Impact of Sea Surface Temperature Front on Stratus-Sea Fog over the Yellow and East China Seas–A Case Study with Implications for Climatology

LI Man1), 2), and ZHANG Suping1), *

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Impact of Sea Surface Temperature Front on Stratus-Sea Fog over the Yellow and East China Seas–A Case Study with Implications for Climatology

LI Man, and ZHANG Suping

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A stratus-sea fog event that occurred over the Yellow and East China Seas on 3 June 2011 is investigated using observations and a numerical model, with a focus on the effects of background circulation and Sea Surface Temperature Front (SSTF) on the transition of stratus into sea fog. Southerly winds of a synoptic high-pressure circulation transport water vapor to the Yellow Sea, creating conditions favorable for sea fog/stratus formation. The subsidence from the high-pressure contributes to the temperature inversion at the top of the stratus. The SSTF forces a secondary circulation within the ABL (Atmospheric Boundary Layer), the sinking branch of which on the cold flank of SSTF helps lower the stratus layer further to reach the sea surface. The cooling effect over the cold sea surface counteracts the adiabatic warming induced by subsidence. The secondary circulation becomes weak and the fog patches are shrunk heavily with the smoothed SSTF. A conceptual model is proposed for the transition of stratus into sea fog over the Yellow and East China Seas. Finally, the analyses suggest that sea fog frequency will probably decrease due to the weakened SSTF and the reduced subsidence of secondary circulation under global warming.

stratus and sea fog; subsidence; sea surface temperature front; the Yellow and East China Seas; global warming

1 Introduction

Fog is water droplets suspended in the atmosphere near the earth’s surface and can reduce atmospheric visibility to less than 1km (0.62 miles). According to the American Meteorological Society’s (AMS) definition, fog differs from cloud only in that the base of fog is at the earth’s surface while clouds are above the surface. It suggests that cloud could convert into fog, and vice versa. Some of the studies on fog formation were conducted along the California coast. Anderson (1931) reported the transition of stratus into fog using aircraft observations. Kora?in(2001) demonstrated the converting process of stratus cloud to fog with a one-dimensional, higher-order, turbulence-closure model aided by observations. Lewis(2003) presented a pictorial summary of fog formation from lowering stratus clouds. The Yellow and East China Seas are heavy fog regions. Sea fog often disrupts marine transportation and other ocean-related activities and causes losses to coastal communities and national economy. Previous studies classified the sea fog over the Yellow and East China Seas as the type of advection fog (Wang, 1983;Fu, 2006, 2008; Zhang and Bao, 2008; Zhang, 2009b; Gao, 2007). Sea fog over the Yellow Sea often forms in June and July, and the key areas that supply moisture for the fog are located from the northern East China Sea to the southern Yellow Sea (Bai, 2010). Wang (1983) described the phenomenon of fog rising up and being converted into stratus in the area around Qingdao, a coastal city bordering the Yellow Sea. The latest statistical analysis also showed that the occurrence of most fog over the Yellow Sea corresponds to the occurrence of low clouds in the East China Sea (Zhang, 2009a; Han, 2012). These studies are suggestive of the probable connection between stratus and sea fog over the Yellow and East China Seas, but, so far, no research has been conducted to investigate the transition of stratus to sea fog.

In this paper, a stratus to sea fog event over the Yellow and East China Seas on 3 June 2011 is investigated with comprehensive observations, reanalysis and satellite datasets, and numerical modeling results. By introducing a new approach to sea fog study over the Yellow and East sea fog and the physical mechanisms involved in the China Seas, the transition process of marine stratus into process are examined. Data and model used in the present study are described in Section 2. Section 3 analyzes the sea fog-stratus case. Section 4 is the numerical study. The potential trend of the fog occurrence frequency under global warming is discussed in Section 5, and Section 6 is a summary.

2 Data and Model

Seven datasets are used in this study. 1) Meteorological Information Comprehensive Analysis and Process System (MICAPS) data from China Meteorological Administration (CMA), including three-hour ground observations and upper air data at 0000 UTC and 1200 UTC. 2) Sea fog observational data in Qingdao from 1971 to 2010 and in Taizhou from 1983 to 2010. 3) Reanalyzed data of the Final Analysis (FNL) issued by the National Centers for Environmental Prediction (NCEP). The dataset has a 1.0?×1.0? horizontal resolution and 26 vertical levels with a six-hour time interval, and includes air temperature, geopotential height, relative humidity, wind,4) Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data that provide vertical structure and optical characteristics of cloud and aerosol (Wang, 2011). 5) Visible satellite images from Meteorological Satellite (MTSAT-2), Japan Meteorological Ad-ministration (JMA). 6) Sea Surface Temperature (SST) data from the Geophysical Fluid Dynamics Laboratory-Earth System Models 2M Version (GFDL-ESM2M RCP- 4.5). The model is developed directly from GFDL’s CM2.1 climate model and uses pressure-based vertical coordinates with a horizontal resolution of approximately 1.0?×1.0?. 7) SST data from the Centre National de Recherches Météorologiques (CNRM-CM5 RCP4.5) Earth System Model with a horizontal resolution of approximately 1.0?×1.0?. The datasets from two coupled-climate-models are used to investigate the variations in the intensity of the SST front that plays an important role in sea fog formation under global warming.

The results of the WRF-ARW (the Advanced Research Weather Research and Forecasting) model are compared with observations. The model has a grid size of 10km and 41 sigma levels in the vertical with a high resolution at the boundary layer (21 levels between=0.8 and 1.0 or below about 2km). The six-hour FNL data and the daily Optimum Interpolation Sea Surface Temperature (OISST) are used as the initial and lateral boundary conditions. A control run (CTL) is conducted to simulate the stratus-fog case and a sensitivity experiment (EXP) to smooth SST. The main physical schemes used in the model are listed in Table 1, which are well suitable for simulating sea fog over the Yellow and East China Seas according to Zhang(2012).

Table 1 Vertical structure and physical schemes in the WRF model

3 Analysis of Sea Fog-Stratus Case

3.1 Observations

Fig.1 displays a sequence of the visible satellite images of MTSAT-2 during the daytime from 2 to 4 June 2011. It was cloudy (very likely stratocumulus) over the Southeastern China and the East China Sea, and the Yellow Sea had scattered stratus or fog (stratus/fog) patches at 0600 UTC (1400 LST) on 2 June (Fig.1a), revealed by their spectrum features (Zhang and Bao, 2008). The fog started to develop over the southern Yellow Sea at 0900 UTC on 2 June (Fig.1b). At 0000 UTC on 3 June (Fig.1d), the Yellow Sea was covered by fog and stratus still existed to the south of the fog area. Note that the clouds and sea fog can be clearly separated near 30?N. This stratus-fog distribution pattern (stratus over the East China Sea and fog over the Yellow Sea) was sustained throughout this stratus-fog process. The fog over the Yellow Sea began to dissipate at 0300 UTC on 4 June as the clouds over the south of the Yellow Sea moved northward (Fig.1e).

Theobservations from nine stations (Hongjia #58665, Xiangshan #58569, Shengsi #58472, Shanghai #58362, Rudong #58265, Sheyang #58150, Lianyungang #58040, Rizhao #54945, and Qingdao #54857) were analyzed along the coast of the Yellow and East China Seas. It can be seen that the stations along the Yellow Sea coast reported sea fog while stations along the East China Sea coast recorded clouds, and clouds moved northward as time elapsed (Fig.2a2). These results are in agreement with the satellite images. In addition, the prevailing southerly wind over the Yellow Sea transports moisture northward.

Using the CALIPSO data, the vertical cloud structure and ice/water phase over the Yellow Sea region are analyzed. It can be clearly seen that northward moving clouds are lowering and reach sea surface near 121?E, 30?N (Fig.2b1, 2b2). The results further indicate the possible transition of stratus into fog over the Yellow and East China Seas.

Fig.1 Visible satellite images from JMA MTSAT-2. (a) 0600 UTC 2, (b) 0900 UTC 2, (c) 0000 UTC 3, (d) 0900 UTC 3, (e) 0300 UTC 4, and (f) 0600 UTC on 4 June 2011.

Fig.2 (a1) The position of MICAPS stations (red points: Hongjia #58665, Xiangshan #58569, Shengsi #58472, Shanghai #58362, Rudong #58265, Sheyang #58150, Lianyungang # 58040, Rizhao #54945 and Qingdao #54857). (a2) Corresponding weather conditions at these stations. The blue color denotes cloud/precipitation, and the yellow color fog. (b1) Vertical feature mask and (b2) ice/water phase of CALIPSO on 3 June 2011. (c) The geopotential height at 1000hPa (contour, greater than 100gpm, interval 2.5gpm) and wind at 10m (vector, ms-1) from FNL, 1200 UTC 2 June 2011.

3.2 Atmospheric and Hydrologic Conditions

A surface high-pressure system controlled the Yellow and the East China Seas at 1200 UTC on 2 June, 2011 (Fig.2c). The southerly wind along the west flank of the system was carrying moisture northward from the East China Sea to the Yellow Sea. The 8 oktas of clouds (Sky is completely covered by clouds) were observed in the coastal regions from the northern East China Sea to the southern Yellow Sea (figure not shown).

Fig.3 shows the spatial-temporal distribution of relative humidity (RH) and the vertical velocity of air masses along 123?E. The wet air (RH>80%) in the lower level of the marine atmospheric boundary layer (MABL) extends northward to the north of 30?N under the influence of southerly wind from 1200 UTC 02 to 0000 UTC 03, providing moisture for sea fog formation (Hu and Zhou, 1997). To the south of 30?N, the wet air layer ascends from 1000hPa to 750hPa, indicating cloud accumulation or precipitation in the region. To the north of 30?N, the thin wet air layer is capped by dry air related to subsidence. The distribution of RH is consistent with the stratus-fog pattern displayed by the satellite data, and reveals possible cloud transition into sea fog. Note that there is a SST front (Sharp SST Gradient, SSTF) off the mouth of the Yangtze River near 30?N, with a warm center to its south and a cold center to its north (Fig.4).

Fig.3 Vertical distribution of relative humidity (shaded, %) and vertical velocity (contour, Pas-1) in a meridional section along 123?E from FNL. (a) 1200 UTC 2, (b) 1800 UTC 2 and (c) 0000 UTC 3 June 2011.

As analyzed above, the observations indicate that there are connections between stratus and fog over the Yellow and East China Seas. Under the influence of high pressure and southerly wind, air masses associated with stratus can descend to sea surface near SSTF as they move northward. What are the contributions of background circulation and SSTF to the occurrence of the stratus-sea fog event? What are the physical processes involved? These questions will be discussed in the following sections using the WRF model.

Fig.4 SST (contour,℃) and SST anomaly from zonal averaged SST between 118? and 127?E (shaded) on 3 June 2011 (the red solid circle denotes the warm center and the blue dashed circle the cold center).

4 Numerical Experiments

4.1 Control Run (CTL)

The WRF modeling is preformed from 0000 UTC 1 to 0000 UTC 6 June 2011. The horizontal atmospheric visibility can be calculated in the model by using an algorithm developed by Stoelinga and Warner (1999):

whereis the horizontal atmospheric visibility (km), β is the extinction conefficients (km) related to cloud water (), cloud ice (), snow (), and rain water (). In the present case, only the cloud water in the atmosphere is considered (Zhang, 2012):

, (2)

in whichis the mixing ratio of cloud water (gkg),is the air density (gm). (.) in Eq. (2) is the liquid water content (LWC) (gm). The simulated fog patches are identified by the mixing ratio of cloud water () greater than 0.016gkgat the model’s lowest level.

4.1.1 Comparison between model results and observations

By comparing the WRF results with the MTSAT-2 visible satellite images for fog patches, it is found that the spatial-temporal distribution from the model results clearly shows the formation, development, and dissipation of the fog process (Figs.5a1–a6). Besides, the calculated vertical profiles of temperature and relative humidity near the Hongjia sounding station (#58665) are in good agreement with the sounding data (Figs.5b1–b2), indicating that the model is suitable for simulating the stratus-sea fog event under consideration.

Fig.5 (a) The calculated mixing ratio of cloud water at the first model level in the control run (shaded, gkg-1). The times in a1–a6 correspond to those in Fig.1a–f, respectively. (b) The vertical profiles of temperature (blue) and relative humidity (red) at (b1) 1900 LST 3 and (b2) 1900 LST 4 June 2011 at the Hongjia sounding station (see Fig.2a). The solid line represents the observations and the line with square symbol the calculations.

4.1.2 Subsidence in the stratus-sea fog case

Using the model results, the air masses at different levels from 1200 UTC 3 to 0600 UTC 2 June are tracked to study the subsidence and physical processes within ABL. From the vertical distribution of potential temperature (Fig.6a) it can be seen that the top of the ABL is at about 250m around 29?N–31?N, and it lowers from south to north. Air parcels near the top of the boundary layer (300m), above the boundary layer (1000m) and within the boundary layer (100m, 10m) are selected for the analyses on subsidence and physical processes.

The air masses within the boundary layer (at 10m and 100m levels) have an ocean-based history over the Yellow and East China Seas. The air paths show that the air masses are driven by the southerlies on the west flank of the high-pressure system over the seas. The air parcels above the boundary layer (at 300m and 1000m levels) have a land-based history moving northeastward from the land of the southeast China into the Yellow Sea (Fig.6b).

Fig.6 (a) Vertical distribution of potential temperature (blue contour, K) and geopotential height (black contour, m) in a meridional section along 123?E and the height of the internal atmospheric boundary layer (red line, m) at 0000 UTC 3 June 2011. (b)Trajectory of air parcels from 0600 UTC 2 to 1200 UTC 3 (the solid circle corresponds to the 10m level, the solid square 100m, the open circle 300m, the open square 1000m) and sea surface tem- perature at 3 June 2011 (contour, ℃, the colored shade has the same scale as in Fig.4).

The air masses at 10m sink from above 60m and are cooled by the cold sea surface through sensible heat ex- change on the cold flank of the SSTF (Fig.7a). The sinking occurs in the ABL when air masses move across the SSTF from warmer to cooler sea surface. Along the trajectory, the relative humidity first decreases slightly (RH is still more than 95%) due to the influence of the adiabatic subsidence and warmer SST, and increases to 100% soon afterward. As warm/moist air parcels move over the cold sea surface, condensation occurs and fog forms. The air masses at 100m sink from above 350m when moving across the SSTF and are dominated by the similar physical processes as those at 10m (figure not shown). Therefore, besides advection, the subsidence forced by the SSTF is one of the vital factors for the sea fog formation.

Themostsignificantsubsidenceoccursfortheairmasses alongthetrajectoryat300mlevel(Fig.7b).Theairmasses sink from about 1600 m to 300m and the subsidence induces about 7℃ of temperature increase and forms temperature inversion (Fig.8). The sink may lead to the decrease in cloud droplets and the increase in dew-point temperature due to evaporation as shown in Fig.7b. Different from the subsidence due to the SSTF, this subsidence occurs in the free atmosphere and is forced by synoptic high pressure. The two types of subsidence are overlapped over the cold flank of the SSTF, thus lower the top of the MABL (Fig.6a) and the base of the stratus, and create favorable conditions for the stratus-to-fog transition.

Fig.7 Variations in temperature (red solid, ℃), SST (blue solid, ℃), dew point temperature (red dashed, ℃), relativity humidity (green solid, %) , the mixing ratio of cloud water (green dashed, gkg-1) and height (black solid, m) of air masses along the trajectories starting at (a) 10m level and (b) 300m level.

As shown in Figs.8a–d, there is an obvious subsidence to the north of SSTF (28?N–30?N). The stratus is gradually approaching to sea surface and eventually converts into sea fog. Figs.8e–g show that the height where temperature inverses decreases from 300m to 150m during 0000 UTC to 1200 UTC 3 June and its intensity is strengthened. In this stratus-sea fog event, the subsidence above the boundary layer generates the temperature inversion and lowers the inversion height, which is similar to the subsidence in the status-to-fog transition along the California coast as reported by Kora?in(2001). However, this study also shows that the sink forced by the SSTF plays an important role in the transition and the fog occurs to the north of status due to the influence of the southerly winds.

4.2 Smoothing SSTF Experiment (EXP)

In order to further investigate the impact of SSTF on the transition of stratus into sea fog, the SSTF is smoothed out in numerical experiments (EXP) (The smoothed SST is shown in Fig.9d). The fog patches over the cold flank of SSTF in the CTL (Figs.9a–c) shrink significantly in EXP as shown in Figs.9d–f.

Fig.8 Vertical distribution of mixing ratio of cloud water (shaded, gkg-1), temperature (red contour, ℃), stream line (blue contour) and height (black contour, m) in a meridional section along 123.5?E. (a) 1500 UTC 2; (b) 2100 UTC 2; (c) 0000 UTC 3 and (d) 0900 UTC 3 June, 2011. The profiles of temperature (red, ℃) and dew point temperature (blue, ℃) at 122?E, 33?N. (e) 1200 UTC 2; (f) 0000 UTC 3 and (g) 1200 UTC 3 June 2011.

Fig.9 Calculated mixing ratio of cloud water (shaded, gkg-1) at the first model level and SST (green contour). (a) 1200 UTC 2; (b) 0000 UTC 3; (c) 1200 UTC 3 June 2011 in the CTL ; (d) 1200 UTC 2; (e) 0000 UTC 3; (f) 1200 UTC 3 June 2011 in the EXP.

4.2.1 The effect of SSTF on horizontal and vertical circulation in the MABL

The air masses in the EXP are selected at the same starting positions as in the CTL. The air masses at 10m and 100m levels have an ocean-based history throughout the tracking time period, while the air parcels at 300m and 1000m levels are from the land of the southeast China and move northeastward into the Yellow Sea (figure not shown), which indicates that smoothing out the SSTF does not change the horizontal circulation.

Fig.10 shows the physical variables along the trajectories at different levels in the EXP. The downward motion on the cold flank of the SSTF in the CTL is turned into an upward motion in the EXP for the air mass at the 10m level (Fig.10a). With the upward motion, temperature decreases and moisture condenses into liquid water, which corresponds to cloud rather than fog formation due to the high elevation of the air masses. As air masses move northward and gradually descend the cold water over the Yellow Sea results in the sea fog formation in the farther north (Figs.9d–f).

The subsidence still exists in the EXP for the 300m trajectory in the free atmosphere. Compared with those in the CTL, the air masses in the EXP move down from a higher level about 3600m in altitude and remain at a relatively higher level (above 600m) during most of the tracking time period (Fig.10b). The differences show that the effect of the SSTF is likely to reach to the free atmosphere. The lack of the SSTF forcing will lead to the weaker sinking and the temperature inversion at a higher level, thus likely resulting in status rather than sea fog formation.

Fig.10 The same as in Fig.7 but for EXP.

The secondary circulation within the MABL forced by the SSTF can be seen clearly in the CTL run (Fig.11a). The upward motion on the warm flank and the subsidence on the cold flank of the SSTF become very weak in EXP (Fig.11b). Because the cloud coverage is basically associated with the warm flank of the SSTF (Liu, 2010), the subsidence on the cold flank of the SSTF will help lower and convert stratus into sea fog as air masses move northward. Thus, the secondary circulation in the MABL directly contributes to the transition of stratus into sea fog over the Yellow and East China Seas.

4.2.2 The effect of SSTF on low-level turbulence and boundary layer height

The low-level turbulence and the height of the MABL are important for sea fog formation and development (Zhang, 2012). A zonal section crossing the cold and warm flanks of SSTF along 30?N is selected to make the comparison between the CTL and EXP runs. In the CTL, the low-level turbulence is stronger over the warm water west of 124?E, and weaker over the cold water east of 124?E (Fig.12a). The height of the boundary layer is compatible with the intensity of turbulence,, the stronger (weaker) the turbulence, the higher (lower) the top of the ABL.

By the contrast analysis, the intensity of low-level turbulence becomes stronger (weaker) over the cooler (warmer) water in the EXP than that in the CTL because the original cooler (warmer) water becomes warmer (cooler) due to without the SSTF. The height of the boundary layer changes accordingly (Fig.12b). Besides, the lack of sinking over the cold flank of the SSTF also contributes to the higher ABL. Much stronger turbulence enhances the fog elevation and promotes the stratus formation as shown in Fig.12b, which is in agreement with previous studies (Zhou and Ferrier, 2008; Zhang, 2012).

Fig.12 Vertical distributions of mixing ratio of cloud water (shaded, gkg-1), ABL height (red solid, m) and Richardson number (contour) in a zonal section along 30?N. (a) 1500 UTC 2 June 2011 in CTL and (b) in EXP.

5 The Potential Trend of Sea Fog Occurrence Under the Conditions of Global Warming

As discussed above, the SSTF plays an important role in the formation of sea fog. The SSTF over the Yellow and East China Seas exists under climatological mean states (Fig.13a). The subsidence induced by the SSTF exists in the secondary circulation as well (Figure not shown), which makes the pronounced difference in the spatial distributions of low cloud and sea fog frequencies. The cloud regime is found south of 28?–30?N, and the fog is dominant north of that region (Fig.13b).

The GFDL-ESM2M RCP4.5 and CNRM-CM5 RCP4.5 suggest that the SSTF over the Yellow and East China Seas will probably become weaker under the conditions of global warming (Figs.14a–b), accompanied by the weaking of the subsidence of secondary circulation. Thus the frequency of sea fog occurrences will probably decrease. According to the observational data in Qingdao from 1971 to 2010 and in Hongjia from 1983 to 2010, the frequency of sea fog occurrences are low (Fig.14c) probably due to the weakened SSTF under the conditions of climate change.

6 Summary

Based onobservations, satellite data, reanalysis data and WRF output, the physical processes are investigated in a stratus-fog case occurring over the Yellow and East China Seas on 3 June 2011 and the focus is on the contributions of the subsidence related to the synoptic circulations and the SSTF to the sea fog formation. Note that the southerly advection, as pointed out in previous studies, is also important in the sea fog formation. As shown in Figs.3 and 7, the wet air masses move northward and descend to the sea surface. The major conclusions are as follows:

1) The synoptic high-pressure system generates southerly winds that transport water vapor from the East China Sea to the Yellow Sea and induce the subsidence of air masses from free atmosphere to the top of the ABL. The persistent downward motion makes the temperature inversion descend and strengthen, providing favorable conditions to the stratus-to-fog transition. The moisture from the evaporation of cloud droplets may also favor the fog formation.

Fig.13 (a) The SST climatological mean (contour) and the SST difference from the zonal average (118?E–127?E) for June (shaded) from 2003–2009 (Meng and Zhang, 2012); (b) The frequency distribution of low cloud (shaded) and sea fog (contours) over the Yellow and East China Seas from 1979 to 2009 (Han et al., 2012).

Fig.14 The difference of SST meridional gradient (10-6, shaded) between the future (2071–2100) and the past (1976–2005) from (a) GFDL-ESM2M and (b) CNRM-CM5. (c) The sea fog frequency during May–July 1971 to 2010 in Qingdao and from 1983 to 2010 in Hongjia and Taizhou.

2) Forced by the SSTF, a secondary circulation occurs within the MABL, and the air masses descend because of the sinking on the cold flank of the SSTF, overlapped with the synoptic subsidence. The cold Yellow Sea surface stimulates the condensation and conversion of the stratus to sea fog. A conceptual model is suggested (Fig.15) based on these results. Sea fog could hardly be detected on the cold flank of the smoothed SSTF in the EXP. The secondary circulation within the MABL over the colder water becomes weaker while the subsidence above the MABL still exists in the EXP.

Fig.15 The conceptual sketch of processes associated with fog formation over the Yellow Sea and East China Seas.

3) The thermal forcing will affect the low-level turbulence and the height of boundary layer. The stronger (weaker) the turbulence is, the higher (lower) the boundary layer is. The stronger turbulence may lead to elevated fog and its conversion to stratus. Transition of stratus into sea fog is also determined by the balance of the subsidence and the low-level turbulence.

4) Under the conditions of global warming, the frequency of sea fog occurrences will probably decrease due to the weakened SSTF. But further investigation is still needed because other factors may also be involved in the sea fog formation. The conceptual model obtained from the present case study needs to be further validated

Acknowledgements

We wish to thank Prof. Qinyu Liu for her constructive comments and Jingchao Long, Geng Han for their help with some of the figures. This work is supported by ‘973’ project 2012CB955602, and NSFC 41175006.

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(Edited by Xie Jun)

① Fog differs from cloud only in that the base of fog is at the earth’s surface while clouds are above the surface. Cloud differs from fog only in that the latter is, by definition, close (a few meters) to the earth’s surface (AMS Glossary).

② The thresholds=0.016gkgcorrespond to the visibility of 1000m according to Eq. (2) (Zhang, 2012).

10.1007/s11802-013-2218-5

ISSN 1672-5182, 2013 12 (2): 301-311

. Tel: 0086-532-66781528 E-mail: zsping@ouc.edu.cn

(November 8, 2012; revised January 4, 2013; accepted February 22, 2013)

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2013