The 3rd Wind Forecast Improvement Project (WFIP3), sponsored by the U.S. Department of Energy, seeks to improve our understanding of the physics of the atmosphere and ocean that dictate the structure and variability of the wind resource within the Marine Atmospheric Boundary Layer (MABL).
Atmospheric cold fronts have elongated along-frontal scales of many 1000s km, but much shorter cross-frontal scales of only 10-100s km. They are accompanied by gale-force surface winds (15-30 m/s) and mark abrupt shifts in the direction between the pre-frontal and post-frontal winds.
Figure 1: Snapshots of peak wave age (shading) and wave peak direction (arrows) on January 8, 2020 at 0600 UTC. Figure 2: Snapshots of 10-m wind speeds (shading, m s−1) and direction (arrows) on January 8, 2020 at 0600 UTC. Sauvage et al. (2023) investigate the impact of ocean surface waves on the momentum flux in the mixed sea state where locally-generated short wind-waves coexist with remotely-generated swell. The dominant wave direction in the mixed seas is not generally aligned with the local wind, whose effect on the surface drag is not currently considered in the COARE wave-based formulation for roughness length formulation, where wave stress is assumed to be a scalar quantity. Sensitivity experiments have been conducted using two different wave roughness parameterizations within COARE 3.5, including one that relies solely on wind speed (called wind-speed-dependent formulation, WSDF) and another that uses spatially varying wave age and wave slope (called wave-based formulation, WBF). Results show that, for sea states dominated by short wind waves under moderate to strong winds, the wave-based formulation increases the surface roughness length on average by 25 % compared to the wind-speed-based approach. On the other hand, for the missed sea states where the dominant wave characteristics are uncorrelated with local winds, the wave-based formulation predicts categorically lower roughness length and surface stress (~15 %). Through the surface layer-PBL coupling, the reduced stress over the mixed sea is also translated into the increased near-surface wind speed above the constant flux layer (~5 %).
Figure 1: Schematic of the main low-level mechanisms responsible for a sample case of HPE in the western Mediterranean region together with geographical locations. (from Ducrocq et al. 2016) Figure 2: Schematic of the main low-level mechanisms occuring during the HPE that happened between the 12 and 14 October 2016 south of France. The western Mediterranean region is regularly affected by heavy precipitation events that are characterized by a large amount of rainfall over a small area in a very short time; these events can lead to flash flooding, causing severe damage and, in some cases, casualties. Mediterranean HPEs are known to be violent events and are quite often associated with strong wind conditions and, thus, a very rough sea state. We investigated the role of the representation of the sea state during the HPE that occurred between the 12 and 14 October 2016 south of France. It led to large amounts of rainfall (up to 300 mm in 24 h) over the Hérault region (southern France). The study case was characterized by a very strong (>20 m s−1) easterly to south-easterly wind at low level that generated very rough seas (significant wave height of up to 6 m) along the French Riviera and the Gulf of Lion