Large-eddy simulation of radiation fog: Part 1: Impact of dynamicson microphysics

Large Eddy Simulations (LES) of a radiation fog event occurring during ParisFog experiment have been studied with a view of analyzing the impact of the dynamics on the microphysics. The LES, performed with the Meso-NH model at 5 m resolution horizontally and 1 m vertically, and with a 2-moment microphysical scheme, included the drag effect of a trees barrier and deposition on vegetation. The model shows a good agreement with the measurements of the near surface dynamic and thermodynamic parameters as well as the cloud water content, but overestimates the cloud droplet sizes and concentration. The blocking effect of the trees induced elevated fog formation, like in the observation, and horizontal heterogeneities, and limited the cooling and the cloud water production. The deposition process was found to exert the most significant impact on the fog prediction, as it not only erodes the fog near the surface, but also modifies the fog life cycle and induces vertical heterogeneities. The comparison with the 2 m horizontal resolution simulation exhibited small differences meaning that the grid convergence was achieved. Conversely, increasing numerical diffusion through a wind advection operator of lower order led to an overestimation of the near surface microphysical fields and had almost a similar effect than removing the effect of the trees barrier. This study allows to establish the major dynamical ingredients necessary to perform correctly the fog life cycle prediction at high resolution.