Study Provides New Way of Looking at Energy Exchange at the Air-Sea Boundary Layer
– AUGUST 8, 2019
Scientists assessed the dynamics of heat and momentum exchange between the ocean and atmosphere to better understand how these factors influence Gulf of Mexico circulation. Specifically, they investigated how these factors constrained the heat dissipation rate (energy transferred to the atmosphere) in non-hurricane conditions during a month-long field experiment studying ocean transport processes. Analysis showed that dissipative heating in low-wind conditions can equal 20%–80% of the heat flux, suggesting that the dissipative heating is not dependent directly upon wind speed. Dissipation rates increased with the steepening of nonbreaking waves, which mechanically enhanced turbulent vertical velocities. Shear or buoyancy (depending on the atmosphere’s vertical structure at any given measurement site) was the dominant mechanism of turbulent kinetic energy (TKE) production and dissipative heating in the atmospheric surface layers sampled. These observations suggest that the traditional formula theorizing that dissipative heating is proportional to wind speed cubed overestimates the magnitude of dissipative heating.
The researchers published their findings in the Journal of the Atmospheric Sciences: Stability and sea state as limiting conditions for TKE dissipation and dissipative heating.
The atmospheric surface layer is directly adjacent to the air–sea interface, where turbulent physical processes can cause momentum transfer to surface waves and currents and, subsequently, a dissipative heat transfer to the atmosphere. This movement of heat influences the development of deep ocean currents, creating flow patterns that affect waters globally. Intensity forecasts for tropical cyclones use dissipative heating parameterizations to incorporate this heat exchange. However, there have been questions raised about the assumptions behind the parameterizations, specifically that all dissipated TKE is transferred as heat and increases the total energy in the atmosphere-ocean system.
“Understanding how the dynamics of the atmosphere and ocean surface structure constrain TKE (the energy associated with the atmosphere’s movement) and its dissipation is an important piece to characterizing the balance of forces and energy on either side of the air-sea interface,” explained study author Andrew Smith. “We were especially interested in how the TKE behaved based on the density structure of the air in the layer of atmosphere near the atmosphere-ocean boundary and the influence of ocean surface waves, rather than just associating the TKE with energy coming from the wind or the wind speed.”
This study challenged the notion that wind speed is the physical process most directly governing and constraining the dissipation rate and dissipative heating. To identify other influential factors, the team analyzed high-resolution ship measurements collected during the Lagrangian Submesoscale Experiment or LASER, studying ocean transport of floating materials, such as happened with the oil slick from Deepwater Horizon.
“Although research vessels have been used to collect scientific data for a long time now, the F.G. Walton Smith was equipped with high-sampling-rate meteorological and oceanographic instruments over a long-term experiment period while providing measurements over a diverse spatial range of the Gulf as well,” said Smith. “Additionally, we also used a unique method for extracting and examining near-surface currents from the vessel’s marine Doppler radar.”
After compiling the first reported dissipative heating measurements in the low-wind atmospheric boundary layer, the team compared their results with aircraft hurricane measurements from the Coupled Boundary Layers Air-Sea Transfer (CBLAST) Experiment. While dissipative heating increased with wind speed regardless of computational method (LASER or CBLAST measurements), the behavior as a function of wind speed was pointedly different.
“The dissipation of TKE and the dissipative heating have a physical limit in terms of rate and efficiency, which are greatest in a non-neutral atmospheric surface layer (vertical structure is stratified in some way) and above a sea surface with steeper wind-waves and slow swells,” explained Smith. “Based on this work, the recent re-evaluation of the original computation of dissipative heating and conventional thinking on the TKE dissipation rate was warranted and useful.”
Smith explained that the data from a non-hurricane environment gave credence to a new way of looking at turbulent energy behavior. “By extending our inquiry to the laboratory for high-wind hurricane conditions, we will shed light on energy transfer and the physics within a hurricane environment. Such an environment resonates with the public, especially residents in hurricane-afflicted locales.”
Data are publicly available through the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at doi:10.7266/N7S75DRP.
By Nilde Maggie Dannreuther and Stephanie Ellis. Contact firstname.lastname@example.org with questions or comments.
This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Consortium for Advanced Research on Transport of Hydrocarbons in the Environment III (CARTHE-III). Other funding sources included the National Oceanic and Atmospheric Administration (NA14NWS4680028) and the National Science Foundation (AGS1822128).
The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit https://gulfresearchinitiative.org/.
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