Land-sea breeze circulation structure on the west coast of the Yellow Sea,China

Yongxing M , , Jinyun Xin , , , Xioling Zhng , Lindong Di , Klus Shefer ,Shigong Wng , Yuesi Wng , Zif Wng , Fngkun Wu , Xinrui Wu , Gungzhou Fn ,

a Plateau Atmosphere and Environment Key Laboratory of Sichuan Province, School of Atmospheric Sciences, Chengdu University of Information Technology, Chengdu,China

b State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics, Chinese Academy of Sciences,Beijing, China

c Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing, China

Keywords:Land-sea breeze Vertical wind speed CCirculation structure Doppler wind lidar Yellow sea

ABSTRACT Land-sea breeze (LSB) is an atmospheric mesoscale circulation that occurs in the vicinity of the coast and is caused by uneven heating resulting from the difference in specific heat capacity between the sea and land surfaces. The circulation structure of LSB was quantitatively investigated with a Doppler wind lidar Windcube100s on the west coast of the Yellow Sea for the first time. The time of observation was 31 August to 28 September 2018. It was found that the height of LSB development was 700 m to 1300 m. The duration of conversion of LSB was between 6 h and 8 h. The biggest average horizontal sea-breeze wind speed at 425 m was 5.6 m s ? 1 , and at 375 m it was 4.5 m s ? 1 . During the conversion process from sea breeze to land breeze, the maximum wind shear exponent was 2.84 at 1300 m altitude. During the conversion process from land breeze to sea breeze, the maximum wind shear exponent was 1.28 at 700 m altitude. The differences in wind shear exponents between sea-breeze and landbreeze systems were between 0.2 and 3.6 at the same altitude. The maximum value of the wind shear exponent can reflect the height of LSB development.

1. Introduction

Land-sea breeze (LSB) is an atmospheric mesoscale circulation that occurs in the vicinity of the coast ( Zhu and Lin, 1981 ). The variability in LSB and its impact on weather are studied to improve the quality of local short-range weather forecasting. Also, studying the impact of LSB on pollutants can service the regional atmospheric environment. LSB observations began in the 1960s, with several researchers having conducted three-dimensional observational studies of LSB in the United States with ships, aircrafts, meteorological towers and balloons ( Fisher, 1960 ; Hsu, 1969 ; Simpson et al., 1977 ). Later, in the 1990s, two-dimensional models and a non-hydrostatic model were developed to study LSB under large-scale stabilized wind field conditions( Bechtold, 1991 ; Koo and Reible, 1995 ). Laser radar and Doppler radar were used by Banta (1995) and Finkele et al. (1995) to monitor the wind profile of LSB.

Fig. 1. (a) Surrounding environment of the Doppler wind lidar; (b) the observation location; (c) the operation principle of DWL; and (d) circulation pattern at 850 hPa.

In recent years, some advances have been made in the study of LSB.Researchers have studied and discussed the characteristics of the patterns of LSB systems and found that observations of sea and land breezes are consistent with model results ( Papanastasiou et al., 2010 ; Rani et al.,2010 ). Additionally, several researchers have carried out linear, numerical and water tank simulations to analyze the LSB structure, revealing that LSB circulation is not symmetrical and the sea-breeze depth scale is controlled by stability ( Porson, Steyn, and Schayes, 2007 ; Young and Zhang 1999 ). Sheng et al. (2009) and Huang and Wang (2014) found that strong sea breezes can reach 1.5 km high and LSB systems can foster the formation of precipitation. Puygrenier et al. (2005) concluded that sea breezes can cause the accumulation of pollutants by affecting the boundary layer height and the conditions for vertical diffusion. There have, however, been few studies carried out to address with high accuracy the vertical structure of LSB.

With its rapid industrialization and urbanization, the air quality along the west coast of the Yellow Sea, China, is deteriorating. Studying the vertical structure of LSB could provide a reference with respect to air pollution transport in this region. This paper aims to address the knowledge gap in China’s observation and research in this area by describing with a high degree of accuracy the structure of LSB as observed by Doppler wind lidar (DWL).

2. Data and methods

In this study, for the first time, a Windcube100s DWL was used to observe LSB on the west coast of the Yellow Sea (35°10 ′ 59 ″ N,119°23 ′ 57 ′ ’E). The study period was from 31 August to 28 September 2018 ( Fig. 1 ). Fig. 1 (d) shows the stable average time background circulation conditions during the LSB. At 850 hPa on the weather map there was no large-scale synoptic system and obvious wind shear affected the observation station. Real-time wind field data with high temporal resolution and high precision were directly obtained by analyzing this measurement information. The system adopts DBS (Doppler beam swing) scanning measurement to obtain wind profiles. The maximum observation range of effective wind speed was 50-3000 m. The vertical resolution was 25 m. The control accuracy of wind speed was 0.1 m sand the temporal resolution was 5 s. In fact, the radial wind is measured directly by the DWL. In Fig. 1 (c) the horizontal wind speed, wind direction, and vertical wind speed can be obtained by analyzing the radial wind speeds at the four azimuths’ triangular functions and the Cone angle

θ

of DBS scan mode. Observations with small signal-to-noise ratio were eliminated during the calculation and the data acquisition rate was guaranteed to be above 90%. The original wind speeds must be more than 40% continuous with each other, or otherwise the hourly average wind will not be calculated. The continuity in time and height of the data was checked every 6 min within 1 h on each beam. The wind speed and direction were calculated by the vector average method. The calculation method of the wind shear exponent (WSE) is shown in the following equation ( Jaramillo and Borja, 2004 ):

v

and

v

are wind speeds measured at height

h

and

h

respectively.

α

is the wind shear exponent.

By applying the governing momentum equations to a turbulent flow it is possible to calculate the tendency of turbulent kinetic energy (TKE)(Markowski, 2010):

u

′

,

v

′

,

w

′

are the kinetic energy of pulsating motion in three-dimensional space.

According to the geographical location of the station and the distribution characteristics of the coastline near the observation station, we defined the sea-breeze direction as between east and southeast, while the land-breeze direction was between southwest and west.

3. Results

3.1. Determination and quantitative analysis of lsb

Fig. 2 (a) shows a typical LSB. Before 1800 LST ( Local Standard Time), the sea breeze was the mainstream. Later, the horizontal wind speed gradually increased at each level. From 1900, the wind direction began to change and deflected to the south. The horizontal wind speed decreased gradually at each altitude. At 2100 the wind direction was mainly northerly. During the period from 2200 to 0000, the wind direction was northerly above 500 m and north-northeasterly below 500 m. At 0100 12 September, the sea breeze completed the conversion to land breeze. The horizontal wind speed increased gradually and the conversion time was about 8 h. The altitude of LSB development was 1300 m. The vertical wind speed difference between bottom and top changed from ? 0.05 m sat 1900 to 0.013 m sat 2000. The average vertical wind speed increased gradually from ? 0.023 m sat 2100 to ? 0.092 m sat 2300. At 0000 UTC 12 September, the vertical wind direction became upwards of 0.066 m s.

As can be seen from Fig. 2 (b), before 2000 27 September, the sea breeze was dominant. Later, the wind direction turned southwards and the horizontal wind speed decreased gradually at each altitude. At 0000 28 September, the wind direction was mainly northerly. The horizontal wind speed at each altitude began to increase gradually. The sea breeze completed the conversion to land breeze at 0400. The height of LSB development was about 1300 m. The average wind speed changed from? 0.048 m sat 2000 to 0.062 m sat 2100. At 2200, the vertical wind speed was 0.044 m sbelow 600 m, and in the range from 600 m to 1000 m it was ? 0.062 m s. From 2300 27 September to 0200 28 September, the average vertical wind speed increased gradually from? 0.062 m sto ? 0.19 m s. At 0400, the vertical wind speed was upwards of 0.06 m s. The vertical wind direction also had an updown-up variation.

As shown in Fig. 2 (c), before 0100 22 ‘September, the land breeze was dominant. At 0100, the horizontal wind began to shift northwards.At 0600, the wind direction shifted to the north, and the horizontal wind speed at each altitude decreased gradually. The sea breeze turned to a land breeze at 0800 22 September. During the entire conversion process of sea breeze to land breeze, the height of LSB development was about 700 m. The conversion time was about 6 h. During the process of LSB development, the vertical direction of the prevailing wind (land breeze) was downwards and the wind speed was 0.1 m sbefore 0100.At 0100, the vertical wind speed shifted to upwards. The wind speed increased from 0.089 m sto 0.173 m sat 0400 22 September. When the land breeze finished turning into a sea breeze, the wind speed was? 0.037 m s. The vertical wind direction had a down-up-down change.

Fig. 3 shows the variation of horizontal wind speed at different heights during the process of three LSB transitions. In Fig. 3 (a), the analysis demonstrates that the average horizontal wind speed of the sea breeze was larger than that of the land breeze below 900 m. The average horizontal wind speed in the LSB conversion process was less than the sea breeze and land breeze. The occurrence of maximum wind speed of the sea breeze and land breeze was at 475 m and 400 m respectively,the values of which were 5.8 m sand 5.1 m s. As Fig. 3 (b) shows,the horizontal wind speed at the time of the sea breeze was less than that of the land breeze below 1300 m. The maximum wind speed of the sea breeze and land breeze appeared at 900 m and 800 m, respectively,the values of which were 4.6 m sand 6.0 m s. As shown in Fig. 3 (c),the maximum wind speeds of the sea breeze and land breeze appeared at 400 m and 300 m respectively, the values of which were 4.9 m sand 4.5 m s. Therefore, the height of maximum horizontal wind speed of the sea breeze was higher than that of the land breeze.

3.2. Wind shear and tke of LSB

Fig. 4 shows the variation of vertical WSE in the conversion of land breeze to sea breeze. As we can see from Fig. 4 (a), the WSE below 500 m was small. During the conversion of sea breeze to land breeze depicted in Fig. 4 (b), warm and humid air masses of the ocean were constantly transported to the land and met dry and cold air masses due to the thermal properties of different surfaces, resulting in the wind speed changes in the vertical direction. During 2000-2200 11 September, as the atmosphere above 1300 m was under the control of easterly circulation,there was a thermal difference at the junction between the easterly circulation and the LSB, thus leading to larger WSE. During this period,the average WSE at the height of 1300 m was 1.49. In the surface layer,wind speed was affected by surface roughness. Therefore, the average WSE at the height of 50 m was 2.2. As Fig. 4 (c) shows, from 0100 21 September, when the land breeze started to convert to the sea breeze,the WSE above 400 m increased. During the conversion, the cold and dry air masses of land wind constantly transported to the ocean and merged with the ocean warm and humid air mass, so the wind speed began to change. From 0400-0800 22 September, the westerly circulation located above 700 m, and the LSB developed below 700 m. Therefore, during the conversion process, the WSE close to 700 m varied greatly with the influence of thermal properties and wind convergence. The WSE at 700 m was 1.28. The results in Fig. 4 (e) are similar to those in Fig. 4 (a). In Fig. 4 (f), from 2100 27 September to 0100 28 September, the westerly circulation located above 1300 m, and the development range of LSB was below 1300 m. During this period, the mean value of the WSE component at 1300 m was 2.84. During the conversion from land breeze to sea breeze, the height of larger absolute values of the WSE can indicate the height of LSB development.

Fig. 5 shows the TKE calculated from the observed data of the DWL at the coastal observatory. It can be seen from Fig. 5 (a) that as the atmospheric boundary layer developed from stable nighttime to unstable daytime at sunrise, atmospheric convection enhanced. From 0930 TKE increased at different heights. As the sea breeze gradually stabilized,the height of strong TKE gradually decreased. After 1800, the boundary layer gradually became stable along with sunset, and the value of TKE remained between 0 and 2 ms. As we can see from Fig. 5 (b),a large value of TKE appeared near the surface layer during the period from 1900-2100 . It was caused by the gradual reduction of the surface temperature to a constant thermal difference after sunset. Fig. 5 (c)shows the LSB development when the atmospheric boundary layer was stable, and the overall TKE was small. From (e), the TKE below 400 m was relatively large, reaching a maximum of 2 ms. The change of TKE in Fig. 5 (e) is similar to that in Fig. 5 (a). As Fig. 5 (f) shows, during the transition from sea breeze to land breeze, a large TKE value at about 1000 m was caused by a large wind shear.

3.3. LSB comparative analysis and conversion process

Table 1 shows the LSB characteristic parameters of different data sources in different regions. The analysis shows that the DWL has a high temporal resolution. The DWL can observe the length of time in the entire LSB conversion process at different altitudes. The comparison shows that the wind speed of the sea breeze and land breeze on the west coast of the Yellow Sea is larger than that in other regions. The LSB height is also higher than that in other regions.

Fig. 2. (a, b) Conversion of sea breeze to land breeze. (c) Conversion of land breeze to sea breeze. Colors represent the vertical wind speed and the arrows represent the horizontal wind speed and direction.

In Fig. 6 , the results are based on LSB observation from 27-28 and from 21-22 September. As Fig. 6 (a) shows, the vertical wind was 0.056 m sin the sea breeze, which then changed to ? 0.124 m sin the conversion process and ultimately reached 0.06 m sin the land breeze. During the conversion process from sea breeze to land breeze,the biggest average WSE at the height of 1300 m was 2.84. As shown in Fig. 6 (b), the vertical wind was ? 0.1 m sin the land breeze, which then changed to 0.09 m sin the conversion process and ultimately reached ? 0.08 m sin the sea breeze. During the conversion process from land breeze to sea breeze, the biggest average WSE at the height of 700 m was 1.28. The temperature and humidity decreased after the land breeze turned into the sea breeze, and increased after the land breeze turned into the sea breeze. During the LSB transition processes, the horizontal wind speed had a variation that manifested first as a decrease, and then an increase. The change in horizontal wind direction was clockwise due to the Coriolis force and the vertical wind direction had changed.The maximum WSE was at the altitude of LSB development.

Fig. 3. Horizontal wind speeds of LSB at different altitudes. SB, sea breeze; CP, conversion process; LB, land breeze.

Fig. 4. Variation of vertical wind shear exponent in the LSB conversion process: (a, c, e) 30-min-average wind shear; (b, d, f) values of instantaneous shear by minute.

Fig. 5. Vertical structure changes of TKE in LSB conversion: (a, c, e) minute average of TKE, ranging from 0-5; (b, d, f) minute average of TKE in LSB conversion,ranging from 0-2.

Table 1 Comparison of observations of the west coast of the Yellow Sea with other regions.

Fig. 6. Vertical structure from the sea breeze to the land breeze and the vertical structure from the land breeze to the sea breeze.

4. Conclusion

The high-resolution structure of LSB can be observed by Doppler wind lidar under a stable background circulation. In September, the altitude of LSB development is relatively high on the west coast of the Yellow Sea, being able to reach 1300 m. In the entire LSB conversion process, the difference in thermal power destroys the original system(sea-breeze or land-breeze system), along with a reduction in horizontal wind speed. After the conversion, a new system (land-breeze or seabreeze system) is formed. Therefore, the horizontal wind speed increases again. The height of the biggest wind speed in the sea-breeze system is higher than that in the land-breeze system. The vertical wind speed changes direction during the conversion process. The change in horizontal wind direction is clockwise during LSB due to the Coriolis force.The temperature and humidity decrease after the sea breeze turns to a land breeze, indicating that the area is impacted by cold and dry land breezes. The temperature and humidity increase after the land breeze turns into the sea breeze, indicating that the area is impacted by warm and humid sea breezes. There is a thermal difference at the junction between the westerly circulation and the LSB at the LSB height, thus leading to larger absolute values of WSE. Therefore, the maximum of WSE can reflect the altitude of land- and sea-breeze development. There is no evident variation of TKE affected by LSB.

Acknowledgments

This study was supported by the National Key Research and Development Program of China [Grant number 2016YFC0202001], the Chinese Academy of Sciences Strategic Priority Research Program [Grant number XDA23020301], and the National Natural Science Foundation of China [Grant number 41375036].