Demystifying the behavior of Coastal Atmospheric Surface Layer Turbulence

For decades, carefully-selected and quality-controlled measurements of atmospheric turbulence and fluxes across a variety of environments – including open ocean, flat farmland, and complex mountainous terrain – has facilitated verification of classical theories of atmospheric turbulence upon which numerical weather model parameterizations, air quality studies and monitoring, and myriad other applications are based. However, measurements of coastal atmospheric boundary layer (ABL) turbulence are limited, in part due to the challenges in securing long-term measurements in addition to the inherent complexities in such environments. Specifically, coastal regions – spanning several tens of kilometers on either side of the coastline – are characterized by strong gradients in surface thermal and roughness properties and can feature complex topography, which modify the ABL beyond the canonical homogeneous theory. Consequently, the applicability of traditional theories and behavior of atmospheric turbulence within coastal environments remains an enigma.

Here, we examine coastal turbulence data obtained at a novel coastal observing site, the Naval Research Laboratory Coastal Environmental Observation Station (NRL-CEOBS), located within the Monterey Bay region of Central California. We address challenges in quality controlling high-frequency turbulent data, and present preliminary eddy-covariance data collected across January to March 2024 collected from a flux tower outfitted with a sonic anemometer and gas, humidity, and temperature sensors located within 0.25 km of the coast. Computed velocity and temperature variance spectra and flux cospectra are compared with classical datasets (e.g., Kaimal et al 1972). Preliminary results reveal that the existence of a true inertial subrange hinges on the direction of the prevailing flow and therefore flux footprint. Additionally, turbulence isotropy is diagnosed using the Reynolds Stress tensor. The data reveal highly anisotropic turbulence is diagnosed more frequently during onshore-oriented flow compared to offshore flow. The proportion of isotropic data increases when decreasing the flux averaging time period, which removes large, energy-producing scales of motion from consideration. We also corroborate the flux tower measurements with upper boundary layer measurements obtained using profiling and scanning LiDAR modes, to elucidate environmental flow attributes which correlate with the near-surface turbulence behavior.