Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2017-292
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
10 May 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
Radiation in fog: Quantification of the impact on fog liquid water based on ground-based remote sensing
Eivind G. Wærsted1, Martial Haeffelin2, Jean-Charles Dupont3, Julien Delanoë4, and Philippe Dubuisson5 1Laboratoire de Météorologie Dynamique, École Polytechnique, Université Paris-Saclay, 91128 Palaiseau, France
2Institut Pierre Simon Laplace, École Polytechnique, CNRS, Université Paris-Saclay, 91128 Palaiseau, France
3Institut Pierre-Simon Laplace, École Polytechnique, UVSQ, Université Paris-Saclay, 91128 Palaiseau, France
4Laboratoire Atmosphères, Milieux, Observations Spatiales/UVSQ/CNRS/UPMC, 78280 Guyancourt, France
5Laboratoire d'Optique Atmosphérique, Univ. Lille – UMR CNRS 8518, F-59000 Lille, France
Abstract. Radiative cooling and heating impact the liquid water balance of fogs and therefore play an important role in determining their persistence or dissipation. We demonstrate that a quantitative analysis of the radiation-driven condensation and evaporation is possible in real-time using ground-based remote sensing observations (cloud radar, ceilometer, microwave radiometer). Seven continental fog events in mid-latitude winter are studied. The longwave (LW) radiative cooling of the fog is able to produce 40–70 g m−2 h−1 of liquid water by condensation when the fog liquid water path exceeds 30 g  m−2 and there are no clouds above the fog, which corresponds to renewing the fog water in 1–2 hours. The variability is related to fog temperature and atmospheric humidity, with warmer fogs below drier atmospheres producing more liquid water. The appearance of a cloud layer above the fog strongly reduces this cooling, especially a low cloud (up to 100 %), thereby perturbing the liquid water balance in the fog, and may therefore induce fog dissipation. Shortwave (SW) radiative heating by absorption by fog droplets is smaller than the LW cooling, but it can contribute significantly, inducing 10–15 g m−2 h−1 of evaporation in thick fogs at (winter) midday. We also find that the absorption of SW radiation by aerosols in the fog may strongly increase this evaporation rate if a large concentration of absorbing aerosols is present, but that this increase likely is below 30 % in most cases. The absorbed radiation at the surface can reach 40–120 W m−2 during daytime depending on the fog thickness. As in situ measurements indicate that 20–40 % of this energy is transferred to the fog as sensible heat, this surface absorption can contribute importantly to heating and evaporation of the fog, up to 30 g m−2 h−1 for thin fogs.

Citation: Wærsted, E. G., Haeffelin, M., Dupont, J.-C., Delanoë, J., and Dubuisson, P.: Radiation in fog: Quantification of the impact on fog liquid water based on ground-based remote sensing, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-292, in review, 2017.
Eivind G. Wærsted et al.
Eivind G. Wærsted et al.
Eivind G. Wærsted et al.

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Short summary
Heating and cooling of fog layers by solar and terrestrial radiation influence importantly the fog life cycle. We quantify these radiative impacts on fog liquid water using detailed cloud radar observations of seven fog events as well as sensitivity studies. We find that the impact of radiation is affected mainly by fog optical thickness, atmospheric humidity and the presence of clouds above the fog. Observing these quantities in real time can therefore be useful for forecasting fog dissipation.
Heating and cooling of fog layers by solar and terrestrial radiation influence importantly the...
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