Analysis of coastal breeze and low-level jets events from numerical modeling and LiDAR measurements

The complexity and variety of phenomena that can occur in coastal areas (e.g. breezes, internal boundary layers, low-level jets (LLJ)) often result in wind profiles strongly deviating from the Monin-Obukhov similarity theory. Moreover, the lack of offshore experimental data leads to gaps in knowledge of the marine coastal atmospheric boundary layer (MCABL). Characterizing the wind resources in MCABL has been identified by Veers et al. (2022) as one “Grand Challenge” for the development of offshore wind farms.

To partially fill these gaps, a joint numerical and experimental study is currently being performed. In 2020, scanning LiDAR measurements were carried out within a few kilometers of the French Atlantic coastline (Conan and Visich, 2023). In parallel, mesoscale to microscale simulations are performed with the Weather Research and Forecasting (WRF) code, using the grid-nesting method to progressively decrease the horizontal mesh size from a few kilometers (RANS modeling) down to a hundred meters (LES modeling). The simulation, giving access to more atmospheric variables in a large area, allows a complementary analysis.

On a particular week of this experiment, complex velocity profiles have been observed in the LiDAR data, highlighting the presence of LLJ and high wind-shear events. Velocity profiles from RANS simulations show good comparison with LiDAR data, which suggests that the mechanisms responsible for the observed phenomena are well reproduced. In addition, these large-scale simulations allow the identification of a complete sea-breeze circulation in the complex coastal area of Brittany.

The marine extent of the sea-breeze can be defined as the isoline where cross-coast velocity component decreases with the distance from the coast to a value of 1 m/s (Arritt, 1989 ; Finkele et al., 1995). The RANS results indicate that the sea-breeze can reach a distance of 70 km offshore during the studied period. A first analysis of the simulations also suggests that the nighttime LLJ observed on the Atlantic coast is related to the residual of a sea-breeze front moving southward from the north coast of Britanny.

Analysis of the LES simulations will permit to study more precisely the onset of the sea-breeze, the evolution of the LLJ across the coastline (related to transition in atmospheric stability and surface roughness) or the turbulence kinetic energy budget in the jet core.

 

Arritt, R.W. 1989. Quarterly Journal of the Royal Meteorological Society 115 (487): 547‑70. https://doi.org/10.1002/qj.49711548707.

Conan, B.,and A. Visich. 2023. Wind Energy Science Discussions, October, 1‑23. https://doi.org/10.5194/wes-2023-141.

Finkele, K., et al.. 1995. Boundary-Layer Meteorology 73 (3): 299‑317. https://doi.org/10.1007/BF00711261.

Veers, P., et al.. Wind Energy Science 7 (6): 2491‑96. https://doi.org/10.5194/wes-7-2491-2022.