Observations and quantification of fog droplet settling made during the SOFOG3D campaign

This paper presents observations collected during the SOFOG3D campaign in South Western France during 2019-2020 to elucidate the processes involved in the deposition of fog droplets to the surface, a process that is often modelled in numerical weather prediction models, and forms an important component of the water budget within fog. The study suggests three main mechanisms cause deposition: turbulent, whereby water drops are transported to the surface in turbulent eddies; aerodynamic, whereby the mean horizontal wind deposits droplets directly onto surface canopy elements that protrude into the airflow, and gravitational, whereby droplets fall under the action of gravity directly onto the canopy. Results indicate that the gravitational settling is on average a minor component of the deposition, amounting to around 14% of the total figure. The study also identified that this proportion is relatively larger at 22%, when the turbulence intensity levels are low (vertical velocity variance < 0.003 m²s²). Consequently, for greater values of turbulence intensity the proportion is smaller at 11%. All three deposition processes are studied further using a multiple linear regression of terms against observed water deposition rates, in an attempt to evaluate various methods to estimate the deposition. Results indicate that a simple regression of liquid water content against observed deposition has some predictive skill, but that a multiple regression including all three terms produces better results with smaller regression errors. A simpler multiple regression, that merged turbulent and aerodynamic terms into a single ‘dynamic’ term produced results almost identical to the three-term multiple regression. Consequences for parametrization of liquid water deposition in fog are discussed.