SOuth west FOGs 3D experiment for processes study – SOFOG3D

Fog strongly perturbs the aviation, marine and land transportation, leading to human losses and high financial costs. Accurate forecastings of fogs are thus required to reduce their impacts on human activities. However there are still many unknowns in the physical mechanisms driving fog variability and how they interact.
The primary objective of SOFOG3D is to advance our understanding of fog processes to improve fog forecasts by numerical weather prediction (NWP) models. Specifically, SOFOG3D conducts process studies on very well documented situations, using synergy between 3D high-resolution Large Eddy Simulation (LES) and unprecedented 3D detailed observations.
A field experiment will take place in the South-West of France which is particularly prone to fog occurrence, to provide 3D mapping of the boundary layer during fog events. The observation strategy is to combine vertical profiles derived from new remote sensing instruments (microwave radiometer (MWR), Doppler cloud radar and Doppler lidars) and balloon-borne in-situ measurements, with local observations provided by a network of surface stations, and a fleet of Unmanned Aerial Vehicles (UAV) to explore fog spatial heterogeneities.
Three nested domains will be instrumented with increasing density to collect observations from regional scale (300x200 km) down to local scale on the super-site (10x10 km). On the super site, detailed measurements of meteorological conditions, aerosol properties, fog microphysics, water deposition, radiation budget, heat and momentum fluxes on flux-masts will be performed on three areas with different characteristics to investigate the impacts of surface heterogeneities (types of vegetation, rivers, orography) on fog processes, and turbulence anisotropy. A technical objective concerns the improvement of temperature, humidity and liquid water retrieval from the combination of cloud radar and MWR measurements. They will be validated with in situ measurements from tethered-balloon, radiosoundings and UAV.
LES of the most documented fog cases will be run at metric resolutions to provide spatio-temporal turbulence and microphysical characteristics of the fog layer and the atmosphere above. Validated by measurements, they will offer a complete 3D description and useful diagnostics to help analysis. They will assess recent advances in parametrizations (two-moment microphysical schemes, turbulence and radiation). Their combination with 3D observations will deliver a comprehensive description of the impact of surface heterogeneities on the fog life cycle.
Process studies based on these hyper documented cases aim to better understand contrasts in key parameters leading to radically different fate in fog life cycles, from formation stage all the way though dissipation, such as shallow stable fog vs deep adiabatic fog, stratus lowering into fog vs stratus persisting aloft, and daytime fog dissipation vs daytime fog persistence. SOFOG3D will particularly focus on the impact of entrainment at fog top, the surface energy budget, the impact of aerosols on radiative cooling and heating and on fog microphysics, and the dissipation period, through radiation, aerosol absorption, droplet sedimentation and deposition processes.
Process studies will provide guidelines for improving physical parametrizations of NWP models. SOFOG3D will also investigate how improving the initial conditions of NWP models can improve fog forecasts. To that end, a ground-based MWR network, combined with cloud radar observations when available, will be assimilated in a convective scale model using an innovative ensemble-based variational data assimilation scheme to reduce fog forecast errors.
Thus, SOFOG3D will provide unprecedented process studies based on 3D observational strategy with innovative techniques combined with very high resolution simulations and advanced data assimilation techniques, paving the way for future improvements in NWP models to provide accurate fog forecasts.