Characteristics of Momentum Flux over Heterogeneous Surfaces

As part of the Coastal Land-Air-Sea Interaction (CLASI) field campaign at the Florida Gulf coast in February 2022, we made high-rate measurements of temperature, water vapor, and turbulence from five towers at variable distances from the shore within the Topsail Hill Preserve State Park and at the inland water bank. Two towers were set nominally at 100 and 200 m from the coastline. Here, we present results on the variability of momentum flux to enhance the understanding of near-surface momentum exchange and aerodynamic roughness at the coastal sites. This research focuses on analyzing wind speed, direction, and the resulting momentum flux across heterogeneous surfaces, with an emphasis on the dependence of momentum fluxes on wind direction at the boundary of ocean and land surfaces. We found that the variability of momentum fluxes with wind direction is associated with the variable terrain height and hence surface roughness upwind of the tower location. A linear momentum flux and wind speed relationship was also identified in some of the onshore wind conditions. The findings suggest that current surface flux parameterizations inadequately represent surface roughness and momentum flux in coastal zones, particularly when accounting for directional wind dependence. The study provides new insights into the dynamics of the coastal atmospheric surface layer, which are essential for improving the accuracy of atmospheric modeling in similar environments.