Early-Morning Adjustment of the Coastal Marine Boundary Layer Under Offshore Flow: Airborne Observations

Abstract

The transition of the atmospheric boundary layer from land to sea remains insufficiently documented by observations, particularly under offshore flow conditions, where land-modified air undergoes rapid thermodynamic and dynamical adjustment over the ocean. This study uses high-resolution airborne observations from a series of research flights conducted over the German Bight in late summer 2024 to investigate the early-morning coastal marine boundary layer. From this dataset, two representative cases from the southern and eastern German Bight are analyzed in detail, capturing contrasting coastal and bathymetric conditions. Combined horizontal transects and vertical profiles reveal a shallow marine boundary layer capped by an inversion typically located between 100 and 250 m. The profiles further indicate the presence of low-level jets (LLJs), suggesting that turbulence during the morning hours was largely sustained by vertical wind shear under near-neutral to weakly stable conditions. Sea surface temperature (SST) exhibits pronounced small-scale variability in shallow nearshore regions, likely driven by tidal modulation and complex bathymetry, particularly over the Wadden Sea. These gradients are associated with variations in air–sea temperature contrasts, boundary-layer stability, turbulence, and wind-speed adjustment with increasing offshore distance. Comparison with the Copernicus North-West Shelf (NWS) ocean model and the Operational SST and Sea Ice Analysis (OSTIA) product reveals predominantly warm biases of approximately 0.2–1.0 K relative to the aircraft observations. Larger deviations occur under nearshore conditions due to unresolved shallow-water processes and strong small-scale SST variability. In addition, both datasets show limited skill in capturing the observed temporal evolution of SST, particularly under nearshore conditions with strong spatiotemporal variability. The observations suggest that offshore flow interacting with thermally heterogeneous coastal waters can produce a shallow, predominantly shear-driven marine boundary layer during the early morning across different coastal environments. These results highlight the importance of resolving nearshore SST variability in both space and time and provide observational constraints for improving the representation of coastal boundary-layer processes in atmospheric models and offshore wind applications.