THE PHYSICAL STRUCTURE OF THE WINTER FOG IN CHONGQING METROPOLITAN AREA AND ITS FORMATION PROCESS

Chongqing is a very famous foggy metropolitan in China.The Chongqing Fog Experiment Group carried out com-prehensive experiments on a large and extensive scale in the Chongqing urban area from 15 December 1989 to 15 Janua-ry 1990.And several items were further observed from 7 December 1990 to 7 January 1991.Based on the analysis of theobservational data,some important characteristics of the Chongqing winter fog and the boundary layer structure havebeen revealed.It is found that such factors as topography,mountain wind,rivers,the urban heat island and airpollution,all contribute to the formation of the Chongqing fog in addition to the radiation conditions.     

Fog Research in China： An Overview

Fog can adversely affect human activity directly and indirectly,resulting in large losses both in terms of the local economy and lives.Much effort has been devoted to studies of fog across many areas of China,and in that context this paper aims to summarize climatic characteristics and review fog field experiments and their major results relating to fog mechanisms,physical properties and chemical characteristics.Progress in the application of remote sensing techniques and numerical simulation in fog research are also discussed.In particular,the effects of urbanization and industrialization on fog are highlighted.To end,perspectives on future fog research are outlined.The goal of this review paper is to introduce fog research in China to the global academic community and thus promote international collaboration on fog research.This is important because most papers on fog in China are published in Chinese,which are unreadable for the vast majority of non-Chinese researchers.     

THE NUMERICAL MODELING OF RADIATION FOG

The formation and development of radiation fog are studied by using a one-dimensional model.Theresults are as follows:(1)The fog can change the ambient wind and temperature fields,on the contrary,theambient fields may also influence the fog;(2)The modeling fog forms initially at a certain level above thesurface,which is detailedly explained in the paper;(3)Turbulence delays the formation of fog but promotesits development;(4)Whether fog liquid water content exchange coefficient equals momentum exchangecoefficient has no influence on the numerical results.In addition,wind,temperature,exchange coefficient,and net radiative flux before and after the formation of radiation fog,are also studied.     

STUDY ON THE CHEMICAL CHARACTERISTICS OF POLLUTING FOG IN GUANGZHOU AREA IN SPRING

Samples of fog water collected in the area of Guangzhou during February, March and April of 2005 are used in this work to study the chemical composition of fog water in polluting fog there. Three typical episodes of polluting fog are analyzed in terms of ionic concentration and their possible sources. It is found that the concentration of various ions in fog water is much higher than those in rainwater. Fog not only blocks visual range but contains liquid particles that result in high degree of pollution and are very harmful to human health. SO₄⁼ is the anion with the highest concentration in fog water, followed by NO₃^-.For the cation, Ca⁺⁺ and NH₄⁺ are the highest in concentration. It is then known that rainwater is more acidic than fog water, indicating that ionic concentration of fog water is much higher than that of rainwater, but there are much more buffering materials in fog water, like NH₄⁺ and Ca⁺⁺. There is significant enrichment of Ca⁺⁺, SO₄⁼, and Mg⁺⁺ in fog water. In the Guangzhou area, fog water from polluting fog is mainly influenced continental environment and human activity. The episodes of serious fog pollution during the time have immediate relationships with the presence of abundant water vapor and large amount of polluting aerosol particles.     

Characteristics of Quasi-Periodic Oscillations During Sea Fog Events

Based on the high-resolution datasets collected in a sea fog field experiment at the northern coast of South China Sea, the variations of liquid water content (LWC) and net longwave radiation flux (NLRF) during two sea fog events on 16th-17th and 18th-19th March, 2008 are exaimeed by wavelet analysis, and the cooling mechanisms for fog formation and persistence are also investigated. The main results are shown as follows. (1) Sea fog may develop and persist whether it is cloudy or not aloft. However, when there is cloud aloft the LWC is less and wind speed in sea fog is higher than that in the clear sky. (2) The quasi-periodic oscillations (QPOs) of NLRF are observed in the formation stage of the two fogs. QPOs of LWC are only found in the developing stage no matter with cloudless or cloudy condition. (3) It is likely that sea fog forms by the cooling effects of longwave radiation and develops through the vertical mixing induced by the radiative cooling at the upper level. (4) During sea fog development and persistence, other mechanisms could also play important roles in fog-layer cooling, such as turbulent heat transport and radiation transport between air-sea interfaces.     

COMPREHENSIVE ANALYSIS OF THE MACRO- AND MICRO-PHYSICAL CHARACTERISTICS OF DENSE FOG IN THE AREA SOUTH OF THE NANLING MOUNTAINS

Using the composite field observational data collected in the area south of the Nanling Mts. and numerical modeling, the seasonal features of dense fog and visibility, fog drop spectrum and physical concept of fog forming have been analyzed. The occurring frequency of low visibility（≤200 m） is very high with a mean of 24.7%, a maximum of 41.8% from the end of autumn to winter and next spring. The fog processes that occur in the area south of the Nanling Mts. in spring and winter result from the interactions of complicated micro-physical processes, the local terrain, water vapor transportation and the influencing weather system. The fog processes are arisen from advection or windward slope, which is much different from the radiation fog. Cooling condensation due to the air lifted by the local mountain plays an important role in fog formation. Windward slope of the mountain is favorable to the fog formation. Dense fog can occur at lower altitudes in the windward slope of mountain, resulting in the lower visibility. The fog is mainly of small-drop spectrum with smaller number-density than that of urban fog, and its drop spectrum has descending trend in the section of smaller diameter. The inverse relationship between fog water content and visibility is the best among several relationships of micro-variables. In addition to micro-physical processes of fog body itself, the motion of irregular climbing and crossing over hillside while the fog body is being transported by the wind are also important reasons for the fluctuation of micro-physical parameters such as fog water content.     

MICROSTRUCTURES AND TEMPORAL VARIATION CHARACTERISTICS DURING A SEA FOG EVENT ALONG THE WEST COAST OF THE TAIWAN STRAIT

Synoptic systems and microphysical properties associated with a sea fog event are analyzed based on the measurements of visibility, meteorological elements and fog droplet spectrum from a comprehensive field campaign in Xiamen, Fujian province during spring 2013. The influences of meteorological elements on the microstructures of the sea fog are also discussed. The results showed that the wind speed and direction changed suddenly during the intermittent and disperse phases of the sea fog. Liquid water content, number concentration and average diameter varied obviously in the development, mature and disperse phases of the sea fog. The burst re-enforcement of sea fog was accompanied by explosive broadening of fog droplet spectrum; average diameter, number concentration and liquid water content increased sharply; and background meteorological conditions also changed significantly. The microstructures fluctuated intensely due to changes in turbulence, radiation and meteorological conditions at different stages, including nucleation, condensation, coagulation, and evaporation, as well as the discontinuity of spatial distribution of droplets.     

A Novel Four-Wavelength Transmissometer for Distinguishing Haze and Fog

Haze and fog exhibit different microphysical and optical properties according to Mie scattering theory. Haze particles are smaller than fog droplets. Light of a shorter wavelength is reduced more than that of a longer wavelength during haze events. In fog, the differences between the extinction coefficients at different wavelengths are not as apparent. On the basis of the different light extinction characteristics of haze and fog, a novel four-wavelength transmissometer based on charge-coupled device （CCD） imaging was designed to distinguish haze from fog with central wavelengths at 415, 516, 650, and 850 nm. The four-wavelength transmissometer was tested in an insitu experiment during the winter of 2009. Fog was determined when the differences of the extinction coefficients at the four wavelengths were not notable, whereas haze was determined when the light at shorter wavelengths was significantly more reduced than that at longer wavelengths. A threshold which describes the relative size of the extinction coefficients at the four wavelengths was defined to distinguish between fog and haze. The four-wavelength transmissometer provided results consistent with the commercial fog monitor during several measurements made in fog and haze events, especially under conditions of low visibility and high relative humidity.     

Numerical Experiment Research of a Marine Fog Event in the Pearl River Estuary Region

A marine fog event that occurred near the Pearl River Estuary region on 26 March 2002 was investigated with the fifth-generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5). The results of numerical experiment are very consistent with the surface observations, especially in the processes of marine fog formation and evolution. Besides, a series of sensitivity numerical experiments were performed by varying the distribution of landuse type and the turbulence exchange process. It is shown that the marine fog forms along the coastal line. Tests have indicated that when the distribution of landuse type is modified, the regions where the marine fog can intrude into are obviously different. The turbulence process in the model is important for fog formation and evolution. If the influence of turbulence process is ignored in numerical simulation experiment, the simulated maximum of the cloud liquid water content and the height of fog top will be decreased greatly.     

STUDY ON THE PHYSICAL PROCESS OF WINTER VALLEY FOG IN XISHUANGBANNA REGION

Xishuangbanna of Yunnan Province is a famous tropical foggy region. A field experiment wascarried out from November 23 to 30 of 1997 during which fogs occurred regularly every day. In thepaper the characteristics of macrostructure of fog are analyzed and the physical processes offormation and dissipation of fog are studied. The results show that the Xishuangbanna valley fogforms firstly in the lower atmosphere with two-layer structures and then develops suddenly in thevertical direction after reaching the ground. Furthermore, the vegetation effect on the formationand dissipation of fog is discussed specially.     

Macro-and Micro-Structures of Heavy Fogs and Visibility in the Dayaoshan Expressway

Belonging to the southern subtropical moist type of monsoon climate,the Nanling mountainous area experiences heavy fogs whenever quasi-stationary fronts appear there from September to May.There can be as many as 15-18 days of heavy fogs per month.Fogs have more serious consequences in the Lechang-Ruyuan section of the Beijing-Zhuhai Expressway(the longest expressway in China)that passes through the main part of the Nanling Mts.,where the road rises from 200m to more than 800m above sea level(ASL).For a major motorway in the mountainous areas of Nanling Mts.,two multidisciplinary integrated field observations were carried out,which measured visibility by the naked eyes,visibility by instrument,spectrum of fog- drops,liquid water content(LWC)of fog,tethered sounding,dual-parameter low-level sounding,turbulence diffusion within fog layers,aerosol spectra of size and composition,sampled fog water compositions,and sampled rainwater compositions.Typical cases were probed for their analyses of synoptics,micro- and macro-structures and microphysics.It is understood that heavy fogs take place with high frequency in the area and bring about serious consequences.Being typical advection and upslope fogs,they are in essence low-lying clouds appearing at high altitudes,which are closely related with the activity of South China frontal processes,especially the South China quasi-stationary fronts,and reflect on the role of local terrain as well.The heavy fogs are characteristic of long duration,extremely low visibility,well-organized lump- shaped structure,large-size fog-drops,moderate concentration,high LWC,and stronger turbulent diffusion within the fog layers than in fine sky.They differ much from radiation fogs,which are better documented in previous study in China.It is found that fog LWC is in significant anti-correlation with visibility so that large LWC is associated with small visual range.It is also noted that one of the reasons for the fluctuation of characteristic quantities of micro-structure such as the LWC of fog in the area is,in addition to the inhomogeneous structure of the fog itself,the effect of advection and inhomogeneous underlying surface; during the translation of fog with the ambient wind,irregular upslope and cross-over movement is another reason for the inhomogeneous structure and fluctuation of fog.The spectrum of the aerosol size displays itself as the power function of monotonous descent.The concentration of submicrometer particles is even higher.The high-concentration sulfate particles found in the aerosols of Nanling Mts.are actually good nuclei for condensation,which are favorable for the formation of fog.The presence of fog can help cleanse the trace compositions in the atmosphere so that fog droplets contain high levels of polluting elements.In the meantime,compared to cloud droplets,fog droplets are easier to be captured by the vertical surfaces of objects on the land surface,such as vegetation and buildings to constitute another kird of cleansing process. In vast stretches of forest like the Nanling Mts.,this kind of cleansing may be quite important.Studying the characteristic variation of fogs in the area realistically assists in setting up a forecast and warning system for local fogs and provides basic information for fog dispersal experiments.     

EVOLUTION OF LIQUID WATER CONTENT IN A SEA FOG CONTROLLED BY A HIGH-PRESSURE PATTERN

On March 16–17, 2008, a sea fog occurred in Dianbai in the west of Guangdong Province and was accompanied by a high-pressure synoptic system. Using comprehensive observation datasets, this study analyzes the evolution of liquid water content during this sea fog and investigates the relationships between liquid water content and the average diameters and count densities of fog droplets, air temperature, wind speed and turbulence exchanges. The main results are presented as follows. (1) The sea fog showed a quasi-periodic oscillation characteristic, i.e., it developed, disappeared and then developed again. (2) During the sea fog, the number of fog droplets changed significantly while the changes in average diameter of the fog droplets were relatively small. The development and disappearance of the sea fog correlated significantly with the fog droplet numbers. (3) The air-cooling mechanism played a significant role in sea fog formation and development. However, the influences of this mechanism were not evident during fog persistence. (4) During sea fog formation, weak turbulence exchanges were helpful for fog formation. During sea fog development and persistence, liquid water content increased when turbulence exchanges weakened, and vice versa. The changes in turbulence exchanges were closely related to the quasi-periodic oscillations observed in sea fog presence.     

A CASE STUDY OF NUMERICAL SIMULATION OF SEA FOG ON THE SOUTHERN CHINA COAST

This study uses numerical simulations to examine a case of sea fog that was observed from 20 to 22 March2011 on the southern China coast. The observation dataset includes observatory data, cloud-top temperature from MODIS, GPS sonde, and data from the Integrated Observation Platform for Marine Meteorology(IOPMM). The simulations are based on the Weather Research and Forecasting(WRF) model with four distinct parameter settings.Both the observations and simulations focus on the characteristics of the fog extent, boundary layer structure, and meteorological elements near the air-sea interface. Our main results are as follows:(1) The extent of mesoscale sea fog can be well simulated when the sea surface temperature has at least 0.5 ×0.5 horizontal resolution.(2) To accurately model the vertical structure of the sea fog, particularly the surface-based inversion, vertical levels must be added in the boundary layer.(3) When these model conditions are met, the simulations faithfully reproduce the measured downward shortwave radiation, downward longwave radiation, and surface sensible heat flux during the sea fog period.     

Examination of Microphysical Relationships and Corresponding Microphysical Processes in Warm Fogs

In this paper, the microphysical relationships of 8 dense fog events collected from a comprehensive fog observation campaign carried out at Pancheng（32.2 N, 118.7 E） in the Nanjing area, China in the winter of 2007 are investigated. Positive correlations are found among key microphysical properties（cloud droplet number concentration, droplet size, spectral standard deviation, and liquid water content） in each case, suggesting that the dominant processes in these fog events are likely droplet nucleation with subsequent condensational growth and/or droplet deactivation via complete evaporation of some droplets. The abrupt broadening of the fog droplet spectra indicates the occurrence of the collision-coalescence processes as well, although not dominating. The combined efects of the dominant processes and collision-coalescence on microphysical relationships are further analyzed by dividing the dataset according to visibility or autoconversion threshold in each case. The result shows that the specific relationships of number concentration to volume-mean radius and spectral standard deviation depend on the competition between the compensation of small droplets due to nucleation-condensation and the loss of small droplets due to collision-coalescence. Generally, positive correlations are found for diferent visibility or autoconversion threshold ranges in most cases, although negative correlations sometimes appear with lower visibility or larger autoconversion threshold. Therefore, the compensation of small droplets is generally stronger than the loss, which is likely related to the sufcient fog condensation nuclei in this polluted area.     

Characteristics of the Boundary Layer Structure of Sea Fog on the Coast of Southern China

Using boundary layer data with regard to sea fog observed at the Science Experiment Base for Marine Meteorology at Bohe,Guangdong Province,the structure of the atmospheric boundary layer and the characteristics of the tops of the fog and the clouds were analyzed.In addition,the effects of advection,radiation,and turbulence during sea fog were also investigated.According to the stability definition of saturated,wet air,the gradient of the potential pseudo-equivalent temperature equal to zero was defined as the thermal turbulence interface.There is evidence to suggest that two layers of turbulence exist in sea fog.Thermal turbulence produced by long-wave radiation is prevalent above the thermal turbulence interface,whereas mechanical turbulence aroused by wind shear is predominant below the interface.The height of the thermal turbulence interface was observed between 180 m and 380 m.Three important factors are closely related to the development of the top of the sea fog:(1) the horizontal advection of the water vapor,(2) the long-wave radiation of the fog top,and(3) the movement of the vertical turbulence.Formation,development,and dissipation are the three possible phases of the evolution of the boundary-layer structure during the sea fog season.In addition,the thermal turbulence interface is the most significant turbulence interface during the formation and development periods;it is maintained after sea fog rises into the stratus layer.     

Analysis and Modeling of a Long-Lasting Fog Event over Beijing in February 2007

The evolution characteristics of a long-lasting fog event over Beijing during 20-22 February 2007 are investigated using the 5-min automatic visibility data and conventional meteorological observations.Data analysis results reveal that there is a close relationship between the development/evolution of this fog event and the weather conditions such as high humidity,light wind,and low temperature in the lower troposphere. Furthermore,numerical simulations of this event are carried out by using the Advanced...     

A New Algorithm for Sea Fog／Stratus Detection Using GMS-5 IR Data

A new algorithm for the detection of fog/stratus over the ocean from the GMS-5 infrared (IR) channel data is presented. The new algorithm uses a clear-sky radiance composite map (CSCM) to compare the hourly observations of the IR radiance. The feasibility of the simple comparison is justified by the theoretical simulations of the fog effect on the measured radiance using a radiative transfer model. The simulation results show that the presence of fog can be detected provided the visibility is worse than 1 km and the background clear-sky radiances are accurate enough with known uncertainties. For the current study, an accurate CSCM is constructed using a modified spatial and temporal coherence method, which takes advantage of the high temporal resolution of the GMS-5 observations. The new algorithm is applied for the period of 10-12 May 1999, when heavy sea fog formed near the southwest coast of the Korean Peninsula. Comparisons of the fog/stratus index, defined as the difference between the measured and clear-sky brightness temperature, from the new algorithm to the results from other methods, such as the dual channel difference of NOAA/AVHRR and the earth albedo method, show a good agreement. The fog/stratus index also compares favorably with the ground observations of visibility and relative humidity. The general characteristics of the fog/stratus index and visibility are relatively well matched, although the relationship among the absolute values, the fog/stratus index, visibility, and relative humidity, varies with time. This variation is thought to be due to the variation of the atmospheric conditions and the characteristics of fog/stratus, which affect the derived fog/stratus index.     

Analysis of the Microphysical Structure of Heavy Fog Ｕsing a Droplet Spectrometer: A Case Study

The microphysical properties of a long-lasting heavy fog event are examined based on the results from a comprehensive field campaign conducted during the winter of 2006 at Pancheng (32.2°N, 118.7°E), Jiangsu Province, China. It is demonstrated that the key microphysical properties (liquid water content, fog droplet concentration, mean radius and standard deviation) exhibited positive correlations with one another in general, and that the 5-min-average maximum value of fog liquid water content was sometimes ...     

On the Evolution and Structure of a Radiation Fog Event in Nanjing

An extremely dense radiation fog event during 10-11 December 2007 was studied to understand its macro-micro-physics in relation to dynamic and thermodynamic structures of the boundary layer, as well as its structural evolution in conjunction with the air-surface exchange of heat and water vapor. The findings are as follows. The extreme radiation fog process was divisible into formation, development, mature, and dissipation phases, depending on microstructure and visibility. This fog event was marked by rapid evolution that occurred after sunrise, when enhanced surface evaporation and cold air intrusion led to a three order of magnitude increase in liquid water content (LWC) in just 20 minutes. The maximum droplet diameter (MDD) increased four-fold during the same period. The fog structure was two-layered, with the top of both the surface-layer and upper-layer components characterized by strong temperature and humidity inversions, and low-level jets existed in the boundary layer above each fog layer. Turbulence intensity, turbulent kinetic energy, and friction velocity differed remarkably from phase to phase: these features increased gradually before the fog formation and decreased during the development phase; during the mature and dissipation phases these characteristics increased and then decreased again. In the development and mature stages, the mean kinetic energy of the lower-level winds decreased pronouncedly, both in the horizontal and vertical directions.     

RESEARCH ON AUTOMATIC FOG IDENTIFICATION TECHNOLOGY BY METEOROLOGICAL SATELLITE REMOTE SENSING

There is an urgent need for the development of a method that can undertake rapid, effective, and accurate monitoring and identification of fog by satellite remote sensing, since heavy fog can cause enormous disasters to China’s national economy and people's lives and property in the urban and coastal areas. In this paper, the correlative relationship between the reflectivity of land surface and clouds in different time phases is found, based on the analysis of the radiative and satellite-based spectral characteristics of fog. Through calculation and analyses of the relative variability of the reflectivity in the images, the threshold to identify quasi-fog areas is generated automatically. Furthermore, using the technique of quick image run-length encoding, and in combination with such practical methods as analyzing texture and shape fractures, smoothness, and template characteristics, the automatic identification of fog and fog-cloud separation using meteorological satellite remote sensing images are studied, with good results in application.