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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D17112, doi:10.1029/2009JD011764, 2009

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Seasonal variation of the land-sea breeze circulation in the Pearl River Delta region Xi Lu,1 Kim-Chiu Chow,1,2 Teng Yao,1 Jimmy C. H. Fung,1,2 and Alexis K. H. Lau1,3 Received 18 January 2009; revised 15 May 2009; accepted 30 June 2009; published 12 September 2009.

[1] The data of a 1-year (2003–2004) simulation with a finest horizontal resolution of

1.5 km, using the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), were analyzed to investigate the seasonal-mean features of the land-sea breeze (LSB) and regional circulation over the Pearl River Delta (PRD) region in southern China. The seasonal-mean diurnal variations reveal the general patterns of the LSB in the four seasons. These small-scale mean flow fields in the region have not been revealed in any previous studies. The results reveal a strong anomalous westerly sea breeze toward the eastern coast of the PRD in the early afternoon that is present in all the four seasons but is particularly strong in autumn and winter and may enhance the low-level convergence in Hong Kong. Furthermore, the condition of the atmosphere in autumn and winter is much more stable when compared with that in spring and summer, which is not favorable for the vertical dispersion of pollutants. The overall effect of these mean meteorological conditions may be an important factor for the generally higher air pollution index observed in Hong Kong during autumn and winter. Citation: Lu, X., K.-C. Chow, T. Yao, J. C. H. Fung, and A. K. H. Lau (2009), Seasonal variation of the land-sea breeze circulation in the Pearl River Delta region, J. Geophys. Res., 114, D17112, doi:10.1029/2009JD011764.

1. Introduction [2] Land-sea breeze (LSB) is a mesoscale phenomenon caused by the difference in diurnal temperature variations between land and sea due to the different heating and cooling rates of the different surfaces. Sea breeze (SB) circulation toward the land usually develops along the coastline in the late morning of a fine sunny day as the land surface heats up and this circulation may expand in both the landward and seaward directions [Abbs and Physick, 1992]. While at night, land breeze (LB) develops because of the cooler land surface. The basic features and structure of LSB have been discussed in numerous previous studies [e.g., Arritt, 1993; Simpson, 1994; Buckley and Kurzeja, 1997]. [3] The Pearl River Delta (PRD) region is located in southern China with a population over 50 million, hosting a number of the major cities in China including Guangzhou, Shenzhen, Hong Kong, Macau, Dongguan, and Zhuhai (Figure 1). The complex shore lines and terrain of the PRD region makes LSB circulation particularly complicated in this area. In particular, most of the urban areas in the PRD region are located near the coastal regions and thus the LSB 1 Atmospheric Research Center, Hong Kong University of Science and Technology Fok Ying Tung Graduate School, Guangzhou, China. 2 Department of Mathematics, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong. 3 Environmental Central Facility, Institute for the Environment, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong.

Copyright 2009 by the American Geophysical Union. 0148-0227/09/2009JD011764$09.00

circulation may have a significant effect on the pollutant transport in these major cities. For example, Li et al. [1999] indicated that the concentrations of air pollutants over the PRD are not only correlated with the synoptic weather patterns but also with the duration of these patterns. Wang et al. [2001] analyzed the observational data from five sites in Hong Kong, and their results show that the averaged ozone levels at most sites generally have their maximum in autumn and early winter and minimum in the summer. On the other hand, it has also been pointed out by Zhang and Zhang [1999] that in the PRD region, the frequency of LSB in autumn and winter is generally higher than the other two seasons. To our knowledge, most previous studies on air pollution in the PRD region have focused on case studies that occurred in autumn and winter. For example, Ding and Wang [2004] simulated the LSB and investigated the transport of pollutants during a prolonged ozone episode observed in Hong Kong in September. Numerical and analytical analyses were also made by Fung et al. [2005] to understand the air pollution episode that occurred over much of the western part of Hong Kong between 28 and 30 December 1999. Feng et al. [2007] used numerical simulations and observational analyses to analyze the aerosol episode that occurred over the PRD region during 1– 3 November 2003. [4] Although some features of LSB and the particular large-scale background flow in specific episodes have been revealed in these case studies, the seasonal-mean features of the LSB in different seasons as well as the mean atmospheric conditions responsible for the higher frequency of air pollution episodes in autumn and winter have not been

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LU ET AL.: LAND-SEA BREEZE OVER PEARL RIVER DELTA

Figure 1. The MM5 domain discussed in this study showing the terrain (m, contours) and locations of some major cities in the Pearl River Delta region. addressed in any previous works. Studying the seasonalmean characteristics of LSB is important for understanding the basic LSB circulation and consequently the basic atmospheric conditions associated with the air pollution transport. Because of the relatively few and sparse distribution of weather stations in this region, observational data are generally not sufficient to resolve a clear picture of the seasonal-mean features of the small-scale LSB circulation in the region. Therefore, it appears that numerical simulation is the ideal tool for this kind of study. However, performing the numerical simulation over such a small scale (highresolution simulation) for a long period of time (e.g., 1 year) requires a large amount of computing resources, which is still a difficult task in practice. [5] In this study, we analyze the seasonal-mean LSB circulation in the PRD region by using the simulation data of Yim et al. [2007] (hereinafter referred to as YIM2007). YIM2007 developed a high-resolution wind map for complex terrain with the MM5 system to investigate the potential of using wind energy in Hong Kong. In their study, hourly wind fields were simulated for 1 year from 2003 to 2004 (more details will be discussed in section 2). On the basis of this limited data, the year of 2003 – 2004 is chosen in this study and three months averaged YIM2007 data in winter (December 2003 to February 2004), spring (March 2004 to May 2004), summer (June 2004 to August 2004), and autumn (September 2004 to November 2004) is used to investigate the seasonal-mean features of the LSB, and in particular the mean diurnal variations of the LSB in different seasons. [6] The details of the YIM2007 data used in this study will be discussed in section 2. The results of the analyses will be discussed in section 3. The main conclusions of this study will be summarized in section 4.

2. Data Sources [7] The meteorological model used by YIM2007 is the Fifth-Generation Pennsylvania State University – National

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Center for Atmospheric Research Mesoscale Model (MM5) version 3.6. In their study, four nested domains with horizontal grid spacing of 40.5 km (D1), 13.5 km (D2), 4.5 km (D3), and 1.5 km (D4) were used. There were 25 sigma levels in the vertical direction with the first 10 layers being concentrated in the atmospheric boundary layer (about 1 km above the ground level) to allow for finer resolution of the planetary boundary layer. [8] The PRD is a rapidly developing region, so the original land use data used by MM5 from the U.S. Geological Survey is not up to date (last updated for 1993). Therefore, YIM2007 used recently updated land use data that were originally compiled by the Hong Kong Planning Department (HKPD) [2003]. The HKPD land use data were reformatted to supersede the original 30-arc second in the MM5 data over the PRD region. [9] The reanalysis data of the European Centre for Medium-Range Weather Forecasts with 2.5° 2.5° horizontal resolution at 6-h intervals were used for the initial and boundary conditions. To enhance the quality of the simulations, four-dimensional data assimilation (FDDA) were applied to each MM5 run, which includes the grid analysis nudging to relax the simulations with the global telecommunication system observational data and the reanalysis data for the outermost domain, D1, at 6-h intervals. Observational nudging for the innermost domain D4 with the surface wind observation from the stations of the Hong Kong Observatory was also applied in the FDDA. [10] The 1-year simulation data (December 2003 to November 2004) are the integration of data from 121 separate MM5 simulations, each of which was a 4-day run with the first day data scrapped for the spin-up period. In the work by YIM2007, the 1-month period of March 2004 data was validated with the surface meteorological station data operated by the Hong Kong Observatory in the same period. Their results showed that the means of the simulated wind speeds at most stations are fairly close to the observed means. In particular, most indices of agreement were close to 1 with an average value of 0.97. The averaged value of the root-mean-square error of the simulated wind speed is around 0.64 m s1. These results suggest that their MM5 simulations of 2004 could successfully reproduce the observed changes in wind direction and that the YIM2007 data are reliable.

3. Seasonal-Mean Circulations in the PRD Region 3.1. Daily-Mean Fields in the Four Seasons [11] The PRD region is located in the subtropical region and is influenced by the Asian monsoon climate in addition to the prevailing easterly trade wind from the northwestern Pacific Ocean. In summer, the Asian Summer Monsoon (ASM) brings warm and moist air from the Indian Ocean and the South China Sea toward the region. Associated with the large-scale southwesterly of the ASM, the prevailing wind direction over the PRD is mainly southerly (Figure 2b). This southerly wind brings clean marine air to the PRD region, and the atmospheric conditions are generally humid and conditionally unstable. [12] Spring is the transition period when the large-scale atmospheric circulation changes from a winter monsoon state to a summer monsoon state and the corresponding

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Figure 2. Daily-mean surface wind and temperature fields averaged in (a) spring, (b) summer, (c) autumn, and (d) winter of 2004. monsoonal wind changes from northeasterly to southwesterly. In this transition period, the wind field in the PRD region (Figure 2a) basically changes from northeasterly in early spring to southerly in late spring, although the seasonalaveraged wind field is southeasterly. On the other hand, the corresponding seasonal-averaged wind field in the open oceanic region southwest of the PRD is mainly easterly (Figure 2a). [13] In autumn, the ASM has already retreated and weak high-pressure systems gradually develop over the Asian continent. The northeasterly winter monsoon begins to control the PRD region in late autumn, and the resulting averaged wind field in the PRD region in autumn is thus northeasterly (Figure 2c). The winter monsoon reaches a mature stage in winter and the dominant surface wind in the PRD region is again northeasterly but of a larger magnitude (Figure 2d). The northeasterly wind from the China mainland carries dry air to the PRD region in autumn and winter. The prevailing northeasterly may also facilitate the long-range transport of pollutants from the China mainland to the PRD region, which is consistent with the generally

higher air pollution index in the PRD region in these two seasons. 3.2. Seasonal-Mean Diurnal Cycles [14] The diurnal variation of the surface wind and temperature fields averaged in the four seasons of 2004 can be observed from the corresponding anomalous fields (local times minus daily mean) shown in Figures 3 – 6. In spring, the land surface is much cooler than the ocean and at 0800 and 0200 LT (Figures 3a and 3d) the LB is well developed over the PRD estuary. At these night periods, a strong anomalous northerly in the PRD estuary can be observed (see Figures 3a and 3d). In the early afternoon period (1400 LT), the increase in surface temperature over the land results in significant southerly SB toward both the east and west coasts of the PRD estuary (Figure 3b). From the corresponding total wind field in this period (Figure 7a), it is apparent that the SB is approximately symmetrical over the estuary. However, it can be seen from the corresponding anomalous field (Figure 3b) that the anomalous westerly wind toward the eastern coast of the PRD is clearly

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Figure 3. Anomalous (LT minus daily mean) surface wind and temperature fields at (a) 0800, (b) 1400, (c) 2000, and (d) 0200 LT in spring of 2004.

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Figure 4. Same as Figure 3 but for summer of 2004.

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Figure 5. Same as Figure 3 but for autumn of 2004.

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Figure 6. Same as Figure 3 but for winter of 2004.

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Figure 7. Surface wind and temperature fields at 1400 LT averaged in (a) spring, (b) summer, (c) autumn, and (d) winter of 2004.

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dominant, possibly related to a larger temperature increase over the land surface in the eastern part of the coastal PRD region (Figure 3b). This significant variation of the SB in the eastern and western coasts of the PRD is also consistent with the findings of Arritt [1993]. From a series of numerical experiments, Arritt [1993] found that the direction and magnitude of the large-scale background wind field may significantly influence the organization of SB in the afternoon. If the background wind is onshore (same direction as the SB), the afternoon SB is generally suppressed because of the weaker convergence zone in the coastal region. On the other hand, if the background wind is offshore (opposite to the direction of the SB), the SB is stronger because of the enhanced convergence zone in the coastal region. In the case of the PRD estuary, the background wind is generally easterly in spring (see Figures 2a and 7a). Therefore, for the eastern coast of the PRD region the background wind is basically onshore, while it is basically offshore for the western coast. And hence, the stronger afternoon SB in the western coast of the PRD estuary in this study generally agrees with the results of Arritt [1993]. [15] The early evening period of 2000 LT is likely a transition period when SB starts changing to LB and the temperature contrast between the land and sea is small (Figure 3c). During this period in spring slight SB can still be recognized around the coastal region (Figure 3c). [16] Similar to that in spring, LB can be observed at 0800 LT in the estuary in summer (northerly anomalous wind in Figure 4a). Since the daily-mean prevailing wind is southerly in the PRD region (Figure 2b), the actual wind over the estuary (figure not shown) is generally weak at 0800 LT. In the early afternoon at 1400 LT, symmetrical SB toward the eastern and western coasts can again be observed in the PRD estuary (Figure 7b) as in spring. However, unlike the case in spring, the corresponding anomalous wind field (Figure 4b) also shows a high degree of symmetry in the estuary, likely related to a similar surface temperature increase in the eastern and western coastal regions of the PRD estuary. Again, on the basis of the finding of Arritt [1993], this result of symmetrical SB may also be explained by the lack of a clear onshore or offshore background wind to the eastern and western coasts of the PRD estuary in summer (see Figures 2b and 4b). [17] The surface wind and temperature fields in autumn are significantly different from those in spring and summer when the winter monsoon starts influencing the region. In this season northeasterly wind is dominant in all time periods (figures not shown). Nevertheless, LB over the PRD estuary can also be identified at night to early morning from the anomalous fields at 0800 and 0200 LT (Figures 5a and 5d). As in spring and summer, early afternoon (1400 LT) SB over the PRD estuary can also be observed (Figures 5b and 7c). The actual SB is also rather symmetrical toward the eastern and western coasts of the PRD estuary (Figure 7c) but is from the north instead of from the south as in spring and summer. It can be observed from the corresponding anomalous fields (Figure 5b) that the SB is dominated by a strong anomalous westerly wind toward the eastern coast of the PRD estuary. This anomalous SB is similar to that in spring (compare Figures 5b and 3b) but is larger in magnitude, likely related to a significant imbalance in temperature increase in the land regions in the western

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and eastern coasts of the estuary (Figure 5b). The stronger SB in autumn may also be related to the larger magnitude of the background wind in autumn (compare Figures 2a and 2c) according to Arritt [1993]. It is worth mentioning that this strong anomalous westerly SB together with the anomalous southerly and easterly around the Hong Kong region may increase the low-level convergence there, which will be further discussed in section 3.3. [18] The patterns of the anomalous surface wind and temperature fields in winter are very similar to that in autumn (compare Figures 5 and 6), although the actual surface temperature is lower and the wind speed is slightly stronger in winter (figures not shown). It is worth noting that although the actual surface temperature in winter is lower than that in autumn, the magnitudes of the diurnal variation in surface temperature are similar in autumn and winter. This and the similar background wind fields may explain the similar magnitudes of the anomalous westerly SB at 1400 LT in the PRD estuary. 3.3. Meteorological Conditions Pertinent to Air Pollution [19] It has been discussed in section 3.2 that anomalous westerly winds over the PRD estuary in the early afternoon period (1400 LT) can be observed in all the four seasons, and the magnitude is particularly strong in autumn and winter. This anomalous afternoon westerly SB may cause a convergence zone in the eastern coastal region of the PRD estuary, near Hong Kong (see Figure 1). The time variation of the area-averaged low-level convergence (average of the three vertical levels below 50 m) over the Hong Kong region (113.8 – 114.4°E, 22.18 – 22.55°N, rectangle in Figure 1) in Figure 8 shows that this convergent effect is very significant in the afternoon period. The maximum convergence occurs around 1400 LT, which is consistent with the strongest anomalous westerly winds over the eastern coast of PRD and is also consistent with the period of maximum land surface temperature, around 1400 LT, for the Hong Kong region (Figure 12). The convergence generally starts to increase from about 0800 LT and attains the maximum around 1400 LT although the time the maximum magnitude occurs is slightly different in the four seasons. The earliest time the maximum convergence occurs is in summer, around 1100 LT, while the latest time is in winter around 1500 LT. This seasonal variation of the maximum convergence period is consistent with the annual cycle of the solar declination angle. The seasonal change of the time period of strongest solar radiation may change the onset period of the SB. This trend is also noted by Zhang and Zhang [1999]. The increase of the convergence is particularly sharp in autumn and winter, which is consistent with the larger surface temperature contrast between the PRD estuary and the land surface in the eastern coastal land region (compare Figures 3b, 4b, 5b, and 6b). It is worth noting that the magnitude of the maximum convergence is largest in autumn, while it is similar in the other three seasons. [ 20 ] The occurrence of the low-level convergence discussed may have a trapping effect, impeding the locally generated pollutants from being dispersed out of the region by environmental winds. This may help explain the observed generally higher air pollution levels in the region

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Figure 8. Diurnal variations of area-averaged divergence over the Hong Kong region (113.8–114.4°E, 22.18–22.55°N) at the lower troposphere (average of the three vertical levels below 50 m) for the four seasons of 2004.

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in autumn. However, winter also has generally high air pollution levels [Wang et al., 2001], but the low-level convergence in winter shown in Figure 8 is not appreciably larger than that in summer and spring. This discrepancy may be explained by the seasonal variation of the atmospheric stability in the region, which is an important meteorological factor when determining the vertical extent to which the pollutants may disperse. [21] The seasonal-mean vertical profiles of equivalent potential temperature average over the land region of Hong Kong (Figure 9) are typical and show that in summer the equivalent potential temperature (EPT) decreases with height up to the 600-hPa level (Figure 9b), denoting a conditional unstable condition. The depth of this instability zone in spring (Figure 9a) is similar to that in summer but is weaker since the decrease in EPT is not as large as in summer. The depth of the instability zone is significantly lower in autumn and winter (Figures 9c and 9d) and this is particularly apparent in winter, when the depth is at its lowest (below the 975-hPa level) among the four seasons because of the coolest ground temperature. The very stable

Figure 9. Vertical profiles of area-averaged equivalent potential temperature (K) over the land of Hong Kong region (113.8 – 114.4°E, 22.18 – 22.55°N) at different periods (0200, 0800, 1400, and 2000 LT) in (a) spring, (b) summer, (c) autumn, and (d) winter of 2004. 10 of 14

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Figure 10. As in Figure 9 but for observational data observed at the Kings Park Station in Hong Kong (station 45004; 22.32°N, 114.17°E) in 2004 at 0800 and 2000 LT. atmosphere in winter may also contribute to the generally higher air pollution level in this season. [22] The corresponding vertical EPT profiles from the data observed in Hong Kong are depicted in Figure 10. The similar magnitudes and trends in Figures 9 and 10 suggest that the seasonal variation of atmospheric stability in the four seasons of 2004 is well simulated by the model. [23] It has been discussed that the westerly winds to the east coast of the PRD in the early afternoon may increase the convergence in the west of Hong Kong. The seasonal features of LSB circulation can be further examined from the anomalous (1400 LT) vertical circulation (Figure 11) in the cross section through Hong Kong at 22.3°N (AA0 in Figure 1). It can be observed that some anomalous circulations developed near the coastal region in autumn (Figure 11c), the most prominent one being located over the PRD estuary (113.6– 113.9°E). Near the west boundary of Hong Kong (113.9°E), the increase of the upward flow may attain up to approximately 1600 m and the maximum anomalous vertical velocity can be over 0.04 m s1. On the other hand, strong downward motion occurs over the PRD estuary (113.9°E). These upward and downward flows are

clearly connected by the strong anomalous westerly SB toward Hong Kong (as discussed in the sections 3.1 and 3.2) to form a semiclosed circulation. The anomalous westerly SB is mainly below the height of 400 m. This U-shaped SB circulation can also be observed in the other three seasons (Figures 11a, 11c, and 11d) but is less well defined and weaker compared with that in autumn. This is consistent with the strongest low-level convergence in Hong Kong occurring in autumn (Figure 8). [24] From the magnitude of the anomalous vertical circulations in these cross sections we can also see that the altitude that the vertical flow may attain is clearly different in the four seasons. The vertical level that the vertical flow is confined below is lowest in winter and is highest in summer, which is generally consistent with the seasonal variation of atmospheric stability as discussed earlier. [25] The afternoon SB circulation in the PRD can be further analyzed by examining the vertical circulation in the cross section through the Pearl River estuary at 113.7°E (BB0 in Figure 1). The anomalous vertical circulations at 1400 LT (Figure 12) show a prominent SB circulation in the

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Figure 11. Vertical cross sections at latitude 22.3°N (AA0 in Figure 1) showing the anomalous (1400 LT minus daily mean) u-w streamlines, vertical velocities (m s1, shadings) in (a) spring, (b) summer, (c) autumn, and (d) winter of 2004. estuary between 22.4 and 23°N that can be observed in all four seasons. The circulation is manifested by the anomalous upward motion over the land region near the northern vertex of the PRD (around 22.9°N) and the anomalous downward motion over the sea surface of the delta in the south. This SB circulation is particularly obvious in autumn and winter (Figures 12c and 12d). Its strength is strongest in autumn (Figure 12c), which is consistent with the strongest anomalous westerly SB toward the western coast of the PRD (see Figures 5b and 10c) and the generally largest land-sea surface temperature contrast in autumn (compare Figures 3b, 4b, 5b, and 6b).

4. Concluding Remarks [26] The seasonal-mean features of the LSB in the PRD region in the four seasons of 2003 – 2004 have been investigated using the data from a high-resolution (horizontal grid size at 1.5 km) numerical simulation by the mesoscale meteorological model MM5 obtained from YIM2007. It has been shown by YIM2007 that this 1-year data can

acceptably replicate the wind speed and wind direction in the region in comparison to the observed data. [27] The results of the analysis presented in this study show that in autumn and winter, the prevailing wind in the PRD region is northeasterly while it is mainly easterly and southerly in spring and summer, respectively. The diurnal variations of the wind and temperature fields indicate that in early afternoon (around 1400 LT), the land-sea temperature contrast in the PRD region is at its maximum and a strong anomalous westerly sea breeze toward the eastern coast of the PRD can be observed in all the four seasons. This characteristic is particularly obvious in autumn and winter and is likely related to the seasonal variation of the direction and intensity of the prevailing large-scale background surface wind as discussed in section 3.2. The enhanced westerly sea breeze is also particularly significant to the low-level convergence in Hong Kong around 1400 LT and has a maximum impact in autumn and winter. This increase in low-level convergence in Hong Kong may have a trapping effect on the locally generated pollutants and may worsen the local air pollution.

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Figure 12. Same as Figure 11 but for cross sections at longitude 113.7°E (BB0 in Figure 1). [28] The presented results suggest that in the PRD region, the higher air pollution levels in autumn and winter compared with that in spring and summer, as indicated in some previous observational studies [e.g., Wang et al., 2001; Lee and Hills, 2003], may be contributed by the seasonal variation of the meteorological conditions in the region, as summarized in the following three points. First, in autumn and winter the prevailing large-scale wind is northeasterly in the region, which may help the long-range transport of pollutants from the Chinese mainland. While in spring and summer, the prevailing large-scale wind is easterly and southerly which is from the open ocean with relatively less pollutants. The higher background pollutants transported to the region in the cool seasons is consistent with the generally higher pollution levels in these seasons. Second, the stability of the atmosphere is generally stable in the cool seasons, and so the level of the instability region is generally much lower in autumn and winter compared with that in spring and summer. The larger volume of the instability region in the atmosphere in spring and summer is favorable to the dispersion of locally generated pollutants. Third, as mentioned in the last paragraph, stronger low-level convergence associated with the stronger afternoon sea

breeze toward the eastern coast of the PRD in autumn and winter may have a trapping effect on the locally generated pollutants. All these seasonal variations in the LSB and meteorological conditions in the PRD region may help explain the generally higher air pollution levels in the region during autumn and winter. Nevertheless, with regard to air pollution it should be pointed out that the seasonal variations of other factors related to the pollutants are also important, such as the seasonal variations of local pollutant emissions and photochemistry in the region. [29] Finally, it is worth mentioning that this study is based on 1 year of simulation data in 2003 – 2004, and so the generality of the results presented is rather limited. Nevertheless, the presented results have revealed some high-resolution seasonal-mean features of the LSB and meteorological conditions in the region that may help elucidate the basic air pollution meteorology in different seasons. Further numerical simulation studies to be performed over a longer time scale are of value so that a more general smallscale climatology for the PRD region can be obtained. [30] Acknowledgments. This research was supported by the Atmospheric Research Centre of the HKUST Fok Ying Tung Graduate School

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and the Hong Kong RGC grant 612807. The authors would also like to thank Zhang Sha for her help in the early data processing stage.

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Lee, Y. C., and P. R. Hills (2003), Cool season pollution episodes in Hong Kong, 1996 – 2002, Atmos. Environ., 37, 2927 – 2939, doi:10.1016/ S1352-2310(03)00296-6. Li, Q., F. G. Li, and Y. X. Liu (1999), The relationship between synoptic patterns and potential pollution and the density of surface air pollution over Pearl River Delta, J. Trop. Meteorol., 15(4), 363 – 369. Simpson, J. E. (1994), Sea Breeze and Local Wind, 234 pp., Cambridge Univ. Press, Cambridge, U. K. Wang, T., Y. Y. Wu, T. F. Cheung, and K. S. Lam (2001), A study of surface ozone and the relation to complex wind flow in Hong Kong, Atmos. Environ., 35, 3203 – 3215, doi:10.1016/S1352-2310(00)00558-6. Yim, S. H. L., J. C. H. Fung, A. K. H. Lau, and S. C. Kot (2007), Developing a high-resolution wind map for a complex terrain with a coupled MM5/ CLAMET system, J. Geophys. Res., 112, D05106, doi:10.1029/ 2006JD007752. Zhang, L. F., and M. Zhang (1999), Study of sea-land breeze system in the mouth area of the Zhujiang River, Chin. J. Atmos. Sci., 23, 581 – 588.

K.-C. Chow and J. C. H. Fung, Department of Mathematics, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong. ([email protected]) A. K. H. Lau, Environmental Central Facility, Institute for the Environment, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong. X. Lu and T. Yao, Atmospheric Research Center, Hong Kong University of Science and Technology Fok Ying Tung Graduate School, Guangzhou 511458, China.

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