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Daniela Nowak, Dominique Ruffieux, Judith L. Agnew, and Laurent Vuilleumier

1. Introduction Precise forecasting of the formation, evolution, and erosion of fog and low stratus is a major challenge for meteorology, especially in complex topography. One of the goals of the COST 720 Temperature, Humidity, and Cloud (TUC) winter experiment undertaken in Switzerland in 2003/04 ( Ruffieux et al. 2006 ) was to provide a dataset for determining the base and top of low clouds using a simple combination of ground-based remote sensing instruments. Frequent and detailed

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A. Ritter, C. M. Regalado, and G. Aschan

1. Introduction Fog water deposition has been widely recognized as an important ecological factor in mountain forests frequently immersed in dense fog ( Azevedo and Morgan 1974 ; Stadtmüller 1987 ; Cavelier and Goldstein 1989 ; Schemenauer and Cereceda 1994 ; Walmsley et al. 1996 ; Bruijnzeel 2001 ). This hydrological process, also known as fog precipitation, occurs when atmospheric water contained in wind-driven fogs impinges upon obstacles in their path. The small droplets impacting on

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Samuel Rémy and Thierry Bergot

, the threshold values are set at 600 m for visibility and 60 m for the ceiling. Various approaches are employed to forecast low-visibility conditions. For airports located in flat terrain, 1D models are suitable for the nowcasting of radiation fog events ( Bergot and Guédalia 1994a , b ). They are currently used in real time to forecast fog at the local scale (e.g., Clark 2002 , 2006 ; Herzegh et al. 2003 ). The 1D boundary layer model Couche Brouillard Eau Liquide (COBEL), coupled with the land

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R. Boers, H. Klein Baltink, H. J. Hemink, F. C. Bosveld, and M. Moerman

1. Introduction Low-level moisture, appropriate low wind speed, and nighttime longwave cooling are the primary prerequisites for the development of radiation fog. Initially, most longwave cooling (longwave radiative flux divergence) originates near the ground, but the level of maximum cooling will rise progressively as the fog layer thickens and the emissivity of cloud droplets becomes dominant. If the fog layer is thick enough and the droplet radius approaches 5–10 μ m, then most longwave

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Yongming Wang, Shanhong Gao, Gang Fu, Jilin Sun, and Suping Zhang

1. Introduction Fog that occurs over the ocean or a coastal region is usually termed sea fog. It significantly reduces low-level visibilities, which can play a role in severe marine accidents ( Trémant 1987 ; Gultepe et al. 2007 ). Sea fog is likely to form over regions where cold sea surface temperatures (SSTs) are adjacent to warm currents ( Lewis et al. 2004 ). The Yellow Sea of China (shown in Fig. 1 ), located north of the warm Kuroshio Current, is exactly such a region and experiences a

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I. Gultepe, M. D. Müller, and Z. Boybeyi

1. Introduction Fog formation is directly related to thermodynamical, dynamical, radiative, aerosol, and microphysical processes and to surface conditions. Extinction of light at visible ranges within the fog results in low visibilities that can affect low-level flight conditions, marine traveling, shipping, and transportation. Fog occurrence more than 10% of time in some regions of Canada ( Whiffen 2001 ) demands that fog nowcasting and/or forecasting models should be improved. In particular

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N. Reuge, P. Fede, J.-F. Berthoumieu, F. Foucoin, and O. Simonin

1. Introduction Coupled evaporation/condensation/coalescence phenomena of droplets combined with particles for seeding are involved in a wide range of various applications from the environmental fields—clouds and fogs ( Jiusto et al. 1968 ) and air pollution ( Van Hop 1989 )—to some industrial processes: spray generation, spray drying, and spray mixing ( Hinds 1982 ; Reuge et al. 2008 ). Cloud or fog particle seeding is an old technique of rain enhancement that began in the 1940s. Various

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Thierry Bergot, Enric Terradellas, Joan Cuxart, Antoni Mira, Olivier Liechti, Mathias Mueller, and Niels Woetmann Nielsen

of 2 in foggy conditions. The flow of air traffic into major airport terminals is reduced by poor visibility conditions, and fog is one of the major causes of aircraft delays. Therefore, more reliable forecasts of fog are needed to allow air traffic managers to regulate traffic more effectively. Fog also has a significant impact on road and marine transportation. Improved numerical weather prediction (NWP) models are needed before improving the analysis and prediction of fog (e.g., Benjamin et

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Huiqin Hu, Juanzhen Sun, and Qinghong Zhang

1. Introduction Fog is a weather phenomenon that affects the atmospheric horizontal visibility (AHV) below 1 km as a result of ice or water droplets suspended in the atmosphere near the surface ( Glickman 2000 ). There recently has been an increased demand for accurate fog prediction because of its high social and economic impacts on aviation, marine, and land transportation ( Gultepe et al. 2007 ). Although the significance of physical parameterization schemes (e.g., turbulence, terrain, and

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A. Philip, T. Bergot, Y. Bouteloup, and F. Bouyssel

1. Introduction Operational short-term fog forecasting is a real challenge and has safety and economic impacts, especially for airports ( Gultepe et al. 2007 ). For security reasons, the air traffic control (ATC) authorities define a low visibility procedure (LVP) for horizontal visibilities of less than 600 m and/or ceilings of less than 60 m. During LVP conditions, the airport’s efficiency for takeoff and landing is reduced, causing flight delays and cancellations. For example, at Paris

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