Abstract
Abstract. Large eddy simulations (LESs) of a radiation fog event occurring during the ParisFog experiment are studied with a view to analyse the impact of the dynamics of the boundary layer on the fog life cycle. The LES, performed with the Meso-NH model at 5 m resolution horizontally and 1 m vertically, and with a 2-moment microphysical scheme, includes the drag effect of a tree barrier and the deposition of droplets on vegetation. The model shows good agreement with measurements of near-surface dynamic and thermodynamic parameters and liquid water path. The blocking effect of the trees induces elevated fog formation, as actually observed, and horizontal heterogeneities during the formation. It also limits cooling and cloud water production. Deposition is found to exert the most significant impact on fog prediction as it not only erodes the fog near the surface but also modifies the fog life cycle and induces vertical heterogeneities. A comparison with the 2 m horizontal resolution simulation reveals small differences, meaning that grid convergence is achieved. Conversely, increasing numerical diffusion through a wind advection operator of lower order leads to an increase in the liquid water path and has a very similar effect to removing the tree barrier. This study allows us to establish the major dynamical ingredients needed to accurately represent the fog life cycle at very high-resolution.
Highlights
Despite a long-standing interest in understanding fog processes, uncertainties still exist in the physical mechanisms driving fog variability
Large eddy simulations of a radiation fog event observed during the ParisFog campaign were performed, with the aim of studying the impact of dynamics on the fog life cycle
In order to study the local structures of the fog depth, simulations were performed at 5 m resolution on the horizontal scale and 1 m on the vertical scale near the ground, and included a tree barrier present near the measurement site, taken into account in the model by means of a drag approach
Summary
Despite a long-standing interest in understanding fog processes, uncertainties still exist in the physical mechanisms driving fog variability. During the formation phase, small banded structures, identified by Bergot (2013) as Kelvin–Helmholtz (KH) billows, occur in the middle of the fog layer in dynamical and thermodynamical fields They are sometimes associated with a burst of turbulent kinetic energy (TKE) (Nakanishi, 2000; Bergot, 2013) but this is not always the case (Porson et al, 2011). In order to establish the main ingredients driving the fog life cycle, and to evaluate how dynamics affects the evolution of fog, sensitivity simulations are conducted To our knowledge, this is the first time that an LES study of radiation fog has been performed at such high-resolution with a sophisticated microphysical parameterization scheme while considering the effect of heterogeneities such as forests on the fog dynamics.
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