Fog Dissipation Research Articles

Surface-to-Atmosphere Exchange in a River Valley Environment

AbstractObservations of the turbulent exchange between a river surface and the atmosphere in a mountainous area in southern Brazil are presented and discussed. A micrometeorological tower was installed directly above the surface of a 60-m-wide river. This paper describes the observed turbulent fluxes over 12 days of observations at this site. Eddy correlation sensible and latent heat fluxes are directed toward the river during daytime and from the river at night, and they are controlled by differences between water and air temperatures. The magnitude of the vertical fluxes between the river and the atmosphere increases during daytime with increasing temperature gradient up to a threshold, beyond which the increasing stability starts to dampen the fluxes. Water and air temperatures show very little variations across the width of the river, indicating that the measurements taken at one margin may be representative of the mean river exchange. Local scalar budgets show that daytime warming and moistening rates above the river are controlled by local transport from the riverbanks. The main vertical fluxes have a very small magnitude: 0.8 W m−2 for sensible heat and 1.1 W m−2 for latent heat. Events of very large sensible heat fluxes from the river to the atmosphere and very large latent heat fluxes from the atmosphere to the river happened on 3 days, following nights with a very deep fog layer in the valley. These events represented the passage of a warm and dry air mass down the river. A process to explain the occurrence of these large fluxes is suggested that is associated with differential fog dissipation over the valley.

Quality Assessment of the Cobel-Isba Numerical Forecast System of Fog and Low Clouds

Short-term forecasting of fog is a difficult issue which can have a large societal impact. Fog appears in the surface boundary layer and is driven by the interactions between land surface and the lower layers of the atmosphere. These interactions are still not well parameterized in current operational NWP models, and a new methodology based on local observations, an adaptive assimilation scheme and a local numerical model is tested. The proposed numerical forecast method of foggy conditions has been run during three years at Paris-CdG international airport. This test over a long-time period allows an in-depth evaluation of the forecast quality. This study demonstrates that detailed 1-D models, including detailed physical parameterizations and high vertical resolution, can reasonably represent the major features of the life cycle of fog (onset, development and dissipation) up to +6 h. The error on the forecast onset and burn-off time is typically 1 h. The major weakness of the methodology is related to the evolution of low clouds (stratus lowering). Even if the occurrence of fog is well forecasted, the value of the horizontal visibility is only crudely forecasted. Improvements in the microphysical parameterization and in the translation algorithm converting NWP prognostic variables into a corresponding horizontal visibility seems necessary to accurately forecast the value of the visibility.

Energy fluxes at a subtropical mountain cloud forest

Results are presented of a 5-day micrometeorological experiment to study the fluxes of radiation and sensible and latent heat above a homogeneous Chamaecyparis forest in northeastern Taiwan at 1650 m a.s.l. The site exhibits a well pronounced diurnal cycle of winds, with valley winds from the SE during daytime, advecting warm and moist air masses and frequently leading to foggy conditions, and with mountain winds from the NW during the nights, typically advecting dryer and slightly cooler air masses, leading to the dissipation of fog. The latent and sensible heat fluxes are high during the days, with Bowen ratios above one and below one (but above zero) occuring with the same frequency of about 38%, respectively, indicating that neither sensible nor latent heat is dominant over the other. During foggy conditions, the short wave radiation is strongly reduced, and the long wave radiation is balanced. Sensible and latent heat fluxes are also strongly limited during foggy conditions. The radiation budget indicates a net gain of energy through radiation in the order of 90 W m −2. The energy balance (as calculated from the one-dimensional approach) exhibits negative values particularly after the onset of the valley winds. This indicates that the advection of warm air plays an important role in the energy balance of the ecosystem. The total net energy flux appears to be slightly negative (−10 W m −2), further supporting the role of advection. During some of the nights, the Bowen ratios were negative with positive (upward) latent heat fluxes and negative fluxes of sensible heat. These conditions occurred during advection of relatively warm and dry air. The energy fluxes were balanced during these conditions, so that the energy for evaporation of water virtually originated from the sensible heat flux.

Formation, Evolution, and Dissipation of Coastal Sea Fog

Evolution of sea fog has been investigated using three-dimensional Mesoscale Model 5 (MM5) simulations. The study focused on widespread fog-cloud layers advected along the California coastal waters during 14–16 April 1999. According to analysis of the simulated trajectories, the intensity of air mass modification during this advection significantly depended on whether there were clouds along the trajectories and whether the modification took place over the land or ocean. The air mass, with its trajectory endpoint in the area where the fog was observed and simulated, gradually cooled despite the gradual increase in sea-surface temperature along the trajectory. Modelling results identified cloud-top cooling as a major determinant of marine-layer cooling and turbulence generation along the trajectories. Scale analysis showed that the radiative cooling term in the thermodynamic equation overpowered surface sensible and latent heat fluxes, and entrainment terms in cases of the transformation of marine clouds along the trajectories. Transformation of air masses along the trajectories without clouds and associated cloud-top cooling led to fog-free conditions at the endpoints of the trajectories over the ocean. The final impact on cloud-fog transition was determined by the interaction of synoptic and boundary-layer processes. Dissipation of sea fog was a consequence of a complex interplay between advection, synoptic evolution, and development of local circulations. Movement of the high-pressure system over land induced weakening of the along-shore advection and synoptic-pressure gradients, and allowed development of offshore flows that facilitated fog dissipation.

The atmospheric electric conductivity and aerosol measurements during fog over the Indian Ocean

The atmospheric electric conductivity and aerosol size-distribution measurements made over the Indian Ocean on two occasions of fog occurrence during the XVI Indian Scientific Expedition, 1996–1997, are reported. The locations and the meteorological conditions associated with two fog episodes, one being a warm fog and the other a cold fog, are widely different. Values of both polar conductivities after showing a slight increase start decreasing about 1 h before the onset of fog, and then start increasing about 0.5–2 h prior to the dissipation of fog. The conductivity values then attain their fair-weather values in case of cold fog but continue to increase and give a maximum ∼2 h after the dissipation of fog in case of warm fog. Total number concentration of aerosol particles (between 13- and 1000-nm diameter) increases during the warm fog period but only just after the dissipation of cold fog. Aerosol size distributions show an increase in concentration of all particle sizes in warm fog but mainly in concentration of the nucleation mode particles in cold fog. Possible causes of the changes observed in conductivity and aerosols and their utility in forecasting the onset and dissipation of fog are discussed.

Aerosol production over remote marine areas‐A new route

We report on enhanced levels of 3–5 nm particles in the remote summer Arctic. Simultaneous increases in particle number occurred in certain size ranges <50nm. Particles >100nm, marine in origin, were also present. Stable airmasses with at least 4 days residence over the ice, a surface mixed layer <140m deep, capped by a temperature inversion and cloud‐free stable layer ∼1km in depth excluded a tropospheric source. Instead a surface source was indicated. The most vigorous nucleation was associated with sudden reductions of humidity (<80%) causing rapid dissipation of fogs. However, particles <50nm contained no detectable H2SO4 implying recent formation or growth from material other than the acid. It is proposed that the marine particles were derived from bubbles bursting on open leads and provided the material for both nucleation and larger particle formation. Nucleation is attributed to oxidation of an amino acid, L‐methionine.

Turbulence, Radiation and fog in Dutch Stable Boundary Layers

The effect of longwave radiation on the structure the clear stable boundary layer (SBL) is examined. Special emphasis is given to radiative cooling near the surface and the top of the boundary layer and its impact on the heat flux profile. Further, the formation, growth and dissipation of fog in the SBL are studied both from observations and from a one-dimensional ensemble averaged turbulence closure model. The model is compared with detailed observations that were made for both a shallow (about 30 m) radiation fog and a deep (about 200 m) fog layer at the 200-m tower at Cabauw in the Netherlands. The model describes adequately the most important mechanisms occurring during the fog evolution. In this study special attention is given to the parameterization of the vegetation, which is important for a good representation of the (minimum) air temperature. The influence of turbulence transport, longwave radiative cooling and gravitational droplet settling on the fog evolution is described. The study demonstrates the need for more accurate measurements of turbulence quantities, especially the master length scale, in a variety of SBLs.

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.

Biological and Agricultural Studies on Application of Discharge Pasma and Electromagnetic Fields. Fog Dissipation and Bad Odor Abatement by Using Corona Discharge Plasma.

The so-called “Yamase” accompanied by very low temperatures and dense fog results in not only bad crop harvests especially rice, but also traffic confusion and accidents during the summer season from the Sanriku Coast to Shimokita Peninsula in the part of northern Japan facing the Pacific Ocean. Effective counterplots against these disasters are strongly expected to be realized by farmers and many citizens. We have invented an electrostatic fog-liquefier in order to solve these problems by dissipating fog artificially. In this paper, we describe the principle of fog dissipation by means of corona discharge, and demonstrate fog and white vapor dissipation experiments not only in a laboratory, but in the field also. Bad odor abatement by means of this newly invented apparatus is also related in detail.

Forecasting Techniques The Use of Hourly Model-Generated Soundings to Forecast Mesoscale Phenomena. Part I: Initial Assessment in Forecasting Warm-Season Phenomena

Since late 1995, NCEP has made available to forecasters hourly model guidance at selected sites in the form of vertical profiles of various forecast fields. These profiles provide forecasters with increased temporal resolution and greater vertical resolution than had been previously available. The hourly forecast profiles are provided for all of NCEP’s short-range models: the Nested Grid Model, Eta Model, and Mesoscale Eta Model. The highresolution forecasts aid in the timing of frontal passages, low-level jets, and convective initiation. In addition, through time‐height cross sections of Richardson numbers, forecasters can alert pilots to the potential for clear air turbulence several hours to a day in advance. Further, the profiles are useful in prediction of cloudiness and the dissipation of low-level stratus and fog. Time‐height cross sections of wind velocity have proven extraordinarily useful in visualizing and forecasting inversion heights, frontal passage timing, boundary layer depth, and available environmental and storm-relative helicity during convective events. The hourly model forecasts were found to be exceptionally helpful when combined with hourly surface observations to produce enhanced real-time analyses of convective parameters for use in very short term forecasting. High-resolution analyses of lifted index, CAPE, convective inhibition, moisture flux convergence, and 2-h changes in these fields aid the forecaster in anticipating convective trends. The introduction of model forecast error into these real-time analyses was minimized by using the latest available Eta or Mesoscale Eta Model runs. Therefore, the model data used to enhance the analyses are typically no more than 6‐12 h old.

三沢漁港における霧の事例研究

Observations of fogs at Misawa Fishing Port in the Aomori district were made with a tethered balloon system and a doppler sodar system from 15 June to 11 July in 1993. Minute analyses were made of time changes of fogs for a long period from the evening on 7 July to the next noon and for a short period from 6 to 13JST on July. The results show that the atmosphere at the cloud top becomes stable from the view point of CTEI (cloud-top entrainment instability) about 2 hours before the intrusion of the fog and becomes neutral (or unstable) just before fog dissipation.

Modeling of the processing and removal of trace gas and aerosol species by Arctic radiation fogs and comparison with measurements

A Lagrangian radiation fog model is applied to a fog event at Summit, Greenland. The model simulates the formation and dissipation of fog. Included in the model are detailed gas and aqueous phase chemistry, and deposition of chemical species with fog droplets. Model predictions of the gas phase concentrations of H2O2, HCOOH, SO2, and HNO3 as well as the fog fluxes of S(VI), N(V), H2O2, and water are compared with measurements. The predicted fluxes of S(VI), N(V), H2O2, and fog water generally agree with measured values. Model results show that heterogeneous SO2 oxidation contributes to approximately 40% of the flux of S(VI) for the modeled fog event, with the other 60% coming from preexisting sulfate aerosol. The deposition of N(V) with fog includes contributions from HNO3 and NO2 initially present in the air mass. HNO3 directly partitions into the aqueous phase to create N(V), and NO2 forms N(V) through reaction with OH and the nighttime chemistry set of reactions which involves N2O5 and water vapor. PAN contributes to N(V) by gas phase decomposition to NO2, and also by direct aqueous phase decomposition. The quantitative contributions from each path are uncertain since direct measurements of PAN and NO2 are not available for the fog event. The relative contributions are discussed based on realistic ranges of atmospheric concentrations. Model results suggest that in addition to the aqueous phase partitioning of the initial HNO3 present in the air mass, the gas phase decomposition of PAN and subsequent reactions of NO2 with OH as well as nighttime nitrate chemistry may play significant roles in depositing N(V) with fog. If a quasi‐liquid layer exists on snow crystals, it is possible that the reactions taking place in fog droplets also occur to some extent in clouds as well as at the snow surface.

Influences on formation and dissipation of high arctic fogs during summer and autumn and their interaction with aerosol

Influences on formation and dissipation of high arctic fogs during summer and autumn and their interaction with aerosol

Influences on formation and dissipation of high arctic fogs during summer and autumn and their interaction with aerosol

Radiosondes established that the air in the near surface mixed layer was very frequently near saturation during the International Arctic Ocean Expedition 1991 which must have been a large factor in the frequent occurrence of fogs. Fogs were divided into groups of summer, transition and winter types depending on whether the advecting air, the ice surface or sea surface respectively was warmest and the source of heat. The probability of summer and transition fogs increased at air temperatures near 0 °C while winter fogs had a maximum probability of occurrence at air temperatures between − 5 and − 10 °C. Advection from the open sea was the primary cause of the summer group, the probability of occurrence being high during the 1st day's travel and appreciable until the end of 3 days. Transition fogs reached its maximum probability of formation on the 4th day of advection. Radiation heating and cooling of the ice both appeared to have influenced summer and transition fogs, while winter fogs were strongly favoured by the long wave radiation loss at clear sky conditions. Another cause of winter fogs was the heat and moisture source of open leads. Wind speed was also a factor in the probability of fog formation, summer and transition fogs being favoured by winds between 2 and 6 ms −1 , while winter fogs were favoured by wind speeds of only 1 ms −1 . Concentrations of fog drops were generally lower than those of the cloud condensation nuclei active at 0.1%, having a median of 3 cm −3 . While a well-defined modal diameter of 20–25 μ m was found in all fogs, a second transient mode at about 100 μ m was also frequently observed. The observation of fog bows with supernumerary arcs pointed to the existence of fog droplets as large as 200–300 µ m in diameter at fog top. It is suggested that the large drops originated from droplets grown near the fog top and were brought to near the surface by an overturning of the fog layer. Shear induced wave motions and roll vortices were found to cause perturbations in the near-surface layer and appeared to influence fog formation and dissipation. The low observed droplet concentration in fogs limits their ability to modify aerosol number concentrations and size distributions, the persistent overlying stratus being a more likely site for effective interactions. It is suggested that variations in the fog formation described in this paper may be a useful indicator of circulation changes in the arctic consequent upon a global warming. DOI: 10.1034/j.1600-0889.1996.t01-1-00008.x

Back to basics: Fog: Part 2 — The formation and dissipation of land fog*

Back to basics: Fog: Part 2 — The formation and dissipation of land fog*

Post-fog nucleation of H2SO4_H2O particles in smog

Abstract Homogenous nucleation of sulfuric acid and water associated with urban fogs was examined. We approached the problem by analyzing the factors determining the gas-phase sulfuric acid concentration, and thereby identifying the ambient conditions where new particle production is expected to occur. We showed that H 2 SO 4 (g) concentration is in steady state with respect to its production and depletion via condensation in urban environment. Based on this, a simple formula was derived for estimating the nucleation probability from temperature, fine dry aerosol mass, and SO 2 (g) concentration and its oxidation rate during the post-fog conditions. The major uncertainty is associated with the value of the accomodation coefficient for sulfuric acid condensation on water. Nucleation was shown to be most favorable around 90% relative humidity after the dissipation of fog. We calculated a very high nucleation probability associated with fog observed in the San Joaquin Valley of California. In the South Coast Air Basin of Los Angeles, post-fog nucleation was predicted to occur frequently in Hawthorne and Long Beach, and occasionally at several other locations.

Radiation Fog: A Comparison of Model Simulation with Detailed Observations

Abstract A one-dimensional model designed to study the formation, growth, and dissipation of radiation fog is described. The model is compared with detailed observations made at the 200-m tower at Cabauw in the Netherlands. This study we use observations made in a shallow radiation fog that formed on the night of 16/17 August 1988. The model seems to be able to describe the most important mechanisms occurring during the fog evolution. In this study special attention is given to the parameterization of the vegetation, which is important for a good representation of the (minimum) air temperature. The influence of turbulence transport, longwave radiative cooling, and gravitational droplet settling on the fog evolution is described.

A field study on chemistry, S(IV) oxidation rates and vertical transport during fog conditions

An extensive fog study was carried out in the central plateu of Switzerland. Ninety-seven fog samples were collected along with aerosol filter and cascade impactor samples, and measurements of O 3, SO 2, NO, NO x , PAN, temperature, and wind speed and direction. Maximum levels in fogwater were 4.3, 4.4., 0.033, 1.7, 0.5, 0.024 and 9.2 mmol ℓ −1 for Cl −, NO 3 −, NO 2 −, SO 4 2−, S(IV), oxalate and NH 4 +, respectively. pH varied between 2.9 and 7.1. Sixteen additional elements were determined in the fog samples by ICP. The sum of the concentrations of SO 4 2− and S(IV) agreed very with the total sulfur concentration as determined by ICP. A substantial excess of S(IV) (up to 0.2 mmol ℓ −1) compared to Henry and acid-base equilibrium calculations was found, which can probably be attributed to complex formations with aldehydes. S(IV) oxidation rates of up to 650 nmol ℓ −1 s −1 with ozone and of up to 100 nmol ℓ −1 s −1 with NO 2 were calculated. S(IV) oxidation due to PAN, NO 2 − and Fe(III) was of minor importance. A substantial fraction of the major ions was present in the intersitial aerosol (aerosol particles < 4 μm) even during fog conditions. High correlations were found for NH 4 +, NO 3 2−. From their ratios in the fog water and the aerosol (< 4 μm) it could be concluded that at least 40% of NO 3 − and 20% of NH 4 + in fog water was due to gas phase scavenging. Increasing concentrations in fog water were found during fog dissipation. Concentrations decreased with increasing height. A vertical transport model including turbulent diffusion and droplet sedimentation is introduced, which matches the experimental data of this vertical profile.

Interactions of aerosols (ammonium sulfate, ammonium nitrate and ammonium chloride) and of gases (HCl, HNO3) with fogwater

The concentrations of aerosols (NH4NO3, (NH4)2SO4 and NH4Cl) and of gases (HCl(g), HNO3(g), NH3(g) were determined by denuder methods under different conditions (in the absence of fog, before, during and after fog events). At this site situated in an urban region, high concentrations of the gaseous strong acids HCl(g) and HNO3(g) are observed. NH4Cl and NH4NO3 aerosols represent a major fraction of the Cl− and NO3− aerosols (<2.4 μm)collected by denuders. During a fog event, very high concentrations of SO42− were found in small aerosols, which are attributed to the aqueous phase oxidation of SO2 under the influence of high pH due to the presence of NH3. Differences in SO42− concentrations measured in aerosols (<2.4 μm) and in fog droplets were probably due to mass-transport limitations of the SO2 oxidation. Ammonium sulfate aerosols represent in some cases a significant fraction of the total S present (SO2(g) + SO42−. Soluble aerosols and gases contribute to the composition of fogwater and are released again after fog dissipation.

Mathematical modeling of acid deposition due to radiation fog

A Lagrangian model has been developed to study acidic deposition due to radiation fog. The model couples submodels describing the development and dissipation of radiation fog, the gas‐phase chemistry and transfer, and the aqueous‐phase chemistry. The model is applied to a radiation fog episode in Bakersfield in the San Joaquin Valley of California over the period January 4–5, 1985. Model predictions for temperature profile, fog development, liquid water content, gas‐phase concentrations of SO2, HNO3, and NH3, pH, aqueous‐phase concentrations of SO42−, NH4+, and NO3−, and finally deposition rates of the above ions are compared with the observed values. The deposition rates of the major ions are predicted to increase significantly during the fog episode, the most notable being the increase of sulfate deposition. Pathways for sulfate production that are of secondary importance in a cloud environment may become significant in a fog. Expressing the mean droplet settling velocity as a function of liquid water content is found to be quite influential in the model's predictions.