• Byun, H.-R., D.-G. Lee, and H.-W. Lee, 1997: Analysis on the characteristics and predictability of the marine fog over and near the East Sea. J. Korean Meteor. Soc.,33, 41–62.

  • Cereceda, P., and R. S. Schemenauer, 1991: The occurrence of fog in Chile. J. Appl. Meteor.,30, 1097–1105.

  • Kim, H.-D., 1997: Numerical experiment on the sea-fog formation at the east coast of Korean peninsula in early summer. J. Korean Meteor. Soc.,33, 337–348.

  • Kim, S.-S., and N.-Y. Lee, 1970: On the classification of the fog regions of Korea. J. Korean Meteor. Soc.,6, 1–15.

  • Korea Ocean Research Development Institute, 1986: Tidal front in the southwest area off Korea. KORDI Rep. BSPE 00058-93-1, 106 pp.

  • Lee, S. W., 1992: Korean Nearshore Oceanology (in Korean). Jipmoondang, 334 pp.

  • Lie, H.-J., 1989: Tidal fronts in the southwestern Hwanghae (Yellow Sea). Contin. Shelf Res.,9, 527–546.

  • National Fisheries Research and Development Agency of Korea, 1986: Mean Oceanographic Charts of the Adjacent Seas of Korea. 186 pp.

  • Pingree, R. D., and D. K. Griffiths, 1978: Tidal fronts on the shelf seas around the British Isles. J. Geophys. Res.,83, 4615–4622.

  • Roach, W. T., 1995: Back to basics: Fog. Part 3—the formation and dissipation of sea fog. Weather,50, 80–84.

  • Simpson, J. H., and J. R. Hunter, 1974: Fronts in the Irish Sea. Nature,250, 404–406.

  • View in gallery

    Bathymetry and schematic circulation pattern of the sea around the Korean Peninsula. The Tsushima Current is TC. Arrows indicate the main path of the Tsushima Current. Numbers represent the depth in meters.

  • View in gallery

    A station map for sea fog observation for the west sea (▪), the south sea (•), and the east sea (▴). Inchon, Kunsan, Mokpo, Chejudo, Yosu, Pusan, Changgigap, and Jumunjin are routine meteorological stations.

  • View in gallery

    Seasonal variation of the mean frequency of sea fog occurrence in the east sea, the west sea, and the south sea of Korea.

  • View in gallery

    Mean frequency days of sea fog occurrence for (a) Jan and (b) Jul.

  • View in gallery

    Seasonal variations of mean air temperature (AT) and mean sea surface temperature (SST) at (a) Jumunjin, (b) Sorido, and (c) Chukdo for 10 yr.

  • View in gallery

    Mean sea surface temperature in (a) Jun and in (b) Dec.

  • View in gallery

    Vertical section of the water temperature in the sea off Mokpo on 18 Jul 1983 (Korea Ocean Research Development Institute 1986).

  • View in gallery

    Frequency of sea fog occurrence according to dewpoint temperature (DPT) and the value of DPT minus the sea surface temperature (DPT–SST) at stations (a) Jumunjin, (b) Sorido, and (c) Chukdo for 10 yr.

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Sea Fog around the Korean Peninsula

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  • 1 Faculty of Earth Systems and Environmental Sciences, Institute of Marine Science, Chonnam National University, Kwangju, South Korea
  • | 2 Division of Weather Forecast, Kwangju Regional Meteorological Office, Kwangju, South Korea
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Abstract

There are many island stations that routinely observe sea fog around the Korean peninsula. Historical daytime sea fog data were used to investigate the relationships between sea fog occurrence and its associated environmental factors. The frequency of sea fog occurrence is at its maximum in July in all seas around the Korean peninsula. The frequency shows a maximum in the west sea and a minimum in the east sea in spite of their similar latitude. The value of the air temperature minus the sea surface temperature is highest in all seas in July, when the frequency of sea fog occurrence is at its maximum. The heavy frequency of sea fog occurrence appears at the strong tidal mixing region in the west sea in summer, when the temperature difference between the air and the sea surface is large. Strong tidal currents provide relatively cold surface water at the mixing region in summer. It is clearly shown that the sea fog occurrence depends on dewpoint temperature (DPT) and sea surface temperature (SST). The frequency of sea fog increases greatly when the DPT is high and the value of DPT minus SST (DPT–SST) is large. A heavy frequency of sea fog of more than 50% appears frequently in cold water regions by strong tidal mixing when DPT is over 12°C and DPT–SST is larger than 2°C in summer.

Corresponding author address: Dr. Yang-Ki Cho, Faculty of Earth Systems and Environmental Sciences, Chonnam National University, Kwangju, South Korea, 500-757.

ykcho@chonnam.chonnam.ac.kr

Abstract

There are many island stations that routinely observe sea fog around the Korean peninsula. Historical daytime sea fog data were used to investigate the relationships between sea fog occurrence and its associated environmental factors. The frequency of sea fog occurrence is at its maximum in July in all seas around the Korean peninsula. The frequency shows a maximum in the west sea and a minimum in the east sea in spite of their similar latitude. The value of the air temperature minus the sea surface temperature is highest in all seas in July, when the frequency of sea fog occurrence is at its maximum. The heavy frequency of sea fog occurrence appears at the strong tidal mixing region in the west sea in summer, when the temperature difference between the air and the sea surface is large. Strong tidal currents provide relatively cold surface water at the mixing region in summer. It is clearly shown that the sea fog occurrence depends on dewpoint temperature (DPT) and sea surface temperature (SST). The frequency of sea fog increases greatly when the DPT is high and the value of DPT minus SST (DPT–SST) is large. A heavy frequency of sea fog of more than 50% appears frequently in cold water regions by strong tidal mixing when DPT is over 12°C and DPT–SST is larger than 2°C in summer.

Corresponding author address: Dr. Yang-Ki Cho, Faculty of Earth Systems and Environmental Sciences, Chonnam National University, Kwangju, South Korea, 500-757.

ykcho@chonnam.chonnam.ac.kr

Introduction

Disasters at sea around the Korean peninsula have greatly increased, because transports by sea have increased with recent industrial development. The disasters usually occur during bad weather, such as storms and sea fog (Lee 1992). In particular, the dense sea fogs have frequently caused the collision of ships. However, there has been no proper forecasting of sea fog occurrence. This is mainly due to the lack of understanding of sea fog under various synoptic conditions in this region. An analysis of historical data on sea fog occurrence and its meteorological and oceanographic environments could provide useful information in understanding the physical processes and character specific to the region. Well-documented data have been rare, however, because it is hard to observe sea fog routinely. Luckily, there are many island stations that routinely observe sea fog around the Korean peninsula.

The west sea (Yellow Sea), the south sea, and the east sea (Japan Sea) of Korea show different characteristics in many oceanographic aspects (Fig. 1). The west sea and the south sea are shelf seas with maximum depths of less than 100 m, while the east sea has a mean depth of over 1000 m. The tidal current is more than 1.5 m s−1 at maximum velocity in the west sea and less than 0.3 m s−1 in the east sea (Lee 1992). The Tsushima Current (TC in Fig. 1) originating from the Kuroshio, supplies heat to the east sea and the south sea throughout the year, while it has little effect on the west sea. These factors create different sea surface temperatures that may be important in the formation of sea fog (Lee 1992).

There have been some reports on sea fog around the Korean peninsula (Byun et al. 1997; Kim and Lee 1970;Kim 1997). Kim and Lee (1970) classified the Korea peninsula into several regions by fog occurrence characteristics. Byun et al. (1997) and Kim (1997) investigated the characteristics and occurrence of sea fog in the east sea. However, there has been a lack of understanding of the structure of sea fog occurrence and its spatial and seasonal variations. Although there have been routine observations of sea fog at island and coastal stations, most of the observation data were not documented. In this study, we documented historical data on sea fog and investigated the relation between sea fog occurrence and its environmental factors around the Korean Peninsula.

Data and method

Figure 2 shows nine coastal stations and 15 island stations for sea fog observations from 1986 to 1995. Most stations are located along the coast of the mainland in the east sea, whereas many stations are located on islands in the west sea. Coastal stations on the mainland are standard meteorological stations that routinely observe meteorological data. The island stations are lighthouses that observe fog, air temperature, and sea surface temperature. The typical elevation of a lighthouse is less than 100 m, except in Sonmido, Chilbaldo, and Ullungdo, South Korea. Ullungdo is highest at 171 m (Table 1).

These stations report fog when the visibility is 1 km or less. During most of this period, observations were made every 3 h from 0600 to 1800 local standard time (LST). The first observation and last observation were taken at 0700 and 1700 LST, respectively, in winter. A fog day was defined as a day where there was one or more observations of fog, as defined by Cereceda and Schemenauer (1991). We excluded the rainy days among fog days to remove fog accompanied by rain. The fog frequency calculated for each site is the number of fog days divided by the number of years of observation.

Air temperatures were observed at approximately 0900 LST at each station. Sea surface temperatures were observed at approximately 1000 LST on the coast near the station. The time difference is due to the different observation agencies for sea surface temperatures and air temperatures, respectively.

Seasonal and spatial variation of sea fog occurrence

Monthly mean frequencies of sea fog occurrence were averaged for groups of stations in each sea to examine their seasonal variations from 1986 to 1995 (Fig. 3). Eleven stations from Sochongdo to Sohuksando, were used for the west sea, eight from Sogwipo to Pusan, for the south sea, and five from Changgigap to Kojin, for the east sea. The average rate of occurrence shows that most sea fog occurs in all seas in summer. Fog days are less than one day from September through February and at their peak in all seas in July. This is similar to the structure of sea fog occurrence around the United Kingdom, where sea fog occurs most commonly in spring and early summer (Roach 1995).

The mean frequency of sea fog occurrence in the west sea is higher than that in the east sea throughout the year and twice (5.3 days) the mean frequency in July than the east sea (2.5 days). The standard deviations of frequency are mostly as large as the mean frequencies because of large year to year variation. However, the order of sea in frequency is unchanged most years, as the frequency fluctuates simultaneously at all seas. Considering that both seas are located at the same latitude, this difference may result from different sea surface temperatures. The frequency in the south sea is similar to that in the east sea in fall and winter, but higher in spring and summer.

Isopleths of sea fog frequency show well the spatial distribution of sea fog occurrence, despite the lack of stations in some areas. We examined the monthly mean of fog frequency in January (Fig. 4a) and July (Fig. 4b) because they could represent the minimum and the maximum frequency, respectively, as shown in Fig. 3.

The frequency at all stations except Inchon (1.6 days) is less than one day. Fog frequency in July is higher by far in the west sea than the east sea (Fig. 4b). It has remarkably higher frequency (more than 6 days) in the seas off Inchon and Mokpo in the west sea. The frequency is lower, less than three days, at most stations in the east sea.

Seasonal variations of air temperature and sea surface temperature

Three stations were selected to examine the seasonal variations of air temperature and sea surface temperature for each sea. Jumunjin in the east sea, Sorido in the south sea and Chukdo in the west sea were selected (see Fig. 1). Selection of sites is somewhat artificial. Chukdo and Sorido are located on islands, and Jumunjin is located on the eastern coast of the mainland.

Figure 5 is the variation of mean air temperatures and mean sea surface temperatures at three stations for 10 years. Air temperatures (solid lines) show not only seasonal variation, but also short-term fluctuation. But the variation of sea surface temperatures (dashed line) is predominantly seasonal. Mean air temperatures were generally higher than sea surface temperatures in spring and summer, and vice versa in fall and winter. Air temperatures were highest in July, and sea surface temperatures were highest in August. Different thermal capacity between the air and the sea makes the temperature difference. This makes the values of air temperature minus sea surface temperature (henceforth AT–SST) greatest in all seas in July, when the frequency of sea fog occurrence is highest, as shown in Fig. 3.

The air temperature at Jumunjin is lowest in winter, because Jumunjin is located at a higher latitude than the other stations, but the air temperature in summer is similar at all stations. It is noticeable that the sea surface temperature at Chukdo is lower than at Jumunjin in summer, although the latitude of Chukdo is lower than Jumunjin. The cold sea surface water makes AT–SST significant around Chukdo in summer.

The horizontal distribution of sea surface temperature

Sea surface temperature could be one of the most important factors in the occurrence of sea fog, because a cold sea surface causes a high AT–SST. Horizontal distributions of sea surface temperature were examined to investigate their relation with sea fog occurrence in summer and winter, respectively. The mean sea surface temperature data for 23 yr reported by the National Fisheries Research and Development Agency of Korea (1986) were used. The sea surface temperatures in June and December were used instead of those in July and January because routine observation of sea surface temperature was taken only in evenly numbered months.

Figure 6a is the horizontal distribution of mean sea surface temperature in June. The Tsushima Current affects the temperature of the east sea and the south sea. Most of the east sea and the south sea are warmer than 17°C and 18°C, respectively, except in some coastal regions. There are two cold water regions colder than 16°C in the west sea: the sea just off Inchon and Mokpo. The cold regions are consistent with the high frequency regions of sea fog occurrence in summer, as shown in Fig. 4b. The cold regions are characterized by shallow water under strong tidal currents (Lie 1989). The structure of sea surface temperature taken by satellite sometime in July shows similar structure to that in June, although its absolute value is higher by 2°∼4°C than the temperature in June.

It is well known that tidal fronts occur in shallow water under the influence of strong tidal currents (Pingree and Griffiths 1978; Simpson and Hunter 1974). The surface warming, due to the increase of heat input into the upper layer by solar radiation, produces stratification. The stratification makes the water column stable. However, strong tidal currents in the shallow area provide enough energy to mix the water column (Pingree and Griffiths 1978). The mixing area provides a relatively cold sea surface, because the whole water column becomes homogeneous by vertical mixing. A tidal front appears at the surface between the mixing area and the stratified area.

The observed section of temperature in the sea off Mokpo shows the tidal front well (Fig. 7). The section was taken along the zonal line of 34°35′N in latitude from 124°45′E to 125°30′E in longitude, on 18 July 1983. The temperature of the coastal area is homogeneous vertically, whereas that in the offshore area is stratified. Therefore, the surface temperature in the coastal region is lower by 2°C than that in the offshore area.

In winter, surface water becomes cold, and stratification is destroyed by convective mixing. The tidal front could not appear in winter because the whole area of the sea becomes vertically homogeneous. There will be no sharp gradient of sea surface temperature in winter. Figure 6b is the horizontal distribution of mean sea surface temperature in December. The east sea and the south sea that are affected by the Tsushima Current, are mostly warmer than 14°C. All of the west sea is colder than 12°C. There is no evidence of tidal fronts in the seas off Inchon and Mokpo.

Sea fog frequency according to dewpoint temperature and sea surface temperature

We saw that the frequency of sea fog occurrence may be related to the temperature difference of the air and the sea surface in previous sections. Humidity might be another important factor that affects the occurrence of sea fog. It is desirable to relate the sea fog occurrence to humidity and AT–SST. We selected dewpoint temperature instead of humidity, because humidity depends on temperature. We made frequency diagrams according to dewpoint temperature (henceforth DPT) and the value of DPT minus sea surface temperature (henceforth DPT–SST).

All daily data at three stations mentioned in the previous section were analyzed to examine the frequency for 10 yr. The humidity for dewpoint temperature was observed at the nearest routine meteorological station for each station. The maximum distance between two stations is less than 60 km. The grids that observed less than three days during the 10-yr period (marked × in Fig. 8) were excluded in this study. The numbers in the uppermost grids and the rightmost grids represent the percentages of sea fog occurrence and total number of observations in each column and row, respectively. The bottom symbols represent the percentages of sea fog occurrence at each grid. That is, the filled rectangular grid represents the frequency of sea fog occurrence greater than 70%, the mesh 50%∼69%, the horizontal lines 30%∼59%, the dots 10%∼29% and the blank less than 10%.

The frequency of sea fog occurrence is lowest at Jumunjin in the east sea (Fig. 8a) as shown in previous sections. No column or row exceeds 10% in frequency except the third row. The frequencies represented in the grid are mostly less than 10% except for six of the grids. Higher frequencies of more than 10% appear frequently when DPT–SST is between −2°C and 2°C. The highest frequency in the grids is 21%, when the DPT is 24.0°∼25.9°C and DPT–SST is 0.0°∼1.9°C.

The frequency at Sorido in the south sea is somewhat higher than that at Jumunjin (Fig. 8b). The frequency is generally higher when the DPT and DPT–SST are high. The frequency in the row and the column is more than 10% when DPT and DPT–SST are higher than 20°C and 0°C, respectively. The highest frequency represented in the grids is 40% when the DPT is 24.0°∼25.9°C and DPT–SST is 4.0°∼5.9°C.

The frequency at Chukdo in the west sea is the highest (Fig. 8c) among the three stations. Most of the heavy frequency (more than 30%) appears when DPT–SST is more than 2°C. There are many observations of high DPT–SST greater than 2°C, whereas there are few observations at Jumunjin and Sorido. High DPT–SST occurs mainly in the cold water at the sea surface, due to strong tidal mixing around Chukdo, as discussed in the previous section.

When DPT–SST is larger than 6°C in the column, the frequency is higher than 50% except one block. When DPT is higher than 12°C and DPT–SST is larger than 2°C, the frequency greater than 50% appears more often. The highest frequency in the grid is 75% when DPT is 22.0°–23.9°C and DPT–SST is 8.0°–9.9°C

High values of air temperature normally mean less chance of fog. However, in the marine atmosphere, dewpoint depressions are usually small and a high temperature usually means a high dewpoint. As shown and as generally accepted, high dewpoints relative to the SST increase the fog probability.

Conclusions

The frequency of sea fog occurrence is highest in July for all seas around the Korean peninsula. The frequency is greatest in the west sea and smallest in the east sea, in spite of their similar latitude. The mean frequency in July is 5.3 days in the west sea and 2.5 days in the east sea, according to a 10-yr average. It is less than 1 day at most seas during fall and winter.

It is shown that the high frequency of sea fog occurrence is greatly related to high AT–SST. AT–SST in all seas is at its maximum in July when the frequency of sea fog occurrence is greatest. The heavy frequency of sea fog occurrence appears at the strong tidal mixing region in the west sea where the AT–SST is large, because strong tidal currents provide relatively cold surface water by vertical mixing in summer.

The frequency diagrams, according to DPT and DPT–SST, show clearly that sea fog occurrence mainly depends on the DPT and SST. The frequency is high when both DPT and DPT–SST are high. The DPT is high in summer at all stations, and DPT–SST is high at strong tidal mixing regions in the west in summer.

A heavy frequency of sea fog of more than 50% appears frequently at the tidal mixing region when the DPT is over 12°C and DPT–SST is higher by more than 2°C. For the special range of DPT and DPT–SST, the frequency of sea fog occurrence exceeds 70%. Although other meteorological conditions such as wind might also affect the occurrence of sea fog, the frequency according to DPT and DPT–SST from historical data could provide useful information in predicting the probability of sea fog occurrence.

Acknowledgments

This research was supported in part by the Korean Science and Engineering Foundation (2000-2-13500-001-2) and Chonnam National University (1998). We are thankful to B.-J. Choi at Rutgers—the State University of New Jersey for processing data, and graduate students in the Physical Oceanography Laboratory of the Chonnam National University for preparing figures.

REFERENCES

  • Byun, H.-R., D.-G. Lee, and H.-W. Lee, 1997: Analysis on the characteristics and predictability of the marine fog over and near the East Sea. J. Korean Meteor. Soc.,33, 41–62.

  • Cereceda, P., and R. S. Schemenauer, 1991: The occurrence of fog in Chile. J. Appl. Meteor.,30, 1097–1105.

  • Kim, H.-D., 1997: Numerical experiment on the sea-fog formation at the east coast of Korean peninsula in early summer. J. Korean Meteor. Soc.,33, 337–348.

  • Kim, S.-S., and N.-Y. Lee, 1970: On the classification of the fog regions of Korea. J. Korean Meteor. Soc.,6, 1–15.

  • Korea Ocean Research Development Institute, 1986: Tidal front in the southwest area off Korea. KORDI Rep. BSPE 00058-93-1, 106 pp.

  • Lee, S. W., 1992: Korean Nearshore Oceanology (in Korean). Jipmoondang, 334 pp.

  • Lie, H.-J., 1989: Tidal fronts in the southwestern Hwanghae (Yellow Sea). Contin. Shelf Res.,9, 527–546.

  • National Fisheries Research and Development Agency of Korea, 1986: Mean Oceanographic Charts of the Adjacent Seas of Korea. 186 pp.

  • Pingree, R. D., and D. K. Griffiths, 1978: Tidal fronts on the shelf seas around the British Isles. J. Geophys. Res.,83, 4615–4622.

  • Roach, W. T., 1995: Back to basics: Fog. Part 3—the formation and dissipation of sea fog. Weather,50, 80–84.

  • Simpson, J. H., and J. R. Hunter, 1974: Fronts in the Irish Sea. Nature,250, 404–406.

Fig. 1.
Fig. 1.

Bathymetry and schematic circulation pattern of the sea around the Korean Peninsula. The Tsushima Current is TC. Arrows indicate the main path of the Tsushima Current. Numbers represent the depth in meters.

Citation: Journal of Applied Meteorology 39, 12; 10.1175/1520-0450(2000)039<2473:SFATKP>2.0.CO;2

Fig. 2.
Fig. 2.

A station map for sea fog observation for the west sea (▪), the south sea (•), and the east sea (▴). Inchon, Kunsan, Mokpo, Chejudo, Yosu, Pusan, Changgigap, and Jumunjin are routine meteorological stations.

Citation: Journal of Applied Meteorology 39, 12; 10.1175/1520-0450(2000)039<2473:SFATKP>2.0.CO;2

Fig. 3.
Fig. 3.

Seasonal variation of the mean frequency of sea fog occurrence in the east sea, the west sea, and the south sea of Korea.

Citation: Journal of Applied Meteorology 39, 12; 10.1175/1520-0450(2000)039<2473:SFATKP>2.0.CO;2

Fig. 4.
Fig. 4.

Mean frequency days of sea fog occurrence for (a) Jan and (b) Jul.

Citation: Journal of Applied Meteorology 39, 12; 10.1175/1520-0450(2000)039<2473:SFATKP>2.0.CO;2

Fig. 5.
Fig. 5.

Seasonal variations of mean air temperature (AT) and mean sea surface temperature (SST) at (a) Jumunjin, (b) Sorido, and (c) Chukdo for 10 yr.

Citation: Journal of Applied Meteorology 39, 12; 10.1175/1520-0450(2000)039<2473:SFATKP>2.0.CO;2

Fig. 6.
Fig. 6.

Mean sea surface temperature in (a) Jun and in (b) Dec.

Citation: Journal of Applied Meteorology 39, 12; 10.1175/1520-0450(2000)039<2473:SFATKP>2.0.CO;2

Fig. 7.
Fig. 7.

Vertical section of the water temperature in the sea off Mokpo on 18 Jul 1983 (Korea Ocean Research Development Institute 1986).

Citation: Journal of Applied Meteorology 39, 12; 10.1175/1520-0450(2000)039<2473:SFATKP>2.0.CO;2

Fig. 8.
Fig. 8.

Frequency of sea fog occurrence according to dewpoint temperature (DPT) and the value of DPT minus the sea surface temperature (DPT–SST) at stations (a) Jumunjin, (b) Sorido, and (c) Chukdo for 10 yr.

Citation: Journal of Applied Meteorology 39, 12; 10.1175/1520-0450(2000)039<2473:SFATKP>2.0.CO;2

Table 1.

Elevations of observed stations.

Table 1.
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