Formation of fog due to stratus lowering: An observational and modelling case study

Abstract

We numerically investigate the processes responsible for a fog event formed by stratus cloud lowering, observed on December 1-2, 2016, during the experimental campaign in the northeast of France. The observations revealed a complex temporal evolution with stratus followed by a relatively drier period and then its reformation leading to fog formation by stratus lowering. Microphysical observations below a tethered balloon exhibit different vertical profiles of liquid water content and droplet concentration between the stratus and the fog formed below. A simulation at 100-m grid spacing reproduced the main observed characteristics of the cloud life cycle despite a time lag in stratus formation due to large-scale conditions. The advection of cloud water in the stratus and at its top appears crucial to feed the stratus lowering, resulting in radiative cooling, vertical transport, droplet sedimentation, evaporation, and cooling of the sub-cloud layer. The advection of cold or warm air in the lowest 250 m, mainly driven by fine-scale orographic circulations, impacts the fog formation due to stratus cloud lowering. When non-local conditions are favourable, the most important microphysical process to favour fog formation is the droplet sedimentation, leading to the cooling and moistening in the sub-cloud layer by evaporation. Droplet sedimentation appears more efficient when the droplet concentration is low, and a two-moment microphysical scheme more appropriate than a one-moment scheme to reproduce the observed variability of the droplet concentration. Given the predominance of non-local processes on this case study, a three-dimensional high-resolution model appears crucial to perform accurate forecasts of fog by stratus lowering.

Effect of the boundary layer low-level jet on fast fog spatial propagation

“…2.2.3) to the downstream area by the BLLJ could also be a potential reason. We hypothesize that the formation of ground fog is partly favored by the stratus lowering, which has been reported by previous studies (e.g., Haeffelin et al, 2010;Liu et al, 2012); the base height of stratus can be smaller than 100 m before fog formation (Dupont et al, 2012;Fathalli et al, 2022), which is basically close to our results (10-66 m in Fig. 10), while in this event the stratuslowering phenomenon remains to be verified by additional high-spatiotemporal-resolution vertical observations.…”

Simulating the Effects of Regional Forest Cover and Windthrow‐Induced Cover Changes on Mid‐Latitude Boundary‐Layer Clouds

“…To perform mesoscale simulations, we use the non-hydrostatic anelastic research model Meso-NH (Lac et al, 2018;Lafore et al, 1998) at 500 m resolution. This model is well adapted to hectometric resolution and has already been largely used to study the evolution of shallow clouds (e.g., Field et al, 2017), deep clouds (Nuissier et al, 2020;Strauss et al, 2019;Verrelle et al, 2017), and fog (Cuxart & Jiménez, 2012;Ducongé et al, 2020;Fathalli et al, 2022) at hectometric resolution.…”

Role of thermodynamic and turbulence processes on the fog life cycle during SOFOG3D experiment

“…It also has a high impact on solar energy, particularly in the mid-latitudes during autumn and winter. Based on in situ measurements, several studies have focused on fog formation at different regions and highlighted the main processes leading to its initiation, allowing the definition of five categories of fog: radiation fog (Price, 2019), advection-radiation fog (Gultepe et al, 2007(Gultepe et al, , 2009Niu et al, 2010a, b;Dupont et al, 2012), advection fog (Koračin et al, 2014;Liu et al, 2016;Fernando et al, 2021), fog by stratus lowering (Koračin et al, 2001;Fathalli et al, 2022), and precipitation fog (Tardif and Rasmussen, 2007;Liu et al, 2012). According to the literature, several processes are identified as driving fog evolution and dissipation depending on each category.…”

“…The dissipation processes are more difficult to study than the fog formation processes due to the complexity of fog's scale. At the state of the art, based on case studies, numerical weather prediction models (Philip et al, 2016;Bell et al, 2022) and high-resolution models (Price et al, 2018;Ducongé et al, 2020;Fathalli et al, 2022) including LESs (Bergot et al, 2015;Mazoyer et al, 2017) have the ability to simulate fog formation in several complex areas. However, they have difficulties in simulating the processes driving fog evolution over land in real time (Steeneveld et al, 2015;Price et al, 2015;Román-Cascón et al, 2016;Waersted et al, 2019;Pithani et al, 2020;Boutle et al, 2022).…”