Fog Days Research Articles

Anomalous Atmospheric Circulation Associated with the Extremely Persistent Dense Fog Events over Eastern China in the Late Autumn of 2018

Under a declining trend of fog days in China, the duration of fog events since the 1990s reached a significant peak in the late autumn of 2018 over Eastern China. The average anomalous fog days were 4.74 d in November 2018 over Jiangsu Province in Eastern China, with a 1.73 standard deviation departure from climatology. Those fogs can thus be identified as a significantly abnormal climatic event with long duration, strong intensity, and extensive coverage. Based on the daily evolutions and correlations of atmospheric parameters, the dense fogs are revealed to be well configured by favorable metrological conditions such as weak dynamic progress, strong inversion in the lower troposphere and saturated air near the surface. If not disturbed, the intensification or duration of these conditions will further promote and maintain the development of fogs. The anomalous atmospheric background associated with those favorable meteorological conditions is revealed by composing the standardized anomalies of circulation fields during the fog days. Over the fog areas, vortex activities or cold air invasion is effectively hampered and the atmosphere inclines to be stable, due to the anomalous circulation pattern composed of the broadened jet stream, weakened jet core over Eastern China, undermined East Asian trough, declined East Asian winter monsoon, and enhanced anomalous southerly flows that transport abnormal warm and wet air to Eastern China. The vapor supplement is intensified by both sustained anomalous northward wind at the lower troposphere and anomalous westward wind in the near-surface. Overall, the numbers of standardized anomalies of 1000–200-hPa height, temperature, wind, and moisture fields during these fog days all significantly depart from climatology for that locale and time of the season, further demonstrating that the persistent dense fogs over Eastern China in the late autumn of 2018 is an unusual weather event with extreme synoptic-scale departures from normal.

Contending with the Elements: The Role of Weather in the Battle of Spotsylvania Court House

Contending with the ElementsThe Role of Weather in the Battle of Spotsylvania Court House Cameron M. Boutin (bio) More than two decades after the conclusion of the American Civil War, Union assistant adjutant general Francis Walker recollected that the climax of the Battle of Spotsylvania Court House was the "closest and fiercest" fighting of the entire conflict, when "for the distance of nearly a mile, amid a cold, drenching rain, the combatants were literally struggling across the breastworks."1 Between May 8 and 19, 1864, the Union Army of the Potomac and the Confederate Army of Northern Virginia battled at the small Virginia crossroads of Spotsylvania Court House in a clash that proved to be one of the bloodiest of the war. As the costliest engagement of the Overland Campaign, the fighting at Spotsylvania has received widespread attention from scholars since the war ended.2 Despite the extensive studies of Spotsylvania, one aspect of the battle has often been overlooked: the role of weather. Similar to most conflicts, especially before the era of mechanized warfare, a main facet of the Civil War was that armies lived, marched, and fought in natural [End Page 200] landscapes while exposed to the vagaries of the weather.3 The interaction between soldiers and weather was unavoidable, intimate, and continuous. The Civil War, like history in general, encompassed the dynamic interplay between humans and nature, and the weather was an active force in the conflict, with a direct impact on human affairs.4 While not hard and fast determinants of events, the weather and other aspects of the natural world affect the course of history through their interaction with human beings. As historians Lisa Brady and Timothy Silver declare, "the natural world moves to its own peculiar rhythms and frequently shapes historical events in ways that humans cannot anticipate."5 Humans react to nature, nature reacts to human actions, and weather is no exception. Weather is experienced all the time by humans, especially soldiers in the midst of war, but in no way can it be controlled, and in the nineteenth century during the Civil War, it could not even be accurately predicted. The cursory discussion of the meteorological conditions at Spotsylvania Court House in most scholarship on the battle may make it seem that weather was simply a passive backdrop to the clash, but it was actually an interactive agent that influenced the course of events.6 During the nearly two-week span of the Battle of Spotsylvania, the weather was frequently extreme, particularly in the first nine days. Scorching, dry heat at the beginning of the battle changed to days of heavy fog and torrential rains that transformed the landscape around Spotsylvania into [End Page 201] "a sea of mud," in the words of one contemporary.7 Only in the final days of the fighting did milder weather occur and become a more minor factor influencing the course of the battle. At the same time that the Federal and Confederate soldiers were engaged in combat operations at Spotsylvania, they were forced to continually struggle against the often severe meteorological conditions. As the weather was omnipresent, the troops needed to contend with its vagaries more regularly than the actual enemy forces. The intense weather significantly affected the physical and mental health, and therefore capabilities, of the Union and Confederate troops. The soldiers themselves recognized its influence over their well-being.8 While the weather was not the overall determinant of the battle's outcome, its direct and at times dramatic impact on the soldiers meant that it was central to the combatants' experience within the engagement. As the progress of the Battle of Spotsylvania as a whole, like all the military events of the Civil War, was determined by the Union and Confederate troops' experiences, the course of the clash was frequently shaped by the extreme weather conditions. For the soldiers battling at Spotsylvania Court House, the weather was a changeable variable, but one that could prove decisive. While the weather sometimes was an ally, helping the armies implement their plans and fulfill their objectives, at other times it was an enemy, causing difficulties for the soldiers and hindering their operations. The role...

Factors determining the diffusion of COVID-19 and suggested strategy to prevent future accelerated viral infectivity similar to COVID

This study has two goals. The first is to explain the geo-environmental determinants of the accelerated diffusion of COVID-19 that is generating a high level of deaths. The second is to suggest a strategy to cope with future epidemic threats similar to COVID-19 having an accelerated viral infectivity in society. Using data on sample of N = 55 Italian province capitals, and data of infected individuals at as of April 7th, 2020, results reveal that the accelerate and vast diffusion of COVID-19 in North Italy has a high association with air pollution of cities measured with days exceeding the limits set for PM10 (particulate matter 10 μm or less in diameter) or ozone. In particular, hinterland cities with average high number of days exceeding the limits set for PM10 (and also having a low wind speed) have a very high number of infected people on 7th April 2020 (arithmetic mean is about 2200 infected individuals, with average polluted days greater than 80 days per year), whereas coastal cities also having days exceeding the limits set for PM10 or ozone but with high wind speed have about 944.70 average infected individuals, with about 60 average polluted days per year; moreover, cities having more than 100 days of air pollution (exceeding the limits set for PM10), they have a very high average number of infected people (about 3350 infected individuals, 7th April 2020), whereas cities having less than 100 days of air pollution per year, they have a lower average number of infected people (about 1014 individuals). The findings here also suggest that to minimize the impact of future epidemics similar to COVID-19, the max number of days per year that Italian provincial capitals or similar industrialized cities can exceed the limits set for PM10 or for ozone, considering their meteorological conditions, is about 48 days. Moreover, results here reveal that the explanatory variable of air pollution in cities seems to be a more important predictor in the initial phase of diffusion of viral infectivity (on 17th March 2020, b1 = 1.27, p < 0.001) than interpersonal contacts (b2 = 0.31, p < 0.05). In the second phase of maturity of the transmission dynamics of COVID-19, air pollution reduces intensity (on 7th April 2020 with b′1 = 0.81, p < 0.001) also because of the indirect effect of lockdown, whereas regression coefficient of transmission based on interpersonal contacts has a stable level (b′2 = 0.31, p < 0.01). This result reveals that accelerated transmission dynamics of COVID-19 is due to mainly to the mechanism of “air pollution-to-human transmission” (airborne viral infectivity) rather than “human-to-human transmission”. Overall, then, transmission dynamics of viral infectivity, such as COVID-19, is due to systemic causes: general factors that are the same for all regions (e.g., biological characteristics of virus, incubation period, etc.) and specific factors which are different for each region and/or city (e.g., complex interaction between air pollution, meteorological conditions and biological characteristics of viral infectivity) and health level of individuals (habits, immune system, age, sex, etc.). Lessons learned for COVID-19 in the case study here suggest that a proactive strategy to cope with future epidemics is also to apply especially an environmental and sustainable policy based on reduction of levels of air pollution mainly in hinterland and polluting cities- (having low wind speed, high percentage of moisture and number of fog days) -that seem to have an environment that foster a fast transmission dynamics of viral infectivity in society. Hence, in the presence of polluting industrialization in regions that can trigger the mechanism of air pollution-to-human transmission dynamics of viral infectivity, this study must conclude that a comprehensive strategy to prevent future epidemics similar to COVID-19 has to be also designed in environmental and socioeconomic terms, that is also based on sustainability science and environmental science, and not only in terms of biology, medicine, healthcare and health sector.

Influence of Arctic Oscillation on Frequency of Wintertime Fog Days in Eastern China

The influence of Arctic Oscillation (AO) on the frequency of wintertime fog days in eastern China is studied based on the winter AO index, the wintertime fog-day data of national stations in China, and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data from 1954 to 2007. The results show that heavy fog and light fog are more likely to occur during winter in eastern China with the strong interannual variability. During the winter with the positive-phase AO, there are more days of heavy fog in North China but less in South China, while light fog days become more in the whole of eastern China. It is mainly because that when AO is in the positive phase, the pressure in the polar region decreases at 500 hPa; the pressure in East Asia increases anomalously; the East Asian trough decreases; and the low-level westerly jet moves northward, preventing the northwesterly cold air from moving southward. Therefore, the whole eastern China gets warmer and wetter air, and there are more light fog days with the enhanced water vapor. However, the atmosphere merely becomes more towards unstable in South China, where the precipitation increases but the heavy fog days decreases. Nevertheless, heavy fog days increase with the water vapor in North China because of moving towards a stable atmosphere, which is formed by the anomalous downdrafts north of the precipitation center in South China. When AO is in the negative phase, the situation is basically opposite to that in the positive phase, but the variations of the corresponding fog days and circulations are weaker than those in the AO-positive-phase winter, which may be related to the nonlinear effect of AO on climate.

Analysis of spatio-temporal distribution characteristics and fog forming conditions of Zhejiang coast in recent ten years

Based on the hydrometeorological observation data of Zhejiang coastal observation station from 2009 to 2018, the temporal and spatial distribution characteristics and fogging conditions of sea fog along the coast of Zhejiang Province were analyzed in this paper. The result shows that the average annual fog day along the coast of Zhejiang Province is 112 days according to the situation that the sea fog occurs as long as there is one station along the coast of Zhejiang Province. The fog season is from March to June and the sea fog occurs in the middle of the night and early in the morning. The spatial distribution of sea fog along the coast of Zhejiang Province is more in the middle of the two ends and the average annual fog days of each station are in the range of 19.2-82.0 days. Under the condition of fog, most of the relative humidity in Zhejiang coastal area is greater than or equal to 90%, the average temperature difference between gas and water is-0.5 - 2.1°C, and the direction of fog wind is mainly eastward. More than 93.6% of the sea fog occurs under the condition that the wind force is less than or equal to level 5. There are four main types of weather situation affecting the occurrence of sea fog in coastal areas of Zhejiang Province, including inverted trough type, low pressure sea exit type, weak cold air type and high-pressure sea entry type. When the continuous sea fog process occurs, the weather situation usually goes through the high-pressure sea entry type, the inverted trough type and the low pressure sea exit type. The wind direction will change with the variation of weather situation and the wind force will be less than or equal to 4 levels. When the wind is greater than level 5, the wind changes from west to southwest, and the sea fog tends to dissipate. Thick fog or strong fog occurs mainly in the middle of the night, early in the morning and around the evening.

哀牢山中山湿性常绿阔叶林木径向生长季节动态及其对气候因子的响应

PDF HTML阅读 XML下载 导出引用 引用提醒 哀牢山中山湿性常绿阔叶林木径向生长季节动态及其对气候因子的响应 DOI: 10.5846/stxb201901070057 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金面上项目（31770533）；国家自然科学基金国际（地区）合作项目（31861133007）；国家重点研发计划第五课题（2016YFC0502105） Intra-annual radial growth of evergreen and deciduous tree species and their response to climatic factors in a montane moist evergreen broad-leaved forest in the Ailao Mountains, Southwest China Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:研究采用树木生长环在哀牢山中山湿性常绿阔叶林持续9年（2009-2017年）监测了2个常绿树种（厚皮香，Ternstroemia gymnanthera；南亚枇杷，Eriobotrya bengalensis）和2个落叶树种（西桦，Betula alnoides；珍珠花，Lyonia ovalifolia）的树干月生长量，采用逻辑斯蒂生长模型（Logistic model）模拟树木径向生长量和物候参数，并分析了年、季尺度上径向生长与主要气候因子的关系。结果表明：1）4个树种年平均生长量为6.3 mm，落叶树种年平均生长量（10.6 mm/a）显著高于常绿树种（3.0 mm/a）；2）雨季（5-10月）是哀牢山中山湿性常绿阔叶林树木生长的主要时期，4个树种雨季平均生长量为5.9 mm，占全年总生长量的93%，其中落叶树种雨季生长量占全年的96%，而常绿树种雨季生长量占全年的86%；3）常绿树种生长季长度为169天，长于落叶树种（137天），而落叶树种最大生长速率（0.14 mm/d）显著高于常绿树种（0.03 mm/d），最大径向生长速率能很好地预测树种年生长量；4）低温、雾日和光合有效辐射是影响哀牢山亚热带常绿阔叶林4个研究树种径向生长的重要环境因子，其中温度对常绿树种径向生长具有显著影响，而雨日、雾日与空气湿度等水分因子对落叶树种径向生长更为重要。常绿树种年生长量对旱季气候因子的响应相比落叶树种更为敏感，树木旱季生长量除了受低温限制外，也受到水分供给的影响。气候变化可能改变不同物候类型树种在哀牢山中山湿性常绿阔叶林中的生长状态与分布格局。 Abstract:This study monitored intra-annual stem diameter growth of two evergreen (Ternstroemia gymnanthera, Eriobotrya bengalensis) and two deciduous tree species (Betula alnoides, Lyonia ovalifolia) for a 9-year period (2009-2017) in a montane moist evergreen broad-leaved forest in the Ailao Mountains, southwest China. The metal band-dendrometer technique was used to measure stem diameter. We modeled growth phenology (i.e., start day, stop day, growth duration, and annual growth) with a logistic model, and analyzed the associations between annual and seasonal growth and climate factors over the study period. The deciduous trees had an average annual growth rate of 10.6 mm over the study period, and the evergreen trees had an average annual growth rate of 3.0 mm. The average annual growth rate of all four species was 6.3 mm. On average, trees grew 5.9 mm during the rainy season, which accounted for nearly 93% of annual growth. Deciduous trees showed higher growth rates than evergreen trees during the rainy season. Compared with deciduous trees, evergreen trees started growth earlier and stopped growth later. Annual total growth of the four studied species were highly related to their maximum growth rates during the rainy season, but less correlated with the length of the growing season. Temperature was the main limiting factor for stem growth of evergreen tree species, while precipitation days, fog days, and relative humidity were more important for stem growth of deciduous tree species. The different growth-climate relationships between evergreen and deciduous tree species implies that ongoing climate change may alter tree growth rates and species composition of subtropical evergreen broad-leaved forests. 参考文献 相似文献 引证文献

Distinction of two kinds of haze

Haze is a phenomenon caused by the suspension of fine particulate matters in the air that leads to impaired visibility. In the meteorological practice, a haze day is determined using the observation of visibility and relative humidity, after screening the special weather days of rain, snow, dust, fog, and others. While in the environmental monitoring practice and relevant studies, the term haze is also commonly used and a haze day is determined using the reading of particulate matters (PM). This has actually caused two different kinds of haze days, i.e., meteorological haze days (MHDs) and environmental haze days (EHDs), each with different definitions and standards. Our analysis shows that although MHDs and EHDs overlap, the proportion of MHDs in EHDS is 43% in Beijing, 44% in Shanghai, and 46% in Guangzhou. Meanwhile, there is large percentage of MHDs that are not EHDs, about 21% in Beijing, 42% in Shanghai and 67% in Guangzhou. Therefore, an explicit distinction of the two haze definitions should be made in the study of PM pollution variation and cause in China, which will have important applications in addressing air pollution, formulating environmental policy, and dealing with the health effect of haze in China.

Climate Change and Its Attribution in Three Gorges Reservoir Area, China

Climate change in dam areas is one of the environmental problems associated with dams. However, the main factors and mechanisms that impact climate change in dam areas remain unclear. In this study, linear regression, the observed minus reanalyzed (OMR) method, and multi-source data are used to assess climate change in the Three Gorges Reservoir Area of China and investigate the main impact factors among the controversial factors (land cover change, environmental climate, and reservoir impoundment). Our results indicate that turning points of trend changes for annual fog days (FD), annual average temperature (T), and annual average relative humidity (RH) occurred at around 1996 during the period 1973–2013, and annual precipitation (PRE) suggested no obvious turning point. The change trends after 1996 were steeper than before 1996. These changes are mainly closely correlated with environmental climate. In particular, temperature was significantly correlated with environmental temperature (1979–2013: r = 0.799, p &lt; 0.01), and their relationship was stronger after 1990 (r = 0.842, p &lt; 0.01). Moreover, the turning point for FD, T, and RH also correlated with land use/cover change. In addition, reservoir impoundment showed an obvious humidification effect (OMR RH correlated with water area: r = 0.566, p &lt; 0.01). Our findings support the view that climate change in dam areas is mainly affected by environmental climate changes.

Influences of synoptic situation and teleconnections on fog‐water collection in the Mediterranean Iberian Peninsula, 2003–2012

AbstractFog‐water collection has been widely analysed for its quantification and potential uses; however, there are few studies assessing the synoptic conditions and large‐scale teleconnection patterns that affect its occurrence. Focusing on the Mediterranean Iberian Peninsula, this work aims to analyse the synoptic patterns, both at surface level and 850 hPa geopotential height, that most likely to favour fog‐water collection, and to quantify the relationship between fog‐water collection and the NAOi (North Atlantic Oscillation index), MOi (Mediterranean Oscillation index) as well as WeMOi (Western Mediterranean Oscillation index) teleconnection patterns. For this purpose, daily fog‐water observations from a dense network of 23 fog‐water collectors located along the Mediterranean Iberian Peninsula for 2003–2012 were analysed in relation to synoptic patterns and the three‐teleconnection indices. The major findings are: (a) The most favourable synoptic patterns for fog‐water collection are maritime advections carrying humidity from the Mediterranean basin, and cyclonic circulations, whereas anti‐cyclonic situations generally led to large number of foggy days with low fog‐collection rates. (b) In terms of winds at 850 hPa, the most favourable low‐level flows for fog‐water collection are associated with strong winds (&gt;5.1 m s−1) from the Mediterranean. Atlantic winds generally cause a greater number of fog days than Mediterranean winds, with less fog‐water collection rates. (c) WeMOi has the greatest influence on fog‐water collection, mainly during winter and spring months, with statistically significant negative relationships for most of the stations. MOi also shows a great influence, with a large number of statistically significant negative correlations, mainly during the same months as WeMOi. Lastly, NAOi presented the lowest and no significant negative correlations with fog‐water collection.

Investigating the effects of monthly weather variations on Connecticut freeway crashes from 2011 to 2015

IntroductionThe objective of this research is to investigate the effects of monthly weather conditions on traffic crash experience on freeways, considering the interactions between weather, traffic volumes, and roadway conditions. Methods: Data from the state of Connecticut from 2011to 2015 were used. Random parameters negative binomial models with first-order, autoregressive covariance were estimated for representative types of freeway crashes (front-to-rear, sideswipe-same-direction, and fixed-object), most severe crashes (i.e., fatal and injury crashes), and non-injury crashes (i.e., property-damage-only crashes). Results: Major findings are that variations in monthly traffic volumes, roadway geometry, and weather conditions explain much of the variations in monthly traffic crashes. Time effects exist in the panel monthly data for all types of crashes. Taking into account this effect improves model prediction results. When the raw weather measures are highly correlated, using dimension reduction techniques helps to extract more interpretable weather factors. By considering the interaction effects between roadway condition variables, additional findings were found. In general, lower temperature, more heavy fog days, decreased precipitation, lower wind speed, higher monthly traffic volumes, and narrower inside shoulder were found to be associated with higher monthly crashes. The effects of area type and outside shoulder width change dramatically as the number of through lanes changes. Practical applications: The findings of this research could help researchers and general readers gain a better understanding of the effects of monthly weather conditions and other roadway factors on freeway crashes and give engineers practical guidelines on improving freeway safety.

Visibility Data Assimilation and Prediction Using an Observation Network in South Korea

There have been many studies to improve visibility forecast skills using numerical models, but their performance still remains behind the forecast skills for other meteorological phenomena. This study attempted to improve visibility forecasts using a newly established automatic visibility observation network composed of 291 forward-scattering sensors in South Korea. In the analysis of recent 3-year visibility observations, clear days (visibility above 20 km) were reported for 46% of the days, and fog cases (visibility less than 1 km) accounted for 1.58% of the total observations. The Very short-range Data Assimilation and Prediction System (VDAPS) of the Korea Meteorological Administration (KMA) assimilated the visibility observations based on the Met Office Unified Model with visibility data assimilation of Clark et al. (Q J R Meteorol Soc 134:1801–1816, 2008). Prior to the data assimilation, a precipitation check eliminated visibility data with precipitation (9.4% in total, 23% for visibility less than 1 km), and a consistency check removed visibility observations that were inappropriate to relative humidity, temperature, and pressure. In a case study on two consecutive fog days, visibility forecast skills were improved by applying visibility data assimilation, mostly through modifications of aerosol concentrations. A 3-month model run in the winter of 2016 showed a positive bias in visibility predictions, especially for the low-visibility cases. Visibility data assimilation improved the prediction skills, but the positive effects were limited within 9 forecast hours and were smaller for extremely low-visibility events. Sensitivity experiments were performed using local aerosol observations with a larger number of smaller aerosol particles. Modifications in aerosol properties made better results in frequency bias for the whole forecast ranges and also improved the equitable threat score (ETS) for relatively longer forecast hours (more than 4 h).

A new method to determine the aerosol optical properties from multiple-wavelength O&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; absorptions by MAX-DOAS observation

Abstract. Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) observation was carried out from November 2016 to February 2017 in Beijing, China, to measure the O4 absorptions in UV and visible bands and further to illustrate its relationship with aerosol optical properties (AOPs) under different weather types. According to relative humidity, visibility, and PM2.5, we classified the observation periods into clear, light-haze, haze, heavy-haze, fog, and rainy weather conditions. There are obvious differences for measured AOPs under different weather conditions, especially scattering coefficient (σsca) and absorption coefficient (σsca). It was also found that both the O4 differential slant column densities (DSCDs) at the UV and visible bands varied in the order of clear days &gt; light-haze days &gt; haze days &gt; heavy-haze days &gt; fog days. The correlation coefficients (R2) between O4 DSCDs at 360.8 and 477.1 nm mainly varied in the order of clear days &gt; light-haze days &gt; haze days &gt; heavy-haze days. Based on the statistics of O4 DSCDs at an elevation angle 1∘ with the corresponding linear regression between UV and visible bands of segmental periods, the relationships between O4 DSCDs and AOPs were established. It should mainly be clear or light-haze days when the correlation slope is greater than 1.0, with a correlation coefficient (R2) greater than 0.9, and O4 DSCDs mainly greater than 2.5×1043 molec. cm−2. Meanwhile, σsca and σabs are less than 45 and 12 Mm−1, respectively. For haze or heavy-haze days, the correlation slope is less than 0.6, with an R2 less than 0.8, and O4 DSCDs mainly less than 1.3×1043 molec. cm−2, under which σsca and σabs are mainly located at 200–900 and 20–60 Mm−1. Additionally, the determination method was well validated based on another MAX-DOAS measurement at Gucheng from 19 to 27 November 2016. For more precise and accurate inversion of AOPs, more detailed look-up tables for O4 multiple-wavelength absorptions need to be developed. Since the ground surface AOPs were determined using MAX-DOAS observation at a 1∘ elevation in this study, we hope to highlight the potential of retrieved vertical spatially resolved AOPs being expected when multiple elevation angles of MAX-DOAS observation are used together.

Vegetation and climate change in the Pro-Namib and Namib Desert based on repeat photography: Insights into climate trends

This paper presents empirical evidence of historical vegetation and climate change in the arid Pro-Namib and hyper-arid Namib Desert spanning the late 19th century to the present based on one hundred archival landscape photographs that have been re-photographed or ‘matched’. Each photo site was evaluated for changes in woody cover and taken together serve as a proxy for how climate has changed in the region. Vegetation change was related to values for precipitation and temperature derived from the Global Land Data Assimilation System for the period 1948–2017 as well as the number of fog days expected at a site. The resulting analysis reveals a trend of increased vegetation cover associated with increased precipitation (fog and rain) in the coastal Fogbelt and the inland Savanna transition with a shrinking of the hyper-arid Minimum zone between the two. These findings accord with projected effects of global warming on the Benguela upwelling system but are at variance with regional climate model forecasts that project widespread aridification. In the absence of long-term climate data, the results of this research are an important contribution of evidence-based knowledge of past climate trends and their relationship to future climate change scenarios for the region.

Long‐Term Fog Variation and Its Impact Factors Over Polluted Regions of East China

AbstractDuring the past five decades, China has witnessed frequent fog and haze events. Previous studies have revealed that in polluted regions of East China, such as the Beijing‐Tianjin‐Hebei (BTH) region, the Yangtze River Delta (YRD) region, and the Pearl River Delta (PRD) region, fog frequencies are impacted not only by climate change but also by fast urbanization and increasing air pollution. In this study, we find that haze days increased markedly in the three regions. However, fog days showed an initially increasing and then decreasing trend, with the turning point in the 1980s; moreover, the turning point occurred 5–15 years earlier in large cities than in small cities. We further attribute the long‐term fog variations over East China to three factors: regional‐scale climate change, urbanization, and air pollution (aerosol pollution). We quantitatively show that urbanization and aerosol pollution contributed at least 1.6 times more than climate change, and their contributions varied with urban development stages. Fog was dominantly promoted by abundant aerosols (45–85%) during the slow urbanization stage, while it was dominantly inhibited by urban heat island and dry island (53–60%) effects during the rapid urbanization stage. In the 53‐year period, aerosol pollution promoted fog formation (20–40%).

Impacts of urban expansion on fog types in Shanghai, China: Numerical experiments by WRF model

Fog is a hazard to transportation activities in Shanghai, China, but it is not known how this fog is influenced by urban expansion. Here we use a numerical model to run, for the first time for Shanghai, sensitivity experiments of the fog response to urban expansion, including the changes of land use and anthropogenic heat. Instead of using ‘fog days’ as a measure, we use the 29-year (1989–2017) meteorological observations of fog events at Hongqiao International Airport in central Shanghai, and classify the fog into radiation, advection, advection–radiation, and precipitation types. The results show that (1) Fog events generally decrease over these 29 years, with the decline in winter accounting for 50.2% of the total reduction. (2) Radiation fog decreases the most, but remains the most common type throughout the period. (3) Numerical sensitivity experiments show that the urban expansion in the past 29 years caused both an increase in surface air temperature and a decrease in water-vapor mixing ratio, resulting in a decrease in relative humidity and an increase in visibility for radiation fog. (4) For advection fog, the increased surface air temperature allowed an increase in water-vapor mixing ratio, but a decrease in liquid water. (5) Due to warmer near-surface air, the inversion layer weakened. Hence, urban expansion in Shanghai has reduced the amount of not only radiation fog, but also advection fog.

Widespread fog over the Indo-Gangetic Plains and possible links to boreal winter teleconnections

Boreal wintertime planetary-scale atmospheric circulations and their possible consequences to widespread fog occurrences over the Indo-Gangetic Plains (IGP) region of the Himalayan valley are investigated in this study. Among the different fog types, radiation fog type seen at night or early morning hours favored by large-scale subsidence aloft and strong near-surface inversion is focused in this study. A composite analysis reveals that upper air circulation associated with 105 fog days over the IGP region show a trail linked to circulation anomalies over the Eurasian continents and the Arctic Circle. The findings suggest that there is a footprint of the Arctic Oscillation (AO) and conventional Eurasian (EU) circulation patterns linked to anticyclonic circulation aloft over the IGP region. Although widespread IGP fog occurrences under the large-scale subsidence environment are seen to occur during both phases of AO, the negative AO phase (high pressure environment over the Arctic Circle) portends a greater likelihood for fog occurrences in the IGP region. A coupling of positive mid-tropospheric height anomalies over western Eurasia and the anticyclonic circulation anomalies over the IGP region is evident during the IGP fog periods concomitant with EU positive (height excess over Siberia) phase. Further, anomalous circulation over the IGP region during the fog periods appears to rely more on the strength of the AO negative phase than the circulation strengths over Eurasia. On the contrary, the Eurasian circulation largely appears to influence the subsidence aloft over the IGP region irrespective of the strength of the AO positive phase. It is also noted that upper-air circulation during non-foggy periods over the IGP region has conformity with positive AO phase and rapidly progressing EU pattern. These planetary-scale teleconnection pathways offer new dynamical insights into comprehending widespread IGP fog scenario, which have been hitherto perceived mostly from a regional context.

Influence of ENSO on frequency of wintertime fog days in Eastern China

The influence of El Nino–Southern Oscillation (ENSO) on wintertime fog days in eastern China was investigated through the analysis of fog station data in China and the National Centers for Environmental Prediction reanalysis data during the period of 1954–2007. A composite analysis shows distinctive anomaly patterns of fog days in early and late winter during ENSO. More frequent fogs appear in Central and North China (around 30°–40°N) in November–December (ND), while more frequent fogs occur over Middle Reach of Yangtze River (27°–32°N) and less frequent fogs appear over 32°–40°N in January–February (JF) during El Nino. The cause of this distinctive sub-seasonal evolution feature is discussed as the following. Actually we analyzed all meteorological fields during El Nino and La Nina, and found that they are approximately in a mirror image. To reduce the number of figures, we presented the difference fields in most of cases. We found that anomalous southwesterlies associated with El Nino penetrate into North China (up to 40°N) in ND. The northward moisture transport leads to near-surface air saturation over the region. Moreover, atmosphere becomes convectively unstable in South China (20°–30°N), which leads to a positive precipitation anomaly there. The local convergence leads to an anomalous divergence and subsidence north of 30°N. Both the saturated air and stable stratification lead to more frequent fog days in 30°–40°N. Compared to ND, anomalous southwesterly flow in JF is weaker, and can only transport moisture up to Central China (near 30°N). As maximum precipitation anomaly is confined near the southeastern coast of China, anomalous divergence and subsidence appear north of 27°N. This stable stratification together with the anomalous moisture transport causes the increase of fog over Central China but decrease of fog in North China (33°–38°N) due to the subsidence induced dryness.

Factors affecting relative humidity and its relationship with the long-term variation of fog-haze events in the Yangtze River Delta

Relative humidity (RH) is one of the most important parameters in the study of fog-haze. This paper first estimates the contributions of the key quantities (temperature, water vapor, air pollutant) and their combinations to the relative change of RH, and then investigates the relationships of RH with the long-term variation of haze and fog days based on the meteorological data in the Yangtze River Delta over the period 1970–2010. The main conclusions are as follows. (1) Temperature is the foremost factor influencing RH, with the effect of specific humidity and direct contribution of air pollutant being second and third. (2) RH shows a prominent descending trend in the Yangtze River Delta, due to global warming and the ‘analogous heat island effect’ (AHIE). (3) Decreasing RH was responsible for the reduction of fog days. The AHIE can explain the phenomenon in which fog days in metropolises are less than in small cities. (4) Granger causality analysis of inter-annual variation further shows that increasing aerosol loading Ganger-causes the increasing haze days, rather than meteorological parameters such as RH. High RH would enhance hygroscopic growth of particle and then suppress the planet boundary layer height (PBLH) and lead to more haze days, lower PBLH further increases aerosols and RH, this positive feedback mechanism can be established under the condition of aerosol loading being relatively stable and beyond the threshold for haze formation. (5) If aerosol emissions maintain the status quo, climate cooling would result in serious fog-haze events occurring more frequently in Yangtze River Delta. The research provide a scientific basis for understanding the influence of meteorological factors on RH and the connection between the variation of long term fog-haze days and RH under the background of climate change.

Characterization of submicron aerosols and CCN over the Yellow Sea measured onboard the Gisang 1 research vessel using the positive matrix factorization analysis method

Aerosol size distributions and cloud condensation nuclei (CCN) number concentrations were measured in spring 2017 over the Yellow Sea on board the research vessel Gisang 1. The average number concentration of particles larger than 10 nm and CCN at 0.65% supersaturation (S) were 7622 ± 4038 cm−3 and 4821 ± 1763 cm−3, respectively. Characteristics of aerosol size distribution data were analyzed using a positive matrix factorization (PMF) method. It was found that only 6 Factors could explain the aerosol size distribution reasonably well. Factors 1 and 2 indicated nucleation mode particles, Factor 3 indicated Aitken mode particles, and Factors 4, 5, and 6 indicated accumulation mode particles. The concentrations of nucleation and Aitken mode particles showed a clear wind direction dependence; high under westerly winds due to the high concentrations of particles and precursor gases in eastern China. Meanwhile, the concentration of larger particles and CCN showed no significant wind direction dependence. Aerosol size distribution was also significantly influenced by meteorology. Small particles were predominant during clear days. In contrast during mist or fog days, the aerosol size distribution shifted to larger sizes. A CCN closure experiment was conducted using results of the PMF analysis. The assumption of internally mixed particles led to overestimation of predicted CCN concentrations but agreement was significantly better when externally mixed particles were considered. The logarithmic curve fit of NCCN(S) = 4825 ∗ log S + 4933 was found to very well explain measured CCN concentrations at a few S over the Yellow Sea, and therefore is recommended as input CCN spectral data for numerical models that explicitly treat CCN activation.

Semivolatile Organic Compounds (SOCs) in Fine Particulate Matter (PM2.5) during Clear, Fog, and Haze Episodes in Winter in Beijing, China

Few efforts have been made to elucidate the influence of weather conditions on the fate of semivolatile organic compounds (SOCs). Here, daily fine particulate matter (PM2.5) during clear, haze, and fog episodes collected in the winter in Beijing, China was analyzed for polycyclic aromatic hydrocarbons (PAHs), brominated flame retardants (BFRs), and organophosphate flame retardants (OPFRs). The total concentrations of PAHs, OPFRs, and BFRs had medians of 45.1 ng/m3 and 1347 and 46.7 pg/m3, respectively. The temporal pattern for PAH concentrations was largely dependent on coal combustion for residential heating. OPFR compositions that change during colder period were related to enhanced indoor emissions due to heating. The mean concentrations of SOCs during haze and fog days were 2-10 times higher than those during clear days. We found that BFRs with lower octanol and air partition coefficients tended to increase during haze and fog episodes, be removed from PM2.5 during clear episodes, or both. For PAHs and OPFRs, pollutants that are more recalcitrant to degradation were prone to accumulate during haze and fog days. The potential source contribution function (PSCF) model indicated that southern and eastern cities were major source regions of SOCs at this site.