Fog Persistence Research Articles

Case Study of Coastal Fog Events in Senegal Using LIDAR Ceilometer

This study aims to examine the atmospheric conditions characterising fog phenomena on the Senegalese coast focusing on two specific instances that occurred on April 3 and April 30, 2023. These events were detected by the LIDAR Ceilometer installed at LPAOSF/ESP/UCAD and confirmed on the METARs of the meteorological stations at Dakar and Diass airports. The LIDAR's backscatter signal showed that the fog of April 3 started around midnight with a vertical extension at 100 m altitude and dissipated around 10 a.m. The April 30 event characterized by a good vertical extension from the surface up to 300 m above sea level, was triggered just after 2 a.m. and lasted around 3 hours. The results showed that a decrease in temperature, accompanied by an increase in humidity and light wind, is favorable for the triggering and persistence of fog. Sea Level Pressure (SLP) anomaly fields show two distinct configurations. The April 3 event was characterized by a zonal dipole of SLP anomalies between the Sahara and the northern Senegalese coast, while the April 30 event was characterized by a meridional dipole between the Sahara and the Gulf of Guinea area as far as the equatorial Atlantic. A weakening of the pressure around the study area was observed in both cases, allowing moisture advection to favor the onset of fog. The hovmoller diagrams of relative humidity and wind show that a good vertical extension of humidity associated with a westerly wind in the lower layers plays an important role in the formation and persistence of fog. The presence of dry air associated with a weak easterly wind in the middle layers could explain the low vertical extension of the fog on April 3. A strong wind in the lower layers would be responsible for the premature dissipation of the April 30 fog.

Developing a long-term high-resolution winter fog climatology over south Asia using satellite observations from 2002 to 2020

The vast Indo-Gangetic Plains (IGP) south of the Himalaya are subject to dense fog every year during winter months (December–January), severely disrupting rail, air and public transport of millions of people living in northern India, Pakistan, Nepal and Bangladesh. Air pollution combined with high moisture availability in the shallow boundary layer, are important factors affecting the persistence and widespread nature of fog over the IGP. Despite the environmental significance and impacts on the public at-large, an in depth understanding of the long-term spatial-temporal distribution of the south Asian fog, is presently not available in the literature. We utilize infrared remote sensing techniques to develop a high-resolution (≈1 km × 1 km) fog/low-stratus detection climatology over the past two decades (2002−2020), using Aqua/MODIS satellite observations. A dynamic brightness temperature difference threshold (involving 3.96 μm and 11.03 μm bands) for nighttime fog/low-stratus detection is constructed based on systematic radiative transfer simulations involving cloud effective radius, cloud top height, liquid water path and satellite viewing geometry. Our satellite-based fog/low-stratus detection is consistent with theoretical simulations of fog characterization and is also found to be well-correlated with near-surface visibility observations of dense fog (r = 0.89, p-value << 0.01). We also provide satellite-derived nighttime estimates of fog/low-cloud effective radius that are comparable to the operational daytime MODIS cloud data product and limited in situ observations. In terms of fog/low-stratus frequency, the IGP is relatively uniformly covered by widespread fog/low-stratus occurrences with the largest frequency found in the low-lying Terai region, bordering India and Nepal, which is also consistently observed in our daytime fog/low-stratus detection results over the last two decades. Additionally, the interannual variations in fog/low-stratus occurrences track closely with that of relative humidity in the IGP, which is associated with shallow boundary layer conditions during winter-time favoring the formation and persistence of fog. Overall, these long-term satellite-derived results present new high-resolution data and insights into the dense and often intense winter fog occurrences which routinely engulf the entire stretch of the Indo-Gangetic Plains and cause significant degradation to ground visibility in one of the world's most densely populated regions.

Weakened Baroclinic Activity Causes an Abrupt Rise in Fog in the Indo‐Gangetic Plain

AbstractDense and widespread winter fog in the Indo‐Gangetic Plain (IGP) causes a significant reduction in visibility and worsened air pollution. Despite extensive observational campaigns, the key processes in the formation and persistence of fog are not yet clear. The global increase in surface temperatures led to a decline in the incidence of fog, whereas the IGP experienced a sudden increase in fog incidence. It is shown here that abruptly reduced activity of the western disturbances (WD) around 1996–1997 resulted in a sudden decline of the cloud cover in the region, with a concomitantly enhanced radiative cooling of the surface and the atmosphere. These abrupt changes, aided by the impacts of human activities, altered the dynamics and thermodynamics of the boundary layer to favor fog formation. Also, the critical role of the extensive deep subsidence in the modulation of stable stratification and turbulence, which are essential for fog formation, is elucidated.

Seasonal dynamics in photosynthesis of woody plants at the northern limit of Asian tropics: potential role of fog in maintaining tropical rainforests and agriculture in Southwest China.

The lowland tropical rainforests in Xishuangbanna, Southwest (SW) China, mark the northern limit of Asian tropics. Fog has been hypothesized to play a role in maintaining rainforests and tropical crop production in this region, but the physiological mechanism has not been studied. The goals of this study were to characterize the seasonal dynamics in photosynthesis and to assess the potential for fog to mitigate chilling-induced photodamage for tropical trees and crops in Xishuangbanna. We measured seasonal dynamics in light-saturated net photosynthetic rate (Aa), stomatal conductance (gs), intercellular CO2 concentration, quantum yield of Photosystem II (Fv/Fm) and maximum P700 changes (Pm; indicates the amount of active PSI complex), as well as chilling resistance and fog (light/shading) effects on low temperature-induced decline in Fv/Fm and Pm for native tree and introduced lower latitude tree or woody shrub species grown in a tropical botanical garden. Despite significant decreases in Aa, gs, Pm and Fv/Fm, most species maintained considerably high Aa during the cool season (2.51-14.6 μmol m(-2) s(-1)). Shaded leaves exposed to seasonal low temperatures had higher Fv/Fm than sun-exposed leaves in the cool season. All species could tolerate 1.4 °C in the dark, whereas a combined treatment of low temperature and high light caused a distinctly faster decline in Pm and Fv/Fm compared with low temperature treatment alone. Because fog persistence avoids or shortens the duration of high light condition in the morning when the temperatures are still low, our results provide support for the hypothesis that fog reduces chilling damage to tropical plants in this region and thus plays a role in maintaining tropical rainforests and agriculture in SW China.

Enhanced persistence of fog under illumination for carbon nanotube fog condensation nuclei

Black carbon (BC) emissions have increased over South Asia and more persistent winter fogs are common during last decade. However, a physical mechanism connecting these two is still unknown. A significant fraction of BC is known to be in the form of carbon-nanotube (CNT). We have investigated the effects of two different sets of fog condensation nuclei, viz., inorganic salts (sodium chloride and ammonium sulphate) and BC (graphite and CNT) on the dissipation of chamber generated fog under dark and halogen lamp illuminated conditions at a controlled temperature, relative humidity, and fog condensation nuclei distribution. We found that, for inorganic salts, fog dissipates at a faster rate under illumination than in dark conditions showing similar trends for both sodium chloride and ammonium chloride. On the contrary, fog is found to be more persistent, for illuminated conditions, in the case of BC. Moreover, fog persistence is found to be more than twice for CNT compared to graphite in illuminated conditions. CNT is known to be an extremely effective absorber of light which leads to microscopic heating and subsequent evaporation which inhibits droplet growth. Small droplets have large residence time leading to longer fog persistence. This hitherto unknown mechanism may be responsible for persistent fog in India during last decade.

Night time fog detection using MODIS data over Northern India

AbstractAn algorithm for night time fog detection using satellite data was used to study 2009 and 2010 fog episodes over the North Indian plains. The algorithm employs a bispectral thresholding technique involving brightness temperature difference (BTD) between two spectral channels: 3.9 and 10.75 µm, combining radiative transfer simulations and satellite data. The prolonged fog episode of January 2010 was analysed in detail using this algorithm in conjunction with the meteorological parameters and it was found that upper tropospheric long waves passed over this region during the fog episode. These waves affect the circulation close to the surface and appear to have played an important role in the formation and persistence of fog during 2010. The present thresholding method can be used to monitor night time fog over the Indian subcontinent on an operational basis using the forthcoming geostationary satellite INSAT‐3D. Copyright © 2011 Royal Meteorological Society

Canopy level fog occurrence in a tropical lowland forest of French Guiana as a prerequisite for high epiphyte diversity

Fog frequency and the meteorological processes leading to fog formation have never been studied in depth in tropical lowland forest areas. This study provides detailed evidence of frequent fog occurrence in lowland valleys of central French Guiana. Fog frequency showed a clear diurnal course, with a maximum before sunrise; average fog duration was 4.6 h. The diurnal course of visibility was positively correlated with the diurnal course of humidity in the above-canopy air. Fog persistence correlated significantly with atmospheric parameters during the dry season, but not during the rainy season. The main trigger of fog development in the lowland forest seemed to be precipitation, leading to higher soil moisture, greater evapotranspiration and, thus, higher water content of air. An increasing temperature difference between valley and hill sites after sunset, together with more frequent down-slope winds during nights with long fog periods, points at some influence of katabatic flows. The frequent occurrence of fog in the valleys correlated with significantly higher epiphyte diversities in valley forests as compared to hill forests, and supported the occurrence of the hitherto undescribed, epiphyte-rich “tropical lowland cloud forest” (LCF) in the valleys. The higher epiphyte diversity in LCF coincided with significantly higher relative air humidity in LCF than in hill forest. The ecological benefits of fog for the epiphytes in LCF are surplus of moisture and delayed onset of the stress period, particularly in the dry season.

Modeling and Forecasting the Onset and Duration of Severe Radiation Fog under Frost Conditions

AbstractA case of a severe radiation fog during frost conditions is analyzed as a benchmark for the development of a very high-resolution NWP model. Results by the Weather Research and Forecasting model (WRF) and the High-Resolution Limited-Area Model (HIRLAM) are evaluated against detailed observations to determine the state-of-the-art in fog forecasting and to derive requirements for further research and development. For this particular difficult case, WRF is unable to correctly simulate the fog for any of the parameterizations and model configurations utilized. Contrary, HIRLAM does model the onset of fog, but is unable to represent it beyond the lowest model layer, which leads to an early dispersal of fog in the morning transition. The sensitivity of fog forecasts to model formulation is further analyzed with a high-resolution single-column version of HIRLAM, and with the Duynkerke single-column model as a reference. The single-column results are found to be sensitive to the proper specification of the external forcings. It is reconfirmed that high vertical resolution is essential for modeling the fog formation, the growth of the fog layer, and when the fog lifts for the maintenance of a stratus deck. The properly configured column models are able to accurately model the onset of fog and its maturation, but fail in the simulation of fog persistence and subsequent dispersal. Details of the turbulence parameterization appear to be important in this process. It is concluded that, despite all of the advances in numerical weather prediction, fog forecasting is still a major challenge.

The boundary layer characteristics in the heavy fog formation process over Beijing and its adjacent areas

By utilizing the Atmospheric Boundary Layer (ABL) observational data made available from the project “973” under the auspices of the Ministry of Science and Technology of the People’s Republic of China – entitled the Beijing City Air Pollution Observation Field Experiment (BECAPEX), including the measurements by a wind profiler, captive airships, tower-based boundary layer wind and temperature gradient observational instruments (ultrasonic anemometers and electronic thermometers), air composition samplers, conventional upper-air, surface and Automatic Weather Stations (AWS) observations, this paper herewith analyzes, in a comprehensive manner, the occurrence of a heavy fog event over Beijing in February 2001, including its formation, development, persistence, dynamic and thermodynamic features as well as evolving stratification structures within the boundary layer at different stages. The results suggested: (i) as a typical case of urban heavy fog, before the fog onset over Beijing, a temperature inversion existed in the lower atmosphere, the smokes and the pollutants like SO 2 and NO 2 had been accumulated at a lower level. Proceeding the fog event, with the increase of SO 2 and NO 2 concentrations, condensability increased sharply. On the contrary, during the fog process, with increasing condensability, SO 2 and NO 2 concentrations decreased. This indicated that, acting as condensation nucleus, these accumulated pollutants were playing a key role in catalyzing the fog condensation. (ii) By analyzing mean gradient-, pulsation- and turbulence-distribution patterns derived from the wind measurements taken by the aforementioned tower-based instruments, they all indicated that about 10 hours before the fog onset, a signal foretelling potential strong disturbances in the lower boundary layer was detected, and a significant rise of both mean and disturbance kinetic energies was observed, revealing that the low-level wind shear was strengthened before the fog onset, consequently creating a favorable condition for the outbreak of turbulences. This strong signal seemed to be very meaningful in monitoring and predicting fog occurrence and its development. (iii) Once the fog was in shape, its condensation feedback effects tended to lift the height of temperature inversion layer within the mid and upper levels of the lower atmosphere, which in return determined the fog persistence and restructuring process.

On the persistence of fog over northern parts of India

In winter there is a spell of about a fortnight, during end of December and beginning of January when persistent fog over plains of northern states of India occurs every year almost regularly . In this paper authors have tried to find the reasons of persistence of this fog particularly over this area. The reason envisaged here is in the large scale circulation . This diagnosis is for making it a forecasting tool and the study is based on satellite observed cloud imagery received in the VIS and water vapour channels and the derived winds.

A satellite-based climatology of fog and low-level stratus in Germany and adjacent areas

A 10-year fog climatology of Germany and adjacent areas, based on NOAA-Advanced Very High-Resolution Radiometer (AVHRR) data, is presented. Maximum fog frequency is obtained for the Lake Constance area; minimum fog occurrence, accounting for basins and river valleys, is located in the Lower Rhine area. This is a transition zone between coastal advection fog and the more continental radiation fog of the lower mountain range. Fog persistence is estimated by a comparison of night and day maps. Using the bispectral approach of Nakajima and Nakajima [J. Atmos. Sci. 52, (1995) 4043], the potential of weather satellite data for climatologies of fog's optical and microphysical properties is examined. Maps of fog optical depth, effective radius and liquid water path (LWP) are computed from well-illuminated noon images and the resulting data set is statistically evaluated. Typical features of advection fog are obtained for the coastal areas of the North and Baltic Seas, while the fog of the pre-Alpine basins reveals characteristics of continental radiation fog. The results are in general accordance with field observations of various sources.

Tracking weather's flight path

Prototypes of systems for helping pilots and air traffic controllers cope better with untoward weather are jockeying for a place aboard critical test flights and in air traffic control and dispatch facilities on the ground. The prototypes are the products of a burst of research in technologies for detecting turbulence in all its varieties, predicting the persistence of fog, and giving pilots a clear picture of the weather ahead. They, along with systems tested last winter for detecting and predicting aircraft icing, will begin impacting commercial flights soon, although few will come into wide use before mid-decade. But all will make flying safer-and a lot more comfortable. Some 30 percent of commercial aircraft accidents have weather as a contributing factor, according to a NASA planning group. The paper discusses turbulence and its detection, the high hopes for LIDAR, tackling the problem of wakes, detection of rain, and icing factors.

Orthotrichum handiense, sp. nov. from Fuerteventura, Canary Islands

A new epiphytic moss, Orthotrichum handiense F. Lara, Garilleti, & Mazimpaka, is described from the Jandia Peninsula (Fuerteventura, Canary Islands, Spain). The new species is included in section Pulchella Vent. and characterized by robustness, lanceolate leaves with recurved margins, immersed capsules, stomata cryptopore located in the neck, well developed prostome, peristome of 16 teeth and 16 segments, and calyptra and vaginula densely covered with slightly papillose hairs. Drawings and SEM micrographs of the main characters are given, and possible confusion with other taxa is discussed. During spring 1992, one of the authors (M. Brugu6s) collected bryophytes in the mountainous area of the Jandfa Peninsula, from the extreme south of Fuerteventura Island, in the eastern part of the Canarian Archipelago. One of the epiphytic samples collected from the beautiful shrub Asteriscus sericeus (L. fil.) DC. corresponds to the moss described here, a robust Orthotrichum that has an exclusive set of characters differentiating it easily from any other member of the genus. The Jandfa Peninsula that gives its name to the new moss, is an area that comprises a phanerogamic flora rich in local endemics; namely, the Zarza Peak (the highest peak) is the locus classicus of, among others, Bupleurum handiense Bolle, Echium handiense Svent., Ononis christii Bolle, and Argyranthemum winteri (Svent.) Humphries. Asteriscus sericeus itself is endemic to the isle, and the most characteristic plant of the Jandia summits, where it forms the dominant scrub. Its occurrence indicates the potential area of the ancient lauroid wood, which is nowadays represented by residual, small trees like Visnea mocanera. L. fil., Maytenus canariensis (Loes.) Kunkel & Sund., and Apollonias barbujana (Cav.) Bornm. The occurrence of lauroid wood (a xeric variant of the Canarian laurisilva), as well as that of the epiphytes that grow on the shrubs, is made possible by fog persistence and horizontal precipitation owing to trade winds, however the annual rainfall average is very low around 250-400 mm (Santos, pers. comm.). At present, the Jandfa peninsula is considered a natural area under protection of the Canarian government, but it is actually subjected to strong and uncontrolled grazing pressure, mainly by goats. Only two Orthotrichum species are known from Fuerteventura O. affine Brid. and O. diaphanum Brid.; both reported by Dirkse et al. (1993) on the basis of Malme's (1988) original records. Both species were found occurring on Asteriscus sericeus, but our gatherings from this shrub at Zarza peak contained only O. diaphanum and the moss described as new here. ORTHOTRICHUM HANDIENSE F. Lara, Garilleti & Mazimpaka, sp. nov. FIGS. 1-15 Planta robusta, ramosa, filidiis lanceolatis acutis vel acuminatis, marginibus variabiliter recurvatis cellulibusque cum papillis humilibus. Capsulae ellipticae, immersae vel vix emergentes, minute sulcatae, stomatis cryptoporis in collo sitis; prostoma praesens, exostoma 16 dentibus longis papilloso-striatisque, endostoma 16 segmentis linearibus, longis papillosisque, membrana connectivali continua; vaginula et calyptra abundantibus pilis longis paulo papillosisque obtectae. TYPE: SPAIN, CANARY ISLANDS. FUERTEVENTURA. Pico de la Zarza, 760 m elev., UTM grid 28RES60, epiphyte on Asteriscus sericeus. M. Bruguds, May 1992. (BCB, holotype; MA and Universidad Aut6noma de Madrid herbarium isotypes). Plants robust, 1-2 cm tall, foliose and branched, olive-green or brown, with rhizoids restricted to lower part, in 3 cm cushions. Leaves erect to erectpatent when dry, upper ones with somewhat flexuose points; erect-patent to patent when moist; 3.00007-2745/99/619-622$0.55/0 This content downloaded from 157.55.39.249 on Wed, 03 Aug 2016 05:55:55 UTC All use subject to http://about.jstor.org/terms 620 THE BRYOLOGIST [VOL. 102

Use of satellite images for fog detection (AVHRR) and forecast of fog dissipation (METEOSAT) over lowland Thessalia, Hellas

In this work, both AVHRR and METEOSAT data are used in order to detect fog areas and to predict fog dissipation. Since fog is created during the night, AVHRR data is used for fog detection at night and METEOSAT data is used for fog observation and fog dissipation forecast during the day over a specific area of Greece, lowland Thessalia. For fog detection at night only the three infrared AVHRR channels are used. The difference between channel 4 and channel 3 is used to distinguish fog or low stratus from land and sea. A cloud classification method applied in this work is based on multispectral box classification. Classification classes are fog and low clouds, sea, land, middle and high clouds. There is also a class for unclassified pixels. For fog dissipation detection, the visible channel of METEOSAT is used, normalized according to the vertical Sun viewing angle. Fog persistence after the first morning calculations is examined versus fog duration. Also, the percentage of the same area affected by fog in the following hours is examined versus fog duration. A case study is presented and the results of the method are discussed.

Environmental appraisal-particulate matter, oxides of sulfur, and sulfuric acid.

The role of particulate matter and oxides of sulfur in degradation of the atmospheric environment is discussed. The Federal Air Quality Criteria for these pollutants are analyzed for their conformance with the requirement of the Clean Air Act of 1967 that they reflect the latest scientific knowledge pertinent to the indication of their effects on health and welfare. Visibility reduction by suspended particulate matter is treated and the bases for the criteria issued in the documents “Air Quality Criteria for Particulate Matter” and “Air Quality Criteria for Sulfur Oxides” are examined. The reactions between particulate matter and gaseous pollutants are discussed, along with the Impact of particulate matter on modifications of weather processes. Local effects, such as precipitation, are considered. The relationship between pollution by particulate matter and cloud formation is discussed, as are persistence of fog and the observation that certain sources of particulate pollution are also sources of ice nuclei.