Pier‐Based Measurements of Air‐Sea Momentum Fluxes Over Shoaling Waves During DUNEX. Issue 11 (16th November 2022)

• Record Type: Journal Article
• Authors: Potter, Henry; Collins, Clarence O.; Ortiz‐Suslow, David G.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-11-16
• Subjects: momentum flux -- drag coefficient -- shoaling waves -- coastal -- nearshore; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2022JC018801
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Open‐ocean homogeneity is violated in the near shore region by wave shoaling and breaking, varying wind‐swell incidence angles, complex currents patterns, rapid bathymetric changes, and shore‐side topographic features. Consequently, existing drag coefficient parameterizations, which were largely developed in deep water, are not effective. This has an adverse impact on marine forecasts, weather and climate models, and coastal morphology prediction. There exists a critical need to identify and systematically quantify the impact of coastal processes and features on the momentum flux over a wide range of wind speeds. As part of the DUring Nearshore Event eXperiment at the Field Research Facility in Duck, NC, six months of direct flux observations were collected from a tower at the end of a 560 m pier. These were used to study the variability of the aerodynamic drag coefficient. Results show that for winds above 3 m s −1 the drag coefficient was between 15% and 50% higher for onshore winds when compared to alongshore. This is attributed to more energy in the downwind component of the flux around the dominant wave frequency for onshore winds, and is independent of wind speed. Holistically, the Coupled Ocean Atmosphere Response Experiment (COARE) algorithm over predicts the drag coefficient for shoaling waves. When separated by wind direction, COARE best predicts the drag coefficient for onshore winds. The results of this study can help the development of more robust nearshore momentum flux parameterizations. This will ultimately lead to improved weather and climate forecasting in the littoral zone, and more physically realistic short‐ and long‐term coastal resilience strategies. Plain Language Summary: When wind blows over the ocean, energy in the form of momentum (motion) is exchanged. This exchange (flux) impacts everything from climate to weather and ecosystem health to sediment transport. Currently, when predicting the rate of air‐sea momentum flux we rely on measurements that were made in deep water where conditions are spatially uniform. Unfortunately, these predictions are not effective nearshore. To help fill this knowledge gap, our research focuses on the momentum flux nearshore where the water is shallow enough for waves to shoal (i.e., get taller, slower, and closer together) as they approach land. We use six months of wind measurements from an anemometer mounted on a pier to quantify the momentum flux. We also used wave data recorded nearby. Our most interesting finding is that the flux is higher when the wind blows onshore when compared to alongshore. This is due to interaction between the wind and waves. However, we couldn't determine if the waves increase the flux for onshore winds due to longer time and space scales of interaction, or if the flux is decreased for alongshore winds due to reduced wind‐wave interaction (or a combination of both). Key Points: Six months of pier‐based direct flux measurements over shoaling waves Above 3 m/s, the drag coefficient is 15%–50% higher for onshore winds when compared to alongshore Higher onshore drag coefficient is attributed to greater flux around the dominant wave frequency and is independent of wind speed