Fog formed by stratus lowering: an observational and modeling case study from the SOFOG3D field campaign

Fog is a difficult meteorological phenomenon to predict due to its high spatial and temporal variability and the complexity of physical processes and their interplay. In this context, the SOFOG3D field campaign, which took place during winter 2019/2020 over the Landes region in the South-West of France, provides a 3D mapping of the boundary layer during fog events. It aims to advance our understanding of fog processes in order to improve forecasts of fog events by numerical weather prediction (NWP) models.

 

The present study focuses on three days between 28 and 30 December 2019 characterized by different fog life cycles between two sites about 100 km apart. In situ and remote sensing measurements, such as microwave radiometer and cloud radar, show that on the supersite a radiative fog that occurred the first night lifted into a stratus in the morning remained all day long, and lowered in the afternoon to form a new fog by stratus lowering. In contrast at the Agen site, the stratus completely dissipated and a radiative fog formed the second night. The widespread radiative fog over the entire domain during the first night developed due to cold air advection from the East. We conduct data analysis to study why the stratus lowering is generalized over the northern part of the domain, while the stratus completely dissipates over the southern part of the domain.

 

This analysis is complemented by a 3D numerical simulation performed with the Meso-NH model applied at 100 m resolution with a downscaling approach from the operational AROME model, using the LIMA 2-moment microphysical scheme with a prognostic representation of a multimodal aerosol population. Preliminary results show that for the second night, the simulation well reproduces the contrasting fog life cycle for both sites with radiative fog on Agen and stratus lowering on the supersite but forming earlier fog on the former.

 

A budget analysis is conducted to investigate the spatial heterogeneity of this fog event at the regional scale and to study the physical mechanisms involved in fog formed by stratus lowering that remains especially difficult to forecast by NWP models.