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Characteristics of Sea Breeze Front Development with Various Synoptic Conditions and Its Impact on Lower Troposphere Ozone Formation

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doi: 10.1007/s00376-013-2256-3

  • To examine the correlation between the sizes of sea breeze fronts and pollutants under the influence of synoptic fields, a numerical simulation was conducted in the southeast coastal area of the Korean Peninsula, where relatively high concentrations of pollutants occur because of the presence of various kinds of industrial developments. Sea breeze and sea breeze front days during the period 2005-09 were identified using wind profiler data and, according to the results, the number of days were 72 and 53, respectively. When synoptic forcing was weak, sea breeze fronts moved fast both in horizontal fields and in terms of wind velocity, while in the case of strong synoptic forcing, sea breeze fronts remained at the coast or moved slowly due to strong opposing flows. In this case, the sea breeze front development function and horizontal potential temperature difference were larger than with weak synoptic forcing. The ozone concentration that moves together with sea breeze fronts was also formed along the frontal surfaces. Ozone advection and diffusion in the case of strong synoptic forcing was suppressed at the frontal surface and the concentration gradient was large. The vertical distribution of ozone was very low due to the Thermal Internal Boundary Layer (TIBL) being low.
    摘要: To examine the correlation between the sizes of sea breeze fronts and pollutants under the influence of synoptic fields, a numerical simulation was conducted in the southeast coastal area of the Korean Peninsula, where relatively high concentrations of pollutants occur because of the presence of various kinds of industrial developments. Sea breeze and sea breeze front days during the period 2005-09 were identified using wind profiler data and, according to the results, the number of days were 72 and 53, respectively. When synoptic forcing was weak, sea breeze fronts moved fast both in horizontal fields and in terms of wind velocity, while in the case of strong synoptic forcing, sea breeze fronts remained at the coast or moved slowly due to strong opposing flows. In this case, the sea breeze front development function and horizontal potential temperature difference were larger than with weak synoptic forcing. The ozone concentration that moves together with sea breeze fronts was also formed along the frontal surfaces. Ozone advection and diffusion in the case of strong synoptic forcing was suppressed at the frontal surface and the concentration gradient was large. The vertical distribution of ozone was very low due to the Thermal Internal Boundary Layer (TIBL) being low.
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Manuscript received: 15 October 2012
Manuscript revised: 07 December 2012
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Characteristics of Sea Breeze Front Development with Various Synoptic Conditions and Its Impact on Lower Troposphere Ozone Formation

    Corresponding author: Soon-Hwan LEE, withshlee@pusan.ac.kr
  • 1. Division of Earth Environmental System, Pusan National University, Busan609-735, South Korea;
  • 2. Department of Earth Science Education, Pusan National University, Busan609-735, South Korea
Fund Project:  This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (Grant No. 2012-0007035).

Abstract: To examine the correlation between the sizes of sea breeze fronts and pollutants under the influence of synoptic fields, a numerical simulation was conducted in the southeast coastal area of the Korean Peninsula, where relatively high concentrations of pollutants occur because of the presence of various kinds of industrial developments. Sea breeze and sea breeze front days during the period 2005-09 were identified using wind profiler data and, according to the results, the number of days were 72 and 53, respectively. When synoptic forcing was weak, sea breeze fronts moved fast both in horizontal fields and in terms of wind velocity, while in the case of strong synoptic forcing, sea breeze fronts remained at the coast or moved slowly due to strong opposing flows. In this case, the sea breeze front development function and horizontal potential temperature difference were larger than with weak synoptic forcing. The ozone concentration that moves together with sea breeze fronts was also formed along the frontal surfaces. Ozone advection and diffusion in the case of strong synoptic forcing was suppressed at the frontal surface and the concentration gradient was large. The vertical distribution of ozone was very low due to the Thermal Internal Boundary Layer (TIBL) being low.

摘要: To examine the correlation between the sizes of sea breeze fronts and pollutants under the influence of synoptic fields, a numerical simulation was conducted in the southeast coastal area of the Korean Peninsula, where relatively high concentrations of pollutants occur because of the presence of various kinds of industrial developments. Sea breeze and sea breeze front days during the period 2005-09 were identified using wind profiler data and, according to the results, the number of days were 72 and 53, respectively. When synoptic forcing was weak, sea breeze fronts moved fast both in horizontal fields and in terms of wind velocity, while in the case of strong synoptic forcing, sea breeze fronts remained at the coast or moved slowly due to strong opposing flows. In this case, the sea breeze front development function and horizontal potential temperature difference were larger than with weak synoptic forcing. The ozone concentration that moves together with sea breeze fronts was also formed along the frontal surfaces. Ozone advection and diffusion in the case of strong synoptic forcing was suppressed at the frontal surface and the concentration gradient was large. The vertical distribution of ozone was very low due to the Thermal Internal Boundary Layer (TIBL) being low.

1 Introduction
  • Land and sea breeze circulations often occur in coastal areas because of the differences in specific heat resulting from local heating in the atmospheric boundary layer (ABL). Since many human activities are based within the ABL, land and sea breezes have various kinds of effects on human life (Kondo and Gambo, 1979; Nielsen, 1989; Helmis et al., 1995; Avissar and Schmidt, 1998) and are therefore important to study. Also, sea breeze fronts relate to large-scale synoptic flows, which are large atmospheric discontinuity phenomena formed by mesoscale coercive power, and are thus meteorologically very interesting.

    Sea breeze fronts refer to the boundary areas between sea breezes and surrounding flows opposing the sea breezes. (Simpson, 1994) and (Miller et al., 2003) stated that many meteorological variables, such as wind velocity, temperature, humidity and water vapor pressure show rapid changes in these areas. (Arritt, 1993) defined sea breeze fronts through 2D numerical simulations, explaining that they might be formed depending on the intensity of opposing large flows, and that even the locations where sea breeze fronts are formed would vary with the intensity of opposing large flows. However, it is difficult to clearly distinguish between sea breezes and sea breeze fronts locally, and areas studied for sea breeze fronts have regional characteristics in that synoptic-scale coercive power and sea breeze fronts appear simultaneously. Therefore, previous studies of sea breeze fronts have been quite limited (Miller and Keim, 2003; Childs and Raman, 2005; Thompson et al., 2007; Dandou et al., 2009).

    Sea breezes and sea breeze fronts are quite closely related with coastal area atmospheric environments. (Gaza, 1998) examined the characteristics of highly concentrated ozone in two situations, the leeside trough and mesoscale sea breeze fronts, in the northeastern coastal area of the USA. According to (Ding et al., 2004), on days when highly concentrated ozone occurs in the Pearl River Delta of China, sea breezes were delayed due to offshore synoptic winds. The delayed sea breezes and offshore synoptic flows contributed to not only low mixed heights during the daytime, but also to the transportation of highly concentrated ozone to the coast.

    When well mixed cold and humid maritime air on the sea is advected to areas below warm air in inland areas, the thermal internal boundary layer (TIBL) is formed to accompany the formation of land and sea breezes to act as an important factor for changes in the concentration of air pollution. Through observation and numerical experiments, (Liu and Chan, 2002) indicated that the height of a TIBL that reached the top of a mountain on an island of complicated topography in Hong Kong was approximately 700 m. TIBLs in coastal areas form dome shapes (Druilhet et al., 1982) and diffuse contaminants are confined within them (Barboto, 1975). The contaminant diffusion occurring within TIBLs is said to be important because of the fumigation phenomenon that occurs when contaminants emitted from stable areas are mixed with the boundary layers of TIBLs, confining the contaminants to the boundary layers and mixing them rapidly with air below the ground surface (Barbato, 1975; Lyons et al., 1981; Hsu, 1988; Abbs and Physick, 1992; Sawford et al., 1998).

    Owing to the fact that the Korean Peninsula is located in the middle latitudes, synoptic-scale pressure systems occur frequently and mesoscale forces act simultaneously because of the geographical characteristics. (Oh et al., 2006) indicated that sea breeze delays occurred in front of high pressure areas or to the rear of pressure troughs, and that these increased ozone concentrations and the frequency of occurrences of high concentrations in the Busan region. (Hwang et al., 2007) studied the distribution of ozone concentrations in Seoul, Korea, and indicated that the distribution of highly concentrated ozone decreased owing to the effects of synoptic weather conditions such as sea breezes under weak westerly synoptic winds and strong easterly opposing synoptic winds, and a sea breeze front under weak opposing synoptic flows. Recently, (Ji et al., 2011) reported the phenomenon that sea breeze fronts are separated because of topographical characteristics in the regions of Busan and Changwon, Korea.

    However, since the generation and development of sea breezes over the Korean Peninsula are quite complicated, because of the complicated topography and distribution of coastal urban areas, studies that analyze only the characteristics of sea breeze fronts, which are combinations of synoptic- and mesoscale-forced powers, are insufficient. In addition, most land and sea breeze analyses are based on surface weather observation equipment, and thus intensive analyses based on vertical observational data are insufficient. Therefore, the study reported in the present paper aimed to examine, through vertical observation data and numerical experiments, the characteristics of changes in weather and air pollutants in relation to the occurrences of sea breezes and sea breeze fronts in the southeastern region of the Korean Peninsula, where very complicated topography and large-scale industrial zones have developed.

2 Data and methodology
  • As shown in Fig. 1c, the city of Jinhae is located close to the coast, and the coastline of this area is in the form of a small bay, and there is a low mountain directly inland. A national industrial complex and a naval base are key emission sources located in the area. Changwon, which is located immediately behind Jinhae, is a basin area surrounded by mountains, and many industrial complexes such as the Korea Industrial Complex and Changwon Industrial Complex are located there. Masan, which is located on the west side of Jinhae is an area located deeper within the peninsula, and this area is characterized by being located on a bay. There is a relatively small mountain named Paryong (328 m) on the east side of this area and there is another mountain named Jangbok (582 m) on the south-east side of this area, beyond a port. Miryang, which is located in the inland area, is marked No. 5 in Fig. 1b. This area is surrounded by high mountains.

    Figure 1.  (a) Simulated coarse domain. (b) Weather staions: 1. Goseong; 2. Hapcheon; 3. Jinhae; 4. Changwon; 5. Miryang; 6. Tongyeong; 7. Ulsan; 8. Busan; 9. Pohang; 10. Daegu; + = wind profiler. (c) Air quality monitoring (yellow makers): 111. Hoewon; 112. Bongam; 141. Myeongseo; 142. Ungnam; 143. Gaeumjeong; 151. Jinhae. Line XY was used to analyze the vertical cross-section from north to south.

  • Using weather data, Automatic Weather Station (AWS) data, wind profilers etc., we proposed the following criteria for the classification of sea breeze and sea breeze front occurrence (Fig. 2). First, the days of sea breeze occurrence were selected based on AWS data from Changwon. Since the coastline is distributed from east to west, these were determined based on prevailing wind directions and their changes. Since upper level synoptic wind is essential for the analysis of sea breeze fronts, the vertical wind distribution was analyzed based on wind profiler data of sea breeze days (Asimakopoulos et al., 1999; Park et al., 2010). When the vertical scale of sea breezes is assumed to be 1-2 km, 500 hPa of atmospheric height is at non-divergence levels, and areas not higher than 850 hPa will be greatly affected by daily temperature radiation at the surface (ground level). Therefore, layers in the range of 1.5-5 km in height were classified into an upper layer and layers below this range were classified into a lower layer.

    To make a final decision regarding sea breeze fronts, wind profiler data and atmospheric pressure patterns at the surface from weather charts were analyzed simultaneously. Sea breeze days were selected as those days on which southerly winds were dominant in the upper/lower layers of the wind profiler data. Sea breeze front days have atmospheric pressure patterns in which northerly winds generated by synoptic-scale coercive power are dominant in the atmosphere of the upper layer, and sea breezes (that is, southerly winds induced by mesoscale forcing), are dominant in the atmosphere of the lower layer. Therefore, wind distributions at high/low levels, which are strongly associated with the synoptic pressure pattern, constitute a suitable method to determine the occurrence of sea breezes and sea breeze fronts. In the target period, sea breezes and sea breeze fronts were observed for 72 and 53 days, respectively.

    Figure 2.  The flowchart for classification of sea breeze day and sea breeze front day.

    Figure 3 shows the numbers of days of main wind directions in the upper/lower layers observed during April and August for four years from 2006 to 2009. In the upper layer, westerly winds were dominant between April and August, occurring 30 times or more. Northerly winds were concentrated in spring (April and May), occurring 50 times or more. As the season shifted from spring, northwesterly winds decreased while southeasterly winds increased. This means that sea breezes gradually increased due to local differential heating. In the lower layer, although various wind directions were evenly distributed between April and August, of them, southerly winds were observed most frequently, on around 40 occasions. This is also because sea breezes, which are southerly winds, are dominant in spring and summer.

    Although not shown here, monthly average wind speeds on sea breeze and sea breeze front days were examined separately for the upper and lower layers. Averaged values of northerly wind speeds in the upper layer were approximately 1.0 m s-1 for sea breeze days, while following sea breeze front development the values were 4-5 m s-1. Average monthly northerly wind speeds at the upper level were 4.6 m s-1, 5.2 m s-1, 4.0 m s-1, 3.8 m s-1, and 3.6 m s-1 for April, May, June, July and August, respectively. Furthermore, northerly wind speeds in the upper layer were high at 4 m s-1 or more in spring, while below 4 m s-1 in summer. The fact that northerly winds in the upper layer were strong in spring in sea breeze fronts was supported by the largest number of occurrences of northerly winds in spring, as shown in the wind direc tion frequencies in Fig. 3.

    Furthermore, we can categorize sea breeze fronts as strong or weak on the basis of northerly wind speed at the upper level and, in this study, sea breeze fronts with upper level wind speeds of over 4.0 m s-1 were treated as "strong" cases. Accordingly, weak sea breeze fronts occurred on 27 days and strong sea breeze fronts on 26 days. The monthly frequency for the two types of sea breeze front and the criteria for their classification are shown in Table 1.

    Figure 5.  Time series plot based on wind profiler data from Changwon Station on (a) 11 April 2008 and (b) 6 May 2009. Broken lines indicate the convergence of wind vectors (m s-1).

  • Since the major purpose of the present study is quantitative analysis of changes in meteorological fields and air pollution when sea breezes and sea breeze fronts occur in complicated coastal manufacturing areas, respective cases were set and analyzed. Accordingly, two cases were selected to clarify the impact of the different large-scale flow intensities on the development of sea breeze fronts. The cases were classified according to the strength of the opposing northerly wind speed mentioned above.

    We selected 11 April 2008 as a day on which synoptic-scale coercive power was weak (Front_W) and northerly winds occupied 92.9% of upper layer winds, with the wind speed identified as 3.0 m s-1. The wind speed of southerly winds in the lower layer was identified as 3.8 m s-1, and thus the speed of the northerly winds in the upper layer and the southerly winds in the lower layer were shown to be similar. For strong synoptic-scale coercive power, we selected 6 May 2009 (Front_S). The share and speed of the northerly winds in the upper layer on this day were identified as 81.6% and 6.5 m s-1 respectively, and the speed of the southerly winds in the lower layer was identified as 2.3 m s-1.

    Figure 4 shows the surface synoptic charts presented by the Korean Meteorological Administration (KMA) at 0900 LST 11 April 2008 for case Front_W and 6 May 2009 for case Front_S. For Front_W (Fig. 4a), the pattern was that high atmospheric pressure had remained above the eastern Mongolia at 0900 LST 11 April 2008, and its influence expanded to cover only the northern part of the Korean Peninsula. On the contrary, on 6 May 2009 different high pressure air masses were located over Shanghai Bay in China and the northern part of Japan, and these air masses were connected to one another. This synoptic air mass distribution provided suitable conditions to induce the strong sea breeze front on 6 May 2009.

    Figure 5 shows vertical vectors observed at Changwon during the two case days using a wind profiler. The wind pattern for Front_W was that northerly winds were blowing at 3 m s-1 at altitudes below 1000 m before 1100 LST, and then these northerly winds changed into southeasterly winds to become sea breezes after 1100 LST. The height of the sea breezes on this day grew up to 1000 m and, after 1400 LST, southerly winds and southeasterly winds converged at altitudes over 1000 m together with northerly winds blowing at speeds exceeding 2 m s-1 to form sea breeze fronts. On the Front_S case day, northerly winds were continuously blowing at altitudes over 1500 m. The directions of surface winds changed at 1000 LST, and thus southerly winds (sea breezes) were observed in the daytime. The height of the sea breezes grew to around 1000 m and surface southerly winds converged with upper-layer northerly winds after 1400 LST to form fronts.

    Therefore, both cases were shown to be obvious sea breeze days with changes in wind directions and wind speeds in the early morning and daytime, and it was proved that synoptic opposing flows, corresponding to northerly winds, existed at altitudes over 1500 m. The conversion of the northerly winds in the upper layer and southerly winds in the lower layer showed the existence of sea breeze fronts through the analysis of wind-profiler observations.

    Figure 6.  Disturbution of emissions of (a) VOC and (b) NO x (CAPSS; 103kgyr-1).

  • The WRF (Weather Research and Forecasting) numerical model version 3.4.2, developed by the National Center for Atmospheric Research (NCAR) and the National Centers for Environmental Prediction (NCEP) of the National Oceanic and Atmospheric Administration (NOAA), was used for the present study. It is a sigma coordinate system mesoscale model that adopts a non-statistical, compressive equation system (Skamarock et al., 2008).

    For the numerical simulations, we set domains to 35 vertical layers at horizontal resolutions of 18 km, 6 km, 2 km and 1 km and grid numbers of 97×97, 85×85, 199×199 and 121×121 through one-way nesting. As shown in Fig. 1, the coarse domain included the entire Korean Peninsula and some Japanese islands, and the finest domain was designed to include part of the southern coast and Gyeongsangnam-do, centering on Changwon and Masan on the Korean Peninsula (Fig. 1b). For the numerical simulations, the final analyses (FNL), which were produced by the NCEP at 1°×1° every six hours, were used as initial boundary data. Physical options used in the present study included the RRTM (Rapid Radiative Transfer Model), long-wave radiation scheme (Mlawer et al., 1997) and the Dudhia short-wave radiation scheme (Dudhia, 1989) as physical processes regarding radiation, as well as the MYJ (Mellor-Yamada-Janjic) scheme (Janjic, 1994), which can analyze turbulence phenomena and calculate turbulence energy as a PBL scheme.

    CMAQ (Community Multiscale Air Quality Model System) version 4.6, developed by the U.S. EPA (United States Environmental Protection Agency) (Byun and Ching, 1999), was used as a model for air quality analysis. This model is the most widely used since it can be used to calculate various contaminants such as aerosol, gas phase materials and photochemical materials using the 3D Eulerian atmospheric chemical and transportation system, and can be implemented for diverse scales ranging from local scales to regional scales. For this model, we used Domain 3 as input data that had been implemented in WRF. As a photochemical mechanism for ozone analysis, CB-IV (Carbon Bond IV) was used.

    For emissions, the 2001 data produced by the CAPSS (Clean Air Policy Support System) were used (KEI, 2003). These data produced point, line and area contamination sources throughout the whole country using a grid of 1×1 km2, and calculated emissions considering coefficients by month, day and hour. The emission distribution used in the present study is shown in Fig. 6. In this figure, industrial zones are mainly located in the Ulsan, Busan, Changwon and Jinhae regions along the southeastern coastline, where volatile organic compounds (VOCs) and nitrogen oxides (NOx) are intensively distributed. Daegu (Fig. 1b), a large city located inland, is a region where NOx emissions are severe because of large traffic volumes, and Pohang (Fig. 1b) is also a region where large amounts of contaminants are emitted due to the Pohang Steel Industrial Complex. The distribution of NOx emissions shown in Fig. 6b is reflective of the pattern of urban expressways.

3 Results
  • To verify the reliability of the models, numerically simulated resultant values for Front_W and Front_S were statistically analyzed, and the results are shown in Table 2. As shown, the RMSEs of Front_W were analyzed, the temperatures of which had smaller errors in Tongyeong (0.85), and wind speeds had smaller errors in Hapcheon (0.78). Regarding the analysis of IOA (Index of Agreement), temperatures showed good results, exceeding 0.9 at six stations. In the results for Front_S, temperatures showed reliable RMSE (0.78) and IOA (0.99) values in Masan. Wind speeds showed reliable RMSE (0.93) values in Tongyeong and good IOA (0.85) values in Masan.

    In particular, in inland areas, the RMSE and IOA of temperatures showed better results for Front_W than Front_S. For instance, whereas the RMSE and IOA of temperatures in Miryang for Front_W were 1.93 and 0.97 respectively, those for Front_S were 3.23 and 0.91 respectively. These statistical results were produced because inland temperatures were numerically simulated to be higher for Front_W due to synoptic opposing flows being weaker compared to those of Front_S. Subsequently, northerly winds did not often persist until the afternoon, allowing sea breezes inland.

  • Figure 7 presents the movements of sea breeze fronts to show the convergence of sea breezes through horizontal wind fields. Differences in moisture content in air around the converged sea breezes were used to analyze front strength. In the case of Front_W (Figs. 7a and b), sea breezes passed Changwon at high speeds exceeding 7 m s-1 to form a sea breeze front at around latitude 35°17’N at 1500 LST, moving to latitude 35°30’N (Miryang) at 1700 LST. Therefore, the sea breeze front moved at high speed. Specific humidity distribution showed 1.5-2 g kg-1 larger differences on the sea breeze frontal surface at 1700 LST compared to two hours earlier. Figures 7c and d show the resultant horizontal wind field for Front_S. The sea breeze front that passed Masan was formed along the coastline with its boundary placed at latitude 35°17’N (outskirts of Changwon) at 1500 LST. However, unlike Front_W, it remained near the coast at 1700 LST. Therefore, the movement speed of the sea breeze front in this case was 1.1 km h-1. Differences in specific humidity on the frontal surface that were 2-3 g kg-1 at 1500 LST increased to 3-4 g kg-1 by 1700 LST.

    Figure 8 shows the locations and times at which the sea breeze front was formed when southerly winds (+) were replaced by northerly winds (-), obtained by analyzing the cross section of wind velocity along the line XY in Fig. 1. Southerly winds were replaced by northerly winds every hour when the winds passed a specific point. This change in winds from onshore to offshore flow means that strong convergence occurs at this point (Stephan et al., 1999). For Front_W, a sea breeze front moved at almost a constant speed during 1100-1700 LST. In this case, the total distance of the sea breeze front's movement toward inland areas was from Y-distance 8 km to 56 km, and thus the sea breeze front moved very fast. However, while the sea breeze front was formed at sea for Front_W, for Front_S it was formed inland. The sea breeze front moved from a 17 km point to a 47 km point during 10 hours. Wind velocity V was at the same position during 1100-1200 LST as during 1400-1500 LST, indicating that the front was static. This is because the development of KHB (Kelvin-Helmholtz billow) (Simpson and Britter, 1980; Atkins et al., 1995) caused frictional force in the upper atmospheric boundary layer to slow down the entry of the sea breeze front into the inland area (Sha et al., 1991). Southerly winds and northerly winds were maintained all day long at an average wind speed of around 4 m s-1 because strong synoptic opposing flows existed to maintain northerly winds in the upper layer, while suppressing southerly winds in the lower layer.

    The next step was to analyze quantitatively the degrees of development of sea breeze fronts on the two case days. To achieve this, a sea breeze front development function formula (Arritt, 1993) was used to analyze the degrees of convergence of sea breeze fronts along line XY in Fig. 1. Sea breeze front development functions are degrees that show the formation or strength of sea breeze fronts which are defined as the ratio of the size of a potential temperature gradient to the distance to cross the coastline. In the case of Front_W (Figs. 9a-c), the value of the sea breeze front development function became largest (2.4×10-1 K m-1 s-1) at around 35.25°N at 1400 LST, and gradually decreased thereafter. Unlike Front_W, for Front_S (Figs. 9d-f) the value of the front development function was static at 35.25°N during 1400-1600 LST. The value of the front development function became largest (3.0×10-6 K m-1 s-1) at 1500 LST and was larger compared to Front_W thereafter. The boundary layer height of the value of the front development function grew, but did not exceed 0.5 km because of large-scale flows.

    Figure 11.  Vertical cross section of TKE along line XY at (a) 1400 LST, (b) 1500 LST and (c) 1600 LST for Front_W, and at (d) 1400 LST, (e) 1500 LST and (f) 1600 LST for Front_S. Arrows mark each figure indicating the sea breeze front's position. The solid line at the bottom indicates topography. Contour interval is 0.5×10-6 K m-1 s-1.

    Figure 10 shows the vertical cross section of potential temperatures and v, w-vectors along line XY in order to examine vertical sea breeze development and mixing heights centering on 1400-1600 LST when sea breeze fronts were formed most strongly. For Front_W at 1500 LST (Figs. 10a-c), the sea breeze front was located at 35.3×N, the head of sea breezes grew up to 0.6 km, and the boundary layer of sea breezes grew up to around 1 km (solid line). The range of horizontal potential temperatures of the sea breeze front was from 286 to 290 K at 1400 LST, and the difference grew to 6 K by 1600 LST. The difference of potential temperatures was large, indicating that the front gradually became stronger. On the contrary, since opposing flows were stronger for Front_S (Figs. 10d-f) than for Front_W, thermal mixing was suppressed, and thus the heights of vertical vectors were lower for Front_S. For instance, the sea breeze head and sea breeze boundary layer heights at 1600 LST were also lower than 0.5 km, which were much lower compared to Front_W. However, the potential temperatures were 289-296 K at 1600 LST, showing a difference of 7 K, which was still larger compared to Front_W. In addition, vertical vectors stayed at 35.25°N for three hours. As mentioned earlier (Fig. 8b), this location coincides with the location where wind velocity V was static due to strong synoptic opposing flows.

    Through the vertical convective development and v, w vectors analyzed earlier, the Turbulence Kinetic Energy (TKE) was analyzed, as shown in Fig. 11, and the correlations between TKE and the development of sea breeze fronts were examined. The locations of sea breeze fronts shown by vertical vectors in Fig. 10 are marked on Fig. 11 with black arrows. At 1500 LST for Front_W (Figs. 11a-c), TKE accumulated and was largest (0.9-1 m2 s-2) at the location of the sea breeze front (35.3°N), after which TKE decreased and was transported to inland areas as the sea breeze front moved. However, the TKE located at latitude 35.2°N could not climb over the mountain and thus became larger as energy gradually accumulated. The energy was increased to confined thermal turbulence because of a developed boundary layer in the city. TKE for Front_S (Figs. 11d-f) had generally smaller turbulence energy values at the location of the front compared to TKE for Front_W. In particular, the values at 1500 LST at the location of the sea breeze front were small, at around 0.5-0.6 m2 s-2. That is, owing to the existence of strong synoptic opposing flows, thermal mixing was gradually suppressed and shear development was weakened so that TKE became smaller at this location. The suppressed thermal mixing brought about increased static stability and thus decreased TKE could be seen.

    Figure 12 shows potential temperature profiles at 1500 LST for intervals of 3 km along line XY in order to analyze the characteristics of the TIBL and ML (Mixed Layer) in relation to sea breeze front structures and locations. For Front_W (Fig. 12a), a sea breeze front was located at Y34 and this location coincided with the location that passed by the front at 1500 LST at wind velocity V shown in Fig. 8a. Section Y16-Y31 is where the TIBL was formed, and the height of this layer was around 0.6 km (broken line). The ML was formed at Y37 as an unstable layer and the height of this layer reached 1.3 km. In MLs, air is mixed due to convective movements to induce substantial turbulence. An ML constitutes the entire atmospheric boundary layer at midday, and its height grows up to 1 km or even higher (Stull, 1988). For Front_S (Fig. 12b), the location of the sea breeze front was Y31, which was closer to the coast because of synoptic opposing flows. The horizontal space of the TIBL was Y19-Y28 and the height of this layer was 0.5 km (broken line), which was lower by 0.1 km than the height for Front_W. In addition, the ML was also reduced to show a height of 0.1 km at location Y37. This is because the synoptic opposing flows contribute to the development of mixing heights, thereby also affecting the formation of MLs and TIBLs.

    In particular, for Front_W, potential temperatures at points Y16-Y31 were around 285-289 K, showing a difference of 4 K for a total of 15 km. However, for Front_S, potential temperatures in the TIBL were 290-294 K, showing a potential temperature difference of 4 K for 9 km. In other words, sea breeze front strength is determined mostly by horizontal potential temperature differences. Therefore, since the potential temperature difference for Front_S was larger than that for Front_W, Fig. 12 clearly proves that the sea breeze front of Front_S was stronger.

  • We analyzed, through numerical simulations, the impact of sea breeze fronts occurring in Changwon and Masan, located in the southeastern coastal area of the Korean Peninsula, where contaminants are emitted from industrial complexes and surrounding cities. Ozone was chosen as a representative contaminant, for which the horizontal and vertical concentration fields and the relationships between them were analyzed, centering on Changwon, Masan and Jinhae.

    Figure 13 shows the horizontal ozone concentration distribution numerically simulated by CMAQ. To eliminate differences in ozone concentration due to differences in solar radiation between the two cases, we scaled ozone concentrations to analyze horizontal ozone concentrations. When a sea breeze front was located at 35°17’N (outskirts of Changwon) at 1500 LST for Front_W, ozone concentrations to the rear of the sea breeze front increased by the largest amount (0.60 ppb) to be distributed in the area. Thereafter, at 1700 LST, as the sea breeze front moved inland to areas around Masan and Changwon, the ozone concentration decreased from 0.6 ppb to 0.4 ppb and the ozone gradient was small. On the contrary, as for horizontal ozone distribution for Front_S, at 1500 LST, concentrations of 0.40-0.65 ppb were clustered along the coast of Masan. This ozone distribution also formed along the sea breeze front shown in Fig. 7c.

    Figure 14 presents the vertical diffusion of ozone in the internal boundary layer of the sea breeze. As shown in Fig. 12, a distribution of ozone concentrations was formed showing the characteristics of TIBLs, which show low heights in coastal areas and greater heights in areas further inland. As for the vertical distribution for Front_W (Figs. 14a-c), from 1400-1600 LST, an ozone concentration of 65 ppb was advected to a latitude of 35.24°-35.35°N, and thus the concentration moved 12.2 km in two hours. In this case, synoptic opposing flows existing in the upper layer in inland areas were not shown, and thus ozone was vertically diffused and rapidly advected horizontally. Vertical ozone concentrations for Front_S (Figs. 14d-f) were almost static in low-formed TIBL (0.5 km) due to synoptic opposing flows. In particular, distributions of ozone gradients and concentrations were shown to be higher than those for Front_W in this layer. At 1400 LST, the advection and diffusion of the concentration were suppressed and delayed, indicating that a 65-ppb contour of concentration was hardly advected from a range of around 35.27°-35.29°N. The vertical height of an ozone concentration of 60 ppb reached 1.2 km at 1400 LST and dropped to 1 km at 1500 LST, and below 0.8 km at 1600 LST. This is because synoptic opposing air currents in the upper layer blown from inland areas to the coast were gradually pushed toward the coast to suppress the vertical diffusion of ozone concentrations.

4 Conclusions
  • The present study investigated the characteristics of sea breeze fronts, as well as patterns of ozone advection and diffusion, by numerically simulating actual case days of sea breeze fronts in order to examine their strengths in relation to the sizes of synoptic opposing flows. The conclusions can be summarized as follows.

    (1) By analyzing wind profile data, in spring, northerly winds were found to be the most frequent in the upper layer, while southerly winds were the most frequent in the lower layer, and thus it could be seen that sea breezes were dominant in spring. When wind speeds on sea breeze and sea breeze front days were separated, the strongest winds were observed for both in spring.

    (2) From the results of statistical analyses conducted in order to verify the reliability of the models, both the RMSE and IOA of temperatures and wind speeds showed high reliability. Also, in the case of Front_W, northerly winds were not maintained until the afternoon, owing to the effects of weak synoptic opposing flows, and thus sea breezes could pass through inland areas. Therefore, temperatures were numerically simulated to be high and showed good reliability.

    (3) As for horizontal wind field analyses, in the case of Front_W, the sea breeze front moved at high speed due to synoptic opposing flows and specific humidity distributions, clearly showing the formation and movements of fronts with differences in moisture content. On the contrary, in the case of Front_S, large-scale flows were strongly developed, and thus the sea breeze front moved at a low speed. In specific humidity fields, the difference in specific humidity was larger at the frontal surface compared to Front_W.

    (4) Based on wind velocity V, in the case of Front_W, a sea breeze front was formed at sea and moved inland at high speed. However, in the case of Front_S, front stagnation appeared above inland areas, such that stronger synoptic opposing flows existed for Front_S compared to Front_W. In addition, southerly winds and northerly winds around front were maintained at constant speeds for Front_S, and because stronger synoptic opposing flows were prevented the increase of southerly wind components, northerly wind components were predominated for longer compared to those of Front_W.

    (5) In a comparison of sea breeze front development functions, it was revealed that the sea breeze front of Front_S developed to become larger than that of front_W. The suppression of vertical movements of the large potential temperature difference of 6 K acted favorably for the development of a strong sea breeze front. The TKE of Front_S at the location of the front was smaller compared to that of Front_W, and the TKE decreased as stability in the boundary layer increased as a result of the suppression of vertical movements. In the case of TIBLs and MLs shown by potential temperature profiles, potential temperature differences were shown to be larger for Front_S than Front_W, indicating that the sea breeze front was stronger and that the TIBL and ML were formed to be low and the sea breeze front located close to the coast was because of the influence of synoptic opposing flow. These features affected ozone fumigation to influence the formation of highly concentrated ozone.

    (6) Regarding ozone concentrations, both cases showed a similar characteristic in that ozone concentrations diffused to the rear of sea breeze fronts. However, this characteristic appeared more evidently for Front_S, indicating that synoptic opposing flows play the role of severely suppressing the diffusion of concentrations. Therefore, under the condition of strong synoptic opposing flows, ozone concentrations showing high concentration gradients could not be widely diffused, and were advected little-by-little along the front in the shape of concentric circles. As for vertical distributions of ozone, in the case of Front_S, owing to the low TIBL and ML, ozone was concentrated in these layers, and thus high concentrations and high concentration gradients were shown there.

    Ozone patterns in relation to the strength of sea breeze fronts were examined. According to the results, strongly developed sea breeze fronts formed low internal boundary layers due to the suppression of vertical development and, owing to the phenomenon of front stagnation, increased ozone concentrations further suppressed ozone diffusion and advection in the internal boundary layer.

Reference

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