Generation and transport of sea spray

Project Overview: The generation of sea spray at the ocean surface in high-wind conditions is a highly complex physical process with a potentially large impact on near-surface momentum, heat, moisture, and gas air-sea transfers. Yet, accurate droplet source functions remain elusive and droplet transport models are challenging. A large, basic uncertainty exists regarding the quantity, size distribution, and ejection velocity of droplets produced at the surface of wind-forced, breaking waves, which greatly impedes the development of models that aim to assess the impact of spray on the near-surface momentum and thermodynamic fluxes. The contributions of sea spray drops to the total air-sea exchanges of momentum, heat, and mass remain an open question. A number of factors obscure any simple quantification of their contribution.

In this project, we combined experimental and theoretical approaches to tackle this issue. Optically-based laboratory experiments will yield droplet concentrations and velocity size spectra at several positions along and downstream of a single breaking event.

Theoretical efforts follow two main avenues of research: 1) Lagrangian point particle transport and 2) Eulerian models of spray transport and evaporation in the moist atmospheric boundary layer.

1) The per-drop contribution to the fluxes is a particularly important factor, as is the number of formed drops. To estimate the per-droplet fluxes, we converge upon the low order statistics from a large number of drop trajectories, which are simulated with a recent Lagrangian Stochastic model adapted for heavy drops within the marine boundary layer. The results from simulations of sea spray drops over the ocean suggest that common simplifications in previous sea spray models, such as the residence time in the marine boundary layer, may not be appropriate.

2) Using a theoretical and mathematical framework inspired by kinetic gas theory, we reconcile the Lagrangian nature of spray transport with the Eulerian description of the atmosphere. In turn, this enables a relatively straightforward inclusion of the spray fluxes and the resulting spray effects on the atmospheric flow. A comprehensive dimensional analysis has led us to identify the spray effects that are most likely to influence the speed, temperature, and moisture of the airflow. We also provide an example application to illustrate the capabilities of the model in specific environmental conditions. Finally, suggestions for future work are offered.

Publications:

• Veron, F., & Mieussens, L. (2020). An Eulerian model for sea spray transport and evaporation. J. Fluid Mech, 897, A6. doi:10.1017/jfm.2020.31
• Richter, D. H., and F. Veron, 2016: Sea Spray: an outsized influence on weather and climate, Physics today., November 2016, 34-39.
• Veron, F., 2015: Ocean spray. Annu. Rev. Fluid Mech., 47:507 38, DOI: 10.1146/annurev-fluid-010814-014651
• Mueller, J., and F. Veron, 2014: Impact of Sea Spray on Air-Sea Fluxes. Part II: Feedback Effects. J. Phys. Oceanogr., 44, DOI: 10.1175/JPO-D-13-0246.1.
• Mueller, J., and F. Veron, 2014: Impact of Sea Spray on Air-Sea Fluxes. Part I: Results from Stochastic Simulations of Sea Spray Drops over the Ocean J. Phys. Oceanogr., 44, DOI: 10.1175/JPO-D-13-0245.1.
• Mueller, J., and F. Veron, 2010b: A Lagrangian stochastic model for sea spray evaporation in the atmospheric marine boundary layer. Boundary-Layer Meteorology. 137: 135-152.
• Mueller, J., and F. Veron, 2010a: Bulk formulation of the heat and water vapor fluxes at the air-sea interface including nonmolecular contributions. J. Atmoph. Sciences., 67(1): 234-247.
• Mueller, J., and F. Veron, 2009c: A Sea State-Dependent Spume Generation Function. J. Phys. Oceanogr., 30(9): 2363-2372
• Mueller, J. and F. Veron, 2009b: Lagrangian Stochastic Model for Heavy Particle Dispersion in the Atmospheric Marine Boundary Layer. Boundary-Layer Meteorology. 130(2): 229-247.
• Mueller, J. and F. Veron, 2009a: Nonlinear Formulation of the Bulk Surface Stress over Breaking Waves: Feedback Mechanisms from Air-flow Separation. Boundary-Layer Meteorology. 130(1): 117-134.

Rainfall on the surface of the ocean

Project Overview: Rain impacts on the ocean surface damping the surface waves and generating intense near-surface mixing. We study the influence of rainfall on the generation of turbulence and the subsequent gas flux between the atmosphere and the ocean.

Small-scale ocean surface dynamics, including waves and turbulence, affect the global climate through their impact on ocean-atmosphere exchange. However, the role of rainfall on surface fluxes, the dynamics of surface waves, and properties of the marine atmospheric boundary layer have not been studied in detail. Rough estimates indicate that a significant percentage of the shear stress at the ocean surface may be provided, at low wind speed, by rainfall. In addition, rainfall produces significant turbulence in the very near-surface layers. This turbulence, in addition to the droplet impacts, disrupts the diffusive molecular surface layers and therefore has the potential to greatly influence air-sea gas fluxes. In this work, laboratory experiments will be performed to directly measure the turbulence generated by rainfall along with the air-water gas, momentum, and heat exchange rates. The role of the dominant parameters such as rain rate, droplet size, wind speed, shear, and turbulence will be measured to quantify the mechanisms by which significant mixing is achieved, and the coupling between the surface fluxes induced by the wind and that induced by the rain will be explored. Secondarily, the dynamic effects of rain-induced stresses on the surface waves will be investigated, specifically with regard to the dependence of the droplet impact angle and velocity (wind speed dependent) on the partition of shear stress (horizontal) and momentum flux (vertical). These effects will further be related to the mechanisms responsible for the surface fluxes. This research is anticipated to lead to a much-needed, improved understanding of the transformation of turbulence within the air boundary layer and fluxes to the upper ocean by rainfall and the consequent enhanced air-sea gas flux.

The figure above shows the turbulent Kinetic energy generated by rainfall on the water surface. One can see both the KE increase as well as the penetration rate increase with higher rain rates.

Publications:

• Harrison, E.L., and F. Veron: 2017: Near-surface turbulence and buoyancy induced by heavy rainfall. J. Fluid Mech. 830, 602-630, DOI: 10.1017/jfm.2017.602
• Veron, F. and L. Mieussens, 2016: A kinetic model for particle-surface interaction applied to rain falling on water waves,. J. Fluid. Mech. – 796, 767-787.
• Harrison, E.L., F. Veron, D.T. Ho, M.C. Reid, P. Orton, and W.R. McGillis, 2012: Nonlinear interaction between rain- ad wind-induced air-water gas exchange. J. Geophys. Res, 117, C03034.
• Ho, D.T., Veron, F., Harrison, E., Bliven, L.F., Scott, N., McGillis, W.R. 2005: The combined effect of rain and wind on air-water gas exchange: A feasibility study. J. Mar. Syst., 66(1-4), 150-160.

Turbulence on both sides of the air-sea interface

Project Overview: In recent years, considerable progress has been made in quantifying the multiple air-sea fluxes. These air-sea transfers indeed play a crucial role in controlling synoptic scale meteorology as well as longer-term climatic trends. In particular, the fluxes of heat and gas rely on exchange processes through the molecular layers, which are usually located within the viscous layer, which is in turn modulated by the waves and the turbulence at the free surface. The understanding of the multiple interactions between, molecular layers, viscous layers, waves, and turbulence is therefore paramount for an adequate parameterization of these fluxes. However, the fundamental processes involved rely on small scale turbulence at the air-sea interface. These processes are poorly resolved and air-sea fluxes are generally parametrized using readily available, bulk variables such as wind speed. In this line of work, we study turbulence on both sides of the interface both in the laboratory and the field.

In a recently funded project, we aim to simultaneously measure the flows on both sides of the air-sea interface to investigate the kinematics and dynamics of the coherent structures and turbulence that drive these interfacial fluxes.

The uppermost layers of the ocean, along with the lower atmospheric boundary layer, play a crucial role in the air-sea fluxes of momentum, heat, and mass, thereby providing important boundary conditions for both the atmosphere and the oceans that control the evolution of weather and climate.

During several field experiments, we have found evidence of a clear coupling between the surface waves, the surface temperature, and the surface turbulence. The modulation of the surface temperature by the waves leads to a measurable wave-coherent air-sea heat flux. When averaged over time scales longer than the wave period, the coupling between the surface temperature and turbulence leads to a spatial relationship between the temperature, divergence, and vorticity fields that are consistent with spatial patterns of Langmuir turbulence. On time scales for which the surface wave field is resolved, we show that the surface turbulence is modulated by the waves in a manner qualitatively consistent with rapid distortion theory.

Publications:

• Hasfi, A. A. Tejada Martinez, and F. Veron, 2020: DNS and LES of small-scale Langmuir circulation and scalar transfer across the air-sea interface. J Fluid Mech., DOI: https://doi.org/10.1017/jfm.2019.802
• Hasfi, A., A.A. Tejada Martinez, and F. Veron, 2017: Direct numerical simulation of scalar transfer across an air-water interface during inception and growth of Langmuir circulation, Computers and Fluids. 158, 49-56.
• Veron, F., W.K. Melville, and L. Lenain, 2009: Measurements of ocean surface waves and surface turbulence interactions. J. Phys. Oceanogr., 39(9): 2310-2323.
• Veron, F., W.K. Melville, and L. Lenain. 2008: Wave-coherent air-sea heat flux. J. Phys. Oceanogr., 38 (4): 788-802.
• Veron, F., W.K. Melville, and L. Lenain. 2008: Infrared techniques for measuring ocean surface processes. J. Atmos. Oceanogr. Technol., 25 (2): 307-326.
• Veron, F., and W. K. Melville, 2001: Experiments on the stability and transition of wind-driven water surfaces. J. Fluid Mech., 446, 25-65.

Airflow above wind-generated waves

Project Overview: The role of surface waves on the air-sea momentum flux (drag) is well established but our physical understanding of the airflow dynamics remains incomplete. This impedes our ability to develop improved physically-based parametrizations for better weather and sea state predictions, especially in high winds and extreme conditions

In this work, we are performing extensive laboratory experiments in which we measure the turbulent airflow above wind-generated waves. Our data covers wave ages C_p/u_∗ varying from 1.4 (very young waves) to 66.7 (old waves) thus encompassing a large range of field-relevant conditions. Please send an email at the address below to request reprints and/or copies of the data.

• Yousefi, K., F. Veron, and M.P. Buckley. 2020: Momentum flux measurements in the airflow over wind-generated surface waves. J. Fluid Mech., 895, A15. doi.org/10.1017/jfm.2020.276
• Yousefi, K. and F. Veron, 2020: Boundary layer formulations in orthogonal curvilinear coordinates for flow over wind-generated surface waves. J. Fluid Mech., 888, A11.
• Yousefi, K., F. Veron, and M.P. Buckley, M. P., 2020: Measurements of airside shear-and wave-induced viscous stresses over strongly forced wind waves. In Recent Advances in the Study of Oceanic Whitecaps (pp. 77-94). Springer, Cham. https://doi.org/10.1007/978-3-030-36371-06
• Husain, N.T., T. Hara, M.P. Buckley, K. Yousefi, F. Veron, and P.P. Sullivan, 2019: Boundary Layer Turbulence over Surface Waves in a Strongly Forced Condition: LES and Observation. J. Phys. Oceanogr., 49, 1997-2015, doi: 10.1175/JPO-D-19-0070.1.
• Buckley M., and F. Veron, 2018: The turbulent airflow over wind generated surface waves, Europ. J. Fluid Mech. https://doi.org/10.1016/j.euromechflu.2018.04.003
• Buckley M., and F. Veron, 2017: Airflow measurements at a wavy air–water interface using PIV and LIF, Exp Fluids. 58: 161. https://doi.org/10.1007/s00348-017-2439-2
• Veron, F., G. Saxena and S Misra. 2007: Measurements of viscous tangential stresses in the separated airflow above wind waves. Geophysical Research Letters, 34, L19603, DOI: 10.1029/2007GL031242.
• Feddersen, F, and F. Veron, 2005: Wind effects on shoaling wave shape, J. Phys. Oceanogr., 35(7), 1223-1228

Air-sea gas exchange in shallow water environments

Project Overview: The exchange of gases across the air-sea interface influences the cycling of climatically important
trace gases such as carbon dioxide (CO₂). In the open ocean, considerable effort has been devoted to parameterizing the gas transfer velocity, k, in terms of wind speed U, which is an easily and widely measured forcing parameter. Recent advancements in field experiments and data analysis techniques have allowed us to assess proposed wind speed/gas exchange parameterizations and narrow the range of the functional dependence of k on U for most circumstances over the ocean. The parameterizations that have been found to work best across multiple datasets are all derived from mass balances conducted in deep water (>20 m) using either deliberate 3He/SF₆ evasion studies or opportunistic 14C invasion from above-ground thermonuclear tests.

However, air-sea gas exchange parameterizations are also needed for coastal and shallow water environments. This is particularly true for coral reef ecosystems, where investigators are interested in issues such as coral reef metabolism or carbon cycling. In fact, coastal ocean ecosystems are susceptible to anthropogenic impacts such as hypoxia or anoxia, and knowledge of biogeochemical budgets and fluxes are needed to understand the causes and predict the future states of these ecosystems.

In this project, A series of experiments over two consecutive years are proposed on the barrier reef of Kaneohe Bay, Hawaii to determine factors that control gas exchange in shallow ocean environments. During the experiment, gas transfer velocities will be measured with tracers (N₂O, SF₆, Rhodamine WT) and eddy covariance of CO₂, along with concurrent measurements of processes such as wind, currents, waves, and turbulence. With the data collected in Kaneohe Bay, the PIs will:

1. Assess when or whether it is valid to apply open ocean wind speed/gas exchange parameterizations to shallow water environments.
2. Improve our understanding of how physical processes such as wind, currents, waves interact with the shallow bottom to produce surface- and bottom-driven turbulence.
3. Examine how surface and bottom generated turbulence control air-sea gas exchange in shallow coastal waters.
4. Parameterize gas exchange in shallow water environments in terms of easily measured environmental variables such as wind speeds, current velocities, and water depth.