Radiation in fog: Quantification of the impact on fog liquid water based on ground-based remote sensing

Eivind G. Wærsted (1) , Martial Haeffelin (1) , Jean-Charles Dupont (1) , Julien Delanoë (2) , Philippe Dubuisson (3)

1 LMD - Laboratoire de Météorologie Dynamique (UMR 8539)
2 SPACE - LATMOS
3 LOA - Laboratoire d’Optique Atmosphérique - UMR 8518

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.

Domaines: Physique Atmosphérique et Océanique [physics.ao-ph]; Météorologie

Eivind G. Wærsted, Martial Haeffelin, Jean-Charles Dupont, Julien Delanoë, Philippe Dubuisson. Radiation in fog: Quantification of the impact on fog liquid water based on ground-based remote sensing. Atmospheric Chemistry and Physics, 2017, 17 (17), pp.10811-10835. ⟨10.5194/acp-17-10811-2017⟩. ⟨insu-01527805⟩