Characterizing Mechanisms Leading to Nocturnal Boundary Layer Turbulence during Clear Sky Conditions at Cape Cod

During the U.S. Department of Energy’s Wind Forecast Improvement Project 3 (WFIP3), a network of instruments for wind measurement including a tower with sonic anemometers at 2 and 10 m and two scanning Doppler lidars were installed at the Highlands Center about 100 m from the coast. In this study, we will explore the drivers behind nocturnal turbulence observed at the Cape Cod Highlands Center (CACO) WFIP3 site about 100 m from the shore of Cape Cod. Using vertical radial velocity observations from a Halo Photonics Doppler lidar installed at the CACO WFIP3 site, from 35 clear air nights from January to May 2024, we identified 8 nights with an unstable boundary layer as indicated by radial vertical velocity variances greater than 0.4 m s-1. Vertical wind profiles from the Velocity Azimuth Display (VAD) scans of the Doppler lidar, first, show that the dominant wind direction at 200 m is from onshore during the turbulent cases and from offshore during the stable cases. In addition, the VAD scans from the turbulent cases show that the 0-1 km vertical wind shear is on the order of 13 m s-1 km-1 during the turbulent cases and 8 m s-1 km-1 for the stable cases, suggesting that this nocturnal turbulence is driven by strong vertical wind shear from low-level jets. In addition, the maximum wind speeds in the lowest 1 km are 15 m s-1 for the turbulent cases and 10 m s-1 for the stable cases.

To better understand the underlying dynamics of these cases, we employ the WRF model in Large Eddy Simulation (LES) mode. The LES simulation is designed to capture the fine-scale turbulent structures and vertical shear associated with the low-level jets observed in the data. By simulating the turbulent and stable nights, we aim to validate the observed differences in the 0-1 km vertical wind shear and wind speed profiles, with a focus on the role of strong vertical wind shear in triggering nocturnal turbulence. These simulations will help elucidate the interaction between coastal topography, wind direction, and low-level jets in driving boundary layer turbulence under clear sky conditions at Cape Cod.