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Eivind Waersted (LMD)

Titre : Description of physical processes driving the life cycle of radiation fog and fog–stratus transitions based on conceptual models

Date et heure : Le 12-10-2018 à 14h00

Type : thèse

Université qui délivre le diplôme : Université Paris-Saclay

Lieu : Amphi Monge, at the campus of École Polytechnique, Route de Saclay, 91128 Palaiseau
Membres du jury :

M. Pierre Gentine, Professeur, Rapporteur

Mme Marie Lothon, Chargé de Recherche, Rapporteur

Mme Christine Lac, Ingénieur, Examinateur

Mme Marjolaine Chiriaco, Maïtre de Conférences, Examinateur

M. Gert-Jan Steeneveld, Associate Professor, Examinateur

M. Philippe Drobinski, Directeur de Recherche, Examinateur

M. Martial Haeffelin, Ingénieur de Recherche, Directeur de thèse

M. Jean-Charles Dupont, Physicien Adjoint, Co-Directeur de thèse

Résumé :

Fog causes hazards to human activities due to the reduction of visibility, and improving the forecasts of fog formation and dissipation is therefore an objective for research. Over a 4-year period, more than 100 fog events are documented at the SIRTA atmospheric observatory by observing fog base (by ceilometer), fog top (CTH) and higher clouds (by cloud radar), and the liquid water path (LWP) (by microwave radiometer (MWR)). We find that the dissipation of fog into stratus can be explained to the first order by the CTH and LWP, and we therefore focus on quantifying the processes driving the evolutions of these two parameters.
Using the radiative transfer code ARTDECO, we find that the radiative cooling at fog top can produce 40–70 g m−2 h−1 of LWP when the fog is opaque (LWP >≈ 30 g m−2) (production is lower for thin fog). Clouds above the fog will strongly reduce this production, especially low clouds. Heating due to solar radiation absorbed at the surface is found to be the dominating process of LWP loss after sunrise, but its magnitude is sensitive to the Bowen ratio.
Using the LES model DALES, we find a strong sensitivity of the fog dissipation to the observed variability in the stratification and humidity above fog top. By enhancing entrainment, a weak stratification can lead to earlier fog dissipation by (1) more depletion of LWP by mixing with unsaturated air, especially if the air is dry, and (2) vertical development of the fog top leading to lifting of the fog base.
A conceptual model which calculates the impacts on LWP and CTH of the six local processes (long-wave and short-wave radiation, surface heat fluxes, entrainment, subsidence and deposition) directly from observations is developed, in order to extend the analysis to more cases. We also find that the shape of the profile of radar reflectivity often changes during dissipation, which might be related to the evaporation of droplets in the lower levels of the fog. Hence, by observing the cloud top development, the stratification, the LWP and the profile of reflectivity, the cloud radar and MWR provide information that has potential for anticipating fog dissipation.

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