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Abstract The behavior and predictability of rip currents (strong, wave‐driven offshore‐directed surfzone currents) have been studied for decades. However, few studies have examined the effects of rip channel morphology on the rip generation or have compared morphodynamic models with observations. Here, simulations conducted with the numerical morphodynamic model MIKE21 reproduce observed trends in flows and bathymetric evolution for two channels dredged across a nearshore sandbar and terrace on an ocean beach near Duck, NC, USA. Channel dimensions, wave conditions, and flows differed between the two cases. In one case, a strong rip current was driven by moderate height, near‐normally incident waves over an approximately 1‐m deep channel with relatively little bathymetric evolution. In the other case, no rip was generated by the large, near‐normally incident waves over the shallower (∼0.5 m) channel, and the channel migrated in the direction of the mean flow and eventually filled in. The model simulated the flow directions, the generation (or not) of rip currents, and the morphological evolution of the channels reasonably well. Model simulations were then conducted for different combinations of the two channel geometries and two wave conditions to examine the relative importance of the waves and morphology to the rip current evolution. The different bathymetries were the dominant factor controlling the flow, whereas both the initial morphology and wave conditions were important for channel evolution. In addition, channel dimensions affected the spatial distribution of rip current forcings and the relative importance of terms.
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Elevated Momentum Flux in the Surfzone during a Storm
Abstract Drag coefficient parameterizations, which are largely developed from homogenous deep ocean data, are ineffective nearshore where conditions are nonuniform. This is problematic because operational forecast accuracy depends upon reliable quantification of air–sea momentum transfer. This is especially important for storms which threaten coastal life and property. To help fill this knowledge gap, direct flux measurements were collected from the beach and pierhead in Duck, North Carolina, as part of the During Nearshore Event Experiment (DUNEX). The footprint analysis shows these fluxes were sourced in the surfzone and offshore, representing very different conditions. During a weeklong storm, wind speeds and significant wave heights were 20 m s−1and 4 m, leading to a broad, vigorous surfzone. The drag coefficient in the surfzone was twice the offshore value, explained by increased roughness due to wind stress and bathymetric changes. The Charnock parameter is well predicted by wave age, but it is expected this is site-specific due to unique bathymetry. A horizontal wind speed gradient was observed and attributed to the high surfzone roughness. The wavelengths of the turbulent eddies in the surfzone were smaller than offshore or predicted by universal scaling. This research offers novel insights that can contribute to a crucial collective effort to develop robust coastal flux models, leading to improved forecasting. Significance StatementWhen wind blows over the ocean, the energy associated with its motion is moved from the air into water. This energy transfer helps grow waves and drive currents which, via many pathways, alter the characteristics of the upper ocean and lower atmosphere. In turn, this affects weather and climate, so it is critical this energy exchange is accounted for in forecasts. Energy transfer is reasonably well understood in the deep ocean, but not nearshore where conditions are nonuniform and change quickly, especially in storms where very few measurements are made. To remedy this, data were collected in May 2022 during a storm in Duck, North Carolina, which had wind speeds of 20 m s−1and 4-m wave heights. The extreme conditions created a very wide and energetic surfzone. Wind measurements were made on the beach and approximately 500 m offshore. Due to the rough surface, twice the energy was transferred from the air into the ocean in the surfzone than offshore and the wind speed decreased as it crossed the surfzone. Finally, the wavelengths of the wind that transfer energy into the ocean are much smaller than offshore or predicted by previous research.
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- Award ID(s):
- 2319548
- PAR ID:
- 10614283
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 82
- Issue:
- 7
- ISSN:
- 0022-4928
- Page Range / eLocation ID:
- 1237 to 1247
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1175/JAS-D-24-0090.1
