Boundary-Layer Meteorology

, Volume 143, Issue 1, pp 207–225 | Cite as

Stratus–Fog Formation and Dissipation: A 6-Day Case Study

  • Jean-Charles Dupont
  • Martial Haeffelin
  • Alain Protat
  • Dominique Bouniol
  • Neda Boyouk
  • Yohann Morille
Open Access


A suite of active and passive remote sensing instruments and in-situ sensors deployed at the SIRTA Observatory (Instrumented Site for Atmospheric Remote Sensing Research), near Paris, France, for a period of six months (October 2006–March 2007) document simultaneously radiative, microphysical and dynamic processes driving the continental-fog life cycle. The study focuses on a 6-day period between 23 and 29 December 2006 characterized by several stratus-cloud lowering and lifting events and almost 18 h of visibility below 1 km. Conceptual models and different possible scenarios are presented here to explain the formation, the development and the dissipation phases of three major stratus–fog events and to quantify the impact of each driving process. For example, slowly evolving large-scale conditions characterized by a slow continuous cloud-base lowering, followed by a rapid transient period conductive to fog formation and dissipation, are observed for cases 1 and 3. During this stable period, continuous cloud-top radiative cooling (≈ −160 Wm−2) induces a progressive and slow lowering of the cloud base: larger droplets at cloud top (cloud reflectivity approximately equals to −20 dBZ) induce slow droplet fall to and beyond cloud base (Doppler velocity ≈ −0.1 ms−1), cooling the sub-cloud layer by evaporation and lowering the saturation level to 100 m (case 1) or to the surface (cases 2 and 3). Suddenly, a significant increase in Doppler velocity magnitude ≈ −0.6 ms−1 and of turbulent kinetic energy dissipation rate around 10−3 m2s−3 occurs at cloud base (case 1). These larger cloud droplets reach the surface leading to fog formation over 1.5 h. The Doppler velocity continues to increase over the entire cloud depth with a maximum value of around −1 ms−1 due to the collection of fog droplets by the drizzle drops with high collection efficiency. As particles become larger, they fall to the ground and lead to fog dissipation. Hence, falling particles play a major role in both the formation and also in the dissipation of the fog. These roles co-exist and the balance is driven by the characteristics of the falling particles, such as the concentration of drizzle drops, the size distribution of drizzle drops compared to fog droplets, Doppler velocity and thermodynamic state close to the surface.


Fog life cycle Fog observations In-cloud processes Stratus cloud 


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Copyright information

© The Author(s) 2012

Authors and Affiliations

  • Jean-Charles Dupont
    • 1
  • Martial Haeffelin
    • 1
  • Alain Protat
    • 2
  • Dominique Bouniol
    • 3
  • Neda Boyouk
    • 4
  • Yohann Morille
    • 4
  1. 1.Institut Pierre Simon LaplaceEcole PolytechniquePalaiseauFrance
  2. 2.Laboratoire Atmospheres Milieux, Observations SpatialesGuyancourtFrance
  3. 3.Groupe d’étude de l’Atmosphère Metéorologique/Centre National de la Recherche en MétéorologieCNRS/Météo-FranceToulouseFrance
  4. 4.Laboratoire de Météorologie DynamiqueInstitut Pierre Simon Laplace, Ecole PolytechniquePalaiseauFrance

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