 6. Numerical experiments (5 km), particles are assumed to be snow and ice, and the fall speed is − considered as the constant value (0.9 m s 1) of the study of snow The reflectivity and wind fields obtained from radar observations particles (Rasmussen et al., 2003). Since snow particles falling below are quite effective in analyzing enhancement mechanisms but are in- 5 km are assumed to have melted (and developed into raindrops), an sufficient to substantiate the mechanism since most of the hydro- − average fall speed of 4.2 m s 1 was applied. meteors are theoretically assumed. Although assumptions have been Tracer numbers 1 to 3 (depicted in Fig. 11) show particles moving stated based on previous studies (Medina and Houze, 2003; Medina northwest on a southeasterly wind. Tracers 1 and 2 fell on the wind- et al., 2005; Houze, 2012; Yuter and Houze, 2003), it cannot clearly ward and mountain summit regions, respectively, but tracer 3 started at explain microphysical processes; their relation with the hydraulic jump 7 km ASL and moved further northwest such that it arrived at the lee- is difficult to verify. In order to alleviate some of these problems, the ward side. Tracer 3 indicated that snow fell slowly before reaching 5 km cloud-resolving model was implemented, and vertical air motion was ASL. In the region wherein a strong upward flow was generated by the analyzed with the hydrometeors. terrain, the particles increased due to a larger vertical velocity rather The simulated results were reliable enough to provide a reasonable than an increased falling speed. Tracers 1 and 4 were sent from the analysis (Fig. 2). The pressure difference between model and observa- southwest part of the island and fell to the windward but 5 and 6 tion is as small as 2–3 hPa. The pathway of the typhoon was quite si- dropped to the lee side. Most of the tracers (except for the tracer 1) milar to the observation, and the track error is < 20 km. The Fr number

Fig. 11. Track of particles as the retrieved wind ﬁeld. Moving track of particles are traced for an hour. No. 1–3 are from 0900 to 1000 UTC and No. 4–6 are from 1030 to 1130 UTC. Starting level of particles fall into three diﬀerent places as the height: the 3 km ASL (No. 1 and 4), 5 km ASL (No. 2 and 5), and 5 km ASL (No. 3 and 6).

68 J.-T. Lee et al. Atmospheric Research 201 (2018) 58–71

Fig. 12. (a) Simulated rainfall amount (shading, solid line contoured every 20 mm) around the Jeju Island with topography (dashed line contoured every 200 m), (b) Simulated radar reﬂectivity with the topography (solid line contoured every 200 m) at 1030 UTC on 18 July 2012. The details of D-DD line are depicted in Fig. 13.

− (model results) is maintained over 1.0 during the typhoon passed. In 6–7ms 1. The volume of cloud water starts increasing at 1.5 km al- addition, the well-simulated rainfall pattern contributes positively to titude (30 km from DD). This is the point wherein convection caused by model's reliability. the terrain and the highest cloud water value appears. The increase in Fig. 12a shows the simulated rainfall for 6 h around Jeju Island. The cloud water is due to the condensation process of the air that rises along model simulated the core of the precipitation near the summit with the terrain. From this point, rain water begins to increase to the more precipitation occurring to the northeast of Jeju Island. This dis- windward and reaches its highest value near the top of the mountain. tribution is quite consistent with the surface observations. Thus, the The background wind are sufficient to overcome the upslope results of the simulation are adequate and sufficient to provide clues (Fr = 1.31; in M1 domain). regarding the development mechanism revealed from the observations. A large amount of cloud water contributed to forming rain water in The distribution is quite similar to the actual track of the typhoon since the windward region, but it also shifted over the mountain to the lee the center of the typhoon is close to the southwestern part of Jeju Island side. The crank-sided green contour well indicates the distribution of (Fig. 12b). The spiral bands of the typhoon are well formed around the cloud water. To the lee side, descending flows due to the terrain oc- typhoon center. When the spiral band reaches the terrain, there is curred, and it accelerated the falling velocity of the hydrometeors. stronger reflectivity (over 45 dBZ) than that with other precipitation Moreover, hydrometeors above 5 km AGL (snow and cloud water) bands. It suggests that convection developed due to influences from the dropped to zero degrees. Rain water increased as the hydrometeors fell land further enhanced precipitation. and precipitated to the lee side. The D-DD line, the vertical velocity (shading), rain water (blue Eventually, the development of lee side precipitation was caused contour), cloud water (green contour), and ice (purple contour) are mainly by the accretion of snow as well as shifting rain water. depicted in Fig. 13. The vertical motion is in clear agreement with the According to Choularton and Perry (1986), snow has a large surface observations. The upward motion at the windward side occurs up to the area, so its accretion process is more efficient than rain water. There- peak of the mountain and has a maximum vertical velocity of fore, the increase of rain water through the accretion of snow occurred

Fig. 13. Vertical cross section (transect D-DD line in Fig. 12b) of − vertical velocity (color shading, m s 1) and hydrometeors (color contours) in simulation results. The terrain of the Jeju Island is masked. The Fr number represented the box M1.

69 J.-T. Lee et al. Atmospheric Research 201 (2018) 58–71 efficiently, explaining the enhanced precipitation at Jeju Island. windward and leeward sides, as the circular change in wind direction. At 5–10 km from DD, a nonlinear upward flow was generated up to Consequently, enhanced rainfall was dependent on the wind direction. 8 km ASL. In addition, the jump in air brings the increase of cloud water However, rainfall on the lee side was enhanced by the seeder-feeder above the lee side (upper level). These dynamic flows combined with an process clearly when the wind was blowing from the southwest. This increase in cloud water to the lee side can provide a favorable condition means that the windward side precipitation process affected the lee side for the enhancement of precipitation. process, which is assumed to be an effect of the upslope gradient or the This distribution of hydrometeors and vertical velocity can support shape of the windward side (Chen et al., 2008; Yu and Cheng, 2013). enhancement mechanisms (upslope condensation and hydraulic jump) In this study, the effect of the enhancement is indicated only in analyzed by the observations. single polarization Doppler radar observation. Therefore, in order to know the contribution of each process on precipitation, further research 7. Summary and conclusion with concrete observation (Dual-pol. Radar) and experiment is necessary. This paper has examined an enhanced mechanism over the windward and leeward sides of the Juju Island during the passage of Acknowledgements Typhoon Khanun (strong background wind fields). Jeju Island has a development mechanism for precipitation on both the windward and We acknowledge Dr. Hyun-Gyu Kang for suggestions regarding the leeward sides. In comparison, Dominica, which is a similarly isolated trajectory. This work was supported by the Korea Meteorological topographical feature, had a dry lee side when the typhoon was landing Industry Promotion Agency by Grants KMIPA 2015-5060. This research (Smith et al., 2009a). The difference in rainfall pattern between these was supported by the National Typhoon Center at Korea Meteorological two islands is caused by the time necessary for a localized convective Administration (NIMS-2016-3100). cell to be generated near the summit. The upslope of Jeju island serves the enough width for forming a cloud, even though it is not much References bigger than Dominica. On the windward side, air is forcibly lifted upslope as it is pushed Bergeron, T., 1965. On the Low-Level Redistribution of Atmospheric Water Caused by against the mountain. The mechanical forcing represented by the Fr Orography, Paper Presented at the International Conference on Cloud Physics. Int. Assoc. of Meteorol. and Atmos. Phys., Tokyo. was always over 1.0 in the dual-Doppler analysis. Consequently, the air Carruthers, D.J., Choularton, T.W., 1983. A model of the feeder–seeder mechanism of ascending due to the forcing underwent the condensation process after orographic rain including stratification and wind-drift effects. Q. J. Roy. Meteorol. LCL. The travel time of lifted air from LCL was theoretically sufficient Soc. 109 (461), 575–588. Chen, S.H., et al., 2008. Effects of unsaturated moist Froude number and orographic as- for a localized precipitation cell to be generated. Therefore, the gradual pect ratio on a conditionally unstable flow over a mesoscale Mountain. J. Meteorol. lifting of air, which generates a precipitation cell, contributed to higher Soc. 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