ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 30, NO. 4, 2013, 1154–1172
Microphysical Characteristics of Sea Fog over the East Coast of Leizhou Peninsula, China
ZHAO Lijuan1 (赵丽娟), NIU Shengjie∗1 (牛生杰), ZHANG Yu2 (张 羽), and XU Feng3 (徐 峰)
1Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control,
School of Atmospheric Physics, Nanjing University of Information Science and Technology, Nanjing 210044
2Zhanjiang Meteorological Bureau, Zhanjiang 524000
3Guangdong Ocean University, Zhanjiang 524009
(Received 19 December 2011; revised 19 September 2012; accepted 23 October 2012)
ABSTRACT Microphysical properties of sea fog and correlations of these properties were analyzed based on the mea- surements from a comprehensive field campaign carried out from 15 March to 18 April 2010 on Donghai Island (21◦3500N, 110◦320500E) in Zhanjiang, Guangdong Province, China. There were four types of circula- tion pattern in favor of sea fog events in this area identified, and the synoptic weather pattern was found to influence the microphysical properties of the sea fogs. Those influenced by a warm sector in front of a cold front or the anterior part of low pressure were found to usually have a much longer duration, lower visibility, greater liquid water content, and bigger fog droplet sizes. A fog droplet number concentration of N>1 cm−3 and liquid water content of L>0.001 g m−3 can be used to define sea fogs in this area. The type of fog droplet size distribution of the sea fog events was mostly monotonically-decreasing, with the spectrum width always being >20 µm. The significant temporal variation of N was due in large part to the number concentration variation of fog droplets with radius <3 µm. A strong collection process appeared when droplet spectrum width was >10 µm, which subsequently led to the sudden increase of droplet spectrum width. The dom- inant physical process during the sea fog events was activation with subsequent condensational growth or reversible evaporation processes, but turbulent mixing also played an important role. The collection process occurred, but was not vital.
Key words: microphysical property, microphysical correlation, fog droplet size distribution, the South China Sea Citation: Zhao, L. J., S. J. Niu, Y. Zhang, and F. Xu, 2013: Microphysical characteristics of sea fog over the east coast of Leizhou Peninsula, China. Adv. Atmos. Sci., 30(4), 1154–1172, doi: 10.1007/s00376-012- 1266-x.
1. Introduction and islands, which is influenced by the ocean. The sea fog described in this paper is fog that occurs over the Fog is a collection of water droplets or ice crystals coast. suspended in the atmosphere near the Earth’s surface, During the last several decades, much effort has which can reduce visibility to below 1 km. Low visibil- been devoted to investigating the ambient conditions, ity caused by fog can bring serious harm to maritime physical factors, and dynamical factors associated with and coastal land traffic, and result in enormous hu- sea fog formation, evolution, and dissipation by com- man casualties and economic losses (Gultepe et al., prehensive field experiments [e.g. CALSPAN (Pili´e 2007). According to the American Meteorological So- et al., 1979), CEWCOM-76 (Lewis et al., 2004), and ciety (AMS) glossary, three types of sea fog are defined, Project Haar (Findlater et al., 1989)] and numerical which are haar, sea fog, and steam fog. Wang (1983) simulations (e.g. Oliver et al., 1978; Nicholls, 1984; defined sea fog as a fog that occurs over the sea, coast Koraˇcinet al., 2001, 2005; Kong, 2002; Fu et al., 2004;
∗Corresponding author: NIU Shengjie, [email protected] © China National Committee for International Association of Meteorology and Atmospheric Sciences (IAMAS), Institute of Atmospheric Physics (IAP) and Science Press and Springer-Verlag Berlin Heidelberg 2013 NO. 4 ZHAO ET AL. 1155
Gao et al., 2007; Heo and Ha, 2010; Yang et al., 2010b). mentioned above, microphysical processes are also im- The spatial variability in sea fog frequency is influ- portant to understand sea fog events, and better mi- enced by SST, and the regions of an extremely high crophysics parameterizations are needed for models in frequency of sea fog are typified by cold SST and close order to improve sea fog forecasting capability (Kong, proximity to warm currents (Lewis et al., 2004). Fur- 2002). Especially, the visibility parameterization used thermore, synoptic weather patterns, advection, tur- in numerical prediction models is usually determined bulence, air–sea temperature differences, atmospheric by microphysical properties of fog, such as fog droplet stability, subsidence, and radiation all have crucial ef- number concentration (N), liquid water content (L), fects on the formation and development of a sea fog effective radius (re), and mean radius (rm), among oth- event (Noonkester, 1979; Pili´eet al., 1979; Wang, 1983; ers (Meyer et al., 1980; Kunkel, 1983; Stoelinga and Leipper, 1994; Lewis et al., 2003, 2004; Fu et al., 2004; Warner, 1999; Kang et al., 2002; Gultepe et al., 2006, Jiang et al., 2005; Hu et al., 2006; Gao et al., 2007; Heo 2009; Gultepe et al., 2007). As early as the mid-1930s, and Ha, 2010). Large-scale circulation can influence Houghton and Radford (1936) investigated the micro- fog formation and its dissipation, and synoptic-scale physical properties of sea fog in Round Hill, Virginia, subsidence is a critically important factor in sea fog U.S., and found that liquid water content was linearly formation (Lewis et al., 2003). Leipper (1994) sum- correlated with horizontal visibility and the variation −3 marized the research on sea fog on the U.S. west coast range of N, L and rm were 1.4–6.0 cm , 0.095–0.222 and pointed out that the advection of warm air pro- g m−3 and 5–45 µm, respectively (Wang, 1983). In vided strong low inversion and dry air aloft, which are 1948, research on microphysical properties of sea fog needed for the formation of dense fog over cool coastal in Qingdao marked the beginning of sea fog micro- waters. Radiation from the fog top is the main factor physical structure investigations in China, and the re- in creating an unstable layer that deepens a fog layer searchers obtained variation ranges of N, L, and rm (Leipper, 1994). Cooling by longwave radiation is the of 0.04–37.4 cm−3, 0.01–0.76 g m−3 and 4–304 µm, main factor in the formation and development of fogs, respectively (Wang, 1983). In the past few decades, whereas heating by solar shortwave radiation plays an several comprehensive experiments have been carried important role in fog dissipation (Dong et al., 2006; out that have substantially improved our understand- He et al., 2009; Liu et al., 2009; Hu et al., 2011). The ing of microphysical characteristics of sea fog (Good- persistence and dissipation of fog are also strongly in- man, 1977; Pili´eet al., 1979; Yang, 1985; Findlater et fluenced by dynamical factors, including wind and tur- al., 1989; Yang et al., 1989; Xu et al., 1994; Gultepe bulence, among others (Sun, 2000; Liu et al., 2010b). et al., 2007, 2009; Huang et al., 2009, 2010). Com- Low-level jets have both positive and negative influ- pared with urban areas and mountain regions, sea fogs ences on fog evolution (Liu et al., 2011a, b). On one in coastal areas usually have lower fog droplet num- hand, when the fog structure is two-layered, the low- ber concentrations, but larger droplet sizes (Niu et level jet between the two fog layers prevents them from al., 2010b; Liu et al., 2011b). Furthermore, the mi- joining. On the other hand, the vertical transportation crophysical structure of sea fog and chemical compo- of heat and moisture caused by the low-level jet is of sition of sea fog water varies systematically with air great benefit to the development and maintenance of parcel trajectory; for example, fogs on a maritime tra- fog. Koraˇcinet al. (2005) simulated the evolution of jectory usually have a smaller average number con- sea fog along the Californian coast and found that ma- centration and higher Cl− concentration (Goodman, rine boundary layer cooling and turbulence generation 1977; Yang et al., 1989). The mean fog droplet diam- were mainly determined by cloud-top cooling, and that eter and L increase with height, and broader droplet the dissipation of sea fog was due to comprehensive size distributions are observed near the inversion layer effects of advection, synoptic evolution, and develop- base (Goodman, 1977). Most average droplet size dis- ment of local circulations. The development of the top tributions in fog are unimodal, but bimodal and mul- of the sea fog is closely related to the advection of wa- timodal distributions have also been observed in some ter vapor, the longwave radiation of the fog top, and fog events (Pili´eet al., 1975; Goodman, 1977; Gerber, the movement of the vertical turbulence, and the evo- 1991; Kang et al., 2002; Gultepe et al., 2007; Niu et lution of sea fog is dependent on complex interactions al., 2010b). The average diameter of fog droplets and of turbulence, radiation and the advection of air (Fu N are inversely correlated with each other, and L has a et al., 2011; Huang et al., 2011; Kim and Yum, 2012). positive correlation with average diameter, especially In short, a delicate balance among different physical the maximum diameter (Huang et al., 2000; Kang et processes and dynamics is needed for accurate sea fog al., 2002; Niu et al., 2010b). However, because of dif- forecasting (Teixeira, 1999; Yang et al., 2010b). ferences in synoptic weather systems, the geography In addition to the physical and dynamical processes of the local environment, and aerosol characteristics, 1156 SEA FOG OVER LEIZHOU PENINSULA VOL. 30
microphysical properties (in terms of their values) and great differences in underlying surface characteristics their evolutions during sea fog events exhibit dramatic and roughness between sea and land, the turbulence is variation. Furthermore, microphysical processes, such extremely strong near the coastal surface, which leads as condensation and collision–coalescence, are difficult to turbulent mixing of foggy air and clear air near the to observe directly using instruments. Yet, accurate ground. In order to obtain representative observations descriptions of different microphysical processes are of sea fog and to minimize the influence of buildings on critical for sea fog simulation by numerical models the sampling, instruments were set up on the east edge (Kong, 2002; Gultepe et al., 2007). Investigating the of the roof of the tallest building about 15 m above sea relationships between microphysical properties sheds level, with no obstacles facing the sea. Thus, our ob- light on the dominant microphysical processes during servations can represent the characteristics of sea fog fog events (Niu et al., 2010a). The main microphys- in this area. ical processes can be deduced based on the evolution The main purpose of the work reported in this pa- of microphysical structure during a fog event. The per was to obtain basic information about the micro- mechanisms of fog formation and dissipation can be physical characteristics of sea fog on Leizhou Penin- revealed by comprehensive analysis of dynamics, ther- sula’s east coast, which can be used to validate the modynamics, and macro- and microphysical structures results from numerical model simulations and satel- (Wang et al., 2006; Pu et al., 2008; Liu et al., 2010a). lite data retrieval, as well as to investigate the dom- The Leizhou Peninsula, jutting out southward from inant microphysical processes during sea fog events, the coast of Guangdong Province, is located between which is important for sea fog forecasting. The rest the South China Sea and the Gulf of Tonkin. The of the paper is organized as follows. Section 2 intro- Leizhou Peninsula together with the Hainan Strait is duces the instruments used for data collection and the one of the five areas in China that experience the major methods used to calculate key properties. Sec- most dense occurrences of sea fog, with fog frequency tion 3 presents the weather conditions, general char- being more than 20 d yr−1 on average (Zhang and acteristics and temporal variation of microphysics and Bao, 2008). The sea fog events on the Leizhou Penin- fog droplet size distribution, and discusses correlations sula mostly occur between January and April, and among microphysical properties. Finally, conclusions especially in February and March (Zhang and Bao, are summarized in section 4. 2008). The Leizhou Peninsula is governed by Zhan- jiang, Guangdong, China. Zhanjiang City, which is 2. Instruments and method now a very important seaport and logistics center in 2.1 Instruments southern China, is located in the northeast part of Leizhou Peninsula and situated along the South China The measurements taken during the experiment in- Sea, with an urban population of 2 940 000. High fog cluded fog droplet spectra, visibility and fog water col- frequency has significant influences on shipping, ma- lection, among others. The size distribution of fog rine fisheries and land transportation, and brings great droplets in the diameter range of 2–50 µm were mea- stress to the local environment as it aims for industri- sured with a FM-100 Fog Monitor (Droplet Measure- alization (Liu et al., 2008; Fan et al., 2009; Wang et al., ment Technologies, Boulder, CO, USA). The instru- 2010; Yang et al., 2010a). With constructions of steel ment sampled at a frequency of 1 Hz and a mode with and petrochemical industry projects, Zhanjiang is in a 20 size classes. This instrument can detect the forward transitional period between an agriculture-based econ- scattered light, and then determine the size of individ- omy and an industrial one. However, the microphysi- ual fog droplets based on the level of correlation be- cal characteristics of sea fog events in Zhanjiang have tween particle diameter and scattering cross section. never been studied. Thus, research on sea fog during More detailed information about the measuring prin- this critical period of regional development is of great ciple can be found in Niu et al. (2010a) and Lu et al. importance for Zhanjiang, because it can provide basic (2010). Because of noise, the data of the first size class, information for future research and applications. which included droplet diameters smaller than 2 µm, In order to elucidate the microphysical properties were not used in this study. of sea fog and to explore the dominant microphysical Visibility and present weather were automatically processes during sea fog events on the Leizhou Penin- measured and recorded every 30 s with a VPF-730 sula, a comprehensive observational study was carried Automated Present Weather Observing System (Biral, out from 15 March to 18 April 2010. The sampling site Bristol, UK). The instrument has an optical transmit- was located on Donghai Island (21◦3500N, 110◦320500E; ter, a forward scatter receiver, and a back scatter re- 15 m above sea level) in Zhanjiang, where the South ceiver, with which fog density, precipitation identity, China Sea lies 200 m to the east (Fig. 1). Because of rain rate, and snowfall rate can be measured. The NO. 4 ZHAO ET AL. 1157
Fig. 1. (a) Map of geographic location and (b) surrounding environment of the ob- servation site on Donghai Island. scattering angle coverage is between 39◦ and 51◦, and 2.2 Method the sample volume is 400 cm3. The transmitter emits According to the Fog Monitor Operator Manual a near-infrared pulse (central wavelength = 0.88 µm) (2004) provided by Droplet Measurement Technolo- in a modulation frequency of 2000 Hz with a band- gies, the true air speed (TAS, in units of m s−1) and width of 0.08 µm. The intensity of scattered light sampling volume per second (V , in units of cm3 s−1) can be measured when small particulates suspended are calculated using the following equations: in, or large particles passing through, the sample vol- 0.5 ume. Given that aerosol absorption can be negligible TAS = 20.06 × M × Ta , (1) in most natural environments, the atmosphere scatter- V = TAS × S, (2) ing coefficient and extinction coefficient were assumed to be the same. Based on the measurements of the at- where M is the Mach number derived from the dy- mospheric extinction coefficient (β), the meteorologi- namic (pitot) pressure and static pressure (in units of cal optical range (MOR) was determined by applying mb), Ta is the actual ambient temperature (in units of 2 a standard relation of MOR=3.00/β. The visual range K), S is the sampling area (0.264 mm ), and the fog was from 10 m to 75 km. The relative error of the in- droplet count of each size class divided by (V ×∆r) −3 −1 strument was ±10% for visibilities of less than 16 km, provides n(r) in cm µm (where r is the droplet and ±20% for visibilities of between 16 and 30 km. radius in units of µm). Mean radius (rm), peak radius Surface meteorological conditions and synoptic (rp), effective radius (re) and maximum radius (rmax) weather data were obtained from the China Meteo- are important in describing fog droplet size distribu- rological Administration’s comprehensive observation tion. Mean radius (rm) indicates the average linear network. scale of fog droplets swarm; peak radius (rp) is the 1158 SEA FOG OVER LEIZHOU PENINSULA VOL. 30
1 1 6 radius corresponding to the maximum of fog droplet −4 6 N rc ≈ 4.09 × 10 χcon 1 , (10) number density; effective radius (re) can be used to L 6 discuss the relationship between optical property and where rc is the critical radius for autoconversion, total volume of fog droplets swarm; maximum radius whose analytical expression was derived by Liu et al. (rmax) is the radius of the biggest fog droplet (Sheng 23 (2004), and χcon = 1.15×10 is an empirical coeffi- et al., 2003; Yang et al., 2011). Fog droplet number −3 cient. concentration (N, in units of cm ), mean radius (rm, in units of µm) and effective radius (re, in units of 3. Results and discussion µm) of fog droplet, liquid water content (L, in units of g m−3), and standard deviation of fog droplet size 3.1 Weather conditions distribution (σ; in units of µm) can be calculated as Based on comprehensive analysis of surface mete- follows: orological observations, relative humidity and visibil- ity, 13 fog events were obtained during the experiment X50 N = n(r)∆r , (3) period. Table 1 shows the basic meteorological infor- r=2 mation of these events. Based on synoptic analysis, there are four types of circulation patterns in favor 1 X50 of sea fog events on the east coast of Leizhou Penin- r = rn(r)∆r , (4) m N sula. They are, a warm sector in front of a cold front r=2 (indicated by the capital letter “W” in Table 1), re- X50 turning flow weather after high pressure has moved r3n(r)∆r eastward (indicated by “R”), a col pressure field or r=2 uniform pressure field (indicated by “C”), and the an- re = , (5) X50 terior part of low pressure (indicated by “A”). During r2n(r)∆r the field experiment, sea fog formed during late night r=2 or before dawn, and dissipated before or after sunrise; the duration of a sea fog event varied from 68 min to X50 4π 762 min. The minimum visibility during a fog event L = 1 × 10−6 × ρ × r3n(r)∆r , (6) 3 could reach 0.12 km, and cases of visibility lower than r=2 0.2 km mainly occurred in fog events of longer dura- " # 1 50 2 tion. Air temperature suitable for fog formation over 1 X ◦ ◦ σ = (r − r )2n(r)∆r , (7) Leizhou Peninsula was between 20 C and 25 C. Rel- N m r=2 ative humidity during the 13 fog events was in the range 98%–100% and changed very little; thus relative −3 where ρ = 1 g cm is the density of water. humidity was not listed in Table 1. Wind directions According to Liu et al. (2005, 2006), all the auto- during these fog events tended to be easterly, and max- conversion parameterizations can be generically writ- imum wind speeds were mostly larger than 3.0 m s−1 ten as: and less than 5.0 m s−1. These data suggested that P = P0fT , (8) most of the sea fogs were advection fog coming from the sea surface. The meteorological conditions during where P is the autoconversion rate, P is the rate func- 0 the sea fog events were closely correlated with syn- tion describing the conversion rate after the onset of optic weather patterns. Because of the weaker pres- the autoconversion process, and f is the threshold T sure gradient and advection, a sea fog event under a function describing the threshold behavior of the au- col pressure field or uniform pressure field had a rel- toconversion process. The autoconversion threshold atively shorter duration, lower temperature and lower function (f ) can be used to examine the strength of T wind speed. Furthermore, sea fog events influenced by the collection process during a fog event. f ranges T returning flow weather were characterized by a shorter from 0 (no collection action) to 1 (full collection ac- duration, lower temperature and higher minimum vis- tion), and a larger value of f indicates a stronger col- T ibility. Meanwhile, when influenced by a warm sector lection process. The expression of f can be generally T in front of a cold front or anterior part of low pressure, described by: the duration of a sea fog event was much longer and Z ∞ Z ∞ the minimum visibility was extremely low, correspond- r6n(r)dr r3n(r)dr ingly showing a higher temperature and wind speed. P f = = Zrc Zrc , (9) Because sea fog events influenced by a warm sector in T P ∞ ∞ 0 r6n(r)dr r3n(r)dr front of a cold front or anterior part of low pressure 0 0 NO. 4 ZHAO ET AL. 1159 ) pattern ◦ ) Wind direction Synoptic 1 − C) Minimum Maximum ( ◦ Time (LST) Duration Minimum visibility Temperature Wind speed (m s 23272231 06181839 09550051 412 05490618 685 04560436 671 07250158 0.16 246 07440233 0.15 68 04042000 0.21 189 05371852 21.6 0.24 127 1041 23.4 0.20 185 0619 24.1 0.31 1.9 882 20.6 0.44 1.0 688 3.5 0.48 1.6 21.2 21.7 3.5 0.12 0 21.3 4.1 0.13 E, NE 0.6 21.3 1.2 ESE 1.2 22.7 2.5 1.5 ESE W 22.7 3.5 2.2 W NE 3.4 2.5 ESE W ENE, 4.4 0.7 E 4.6 C E C 5.1 R E ESE, E R ESE A R A Meteorological elements during the fog events. 16 15–16 March 2010 8 23–24 March 2010 2 01–02 April 2010 3 20 March4 2010 21 March9 2010 22 March 2010 045 April 2010 7 22–23 March 2010 31 March–01 April 2010 12 08 April 2010 1011 0513 April 2010 06 April 2010 13 April 2010 0254 0304 0717 0511 0544 264 0628 161 78 0.3 0.15 0.33 23.3 24.0 23.9 1.7 2.1 3.1 2.6 3.6 4.5 E ESE E A A A Table 1. Case Date Formation Dissipation (min) (km) ( The capital letters “W”,of “R”, a “C” cold and front; “A” R, in returning column flow “synoptic weather pattern” after respectively high represent pressure the four has synoptic moved patterns eastward; C, in favor col of pressure the field sea or fog uniform events: pressure W, field; warm A, sector anterior in part front of low pressure. 1160 SEA FOG OVER LEIZHOU PENINSULA VOL. 30
−3 −3 are more dangerous than the other types, research on rmax and rp were 54.3 cm , 0.021 g m , 2.5 µm, these sea fog events is very important. In order to thor- 13.2 µm and 1.4 µm, respectively. The peak radius oughly analyze the microphysical structure of these (rp) was mostly at 1.4 µm, indicating that most of the two types of sea fog events, case 5 (influenced by the fog droplets were very small and led to the low value of anterior part of low pressure) and case 6 (influenced rm. Similarly, the microphysical properties of sea fog by a warm sector in front of a cold front) were selected were influenced by the synoptic weather pattern. In for the analysis of temporal variation of microphysical general, the mean values of L, rm and rmax of sea fogs characteristics in subsection 3.4. influenced by a warm sector in front of a cold front, by the anterior part of low pressure, by a col pressure 3.2 General characteristics of fog micro- field or uniform pressure field, and by returning flow physics weather ranked first, second, third and fourth, respec- To determine the threshold values of microphysical tively. The most likely reason for this is that the higher properties for fog events, we calculated the mean and temperature and stronger advection under a warm sec- median values of L during those sea fog events with tor in front of a cold front and the anterior part of visibility in the range of 0.98–1.0 km, and the values low pressure can bring more moisture for activation obtained were 0.004 g m−3 and 0.001 g m−3, respec- and condensation growth of fog droplets, which conse- tively. For the data with L>0.001 g m−3 and visibility quently produce larger fog droplets and higher liquid in the range 0.98–1.0 km, the percentage of the data water content. The mean value of N varied widely un- with N>1 cm−3 was 99.3%. Having comprehensively der different synoptic weather patterns, which suggests considered the role of L and N, it is reasonable to de- the synoptic weather pattern may not be the decisive fine sea fog with N>1 cm−3 and L>0.001 g m−3 in factor affecting fog droplet number concentration. this area based on the microphysical properties. The mean values of N, L, rm, rmax and rp of the Table 2 shows the means and variations of key mi- sea fogs in Zhanjiang are consistent with those ob- crophysical properties of sea fog over the east coast served in 2007 at Maoming, which is located on the of Leizhou Peninsula. The mean values of N, L, rm, South China Sea coast (Huang et al., 2009), while the Table 2. Mean values and variations (in parentheses) of key microphysical variables of the sea fog over Leizhou Peninsula.
−3 −3 Case Observation time (LST) N (cm ) L (g m ) rm (µm) rmax (µm) rp (µm) 28.2 0.012 3.1 12.3 1.5 1 15–16 March 2010 (1.2–343.6) (0.001–0.143) (1.5–9.5) (3.5–24.5) (1.4–12.2) 72.1 0.031 2.3 15.5 1.4) 6 23–24 March 2010 (1.0–401.9) (0.001–0.205) (1.5–10.5) (3.5–24.5) (1.4–12.2 40.4 0.022 3.1 12.9 1.4 8 01–02 April 2010 (1.0–322.0) (0.001–0.219) (1.6–9.8) (4.5–24.5) (1.4–10.7) 28.4 0.009 2.5 13.1 1.4 2 20 March 2010 (1.1–146.8) (0.001–0.096) (1.5–10.6) (4.5–24.5) (1.4–13.7) 99.8 0.015 1.9 11.0 1.4 3 21 March 2010 (6.6–540.9) (0.001–0.086) (1.5–4.0) (3.5–19.7) (1.4–1.4) 47.2 0.008 2.2 11.1 1.4 4 22 March 2010 (1.3–240.4) (0.001–0.052) (1.5–8.4) (3.5–23.2) (1.4–8.5) 13.9 0.003 2.3 8.8 1.4 9 04 April 2010 (5.3–30.7) (0.001–0.030) (1.7–3.6) (4.5–24.5) (1.4–1.4) 17.5 0.004 2.1 9.6 1.4 12 08 April 2010 (2.3–78.6) (0.001–0.049) (1.5–4.8) (4.5–24.5) (1.4–1.4) 39.6 0.013 2.4 11.8 1.4 5 22–23 March 2010 (1.2–195.4) (0.001–0.154) (1.6–8.6) (4.5–24.5) (1.4–8.5) 73.0 0.029 2.5 14.1 1.4 7 31 March–01 April 2010 (1.4–470.3) (0.001–0.232) (1.5–9.6) (4.5–24.5) (1.4–8.5) 38.8 0.003 1.8 8.2 1.4 10 05 April 2010 (2.3–173.3) (0.001–0.034) (1.5–4.2) (3.5–24.5) (1.4–1.4) 86.1 0.026 2.1 14.6 1.4 11 06 April 2010 (2.7–245.2) (0.001–0.127) (1.5–5.9) (4.5–24.5) (1.4–1.4) 12.1 0.009 3.9 11.9 1.7 13 13 April 2010 (1.0–91.6) (0.001–0.068) (1.6–11.7) (4.5–24.5) (1.4–16.7) 54.3 0.021 2.5 13.2 1.4 Total March– April 2010 (1.0–540.9) (0.001–0.232) (1.5–11.7) (3.5–24.5) (1.4–16.7) NO. 4 ZHAO ET AL. 1161 variation ranges of N, L, rm and rp in Zhanjiang were slightly larger than those in Maoming (Table 3). This suggests that our results represent the microphysical characteristics of sea fog not only on the east coast of Leizhou Peninsula, but also in the northern part of the South China Sea, and that the new instrument with high sampling frequency can accurately measure the fine-scale microphysical structure of sea fog. Con- strained by the measuring range of the instrument, fog droplets with a radius larger than 25 µm could not be measured during this field experiment, and the max- imum value of rmax in Zhanjiang was slightly smaller than that in Maoming. Considering the extremely low number concentration of large droplets with a radius larger than 20 µm, and the small difference in the max- imum value of rmax in Zhanjiang and Maoming, we Fig. 2. Average droplet size distribution of each case conclude that the measuring range of the instrument (cases 1–13) and all cases (total). did not affect the representativeness of our observa- tions. especially for the fog droplets with radius >20 µm Compared with previous studies of other represen- (Fig. 2). The maximum fog droplet number concentra- tative sea fogs in China (Table 3), the sea fogs along tion appeared in the first bin, and n(r) dropped dra- the South China Sea (e.g. Zhanjiang and Maoming) matically while r was less than 3.5 µm for all cases. are characterized by the highest fog droplet number The shape of each spectrum was approximately the concentration, the sea fogs along the East China Sea same in spite of the fact that the fog droplet size dis- (e.g. Zhoushan) have the largest L and fog droplet tribution differed from case to case. The type of fog size, and the values of N, L, rm and rmax are small- droplet size distribution for each sea fog event and to- est near the Yellow Sea (e.g. Qingdao). In general, tal data was mostly monotonically-decreasing, and the sea fogs have less but larger fog droplets than urban trend of decrease of n(r) became slow with increasing fogs, and the liquid water content in sea fog is smaller r. The synoptic weather pattern had a small impact on than that in mountain fog. This is because plenty of fog droplet size distribution, which may suggest that aerosols in urban areas provide enough cloud conden- the droplet size distribution is determined by other sation nuclei for fog droplets, which results in the high factors, such as size distribution and chemical compo- number concentration of fog droplets in urban fogs. sition of aerosols. Furthermore, the condensation caused by orographic 3.4 Temporal variation of microphysical char- lifting may be a possible reason for the large liquid acteristics water content of fogs in mountain areas. The possible reasons for low liquid water content of sea fogs are as The general description of microphysical charac- follows. The gravitational settling of large droplets is teristics provides an overall impression of sea fog on one reason. However, given the extremely low number Leizhou Peninsula. For a better understanding of sea concentration of large droplets (r>20µm), the gravity fog microphysical structures, temporal variations of deposition of a few big droplets may not be the main microphysical properties and fog droplet size distri- reason. The vertical shear of wind speed and the great bution are discussed next. Considering their represen- difference of surface characteristics between sea and tativeness, case 5 (influenced by the anterior part of land can lead to extremely strong turbulence transfer low pressure) and case 6 (influenced by a warm sector of droplets to the ground and turbulent mixing of fog in front of a cold front) were selected for this purpose. and unsaturated air, which consequently leads to evap- The temporal variations of visibility and micro- oration of fog droplets and lower liquid water content physical properties in case 5 showed obvious quasiperi- (Sheng et al., 2003; Xu, 2011). odic characteristics, whereas the dense fog (visibility lower than 500 m) period in case 6 lasted 7.5 h af- 3.3 Fog droplet size distribution ter a long induction period (Figs. 3a and b). Visibility The spectrum width of the sea fog events was changed with an opposite tendency to N, rm, L, σ, fT broad, with the spectrum width of all cases being and droplet spectrum width, except for rm in case 6 larger than 20 µm. However, the number concentra- (Figs. 3a–l, 4a, b). Because most fog droplets had a tion of fog droplets with radius >3 µm was very low, small radius, rm changed very little (Figs. 3e and f). 1162 SEA FOG OVER LEIZHOU PENINSULA VOL. 30 fog fog radiation fog rainfall fog atmospheric instability Advection Advection– With Radiation Fog with Urban Radiation Geographic Coastal Coastal Island Mountain Mountain Rain-free Slope of a moun- tain p m) environment remarks r µ 1.4 (1.4–16.7) 1.5 (1.4–2.7) 10.0 (0.75–20.1) (1.0–5.0) Coastal m) ( max µ r 24.0) Coastal ∼ 13.2 (3.5–24.5) 12.7 (6.7–28.1) 29.8 (7.1–169.1) 6.35 (2.0–25.0) 8.70 (2.0–24.0) 13.57 (2.0–25.0) ( m) ( m µ 1.83 (1.5–4.28) 1.80 (1.5–2.77) 2.50 (1.5–4.74) 2.5 (1.5–11.7) 2.4 (1.6–7.7) 11.0 (8.9–13.9) 2.2 (2.1–2.5) # )( 3 − # # –0.476) –0.982) –0.903) 5 5 5 − − − 2.68) 0.14) Urban ∼ ∼ 0.021 (0.001–0.232) 0.018 (0.0024–0.18) 0.37 ( 0.055 (0.012–0.20) (0.001–0.19) 0.023 (10 0.021 (10 0.121 (10 0.027–0.541 0.036 (0.007–0.099) 0.080 (0.030–0.127) 0.144 0.063 ) (g m 3 − N L r 565) ( ∼ 54.3 (1.0–540.9) 57 (17–112) 37 (7.6–122) (0.6–43) 28 77.9 (1–993.2) 107.8 (1–1914.7) 211.0 (1–1213.8) ( ∗ ∗ ∗ ∗ ∗ ∗ 2010 2007 1985 2006–2009 2009 2009 1993 (5.4–249) 0.016 March–April March–April April–May May 1980 (June–July) Winter of 26 November 07–12 July 25 June–07 August 2002 2001–2002 Observation time ” is measured by FM-100. ∗ Microphysical characteristics of some representative fog events. Province Province (Huang et al., 2009) Province (Yang et al., 1989) Province (Yang, 1985; Xu et al., 1994) (Niu et al, 2011) et al., 2011) 2011) Rico (Holwerda et al., 2006) (Burkard et al., 2003) The observation labeled with “#” is the median value. Table 3. Obsevation siteZhanjiang, Guangdong Maoming Guangdong Zhoushan, (LST) Zhejiang Qingdao, Shandong (cm Nanjing, China Shanghai, China (Li Taiwan (Gonser et al., Pico del Este, Puerto L¨ageren,Switzerland The observation labeled with “ NO. 4 ZHAO ET AL. 1163
Fig. 3. Temporal variation of 1-s data (gray line) and 1-min average data (black line) of visi- bility (a, b), fog droplet number concentration (N) (c, d), mean radius (rm) (e, f), liquid water content (L) (g, h), standard deviation of droplet size distribution (σ) (i, j), and autoconversion threshold function (fT) (k, l) during case 5 (left panels) and case 6 (right panels).
The significant temporal variation of N was due in threshold function (fT) showed a good agreement with large part to the number concentration variation of droplet spectrum width (Figs. 3k, l, 4a, b). The small droplets, especially fog droplets with radius <3 large value of fT appeared when droplet spectrum µm (Figs. 3c, d, 4a, b). The drastic changes of micro- width was >10 µm, which may suggest that the crit- physical properties indicated that the spatial distribu- ical droplet radius of the collection process was 10 tion of microphysical structures of sea fog is inhomo- µm. Throughout these two cases, we can see that geneous. the fog droplet size distribution was dominated by The variation tendency of the autoconversion the monotonically-decreasing type, with few bimodal 1164 SEA FOG OVER LEIZHOU PENINSULA VOL. 30
Fig. 4. Temporal variation of fog droplet size distribution during case 5 (a) and case 6 (b).
or multimodal distributions during the mature stage sational growth (or evaporation) was the most impor- of fog events (Figs. 4a and b). A sudden increase in tant process throughout the sea fog events, and the droplet spectrum width was observed, which occurred collection process mostly occurred in the development once the droplet spectrum width was >10 µm. This and mature stages. However, the influence of the tur- may suggest that the collection process is the major bulent mixing process on the sea fog events could not cause of this phenomenon. be determined by simply analyzing the temporal vari- Based on the evolution of macro- and microphys- ation of microphysical properties. To further discuss ical characteristics, the sea fog events can be divided the microphysical processes during the sea fog events, into four stages: the formation stage, the development we analyze microphysics correlations in the following stage, the mature stage, and the dissipation stage. subsection. In the formation and dissipation stages, N, L and σ 3.5 Microphysics correlations were usually small and fluctuated slightly, and these microphysical properties had no significant correla- In order to further improve our understanding of tion with each other (Fig. 3). With the contribution microphysical characteristics of sea fog, relationships of big droplets increasing, the collection process be- between microphysical properties are analyzed in this came stronger (the value of fT increased), especially subsection. The fT is used to explore the strength of in the mature stages (Figs. 3 and 4). However, rm the collection process during the sea fog events and was mostly concentrated in the range 1–3 µm under the influence of the collection process on microphys- different stages of the sea fog events, suggesting that ical correlations. To compare these results with the continuous activation and condensational growth (or urban fog events in Nanjing (Niu et al., 2010a), the evaporation) occurred during the whole of the sea fog dataset was categorized into three groups according events (Figs. 3). In conclusion, activation and conden- to the value of fT: 06fT60.2 (weak); 0.2 Table 4. Autoconversion threshold function (fT) during the sea fog events over Leizhou Peninsula. 06fT60.2 0.2 (medium); and 0.66fT61.0 (strong). In spite of the activation via droplet evaporation). Because satura- broad spectrum width, the strength of the collection tion vapor pressure is a function of air temperature, process was weak in the sea fog events due to the low and relative humidity was mostly around 100% during concentration of big droplets (r>10µm). Even during the sea fog events, moisture content could have been the mature stage (visibility <0.5 km) of the sea fog closely related to air temperature during the sea fog events, there were only 1.6% of samples with fT>0.6, events. In other words, higher air temperature meant but the weak collection process appeared in >89% of there was more water vapor in the atmosphere. During the samples (Table 4). the experiment, air temperature was mostly >20◦C, In general, L showed a rising tendency with in- so moisture was in abundant supply near the sea sur- creasing N, and the collection process made L grow face, which was necessary for activation and conden- faster with the increase in N (Figs. 5a, 6a, 7a and sational growth. The collection process usually occurs 8a). This was similar to the urban fog events in Nan- under the participation of big droplets (r>10µm), and jing (Niu et al., 2010a). The concurrent increase in the growth of big droplets consumes a large amount both N and L was probably caused by fog droplet of small droplets. Considering the collection process activation and subsequent condensational growth (de- likely resulted in a negative correlation between N and Fig. 5. Correlations among microphysical properties while 06fT60.2. Shading in units of % represent the scatter distribution. 1166 SEA FOG OVER LEIZHOU PENINSULA VOL. 30 Fig. 6. The same as Fig. 5, but for 0.2 L, the positive N − L correlation indicates that the growth occurred during the sea fog events, but was not most dominant process during the sea fog events was the dominant process. There existed another impor- fog droplet activation, with subsequent condensational tant physical process besides condensational growth growth and/or droplet deactivation via droplet evap- and collision–coalescence growth. As previously men- oration. A stronger collection process led to a faster tioned, turbulent mixing is extremely strong near the growth of L with the increase in N, which also sug- coastal surface, and the mixing of clear air and foggy gests that big droplets provided a greater contribution air may lead to continuous activation and condensa- to L. tion processes and/or droplet evaporation, which re- By activation with subsequent condensational sults in most fog droplets concentrated in the range growth and/or droplet deactivation via droplet evapo- 2–3 µm (Fig. 4). In a homogeneous mixing model, the ration, rm is small and increases slowly; thus, rm is ex- mean radius is positively correlated to droplet number pected to have a weak positive correlation with N and concentration under the influence of evaporation and L. However, under collision–coalescence growth, rm dilution (Baker et al., 1980). In an extreme inhomoge- can increase effectively by consuming small droplets, neous mixing model, some droplets completely evapo- which may lead to a negative relationship between rm rate and other droplets remain unchanged, which may and N and a significant positive correlation between lead to mean radius not changing with the increas- rm and L. During the sea fog events, rm showed a weak ing droplet number concentration (Knollenberg, 1976; negative correlation with N, and the negative corre- Latham and Reed, 1977). While under the influence lation became more significant with the increase of of inhomogeneous mixing and uplift, mean radius is fT, but rm was always irrelevant to L, no matter how negatively correlated to droplet number concentration strong the collection process was (Figs. 5b, c, 6b, c, 7b, (Baker and Latham, 1979). Furthermore, the mixing c, and 8b, c). The correlations between rm and N and mode is dependent on the time scales of evaporation between rm and L indicated that collision–coalescence and mixing, which are related to air pressure, air tem- NO. 4 ZHAO ET AL. 1167 0.25 15 15 c a b 0.20 12 12 ) ) ) 3 - 0.15 9 9 m m m ( ( m m (g 0.10 r 6 r 6 L 0.05 3 3 0.00 0 0 0 100 200 300 400 0 100 200 300 400 0.00 0.05 0.10 0.15 0.20 0.25 -3 -3 -3 N (cm ) L (g m ) N (cm ) 0 2 4 5 7 0 3 6 9 12 0 2 6 10 14 18 10 10 10 d e f 8 8 8 ) ) ) 6 6 6 m m m ( ( ( 4 4 4 2 2 2 0 0 0 0 100 200 300 400 0.00 0.05 0.10 0.15 0.20 0.25 0 3 6 9 12 15 -3 -3 r ( m) N (cm ) L (g m ) m 0 3 6 9 12 01234567 0 1 2 3 Fig. 7. The same as Fig. 5, but for 0.66fT61.0. perature, supersaturation, and turbulent dissipation reversible evaporation processes, there are plenty of rate (Baker and Latham, 1979; Baker et al., 1980). Ac- small droplets in the first bin that are produced by cording to Wang et al. (2009), the shape of the droplet activation and consequent condensational growth, and size spectrum is maintained following inhomogeneous the condensation (or evaporation) process always re- mixing, and the droplet mean volume diameter ap- sults in an increase (or decrease) of N, σ, L and rm. pears almost constant. Therefore, under the influence However, for the turbulent mixing process, the shape of condensational growth (or evaporation) and turbu- of the droplet size spectrum is maintained, which may lent mixing, rm may have a positive, irrelevant or a result in a constant σ and rm. According to Niu et negative correlation with N and L, which relates to al. (2010a), the collection process can lead to a neg- the ambient conditions. The weak negative correlation ative correlation between σ and N, whereas activa- between rm and N and the irrelevance correlation be- tion and condensation or reversible evaporation are tween rm and L indicated that turbulent mixing played responsible for a positive correlation between σ and an important role during the sea fogs. N. Furthermore, the turbulent mixing process can ac- To further explore the microphysical processes dur- count for an irrelevant relationship of σ and N (Liu ing the sea fog events, the relationship of σ with N, et al., 2002; Wang et al., 2009). With the increase of L, and rm (Figs. 5d–f, 6d–f, 7d–f, and 8d–f) were ex- fT, the relationship of σ and N changed from non- amined. As is well known, the collection process is a existent to negative, and the relationship of σ and L process of big droplets consuming smaller ones, and changed from positive to irrelevant, with the positive this process leads to an increase in the big droplet correlation between σ and rm becoming strengthened number concentration and an obvious reduction in the (Figs. 5d–f, 6d–f, and 7d–f). For all data, the rela- small droplet number concentration, which in turn re- tionship of σ and N was irrelevant, the relationship sults in a decrease of N and increases of σ, L and of σ and L was a weak positive correlation, and the rm. Furthermore, in activation and condensation or relationship of σ and rm was a significant positive cor- 1168 SEA FOG OVER LEIZHOU PENINSULA VOL. 30 0.25 15 15 b a c 0.20 12 12 ) ) ) 3 - 0.15 9 9 m m m ( ( (g m m r r 0.10 6 6 L 0.05 3 3 0.00 0 0 0 100 200 300 400 0 100 200 300 400 0.00 0.05 0.10 0.15 0.20 0.25 -3 -3 -3 N (cm ) L (g m ) N (cm ) 0 1 2 6 12 18 24 30 0 2 6 12 18 21 0 3 6 9 12 15 10 10 10 d e f 8 8 8 ) ) ) 6 6 6 m m m ( ( ( 4 4 4 2 2 2 0 0 0 0 100 200 300 400 0.00 0.05 0.10 0.15 0.20 0.25 0 3 6 9 12 15 -3 -3 r ( m) N (cm ) L (g m ) m 0 1 2 3 4 56 0 2 4 6 8 1011 0 4 8 12 16 Fig. 8. The same as Fig. 5, but for correlations among microphysical properties using all data. relation (Figs. 8d–f). These results proved that collec- sured during the field campaign. The work reported tion process occurred during the sea fog events, but in this paper mainly focused on the general character- the dominant physical process was activation and con- istics and temporal variation of sea fog microphysical densation or reversible evaporation, and the turbulent structures and the correlations of microphysical prop- mixing process also played an important role. erties. In summary, during the sea fog events, the domi- Thirteen sea fog events were observed during the nant physical process was activation with subsequent field campaign. It was found that easterly wind was condensational growth or reversible evaporation pro- favorable for the formation of sea fogs in Zhanjiang. cesses; the turbulent mixing process played an impor- There are four types of circulation pattern in favor of tant role; the collection process occurred, but was not sea fog events in this area, which are: a warm sec- vital. With an increasing value of fT, the positive cor- tor in front of a cold front; returning flow weather relations between microphysical properties were weak- after high pressure has moved eastward; a col pressure ened under the influence of the collection process, or field or uniform pressure field; and the anterior part even showed irrelevance, or negative correlation. of low pressure. In general, sea fog was found to form late at night or before dawn, dissipating before or af- 4. Conclusion ter sunrise. The sea fog events influenced by a warm sector in front of a cold front or anterior part of low A comprehensive field campaign was carried out pressure were found to be very important for sea fog from 15 March to 18 April 2010 on Donghai Island disaster risk reduction, because of their much longer (21◦3500N, 110◦320500E; 15 m above the sea level) in duration, lower minimum visibility and greater liquid Zhanjiang, Guangdong Province, China. Fog droplet water content. The microphysical properties of sea fog size distribution, visibility, conventional meteorologi- were also influenced by the synoptic weather pattern. cal variables, and boundary layer structure were mea- Sea fogs influenced by a warm sector in front of a cold NO. 4 ZHAO ET AL. 1169 front usually had the largest L, rm and rmax values, important role in determining the microphysics during followed respectively by sea fogs influenced by the an- the sea fog events, and the collection process was too terior part of low pressure, by a col pressure field or weak to affect the microphysics of the fog events. uniform pressure field, and by returning flow weather. Based on a comprehensive analysis of L and N, it Acknowledgements. Funding for this work was was reasonable to define sea fogs in this area as N>1 mainly provided by the Meteorology Fund of the Ministry cm−3 and L>0.001 g m−3. The mean values of N, L, of Science and Technology (Grant No. GYHY[QX] 2007- rm, rmax and rp of the sea fogs over Leizhou Peninsula 6-26), the National Natural Science Foundation of China were 54.3 cm−3, 0.021 g m−3, 2.5 µm, 13.2 µm and 1.4 (Grant No. 41275151), the Qing-Lan Project for Cloud- µm, respectively. The type of fog droplet size distri- Fog-Precipitation-Aerosol Study in Jiangsu Province, the bution of the sea fog events was mostly monotonically- Graduate Student Innovation Plan for the Universities of decreasing, and the spectrum width was always >20 Jiangsu Province (Grant No. CX10B 292Z), and a project µm. The values and variation ranges of microphysi- funded by the Priority Academic Development of Jiangsu cal properties obtained in this study were consistent Higher Education Institutions. with observations in the same sea waters of the South China Sea. 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