Fog physics

While current numerical weather prediction models are able to forecast situations that are favorable to fog events, these forecasts are usually unable to determine the exact location and time of formation or dissipation. Improved understanding of complex atmospheric boundary layer interactions are obtained through careful analysis of measurements gathered during detailed field experiments and through modeling studies.

The occurrence, development and dissipation of fog result from multiple processes (thermodynamical, radiative, dynamical, and microphysical) that occur simultaneously, through a wide range of conditions, and that feed back on each other inducing non-linear behaviors. Hence to advance our ability to forecast fog processes, we must gain better understanding on how critical physical processes interact with each other. To provide a dataset suitable to study these processes simultaneously in continental fog, a suite of active and passive remote sensing instruments and in-situ sensors were deployed during ParisFog (winter 2006-2007) and ToulouseFog experiment (winter 2007-2008).

 Field experiments : ParisFog and ToulouseFog

To provide a dataset suitable to study simultanously processes involved in fog, a set of active and passive remote sensing instruments and in-situ sensors were deployed near Paris, France, for 6 months (winter 2006-2007) and near Toulouse, France, for 6 months (winter 2007-2008). These observations allow to monitor profiles of wind, turbulence, microphysical and radiative properties, as well as temperature, humidity, aerosol and fog microphysics and chemistry in the surface layer. These field experiments, called ParisFog and ToulouseFog, provides a comprehensive characterization of over 200 fog and mist events. The dataset contains contrasted events of stratus lowering fog and radiative cooling fog, as well as a large number of situations considered as favorable to fog formation, but where fog droplets did not materialize. The effect of hydrated aerosols on visibility, the role of aerosols microphysical and chemical properties on supersaturation and droplet activation, and the role of turbulence and sedimentation on fog life cycle have been investigated using the ParisFog and ToulouseFog dataset.

 Microphysical processes

Research is done on the microphysical parameterization, in order to take into account the specificity of the fog granulometric spectra. This work is based on data coming from the Paris-Fog and Toulouse-Fog field experiments. Preliminary results show a non-trivial dependance between the aerosol particle number and the fog. In very polluted atmosphere, it is very difficult to form dense fog, however haze are very frequent (Jérome Rangognio PhD).

 Large Eddy Simulations

Strong horizontal and vertical gradient exist inside a fog layer. In order to accurately represent the interactions between the physical processes involved inside a fog layer a fine resolution is necessary. The increased computer power allows the use of sophisticated 3D models in research environment. Large Eddy Simulations are produced to gain understanding into boundary layer processes involved during the life cycle of a fog layer. These simulations are done with the Meso-Nh model with an horizontal resolution of 2m and a vertical resolution of 1m, and allow to study the heterogeneities inside fog layer at small scale.