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  • View in gallery

    Climatology of total precipitation in winter (mm; contour) between 2004 and 2015 from AWS observations. The accumulated precipitation thresholds of 60 (thin red), 80 (thick red), and 100 mm (thick dark red), and highlighted by the hatched area. The topographic features are indicated by color shading. The locations of the TMR, PCB, SBB, Pyeongchang, and Gangneung are also marked.

  • View in gallery

    (a) As in Fig. 1, but for the observations used in this study and the topographic features around the Pyeongchang area. The locations of the GDK, GNG, WNJ, JWN, KAN, and SBS are denoted by triangles. The location of the sounding site at SC is denoted by an asterisk. The locations of the wind profiler sites at WJ and GN are denoted by squares. The three selected AWS sites (JC, SJ, and GM) are marked by circles. The area highlighted by the dashed box is the extended WISSDOM synthesis domain. (b) As in (a), but for the main WISSDOM synthesis domain adopted in this study.

  • View in gallery

    KMA surface analysis map obtained from 1800 UTC 20 Jan to 0000 UTC 22 Jan 2013 at 6-h intervals. Full wind barbs correspond to 5 m s−1; half barbs correspond to 2.5 m s−1.

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    The satellite images in the IR channel obtained from 1800 UTC 20 Jan to 0000 UTC 22 Jan 2013 at 6-h intervals.

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    (a) Horizontal distribution of the accumulated precipitation (mm; color shading) observed by the AWS in the WISSDOM domain from 2000 UTC 20 Jan to 0000 UTC 22 Jan 2013, during the study case (28 h). (b) As in (a), but for the frequency when the lowest available radar reflectivity >20 dBZ (%; contour). The marks indicate the locations of various observations as in Fig. 2b. The topographic height thresholds of 400 (thin) and 800 m MSL (thick) are shown.

  • View in gallery

    (a) Time series of precipitation rate (mm h−1; thick) and accumulated precipitation (mm; thin line) with 1-min temporal resolution observed from GM (red), SJ (blue), and JC (green) from 1500 UTC 20 Jan to 0600 UTC 22 Jan 2013. (b) As in (a), but for profiles of horizontal winds observed from GN (red wind barbs) and WJ (blue wind bards). Full wind barbs correspond to 5 m s−1; half barbs correspond to 2.5 m s−1. The time windows of stages I and II are also marked.

  • View in gallery

    Vertical profiles of temperature T (dash–dotted), equivalent potential temperature (solid), and wind speed Ws (dashed) observed from the SC sounding at 0000 (red) and 1200 UTC (blue) 21 Jan 2013. Corresponding winds are also shown. Full wind barbs correspond to 5 m s−1; half barbs correspond to 2.5 m s−1.

  • View in gallery

    (a) Horizontal winds (vectors) and temperature (K; color shading) obtained from ERA-Interim at the 925-hPa level at 0600 UTC 21 Jan 2013. (b) As in (a), but for the 800-hPa level. (c) Horizontal distribution of the maximum radar reflectivity (dBZ; color shading) and (d) averaged ground-relative winds below 1 km MSL (vectors) and topographic feature (m; color shading) at 0600 UTC. The topographic height thresholds of 400 (thin), 800 (thick), and 1200 m MSL (thick dark). The inserted box (dashed box) indicates the main (extended) WISSDOM synthesis domain.

  • View in gallery

    (a) Horizontal distribution of the maximum radar reflectivity (dBZ; color shading) observed in the WISSDOM domain at 0600 UTC 21 Jan 2013. The black line of A–A′ marks the locations of the vertical cross sections shown in Fig. 10. The inserted box indicates the averaged area of vertical cross sections along the southwestern end (SW) to northeastern coast (NE) of the PCB corresponding to Fig. 11. (b) As in (a), but for the averaged ground-relative winds below 1 km MSL (vectors) and topographic feature (m; color shading). (c) As in (b), but for horizontal divergence (10−4 s−1; color shading). The topographic height thresholds of 400 (thin), 800 (thick), and 1200 m MSL (thick dark).

  • View in gallery

    (a) Vertical cross section of the WISSDOM-derived radar reflectivity (dBZ; color shading) and wind vectors (combined cross-barrier flow and quadruple vertical velocity) along the solid line segment A–A′ indicated in Fig. 9a at 0600 UTC 21 Jan 2013. (b) As in (a), but for the vertical velocity (m s−1; color shading) with the cross-barrier (along barrier) flow indicated by black (purple) contours with an interval of 3 m s−1. Positive values of the cross-barrier (along barrier) flow mean that the flow blew toward A′ (into the paper) and vice versa. The brown shading in the lower portion indicates the topography along the cross section.

  • View in gallery

    As in Fig. 10, but showing the averaged vertical cross section along the SW–NE box of Fig. 9a. Positive values of the averaged cross-barrier (along barrier) flow indicate that the flow blew toward the NE (into the paper) and vice versa. The brown shading in the lower portion indicates the averaged topography along the box. The backward trajectories of precipitation particles from the surface at X = 70–150 km (thick black).

  • View in gallery

    As in Fig. 9, but (a),(b) at 1530 and (c),(d) 1830 UTC 21 Jan 2013. The inserted boxes indicate the averaged area of vertical cross sections along the SW to NE of the PCB corresponding to Figs. 14 and 15. The topographic height thresholds of 400 (thin), 800 (thick), and 1200 m MSL (thick dark) are shown.

  • View in gallery

    The horizontal divergence (10−4 s−1; color shading) from 1530 to 1830 UTC in every hour interval. The topographic height thresholds of 400 (thin), 800 (thick), and 1200 m MSL (thick dark) are shown.

  • View in gallery

    As in Fig. 11, but along the SW–NE box in Fig. 12a at 1530 UTC 21 Jan 2013.

  • View in gallery

    As in Fig. 11, but along the SW–NE box in Fig. 12c at 1830 UTC 21 Jan 2013.

  • View in gallery

    Temporal variation of the average precipitation (color shading) and the horizontal winds (wind barbs) from WISSDOM derived in the vicinity of the Pyeongchang area from 2000 UTC 20 Jan to 0000 UTC 22 Jan 2013. The low-level radar reflectivity and winds (below 1 km MSL) within the SW–NE boxes (shown in Figs. 9a, 12a, and 12c) were averaged in a direction normal to the orientation of the box and plotted as a function of time and along-box distance. The brown shading in the lower portion indicates the averaged topography along the box.

  • View in gallery

    Schematic diagram illustrating the structural characteristics of low-level precipitation and airflow for (a) stage I, and (b) the developing (1400–1600 UTC) and (c) moving periods (1600–2000 UTC 21 Jan 2013) of the precipitation band in stage II in horizontal and vertical views. The environmental wind is indicated by wind barbs with a full (half) wind barb corresponding to 5 (2.5) m s−1. Red solid arrows indicate the topographically modified airflow observed from the WISSDOM synthesis, and blue shading denotes the generalized precipitation pattern, with darker shading denoting regions of heavier precipitation. Black shading indicates the gross features of topography in the studied domain. The area of convergence and the blocked zone are also indicated in (a) and (b). The locations of the TMR, PCB, and GNC are also marked.

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Impacts of Topography on Airflow and Precipitation in the Pyeongchang Area Seen from Multiple-Doppler Radar Observations

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  • 1 Department of Astronomy and Atmospheric Sciences, and Center for Atmospheric Remote Sensing (CARE), Kyungpook National University, Daegu, South Korea
  • | 2 Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu, South Korea
  • | 3 Department of Atmospheric Sciences, National Central University, Jhongli, Taiwan
  • | 4 Department of Astronomy and Atmospheric Sciences, and Center for Atmospheric Remote Sensing (CARE), Kyungpook National University, Daegu, South Korea
  • | 5 Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan
Open access

Abstract

This study uses high-resolution radar and surface observations to investigate the finescale structural evolution of airflow and precipitation over complex terrain in the Pyeongchang area, South Korea. The Taebaek Mountain range (TMR) runs parallel to the northeastern coast of South Korea, with a perpendicular ridge known as the Pyeongchang branch (PCB). The objective of this study was to identify the mechanisms of wintertime precipitation over these topographic features during the passage of a low pressure system (LPS) through the southern Korean Peninsula. The analysis indicates that intense precipitation occurred over the southwestern and northeastern sides of the TMR during stage I but only over the northeastern side during stage II. The prevailing southwesterly winds were dominated by warm advection associated with the LPS over the PCB during stage I. These prevailing southwesterly winds locally enhanced precipitation on the southwestern end of the PCB; multiple influences of mountain waves, airflow convergence, and drifted particles are possible factors for causing precipitation on the northeastern side of the TMR. During stage II, the prevailing winds changed from easterlies to northeasterlies offshore from Gangneung. The easterly winds decelerated and were deflected locally along the mountainous coast, and this blocked zone interacted with the oncoming flow to trigger a precipitation band. Consequently, the northeasterly winds helped push the precipitation band toward the coast, causing heavy precipitation in Gangneung. The observational evidence presented shows that the interaction of temporally changing winds accompanying the movement of an LPS over topography is a critical factor for determining the distribution and intensity of precipitation.

Denotes content that is immediately available upon publication as open access.

Current affiliation: Laboratoire de l’Atmosphère et des Cyclones (LACy, UMR-8105, CNRS/Météo France/Université de La Réunion), Saint-Denis, La Réunion, France.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Dr. Chia-Lun Tsai, chialuntsai@me.com

Abstract

This study uses high-resolution radar and surface observations to investigate the finescale structural evolution of airflow and precipitation over complex terrain in the Pyeongchang area, South Korea. The Taebaek Mountain range (TMR) runs parallel to the northeastern coast of South Korea, with a perpendicular ridge known as the Pyeongchang branch (PCB). The objective of this study was to identify the mechanisms of wintertime precipitation over these topographic features during the passage of a low pressure system (LPS) through the southern Korean Peninsula. The analysis indicates that intense precipitation occurred over the southwestern and northeastern sides of the TMR during stage I but only over the northeastern side during stage II. The prevailing southwesterly winds were dominated by warm advection associated with the LPS over the PCB during stage I. These prevailing southwesterly winds locally enhanced precipitation on the southwestern end of the PCB; multiple influences of mountain waves, airflow convergence, and drifted particles are possible factors for causing precipitation on the northeastern side of the TMR. During stage II, the prevailing winds changed from easterlies to northeasterlies offshore from Gangneung. The easterly winds decelerated and were deflected locally along the mountainous coast, and this blocked zone interacted with the oncoming flow to trigger a precipitation band. Consequently, the northeasterly winds helped push the precipitation band toward the coast, causing heavy precipitation in Gangneung. The observational evidence presented shows that the interaction of temporally changing winds accompanying the movement of an LPS over topography is a critical factor for determining the distribution and intensity of precipitation.

Denotes content that is immediately available upon publication as open access.

Current affiliation: Laboratoire de l’Atmosphère et des Cyclones (LACy, UMR-8105, CNRS/Météo France/Université de La Réunion), Saint-Denis, La Réunion, France.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Dr. Chia-Lun Tsai, chialuntsai@me.com

1. Introduction

Topography is one of the critical factors that determine the intensity and distribution of precipitation (Medina and Houze 2003; Rotunno and Ferretti 2003; Houze and Medina 2005; Rotunno and Houze 2007). Houze (2012) concluded that the nature of orographic precipitation is determined largely by topographic features, the direction and strength of airflow, environmental conditions, stability, and microphysical processes. The main topographic features in South Korea are associated with the Taebaek Mountain range (TMR) along the northeastern coast, which has several connected ridges, such as the Pyeongchang branch (PCB) and the Sobaek branch (SBB; see Fig. 1). These two ridges are oriented northeast–southwest, approximately perpendicular to the TMR. A study of orographic precipitation over complex features of the TMR using ground-based radar would have required a significant increase in the already numerous radars in this area. Hence, only a limited number of previous modeling studies have indicated that heavy rainfall and snowfall events occurred frequently near the TMR during winter (Lee and Park 1996; Lee and Kim 2008; Jung et al. 2012; Lee and Xue 2013). Pyeongchang, South Korea, hosted the twenty-third Winter Olympics in 2018 (most venues were located in Gangneung, South Korea, and in higher-altitude areas along the PCB, indicated by arrows in Fig. 1). Our work was completed prior to the Olympic Games as part of the effort to ensure adequate forecasts, such as high-quality wind and precipitation information, were available to ensure the safety of all participants. Furthermore, these observational data are important for improving numerical weather forecasting over the Taebaek Mountains. Detailed observations are necessary to increase our understanding of orographic precipitation in the Pyeongchang area, especially in winter.

Fig. 1.
Fig. 1.

Climatology of total precipitation in winter (mm; contour) between 2004 and 2015 from AWS observations. The accumulated precipitation thresholds of 60 (thin red), 80 (thick red), and 100 mm (thick dark red), and highlighted by the hatched area. The topographic features are indicated by color shading. The locations of the TMR, PCB, SBB, Pyeongchang, and Gangneung are also marked.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

Topographically enhanced precipitation usually occurs during the passage of preexisting weather disturbances, and the interactions of such weather systems with topography are complex because of temporally changing environmental conditions (Roe 2005; Smith 2006; Houze 2012). Orographic snow clouds may be generated under conditions of weak cold-air advection (Kusunoki et al. 2004). Bousquet and Smull (2003) combined airborne and ground-based radar measurements to investigate a case of widespread orographic precipitation during the passage of a deep baroclinic trough during the Mesoscale Alpine Programme (MAP; Bougeault et al. 2001). Medina et al. (2007) showed that enhanced precipitation could be produced by interactions between turbulence and orography as extratropical cyclones move over the Cascade Range in Oregon. The airflow and precipitation associated with tropical cyclones can also be modified significantly as they pass over topography in Taiwan (Yu and Cheng 2008; Yu and Tsai 2017). Changes in precipitation patterns and intensities are strongly affected by interactions between various airflow features associated with different weather systems and topography.

Over the past decade, an automatic weather system (AWS) with more than 600 stations has been established in South Korea; such a high density of surface observations can be considered as representative for surface precipitation analysis. Figure 1 illustrates the climatology of total precipitation that was observed by the AWS in the winter season (December–February) between 2004 and 2015, near Pyeongchang. The intensity of precipitation decreased from the coast to the TMR, with a corridor of higher amounts of precipitation extending southwestward over the PCB (highlighted by the hatched area in Fig. 1). This pattern of precipitation (the decrease from coastal to mountainous areas) was observed only near the TMR and PCB. This observational evidence indicates that the complex topography in the northeastern part of South Korea may have high potential to modify the intensity and distribution of precipitation in winter.

The region of the TMR is usually influenced by a northeasterly airflow in winter, with stronger cross-barrier flow and stable environmental conditions (Lee and Kim 2008; Jung et al. 2012). Conversely, for a weak cross-barrier flow scenario, a topographically blocked zone may trigger moist convection near the coast or offshore; the specific location of the convection is determined largely by the upstream extent of orographic and diurnal influences (Yu and Jou 2005; Yu and Hsieh 2009; Yu and Lin 2017). Based on the numerical simulations of Lee and Park (1996), the topographic effect may dominate the characteristics of precipitation around the Pyeongchang area because of airflow being influenced by the TMR, which affects snowfall along the northeastern coast of South Korea. Lee and Kim (2008) utilized the Weather Research and Forecasting (WRF) Model to investigate topographic effects on snowfall distribution near the TMR. Stronger northeasterly winds were lifted over the windward side, causing heavy snowfall on the northeastern slope of the TMR. Another numerical study by Jung et al. (2012) found that the locations of snowfall near the TMR were related to the direction of the prevailing wind of a low pressure system (LPS). Intense snowfall tended to occur near Gangneung when the prevailing wind (i.e., northerly or northwesterly flow) was obliquely to the orientation of the TMR. However, locations of heavy snowfall shifted from Gangneung to the inland mountainous area when the prevailing winds changed to northeasterlies (i.e., closer to being perpendicular to the axis of the TMR).

Results from previous numerical studies provide some explanations for the occurrence of orographic precipitation under a steady wind direction near the TMR. However, detailed precipitation and airflow structures and their causative mechanisms derived from reliable observations in variable airflow conditions have not been well documented. Park and Lee (2009) used ground-based multiple-Doppler radar synthesis to retrieve the wind field over the southern Korean Peninsula. Although Doppler radar can provide high temporal and spatial resolution with wide coverage of radar reflectivity and radial velocity, the limitations of ground-based Doppler radar should be considered, particularly in terms of beam blockage over complex mountainous areas in the Pyeongchang area. Recently, to address these needs, a new scheme—Wind Synthesis System using Doppler Measurements (WISSDOM; Liou and Chang 2009; Liou et al. 2012, 2014)—was adopted to construct a more complete analysis of 3D precipitation and wind fields. WISSDOM can optimally combine the wind information from radar measurements, soundings, model forecasts, and other sources, such as surface observations, to provide a more realistic description of the precipitation and flow field (details of which will be introduced in section 2) over topography in Pyeongchang.

The primary objective of this study was to use high-temporal- and high-spatial-resolution observational data to investigate the finescale structural evolution of airflow and precipitation over the complex terrain in the Pyeongchang area during the period 20–22 January 2013. Furthermore, the results of this case study provided an important reference for the weather forecast of the 2018 Winter Olympics/2018 Paralympic Winter Games. Multiple radars and the AWS were used to derive 3D airflow and precipitation information over the mountainous regions using WISSDOM. In particular, this study attempted to identify the possible mechanisms of precipitation over topography during the passage of an LPS moving from the southwestern to the southeastern side of the Korean Peninsula. During the passage of the LPS, the prevailing winds changed from southeasterly to easterly/northeasterly over the ocean and were composed of veering winds (from southeasterly to southwesterly) under the influence of warm advection in Pyeongchang. The main reason this event was chosen for detailed analysis in this study is not only because it is a typical case causing significant winter precipitation (exceeding 40 mm) but also because it features alternate changes in prevailing flow. Interactions between complex topography and changing upstream airflow can be explored in this case study. In addition, six ground-based radars were established around the Pyeongchang area, which minimized the influence of beam blockage by the complex topography.

2. Data and methodology

a. Observational data

The locations of selected ground-based Doppler radars, soundings, wind profilers, AWS sites, and the topographic features in the vicinity of Pyeongchang are indicated in Fig. 2a. The main WISSDOM domain is centered at 37.75°N, 128.75°E, which covers most of the TMR, and has a horizontal coverage of 1.5° × 1.5°, as indicated by the inset box in Fig. 2b. In addition, to better understand the upstream conditions of the Pyeongchang area, an extended WISSDOM synthesis was also created in this study as shown by the area in the dashed box of Fig. 2a. The center of this extended domain is 37.25°N, 128.25°E with a horizontal coverage of 2.5° × 2.5°. The TMR has typical elevations of approximately 1.5 km above mean sea level (MSL). Two ridges (i.e., the PCB and SBB) extend southwestward with a gentle slope but exhibit a relatively steep slope on the northeastern side (Fig. 2b). Two Korea Meteorological Administration (KMA) operational S-band (10 cm) Doppler radars are located at Gwangdeoksan (GDK) west of the WISSDOM domain and in Gangneung (GNG) near the northeastern coast. Three C-band (5 cm) Doppler radars of the Korean Air Force (KAF) are located at Wonju (WNJ), Jungwon (JWN), and Gangneung (named KAN). The Ministry of Land, Infrastructure and Transport (MOLIT) S-band dual-polarization radar at Sobaek-san (SBS) is located just outside the southern edge of the WISSDOM domain. Radar reflectivity and radial velocity measurements from these six radars were used for analysis in this study.

Fig. 2.
Fig. 2.

(a) As in Fig. 1, but for the observations used in this study and the topographic features around the Pyeongchang area. The locations of the GDK, GNG, WNJ, JWN, KAN, and SBS are denoted by triangles. The location of the sounding site at SC is denoted by an asterisk. The locations of the wind profiler sites at WJ and GN are denoted by squares. The three selected AWS sites (JC, SJ, and GM) are marked by circles. The area highlighted by the dashed box is the extended WISSDOM synthesis domain. (b) As in (a), but for the main WISSDOM synthesis domain adopted in this study.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

The GDK, GNG, WNJ, JWN, and KAN radars are operated with a temporal interval of 10 min between each volume, while the SBS radar is operated with a temporal interval of 2 min. If there is a discrepancy greater than 2 min between the recorded and retrieved times, the WISSDOM synthesis will eliminate them to ensure coherence of the data. The range of scanning elevations (maximum observational range) for all the radars is −0.5° to 80° (90–250 km). The volumetric distributions of these radar data cover the WISSDOM domain sufficiently to minimize data gaps in the mountainous areas. Details about the characteristics of each of the radar are described in Table 1. To obtain correct and useful radar measurements over the complex topography, especially for reducing beam blockage, the Kyungpook National University hybrid surface rainfall (KHSR; Kwon et al. 2015; Lyu et al. 2015; Kwon 2016) method was adopted in this study. KHSR can construct a dataset from the lowest available radar reflectivity to analyze the horizontal distribution of the precipitation. The lowest radar reflectivity indicated that the effective data located just above topography (i.e., the lowest height) and composited by observing maximum reflectivity form each radar. Nonmeteorological radar echoes were eliminated for each radar station by a fuzzy logic algorithm (Cho et al. 2006; Ye et al. 2015) before performing WISSDOM. As part of the quality control scheme for all radar, observations were screened using a relatively high discrimination threshold (to eliminate data if there was an eclipse of topography of more than 10%) to retain only reliable data. Thus, some meteorological radar echoes may be removed near topography.

Table 1.

Characteristics of adopted radars.

Table 1.

Vertical profiles of temperature, dewpoint, pressure, wind speed, and wind direction from the sounding site at Sokcho (SC; Fig. 2) observed every 12 h were used to represent the environmental conditions. Two wind profiler stations are located on the southwestern slopes of the TMR, one near Wonju (WJ) and the other near Gangneung (GN) (cf. Fig. 2b). The wind profilers provide high-temporal-resolution (10-min interval) and high-spatial-resolution (approximately 70 m for each gate, up to 5 km MSL) wind information. The AWS surface observations provide high-temporal-resolution (1-min interval) meteorological parameters, and the average distance between AWS stations over the mountain area is approximately 12 km.

b. WISSDOM

WISSDOM uses a mathematical variational-based method to minimize the cost function
e1
at the retrieval time. The main differences between WISSDOM and the traditional approach for this particular Pyeongchang case are that WISSDOM can use the data from all radars in that area simultaneously and that the retrievals can be done directly over complex terrain. The wind field (u, υ, w components) is obtained by variationally adjusting the solutions to simultaneously satisfy five constraints [i.e., in Eq. (1)] in the cost function at the same time. The first constraint is the geometric relation between multiple-radar radial velocity observations and Cartesian winds (u, υ, w) and is defined as
e2a
e2b
e2c
where the subscripts (from 1 to ) and (from 1 to 2) are indices for each radar and two time levels (i.e., the retrieved time and 10 min before the retrieved time), respectively; is the radial winds observed by the ith radar at time ; denote the 3D wind at location at ; depict the coordinates of the ith radar; stands for the distance from each grid point to the ith radar; and represents the terminal velocity. The second constraint, which is the difference between the retrieved wind field () and the background winds (), is defined in Eq. (3),
e3
The third constraint is the anelastic continuity equation given by
e4
where is the air density. The fourth constraint comes from the vertical vorticity equation and is added to the cost function with this form:
e5
where is the Coriolis parameter and the overbar is a temporal average of two time levels. The fifth constraint is a Laplacian smoothing filter, denoted as
e6
The procedure of variational analysis is repeated until the cost function reaches a minimum and a more realistic 3D wind field is synthesized.

The primary advantages of WISSDOM are (i) the wind field along the radar baseline can also be well recovered; (ii) the retrieved wind fields satisfy the vertical vorticity equation and thus can be used directly for vorticity budget analysis; (iii) data from any number of radars can be easily merged; (iv) topographic forcing is taken into account by adopting the immersed boundary method (IBM) of Tseng and Ferziger (2003) so that wind retrieval can be conducted directly over complex terrain; and (v) a background wind field, which can be prepared from sounding observations or the outputs of a mesoscale model, is provided to cover the radar-data-void region. The resulting high-quality synthesized 3D wind field has been applied in several previous studies, such as those by Liou and Chang (2009), Liou et al. (2012, 2013, 2014, 2016), and Lee et al. (2018).

The main (extended) WISSDOM synthesis was performed over the northeastern part of South Korea with a spatial coverage of 1.5° × 1.5° (2.5° × 2.5°), as shown in Fig. 2. However, since all the input data were interpolated into a Cartesian coordinate system, we also transferred the original domain into the Cartesian system as 150 × 150 km2 (250 × 250 km2) for the main (extended) WISSDOM domain. The horizontal grid size was set to 1 km, and the vertical grid size was set to 0.25 km from 0- to 10-km height (MSL). High-temporal-frequency radar observations allowed WISSDOM to generate 3D reflectivity and wind fields every 10 min. There are 169 synthesis times available, from 2000 UTC 20 January to 0000 UTC 22 January 2013 (28 h).

3. Overview of the studied case

a. LPS and precipitation

The center of the LPS initially moved northeastward after it formed over the Yellow Sea near 34°N, 122°E at 1800 UTC 20 January 2013 (Fig. 3a). The central surface pressure of the LPS was about 1019 hPa, and the LPS continued to move toward the western coast of South Korea with lower surface pressure (from 1019 to 1015 hPa) in the next 12 h (Figs. 3b and 3c). Note that it appears that a new low center develops south of the Korean Peninsula as the one that moved northeast toward the western coast of South Korea decayed. Subsequently, the LPS started to move eastward after passing over the southern part of the Korean Peninsula and approached land in Japan between 1200 UTC 21 January and 0000 UTC 22 January 2013 (Figs. 3c–f). Figures 4a–f show satellite images in the infrared (IR) channel; cloud shields (relatively bright areas) can be observed ahead of the moving LPS. Those observed cloud shields suggest that the study area is influenced by warm advection and potentially warm front precipitation. The warm advection may provide enough water vapor with larger-scale ascent near the leading edge of the warm front to produce precipitation in the Pyeongchang area. The evidence of warm advection can be validated by wind profiler, sounding observations, and model reanalysis data as presented in the following analysis.

Fig. 3.
Fig. 3.

KMA surface analysis map obtained from 1800 UTC 20 Jan to 0000 UTC 22 Jan 2013 at 6-h intervals. Full wind barbs correspond to 5 m s−1; half barbs correspond to 2.5 m s−1.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

Fig. 4.
Fig. 4.

The satellite images in the IR channel obtained from 1800 UTC 20 Jan to 0000 UTC 22 Jan 2013 at 6-h intervals.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

Furthermore, the accumulated surface precipitation (Fig. 5a) shows that heavy snowfall and rainfall occurred along the northeastern coast (30–40 mm) and along the PCB (20–30 mm) during the 28 h from 2000 UTC 20 January to 0000 UTC 22 January 2013 (i.e., the study period). This precipitation distribution is similar to the climatological precipitation pattern in winter; that is, the majority of precipitation was found on the northeastern coast of South Korea exceeding 100 mm. A large precipitation amount was also observed along the TMR and PCB (60–80 mm) as shown in Fig. 1. To evaluate whether the radar observations can reasonably represent the distribution and intensity of precipitation, the frequency of heavy precipitation during the study period (i.e., 28 h) when the lowest available radar reflectivity (processed by KHSR) was greater than 20 dBZ was adopted here to compare the distribution of precipitation with surface observations. Figure 5b shows that the frequency of heavy precipitation is qualitatively consistent with the pattern of accumulated surface precipitation. Relatively higher frequency (approximately 40%) was observed along the PCB with the maximum values (exceeding 80%) detected around Gangneung. Lower frequency was observed near southeastern regions of Gangneung, possibly as a result of topographic blockage.

Fig. 5.
Fig. 5.

(a) Horizontal distribution of the accumulated precipitation (mm; color shading) observed by the AWS in the WISSDOM domain from 2000 UTC 20 Jan to 0000 UTC 22 Jan 2013, during the study case (28 h). (b) As in (a), but for the frequency when the lowest available radar reflectivity >20 dBZ (%; contour). The marks indicate the locations of various observations as in Fig. 2b. The topographic height thresholds of 400 (thin) and 800 m MSL (thick) are shown.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

To understand the fluctuation of the surface precipitation along the PCB and the northeastern coast under the influence of the LPS, three AWS stations were selected to represent the fluctuations of surface precipitation on the southwestern slope of the topography (JC), the mountain crest (SJ), and the northeastern coast (GM) (locations indicated in Figs. 2b and 5b). Figure 6a shows the time series of surface precipitation observed by the selected AWS during the study period. Fluctuations in surface precipitation rate show different patterns before and after 1200 UTC 21 January 2013; on this basis, the study period is divided into stages I and II, as indicated in Fig. 6a. In stage I, relatively low precipitation rates (between 0.5 and 1 mm h−1) were observed at these three stations in the first 5 h (from 2000 UTC 20 January to 0100 UTC 21 January 2013). However, the precipitation rate rose sharply from around 1 to approximately 3 mm h−1 over a 9-h period, beginning at 0100 UTC at JC and a relatively small increase in precipitation rate (approximately 1–2 mm h−1) also occurred at SJ. In comparison to the relatively minor change in the precipitation rate at GM, stronger (weaker) precipitation clearly occurred at the southwestern slope (mountain crest). Subsequently, only GM showed the dramatic increased precipitation rate in 5 mm h−1 at around 1900 UTC in stage II. For stage I, the highest value of accumulated precipitation was observed at JC (30 mm), a relatively smaller value was detected at SJ (22 mm), and the lowest value was observed at GM (16 mm). However, the highest value of accumulated precipitation at the end of stage II was observed at GM (33 mm) because of the contribution of a sharp increase in precipitation at 1900 UTC 21 January (>5 mm h−1).

Fig. 6.
Fig. 6.

(a) Time series of precipitation rate (mm h−1; thick) and accumulated precipitation (mm; thin line) with 1-min temporal resolution observed from GM (red), SJ (blue), and JC (green) from 1500 UTC 20 Jan to 0600 UTC 22 Jan 2013. (b) As in (a), but for profiles of horizontal winds observed from GN (red wind barbs) and WJ (blue wind bards). Full wind barbs correspond to 5 m s−1; half barbs correspond to 2.5 m s−1. The time windows of stages I and II are also marked.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

Time series of the wind profiles at GN and WJ (locations in Fig. 2a) are shown in Fig. 6b. GN is located at Gangneung near the northeastern coast, whereas WJ has a more inland location, close to the southwestern end of the PCB. The results at GN indicate a general veering of winds with height, from southeasterly below approximately 2 km MSL to stronger westerlies aloft, from 0000 to 1000 UTC 21 January. The southeasterly winds became easterly at an altitude of around 1 km MSL after 1000 UTC; northerly winds existed only locally at around 0.5 km MSL, with easterly winds aloft from 1200 to 1700 UTC. Such low-level (below 1 km MSL) northerly winds are more aligned with the TMR, consistent with the potential influences from the topography. The low-level winds started to become northeasterly after 1700 UTC. Prior to 1200 UTC 21 January, WJ observed relatively weak southeasterly winds below 1 km MSL and veered to stronger west-southwesterlies with height. After 1200 UTC, WJ recorded quite weak and indefinite winds below 2 km MSL, associated with the distant LPS and the obstacle of the mountain. According to the results from the wind profiler observations, the winds below around 2 km also can be separated into two stages. In stage I, the wind patterns may have veered from southeasterly (below 1 km MSL) to southwesterly winds (around 2 km MSL) at both sites; hence, the prevailing winds were southwesterly over the Pyeongchang area. The winds below approximately 2 km observed at GN changed from southeasterly to easterly after 1200 UTC during stage II.

The observational data of wind profilers indicate that the prevailing winds exhibited veering winds with height, consistent with warm advection. The changes in wind direction were observed by wind profilers and depicted by synoptic analysis; they suggested a strong link with the fluctuation of surface precipitation. The observations of the wind profilers support the importance of warm advection/front accompanying the mesoscale precipitation region and orographic enhancement, which almost doubled the precipitation between the northeastern coast and the southwestern slope in stage I. The fluctuations in surface precipitation and prevailing wind imply that the significant temporal and spatial variations in these two stages may be the result of different mechanisms. In stage I, inland airflow interacted with topography to enhance the precipitation associated with warm advection along the PCB and at Gangneung; in stage II, the oncoming airflow from the ocean triggered a precipitation band that moved inland to cause heavy precipitation along the northeastern coast. A detailed analysis and a discussion of precipitation and airflow structures associated with topography will be presented in section 4.

b. Environmental conditions

The sounding station located at SC (Fig. 2) can be used to represent the environmental stability and characteristics of the prevailing winds in the Pyeongchang area. Figure 7 shows a convectively stable environment (i.e., equivalent potential temperature increases with height) during the study period with the exception of a very shallow layer of neutral or convective instability that was found near the surface at 0000 and 1200 UTC 21 January. The prevailing winds were clearly affected by topography below 500 m MSL because they demonstrate no obvious changes in direction from 0000 to 1200 UTC, with persistent northwesterly or northerly flow. In contrast, the LPS dominated the prevailing winds between 1 and 2 km MSL, causing them to be more southeasterly (easterly) at 0000 (1200) UTC 21 January. Stronger westerly flow was detected above ~2 km MSL, indicating a relatively shallow influence from the LPS in this case. The strong veering of the airflow from ~500 to 2250 m is consistent with warm advection; note that the similarity of the wind observations between GN (cf. Fig. 6b) and the sounding suggests that the environmental wind can be described appropriately by the wind profiler observations.

Fig. 7.
Fig. 7.

Vertical profiles of temperature T (dash–dotted), equivalent potential temperature (solid), and wind speed Ws (dashed) observed from the SC sounding at 0000 (red) and 1200 UTC (blue) 21 Jan 2013. Corresponding winds are also shown. Full wind barbs correspond to 5 m s−1; half barbs correspond to 2.5 m s−1.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

The topographic influence on the low-level prevailing winds can be preliminarily evaluated by Froude number analysis (Smith 1979; Overland and Bond 1993)
e7
where is the flow toward the mountain barrier, is the dry Brunt–Väisälä frequency, and is the representative mountain height. The Froude number is an important nondimensional parameter determining whether flow tends to climb over () or be deflected near () topography. Although there is no single best method to estimate the upstream static stability (Reinecke and Durran 2008) for calculating , the sounding observations are the preferred dataset to simply represent the environmental conditions in this case. The average mountain height over the study area is 1 km (MSL). The dry Brunt–Väisälä frequency calculated below the average mountain height was 0.0139 (0.0126) s−1 at 0000 (1200) UTC 21 January. The prevailing winds can be used to represent when evaluating the maximum magnitude of the Froude number because they are influenced by the LPS and topography at heights below 2 km MSL (cf. Fig. 7). The strongest wind speed below 2 km MSL was ~12 and ~10 m s−1 at 0000 and 1200 UTC, respectively, and the estimated values of the maximum Froude number associated with the observed wind speeds were 0.79–0.86. It is appropriate to apply the dry Brunt–Väisälä frequency because the environmental relative humidity in this case study was only 80%–90% (an insufficiently saturated environment below the crest). The maximum value of is 1.3 (the mean values of is 0.8) after sensitivity testing using the dry/saturated Brunt–Väisälä frequency and increasing the value of from 800 to 2000 m during the study period. These relatively low Froude numbers indicate a stable atmosphere that prevented airflow from climbing over the topography; hence, the airflow tended to be deflected by the mountain barrier, which would have modified the precipitation in the Pyeongchang area.

4. Structural evolution of airflow and precipitation over topography

Considering the fluctuations of the observed precipitation and the evolution of the wind profile described above, three times—0600, 1530, and 1830 UTC 21 January—were chosen to illustrate the detailed airflow and precipitation structures along the PCB. These particular times were chosen because 0600 UTC is the intermediate time of stage I and 1530 and 1830 UTC are times before and during the dramatic increase in precipitation observed in stage II (cf. Fig. 6a).

a. Stage I (0600 UTC 21 January 2013)

ERA-Interim (Dee et al. 2011) depicts the larger-scale circulation and temperature associated with the LPS at 0600 UTC in stage I (see Figs. 8a and 8b); this reanalysis dataset can be downloaded from the European Centre for Medium-Range Weather Forecasts (ECMWF) web interface. Six-hourly temporal-resolution data and 0.125° spatial-resolution data were applied in this study. A cyclonic circulation surrounded the center of the LPS (near 36.5°N, 124.5°E) at the 925-hPa layer. Most of South Korea was covered by southeasterly winds accompanying warmer air under the influence of the LPS, except for the southwestern part between 34° and 36°N, 126° and 128°E (Fig. 8a). Figure 8b shows that the southwesterly and westerly winds covered all of South Korea at 800 hPa. The veering winds with height illustrated in Fig. 8 imply warm advection occurring at these lower levels. The location of the extended WISSDOM domain is also marked in Figs. 8a and 8b. The horizontal distribution of maximum radar reflectivity (i.e., the maximum values were composited from all layers from 0.25 to 10 km MSL for each grid in the studied domain) and low-level averaged horizontal wind (<1 km MSL) are shown in Figs. 8c and 8d, respectively. A strong radar echo was observed at the southwestern end of the PCB and SBB, and another one was detected near Gangneung.

Fig. 8.
Fig. 8.

(a) Horizontal winds (vectors) and temperature (K; color shading) obtained from ERA-Interim at the 925-hPa level at 0600 UTC 21 Jan 2013. (b) As in (a), but for the 800-hPa level. (c) Horizontal distribution of the maximum radar reflectivity (dBZ; color shading) and (d) averaged ground-relative winds below 1 km MSL (vectors) and topographic feature (m; color shading) at 0600 UTC. The topographic height thresholds of 400 (thin), 800 (thick), and 1200 m MSL (thick dark). The inserted box (dashed box) indicates the main (extended) WISSDOM synthesis domain.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

The strong radar echoes were embedded within a banded/mesoscale precipitation region near the southwestern end of the PCB (as Fig. 8c). This mesoscale precipitation was oriented quasi parallel to the warm-frontal isotherms, and it may have contributed a great amount of surface precipitation at JC during stage I (cf. Fig. 6a). Although an analysis of ERA-Interim data shows that larger-scale winds were predominantly southeasterly at 925 hPa, the averaged low-level winds (<1 km MSL) retrieved by WISSDOM exhibit southwesterly or southerly flow across most of the domain, except for the southeasterly winds over the ocean (Fig. 8d). In particular, the warm advection will have dominated the prevailing winds to be more southwesterly in this extended WISSDOM domain, especially in the southwestern part (approximately 36°−37°N, 127°−128°E) and mountainous areas exhibiting stronger southwesterly winds.

Figure 9a shows the maximum radar reflectivity from the results of the main WISSDOM synthesis at 0600 UTC. One region of strong reflectivity (>35 dBZ) occupied the southwestern end of the PCB and extended northeastward with relatively weak reflectivity of 15–25 dBZ, coincident with more southerly and southwesterly winds below 1 km MSL (Fig. 9b). Strong echoes also occurred around Gangneung, with a stratiform near coastal and offshore regions. The stratiform area appears to have extended northward as a result of the southeasterly winds over the ocean. Figures 9a and 9c also illustrate that stronger reflectivity along the coast was collocated with the horizontal convergence of the inland southwesterly flow and the coastal southeasterly flow.

Fig. 9.
Fig. 9.

(a) Horizontal distribution of the maximum radar reflectivity (dBZ; color shading) observed in the WISSDOM domain at 0600 UTC 21 Jan 2013. The black line of A–A′ marks the locations of the vertical cross sections shown in Fig. 10. The inserted box indicates the averaged area of vertical cross sections along the southwestern end (SW) to northeastern coast (NE) of the PCB corresponding to Fig. 11. (b) As in (a), but for the averaged ground-relative winds below 1 km MSL (vectors) and topographic feature (m; color shading). (c) As in (b), but for horizontal divergence (10−4 s−1; color shading). The topographic height thresholds of 400 (thin), 800 (thick), and 1200 m MSL (thick dark).

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

A vertical cross section (marked by A–A′ in Fig. 9a) perpendicular to the orientation of the TMR (parallel to the PCB) was chosen to illustrate the structural characteristics of precipitation and winds at 0600 UTC (Fig. 10). The mesoscale precipitation region and enhanced precipitation exhibited a vertical extent with the radar reflectivity in 20-dBZ contour reaching approximately 2.5 km MSL near the southwestern end of the PCB (i.e., X = 0–30 km in Fig. 10a). The Doppler-derived winds can be projected as cross-barrier flow (positive values indicate airflow blowing from southwest to northeast across the TMR) and along-barrier flow (positive values indicate airflow blowing from southeast to northwest along the TMR); they are equivalent to the wind components that are roughly perpendicular (northeast–southwest; 50°–230°) and parallel (northwest–southeast; 320°–140°) to the orientation of the TMR, respectively. The variables and are more intuitive parameters for understanding the interactions between topography and prevailing winds. Although the relatively stable environment may have restrained the airflow from climbing over the mountain, the updraft can still be triggered locally by mechanical lifting on the first slope when the positive cross-barrier flow (i.e., southwesterly) encounters the southwestern end of the PCB (Fig. 10b), inducing precipitation. Moreover, intense radar reflectivities associated with stronger (15 m s−1) or synoptic precipitation shields were observed at midlevel heights (around 1.5–2.5 km MSL) above the PCB at X = 50–80 km, and these intense radar reflectivities also extended to the northeastern coast.

Fig. 10.
Fig. 10.

(a) Vertical cross section of the WISSDOM-derived radar reflectivity (dBZ; color shading) and wind vectors (combined cross-barrier flow and quadruple vertical velocity) along the solid line segment A–A′ indicated in Fig. 9a at 0600 UTC 21 Jan 2013. (b) As in (a), but for the vertical velocity (m s−1; color shading) with the cross-barrier (along barrier) flow indicated by black (purple) contours with an interval of 3 m s−1. Positive values of the cross-barrier (along barrier) flow mean that the flow blew toward A′ (into the paper) and vice versa. The brown shading in the lower portion indicates the topography along the cross section.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

The selected areas indicated by the inset box in Fig. 9 are used to obtain the averaged representative airflow and precipitation structures at 0600 UTC in a southwest–northeast direction (i.e., along the PCB), corresponding to 0–150-km distance (from southwest to northeast), as shown in Fig. 11. Figure 11a shows that stronger radar reflectivity over the PCB was elevated at the midlevel and extended northeastward and connected to the region of low-level intense precipitation near the coastal region. This elevated characteristic was possibly due to warm advection aloft over a drier low-level air mass. Despite Fig. 9a indicating intense precipitation in the vicinity of the northeastern side of the TMR, a strong updraft was observed downstream of the PCB. This upward motion can exceed 1 m s−1 above 1−3 km MSL coincident with strong radar echoes (Figs. 10 and 11). Such elevated updraft features on the downstream side of the mountain are consistent with a mountain wave, and it will play an important role in the production of heavy precipitation on the northwestern side of the PCB (Bruintjes et al. 1994; Bousquet and Smull 2003; Garvert et al. 2007). The existence of the mountain wave is also supported by observed stable and stratified environmental conditions in stage I.

Fig. 11.
Fig. 11.

As in Fig. 10, but showing the averaged vertical cross section along the SW–NE box of Fig. 9a. Positive values of the averaged cross-barrier (along barrier) flow indicate that the flow blew toward the NE (into the paper) and vice versa. The brown shading in the lower portion indicates the averaged topography along the box. The backward trajectories of precipitation particles from the surface at X = 70–150 km (thick black).

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

The mean backward trajectories of precipitation particles from Gangneung are presented in Fig. 11a. The backward trajectories are considered the retrieved winds from WISSDOM and solid particles can be transported while wind speeds exceed 15 m s−1 (Li and Pomeroy 1997). Moreover, the terminal velocity of snow particles was estimated by the empirical relationship between mean terminal velocity and radar reflectivity (Atlas et al. 1973). Most particles drifted from X = 0 km around 2−3 km MSL (i.e., above the PCB) and they fell to the surface downstream of the PCB. The results indicated that occurrences of strong radar echoes near the northeastern side of the TMR are potentially strengthened by the drifted particles. Figure 11b also shows that significant deceleration of coincided with stronger and weak updrafts (<0.5 m s−1) near the surface at X = 100–120 km, where convergence and reflectivity were also observed at low levels (below 1 km MSL). The mountain wave, airflow convergence, and the drift of the particles associated with the prevailing southwesterly wind are possible factors to enhance locally heavy precipitation near Gangneung. The similarity between an individual (Fig. 10) and the averaged cross section (Fig. 11) suggests that the averaged cross section can appropriately represent the precipitation and airflow structures along the PCB for stage I.

b. Stage II (1530, 1830 UTC 21 January 2013)

The radar reflectivities and low-level winds observed along the PCB at 1530 and 1830 UTC were weaker than those at 0600 UTC. In contrast to the strong reflectivities in the inland area, the primary precipitation was confined to an elongated zone located off the northeastern coast (~20 km) and exhibited an obvious banded pattern at 1530 UTC (Fig. 12a). The precipitation band was oriented parallel to the TMR with a length (width) of ~120 (25) km. The strongest radar reflectivity associated with the precipitation band exceeded 35 dBZ. The band was characterized by a pronounced horizontal gradient of radar reflectivity, with stratiform precipitation mostly on its western side (i.e., over inland, nearshore regions). Because the LPS was moving toward the southeastern side of South Korea during stage II (cf. Fig. 3d), the low-level winds changed from southeasterly to more easterly winds over the ocean (Fig. 12b). With time, the low-level winds changed further to become northeasterly and the precipitation band moved toward the northeastern coast, causing heavy precipitation in Gangneung approximately 3 h later, at 1830 UTC (Figs. 12c and 12d). The analysis of horizontal divergence (Figs. 13a–d) found a semistationary convergence band located along the northeastern coast in stage II. This area of convergence also indicates that the behavior of precipitation band was related to the orographically modified airflow. The precipitation band produced heavy precipitation along the coast in stage II when the band moved inland. At this time, convective (stratiform) precipitation occurred at the western (eastern) side of the precipitation band. These characteristics reflect not only the structural evolution of the precipitation band but also its potential impact on the distribution of heavy precipitation in the Pyeongchang area.

Fig. 12.
Fig. 12.

As in Fig. 9, but (a),(b) at 1530 and (c),(d) 1830 UTC 21 Jan 2013. The inserted boxes indicate the averaged area of vertical cross sections along the SW to NE of the PCB corresponding to Figs. 14 and 15. The topographic height thresholds of 400 (thin), 800 (thick), and 1200 m MSL (thick dark) are shown.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

Fig. 13.
Fig. 13.

The horizontal divergence (10−4 s−1; color shading) from 1530 to 1830 UTC in every hour interval. The topographic height thresholds of 400 (thin), 800 (thick), and 1200 m MSL (thick dark) are shown.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

The selected areas indicated by the inset boxes in Fig. 12 were used to obtain the averaged representative airflow and precipitation structures at 1530 and 1830 UTC along the PCB, as shown in Figs. 14 and 15, respectively. In stage II, distinctly different precipitation and airflow patterns occurred below 2 km MSL at 1530 UTC. More convective precipitation generally associated with significant upward motion was observed off the northeastern coast, near X = ~110 km (approximately 20 km from the coast), as shown in Fig. 14. However, the regions over the PCB were characterized by lower reflectivities and weak upward or downward motions. The primary precipitation (i.e., radar reflectivity > 20 dBZ) and convective motion (>0.5 m s−1) associated with the offshore line were confined to the lowest 1.5 km MSL, which approximately correspond to the topographic elevation. Negative (of magnitude greater than ~6 m s−1) was detected on the northeastern edge of the precipitation band below ~1.5 km MSL. It clearly decelerated and lifted coincident with the stronger updraft near the coastal region with negative (X = ~100 km). Note that although the sounding at SC indicates a relatively stable environment to prevent triggering convection, the environment changed gradually to become conditional instability near the surface (below around 250 m MSL) at 1200 UTC 21 January (cf. Fig. 7) and a more neutral environment can be found below 2 km MSL at 0000 UTC 22 January (not show). The changed environmental condition favored the development of shallow convection near the northeastern coast. These signatures are similar to those of the topographic blocking that induces offshore precipitation bands along the mountainous coast of southeastern Taiwan described in previous studies (Yu and Lin 2008, 2017; Yu and Hsieh 2009). Figure 15a indicates that the precipitation band made landfall near Gangneung (X = ~90 km) at 1830 UTC with a velocity exceeding ~2 m s−1. Compared to the earlier period at 1530 UTC, there were some structural changes to the moving precipitation band, such as decelerated along-barrier flow and weaker upward motions.

Fig. 14.
Fig. 14.

As in Fig. 11, but along the SW–NE box in Fig. 12a at 1530 UTC 21 Jan 2013.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

Fig. 15.
Fig. 15.

As in Fig. 11, but along the SW–NE box in Fig. 12c at 1830 UTC 21 Jan 2013.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

The impacts of the topography on the airflow and precipitation structures associated with the moving LPS, as evident above, can be further elaborated by detailed time series analysis of the sequence of radar reflectivity and horizontal winds from the WISSDOM synthesis averaged within elongated boxes along the PCB that are indicated in Figs. 9 and 12. During the 28-h study period, the LPS moved from the southwestern side of the Korean Peninsula to central Japan. To highlight the impact of topographic modifications on airflow, which were expected to be more significant below the mountain height (cf. sounding observations in Fig. 7 and blocking zone in Fig. 14b), the analysis fields below 1 km MSL were averaged. The characteristics of the radar reflectivity can also be divided into stages I (before 1200 UTC) and II (after 1200UTC), as shown in Fig. 16. In stage I, more continuous reflectivities (15 dBZ) coincided with southerly winds in the inland area (X = 0–20 km before 0900 UTC 21 January). Relatively weak reflectivities (<10 dBZ) were associated with the southwesterly winds over the PCB (X = 30–70 km). In this case, the observed reflectivities were lower near the surface at the crest of the PCB, which is consistent with the horizontal radar analysis and surface precipitation observations shown in Figs. 5 and 6a. Precipitation intensity was obviously stronger, with radar reflectivities exceeding 18 dBZ near the coast and the offshore area (i.e., X = 85–120 km) in association with southeasterly winds.

Fig. 16.
Fig. 16.

Temporal variation of the average precipitation (color shading) and the horizontal winds (wind barbs) from WISSDOM derived in the vicinity of the Pyeongchang area from 2000 UTC 20 Jan to 0000 UTC 22 Jan 2013. The low-level radar reflectivity and winds (below 1 km MSL) within the SW–NE boxes (shown in Figs. 9a, 12a, and 12c) were averaged in a direction normal to the orientation of the box and plotted as a function of time and along-box distance. The brown shading in the lower portion indicates the averaged topography along the box.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

During stage II, a precipitation band started to organize at 1400 UTC as the environmental wind changed from southeasterlies to easterlies. This precipitation band intensified (exceeding 30 dBZ), coincident with a significant deflection of winds from the easterlies offshore to northeasterlies near the foot of the mountains (X = 90 km) from 1500 to 1600 UTC. The development of this precipitation band was strongly related to changes in the environmental winds and their interactions with topography, since its onset occurred after the change in wind direction and intensity, which were observed when the blocking zone was building, between 1400 and 1600 UTC (i.e., developing period in stage II). Between 1600 and 2000 UTC, the precipitation band started to move toward the coast (i.e., moving period in stage II) when the environmental winds changed from easterlies to northeasterlies. Note that there were quite weak winds and precipitation over the PCB after 1200 UTC, indicating that the development of the precipitation was fundamentally influenced by the changes in the environmental winds in this case. The strong blocking may have occurred unobserved below 500 m MSL (cf. Fig. 14b), since the relatively lower vertical resolution of the data leads to WISSDOM underestimating the averaged blocking response below 1 km MSL.

5. Conclusions

This study used high-temporal- and high-spatial-resolution data to investigate the finescale structural evolutions of airflow and precipitation over complex terrain in the Pyeongchang area. Multiple radars and AWS observations were used to derive the 3D wind field and precipitation features through WISSDOM. The possible mechanisms of precipitation over the topography were documented during the passage of a low pressure system (LPS) from the southwestern side of the Korean Peninsula to Japan during the period 20–22 January 2013. The primary characteristics of the airflow and precipitation during the study period are summarized in the schematic diagram of Fig. 17.

Fig. 17.
Fig. 17.

Schematic diagram illustrating the structural characteristics of low-level precipitation and airflow for (a) stage I, and (b) the developing (1400–1600 UTC) and (c) moving periods (1600–2000 UTC 21 Jan 2013) of the precipitation band in stage II in horizontal and vertical views. The environmental wind is indicated by wind barbs with a full (half) wind barb corresponding to 5 (2.5) m s−1. Red solid arrows indicate the topographically modified airflow observed from the WISSDOM synthesis, and blue shading denotes the generalized precipitation pattern, with darker shading denoting regions of heavier precipitation. Black shading indicates the gross features of topography in the studied domain. The area of convergence and the blocked zone are also indicated in (a) and (b). The locations of the TMR, PCB, and GNC are also marked.

Citation: Monthly Weather Review 146, 10; 10.1175/MWR-D-17-0394.1

Widespread precipitation was associated with the circulation of the LPS during stage I, and the mesoscale precipitation region and locally orographic enhancement were found over the southwestern end of the PCB (i.e., windward slope in this stage), as shown in the horizontal view (left panel) of Fig. 17a. Low-level southwesterly and southeasterly flow prevailed in the Pyeongchang area and over the East Sea, respectively. The prevailing winds exhibited veering changes from southeasterly to southwesterly with the height, consistent with warm advection (right panel of Fig. 17a). The southwesterlies lifted over the terrain at the southwestern end of the PCB to enhance precipitation, and this enhanced precipitation was embedded within the mesoscale precipitation region of the warm advection. A zone of heavier precipitation associated with prevailing southwesterly winds, extending from the southwestern end to the northeastern side of the PCB, was also evident. A second region of enhanced precipitation was documented downstream of the PCB, coincident with the multiple effects of mountain wave inducing upward motion, drifted precipitation particles, and low-level convergence zone near the coast, which were generated by the prevailing southwesterlies and southeasterly winds over the East Sea.

For stage II, following the eastward movement of the LPS, the prevailing winds became more easterly, decelerated, turned parallel to the axis of the TMR, and developed a blocked zone along the coast (Fig. 17b). A precipitation band was triggered by the convergence produced as the low-level oncoming flow encountered the nearshore blocked zone. With time, the prevailing winds became northeasterly (i.e., stronger onshore flow), which may have helped push the precipitation band inland, where it induced heavy precipitation along the northeastern coast and in Gangneung, as shown in Fig. 17c. The observational evidence presented shows that the interaction of temporally changing winds accompanying the movement of an LPS over complex topography is a critical factor in determining the distribution and intensity of precipitation in the Pyeongchang area.

Although the topographically modified winds and resultant enhanced precipitation were documented by the WISSDOM analysis, some ambiguous relations existed between the surface precipitation and the observed radar reflectivity (e.g., intense reflectivity observed in Gangneung with relatively low precipitation). Because various types of particles may cause different responses in radar reflectivity, a more detailed analysis of the microphysics process is necessary to improve the accuracy of the precipitation estimate. For the winter case studied here, hydrometeor information can be provided by the dual-polarization radar at SBS, vertical pointing precipitation radar (Micro Rain Radar), and the optical disdrometer [Particle Size Velocity (PARSIVEL)] near the mountain crest and northeastern slope of the PCB. The impacts of the topographically modified winds on the microphysics processes will be investigated using these observations in the future.

Acknowledgments

We thank Dr. Daniel Kirshbaum and three anonymous reviewers for their constructive and insightful comments that improved the manuscript. We also thank Mr. Geunsu Lyu and Hong-Mok Park for providing the postprocessing of the Doppler radar data. This research was supported by the “Development and Application of Cross Governmental Dual-Pol. Radar Harmonization (WRC-2013-A-1)” project of the Weather Radar Center, Korea Meteorological Administration and Ministry of Science and Technology of Taiwan under Research Grants MOST106-2111-M-002-002-MY3, MOST106-2119-M-008-009-, and MOST106-2625-M-008-012-.

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