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Meteorology & Atmospheric Sciences; Radiation fog; Stratus lowering fog; Turbulent fluxes; Fog microphysics;
Abstract:
During the PARISFOG campaign in winter 2012/2013, microphysical properties and turbulent fluxes of fog droplets (liquid water), water vapor, and energy were characterized and quantified during fog events of various types that occurred at the SIRTA (Site Instrumental de Recherche par Teledetection Atmospherique) atmospheric observatory outside Paris. The eddy covariance technique was applied, employing a fast (10 Hz) fog droplet spectrometer, a three-dimensional ultrasonic anemometer, and a fast response gas analyzer, which were operated at an altitude of 2.5 m above ground. A visibility-meter was used to detect the occurrence and density of fog. A total of twenty-one fog events were measured during the field campaign. After applying quality criteria, six events remained. For this study, two fog events out of the six, representing a radiation fog and stratus lowering fog, respectively, are analyzed in detail. The two fog events exhibited very distinct patterns in terms of fog droplet size distribution, fog number concentration, and liquid water content. The evolution of these microphysical properties is elucidated through combined analysis of the turbulent fluxes of fog droplets (liquid water), water vapor and energy as well as reasoning of microphysical processes like, condensation, collision-coalescence, and droplet evaporation. Downward droplet number fluxes and liquid water fluxes were mostly observed in stratus lowering fog, however, upward fluxes were also observed in response to downward water vapor fluxes. In radiation fog, both upward and downward droplet number fluxes and liquid water fluxes were observed depending on the position at which the microphysical process was observed with respect to the measurement height. Bi-directional fog droplet fluxes with different flux directions of smaller and larger droplets were observed. In both fog events, the downward water vapor fluxes were the major cause for (I) the broadening of the fog droplet size distribution and (II) the largest upward fog water fluxes throughout the whole event. (C) 2014 Elsevier B.V. All rights reserved.
