Seastate-Dependent Air-Sea Heat Fluxes with Sea Spray in High Winds: Physical Processes, Multiscale Interactions, and Future Possibilities for Observations and Modeling

Benjamin Barr

Woods Hole Oceanographic Institution

Tuesday, Sep 24, 2024, 2:00 pm MT
DSRC Room GC402

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Abstract

Air-sea fluxes of sensible and latent heat are fundamental to the structure and energetics of coupled atmosphere-ocean boundary layers (BLs) and storm systems in extreme conditions. The contributions of sea spray to high-wind air-sea heat fluxes and corresponding impacts on storm structure and intensity are not well understood due to the difficulty of obtaining simultaneous, long-term measurements of spray, heat fluxes, and surface conditions in high-wind environments and to the challenge of representing complex seastate-dependent spray physics in weather forecast models. Here we present a new parameterization for seastate-dependent air-sea heat fluxes with spray, explore the multiscale interactions that it produces in coupled atmosphere-wave-ocean tropical cyclone (TC) simulations, and discuss ongoing work and future opportunities to constrain spray physics and air-sea fluxes using in situ observations.

Sea spray generation in the new seastate-dependent parameterization differs from traditional wind-based methods in terms of both total spray mass flux and the droplet size distribution. Spray heat fluxes from the new model behave differently than interfacial (bulk) heat fluxes, including a sharp transition of the spray sensible heat flux from negative (i.e., cooling, due to droplet evaporative cooling) to positive (i.e., warming, due to direct heating of the air by warmer droplets) when the 10-m windspeed U10 exceeds roughly 30 to 40 m s-1, which appears to control spray’s influence on TC intensity. For tropical storms and weak hurricanes, spray evaporation in the atmospheric BL cools inflow, suppressing eyewall deep convection and causing spray to oppose intensification. Continued intensification with stronger wave breaking increases spray production under the eyewall, producing surface warming by spray there. Further intensification amplifies this warming, allowing it to eventually overwhelm the negative influence of the cooled BL inflow and ultimately causing spray to promote intensification in strong hurricanes (i.e., Category 4 and 5 storms).

Ongoing work includes comparing parameterized heat fluxes to direct covariance sensible and latent heat flux measurements, which suggests the presence of spray heat fluxes in currently available measurements for U10 < 30 m s-1. Future improvement of the model will benefit from ongoing community efforts to observe waves, spray, and fluxes at the high-wind air-sea interface using coordinated measurements by buoys, drifters, autonomous vehicles, crewed aircraft and other systems.

Bio : My research focuses on fluxes and physical processes in the air-sea transition zone and how they influence atmospheric systems such as hurricanes and extratropical cyclones. I am particularly interested in the physics of air-sea interaction in high winds, where phenomena such as wave breaking and sea spray ejection push the limits of current understanding and modeling capability. I work to extend scientific knowledge and predictive power for these challenging topics by developing process-based parameterizations and model coupling strategies and performing numerical experiments to explore the complex interactions connecting storms to air-sea processes. While at WHOI for my postdoc, I am also exploring techniques to make the difficult observations critically needed to understand high-wind air-sea interface physics and to calibrate models. Additionally, I am investigating strategies for using high-resolution simulations, such as Large Eddy and Direct Numerical Simulations, to calibrate the complex physics in parameterizations for weather and climate models. I view improvement of process parameterizations and atmosphere-wave-ocean model coupling strategies as a critical opportunity for enhancing our understanding of the air-sea transition zone in severe weather systems and our ability to protect the people and built environments that are impacted by severe weather events.