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PARISFOG: Shedding New Light on Fog Physical Processes See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/228412535 PARISFOG: Shedding new light on fog physical processes Article in Bulletin of the American Meteorological Society · June 2010 DOI: 10.1175/2009BAMS2671.1 CITATIONS READS 60 202 21 authors, including: Martial Haeffelin Thierry Bergot French National Centre for Scientific Research Centre National de Recherches Météorologiques 172 PUBLICATIONS 2,878 CITATIONS 54 PUBLICATIONS 1,110 CITATIONS SEE PROFILE SEE PROFILE Thierry Elias Robert Tardif Hygeos University of Washington Seattle 73 PUBLICATIONS 653 CITATIONS 38 PUBLICATIONS 778 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Megha-Tropiques View project EMME-CARE: Eastern Mediterranean Middle East - Climate & Atmosphere Research Centre View project All content following this page was uploaded by Robert Tardif on 21 May 2014. The user has requested enhancement of the downloaded file. PARISFOG Shedding New Light on Fog Physical Processes BY M. HAEFFELIN , T. BERGOT , T. ELIAS , R. TARDIF , D. CARRER , P. CHAZETTE , M. COLOMB , P. D ROBINSKI , E. DUPONT , J.-C. DUPONT , L. GOMES , L. MUSSON -GENON , C. PIETRAS , A. PLANA -FATTORI , A. PROTAT , J. RANGOGNIO , J.-C. RAUT , S. RÉMY , D. RICHARD , J. SCIARE , AND X. ZHANG A field experiment covering more than 100 fog and near-fog situations during the winter of 2006–07 investigated the dynamical, microphysical, and radiative processes that drive the life cycle of fog. ow-visibility meteorological conditions, such as fog, are not necessarily considered extreme weather conditions, such as L those encountered in storms, but their effects on society can be just as significant. Fog creates situations where our transporta- tion systems on roads, rails, sea, and air become more hazardous, requiring specific safety measures to prevent accidents that lead to delays or cancellation of transport. While the meteorological event is inevitable, there is significant pressure from airport and road transport authorities to obtain more reliable forecasts. Local short-term fog forecasts relying on 1D assimilation-forecast high- resolution models (e.g., Cobel-Isba model; Bergot et al. 2005) have been implemented at airports in Paris and Lyon, France (Bergot 2007), and San Francisco, California (Ivaldi et al. 2006). These models include precise parameterizations of radiative, turbulent, and surface processes and rely on detailed and continuous near- surface observations of temperature, humidity, wind, radiation, and visibility. They produce more accurate fog forecasts than current NWP models (Bergot 2007), but their application remains local. Hence further improvements in fog forecast rely on better understanding of physical processes at play in the fog life cycle. Fog formation results from condensation of water vapor into liquid droplets or ice crystals, as a result of air cooling, moisten- ing, and/or through mixing of contrasting air parcels. The most common scenario considered when invoking fog formation over land involves Aerosol and fog microphysics sensors are used to further document particular events of fog and near-fog. For more information see Fig. 3. nocturnal radiative cooling under light wind condi- that the development of radiation fog results from tions (Roach 1995), while dissipation typically occurs the balance between radiative cooling and turbulent a few hours after sunrise as a result of warming from mixing [e.g., Roach et al. (1976) based on observations sensible heat fluxes over a surface heated by solar performed in Cardington, United Kingdom]. Other radiation (the so-called fog burn-off). However, this datasets were put together to focus on radiation fog statement hides a more complex reality, with regions such as the Fog-82 campaign in Albany, New York experiencing fog events due to conditions such as (Meyer et al. 1986), and the Lille-88 and Lille-91 field advection fog or stratus lowering rather than the typi- experiments in northern France (Guédalia and Bergot cal radiative fog event (Croft et al. 1997; Tardif and 1994). The role of turbulence was investigated using Rasmussen 2007). Furthermore, the nature and con- measurements performed at the Cabauw experimen- centration of aerosols present in the surface layer are tal site in the Netherlands (Duynkerke 1991, 1999). known to be critical parameters throughout the fog In the same period, the Po Valley in northern Italy life cycle as their chemical and microphysical proper- received considerable attention, with two field cam- ties control the activation process (Rangognio et al. paigns (1989 and 1994) focused on fog microphysical 2009), and their optical properties affect radiative processes and evolution of chemical species (Fuzzi cooling and heating (Elias et al. 2009). In addition, et al. 1992, 1998). turbulent mixing is known to be a key but ambigu- However, the occurrence and development of fog is ous factor in influencing fog formation. If turbulent the result of multiple processes occurring simultane- mixing is too low, dew deposition at the surface will ously that interact nonlinearly with each other. These inhibit condensation in the atmosphere and hence interactions likely result in nontrivial sets of key fog inhibit fog formation. If turbulence is strong enough, parameter values leading to fog formation, while it may promote condensation in a supersaturated other combinations of values prevent fog formation. surface layer of sufficient depth and hence lead to fog Today key remaining questions are the following: formation and development (Bergot et al. 2008). How do competing radiative, thermodynamic, As reviewed in Gultepe et al. (2007), several field microphysical, dynamical, and chemical processes campaigns carried out in Europe and North America interact with each other? Do key parameters such have focused on physical and chemical processes as aerosol concentration, supersaturation, radiative involved in continental fog. Early studies revealed cooling rates, and turbulent mixing take on critical values to reach a particular balance that result in fog formation? Is there a hierarchy in these processes, or AFFILIATIONS: HAEFFELIN AND DUPONT —Institut Pierre- a single dominating process whose behavior must be Simon Laplace, Ecole Polytechnique, Palaiseau, France; BERGOT , better quantified? The significant variability of local TARDIF , CARRER , GOMES , RANGOGNIO , AND RÉMY —CNRM-GAME, conditions in which fog formation, vertical develop- Météo-France, Toulouse, France; DRO B INSKI AND PIETRAS — ment, and dissipation typically occur emphasizes Laboratoire de Météorologie Dynamique, Institut Pierre- the difficulty of giving complete answers to these Simon Laplace, Palaiseau, France; MUSSON -GENON , DUPONT , questions. AND ZH ANG —Centre d’Enseignement et de Recherches en Environnement Atmosphérique (Laboratoire Commun ENPC— The ParisFog field experiment was designed to EDF R&D), Chatou, France; COLOM B —Laboratoire Régional shed some light on these questions by 1) monitoring des Ponts et Chaussées, Clermont-Ferrand, France; CH A Z ETTE , simultaneously all important processes and 2) sam- ELIAS , AND SC IARE —Laboratoire des Sciences du Climat et de pling a large range of conditions during a 6-month l’Environnment/IPSL, Saclay, France; PLANA -FATTORI , PROTAT , winter season (October 2006–March 2007). To do so, AND RAUT —Laboratoire Atmosphères, Milieux, Observations the experimental setup was designed to monitor on Spatiales, Guyancourt, France; RI ch ARD —Institut Physique du a routine basis surface conditions, large- and small- Globe de Paris, Paris, France scale dynamics, radiation, turbulence, precipita- CORRESPONDING AUTHOR: Martial Haeffelin, Institut Pierre- Simon Laplace, LMD/IPSL, Ecole Polytechnique, 91128 Palaiseau tion, droplet and aerosol microphysics, and aerosol CEDEX, France chemistry, combining in situ and remote sensing E-mail: [email protected] instruments on a long-term basis to describe the com- The abstract for this article can be found in this issue, following the plete environment in which fog develops. The long table of contents. observing period was intended to sample processes DOI:10.1175/2009BAMS2671.1 taking place during contrasting scenarios, such as In final form 1 December 2009 fog formation versus nonformation in similar condi- ©2010 American Meteorological Society tions (quasi fog), formation in clean and polluted air masses, and evolution of different fog types. 768 | JUNE 2010 This paper presents the 6-month ParisFog field picted in Fig. 2). Particular events of fog and near-fog experiment and provides information on the ParisFog were further documented by deploying additional, database. It describes the noteworthy meteorological albeit more user-intensive, sensors during intensive and physical conditions encountered and illustrates observation periods (IOPs), as shown in Fig. 3. key processes involved in various fog types using Two 30-m masts, located in zones 1 and 3, ParisFog observations. hosted standard weather sensors to monitor the vertical thermodynamic structure in the surface PARISFOG OBSERVATIONS AND DATA- layer. Measurements were extended vertically by BASE. The geographical location of Paris, France, radiosonde profiles performed routinely at 0000 was chosen because fog creates strong constraints on and 1200 UTC 15 km west of SIRTA as part of the transport in an area of 12 million inhabitants with Météo-France
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