Analyzing the footprints of turbulence producing mechanisms at the free water surface

At the free wind-sheared water surface, various turbulence producing mechanisms contribute significantly to enhanced transfer rates of heat and gas across the air-water boundary. As a result, surface convergence and divergence form and can be visualized in infrared images of the water surface. Within this work the footprints of turbulent processes in the surface temperature pattern are analyzed. Image processing techniques, such as motion estimation and classification, are adapted and applied to the infrared images of the water surface. Dense flow fields are estimated without the suppression of surface divergence and up- and down-welling sites are identified. Data from a range of laboratory facilities is evaluated with a focus on small spatial scales, low to moderate wind stress and the impact of surfactants on the surface dynamics. In this wind regime shear induced turbulence, small Langmuir circulations and microscale breaking waves are identified as the dominant processes that drive heat exchange. The turbulent cell size is suggested as a characteristic feature of shear and Langmuir turbulence and is related against friction velocity and wave field. Within this work, a novel method is suggested to assess the impact of individual processes on the overall heat transfer rate and tested on the measured laboratory data also with respect to the influence of increasing wind stress and surfactant coverage. The following key points are found: Shear induced turbulence is a major contributor to enhanced transfer rates at low wind stress. Its relevance increases if the surface is covered by surfactants as the onset of waves is delayed. Langmuir circulations play a major role at intermediate wind stress and cause a significant increase of transfer rates. Surfactants on the water surface delay the evolution of Langmuir circulation and intensity considerably. Microscale breaking is a dominant contributor at moderate wind stress and is responsible for approximately 50% and more of the transfer rate

A study of the statistics of the air/water interfacial temperature field during mixed convection heat transfer

Heat transfer across an air/water interface is of particular importance to limnology, oceanography and some industrial applications. The relationship between the statistics of the air/water interfacial temperature field and the interfacial heat flux is poorly understood, particularly for the mixed convection condition, which is a common heat transfer mechanism for small inland lakes. The few studies that have been conducted under mixed convection conditions have been limited to an uncontrolled surfactant condition (tap water). Therefore, in this dissertation research two sets of experiments for wind speeds from 0 to 4 m/s were conducted: controlled surfactant contaminated conditions (with oleyl alcohol) and clean water surface conditions. The air/water interfacial heat flux and the surface temperature field statistics (root mean square (RMS) and skewness) were computed to study the relationship between them, and the results under different surface conditions were presented and compared. It was found that, for a given wind speed and surface condition, the RMS of the interfacial temperature field increased linearly with heat flux, and the RMS of clean surfaces was greater than that of a surfactant-covered surface. The surface skewness for clean surfaces was found to be more negatively skewed than that under surfactant-covered surfaces. There was almost no wind speed effect on the surfactant-covered surface skewness. However, the clean surface skewness became less skewed when the wind speed increased. The RMS was scaled by water bulk/surface temperature difference, which is the maximum possible value of RMS. The scaled RMS decreased by a factor of two in the presence of a surfactant monolayer. A parameterization study was also carried out to find the relationship between the scaled RMS and the Rayleigh number, the Reynolds number and the Prandtl number. The effect of the surfactant monolayer on the relationship between the scaled RMS and the Rayleigh number, Reynolds number and Prandtl number was parameterized by introducing a new dimensionless group. The probability density functions (PDFs) of the surface temperature fields were also determined. The wind speed, heat flux, and surface condition were all found to affect the temperature PDFs. Finally, the presence of longitudinal vortices, which are an air-side phenomenon, oriented in the wind direction, were observed under certain wind speeds and air/water temperature differences. Experiments were conducted to investigate their onset instability mechanism. The streak spacing and the onset position varied with the Reynolds number and the Grashof number. This research provides an improved understanding of turbulence using an experimental model that is more relevant to lakes than is the case for Rayleigh-Benard convection, which is often used as a model of lakes and oceans. This research also finds application in small lake thermal modeling, atmospheric modeling, volcanic lake modeling, treaty verification, the prediction of ice formation, gas/mass transfer studies, and metal surface solidification

Transfer across the air-sea interface

The efficiency of transfer of gases and particles across the air-sea interface is controlled by several physical, biological and chemical processes in the atmosphere and water which are described here (including waves, large- and small-scale turbulence, bubbles, sea spray, rain and surface films). For a deeper understanding of relevant transport mechanisms, several models have been developed, ranging from conceptual models to numerical models. Most frequently the transfer is described by various functional dependencies of the wind speed, but more detailed descriptions need additional information. The study of gas transfer mechanisms uses a variety of experimental methods ranging from laboratory studies to carbon budgets, mass balance methods, micrometeorological techniques and thermographic techniques. Different methods resolve the transfer at different scales of time and space; this is important to take into account when comparing different results. Air-sea transfer is relevant in a wide range of applications, for example, local and regional fluxes, global models, remote sensing and computations of global inventories. The sensitivity of global models to the description of transfer velocity is limited; it is however likely that the formulations are more important when the resolution increases and other processes in models are improved. For global flux estimates using inventories or remote sensing products the accuracy of the transfer formulation as well as the accuracy of the wind field is crucial

Visualization of air-water gas exchange

The mechanisms of air-sea gas transfer are studied in a large annular air-sea interaction facility, the Heidelberg Aeolotron. A novel visualization technique is used, in which an alkaline gas (ammonia) in ppm concentration levels is absorbed into slightly acid water (pH = 4). The concentration gradient in the aqueous mass boundary layer is transformed into a sharp pH gradient, which is made visible by a fluorescent pH indicator (Pyranine). Regulating the gas flux into the water allows controlling the thickness of the fluorescent layer. By observing the fluorescence induced by high power LEDs on a 2D surface patch ((21 x 25) cm²) with a camera from above at 100 fps and a spatial resolution of 150 μm, the processes creating near-surface turbulence can be studied even at a wavy water surface. Results from an extensive set of experiments are presented, covering wind speeds from 0.4 - 8.6 m/s, different concentrations of a soluble surfactant, and waves with limited and unlimited fetch. The fluorescence footprints of different processes known to contribute to gas exchange, e.g. Langmuir circulations and microscale breaking, are identified. It is found that the transition of the Schmidt number exponent, which is essential for the scaling of transfer velocities of different gases, from 2/3 to 1/2 is correlated with the frequency of microscale breaking. Surfactants shift the onset of surface renewal events to higher wind speeds. Direct comparison to simultaneously captured thermal imagery shows that despite the large difference in diffusion coefficients between ammonia and heat by a factor of 100, the transport of both tracers is governed by the same mechanisms

Microscale to Mesoscale Modeling of the Ocean Under Tropical Cyclones: Effects of Sea Spray and Surfactants on Tropical Cyclone Intensity and Air-Sea Gas Exchange

Tropical cyclone intensity prediction remains a challenge despite computational and observational developments because successful intensity forecasting requires implementing a multitude of atmospheric and oceanic processes. Hurricane Maria 2017 and Hurricane Dorian 2019 serve as prime examples of rapidly intensifying storms that devastated communities in the Caribbean. A lack of understanding and parameterization of crucial physics involved in tropical cyclone intensity in existing forecast models may have led to these and other forecasting errors. Microscale physical processes at the air-sea interface are a major factor in intensification of tropical cyclones that are often unaccounted for in forecasting models since they are difficult to study in the field and laboratory and are therefore not well understood. An ongoing uncertainty in tropical cyclone dynamics is the sea spray generation function (SSGF). While multiple estimates of the SSGF have been produced, a lack of experimental data in high wind conditions makes it difficult to establish a confident SSGF for tropical cyclones. Surface active agents impact spray generation, causing variation in spray diameter and an increase in generation that may influence heat, momentum, and gas exchanges during tropical cyclones. To better understand these processes, a computational fluid dynamics model was developed that simulates spray generation under all five tropical cyclone category conditions and resolves spray with radii starting from 100-mm. The numerical results were validated with Category 1 data from a laboratory experiment at the University of Miami. SSGFs calculated from the model revealed an increase in the spray generation under all categories of tropical cyclone conditions except Category 4 and Category 5 conditions, where little to no impact of surfactants on spray generation was found. This phenomenon might be explained by a change in regime under major tropical cyclones. Additionally, small to mesoscale ocean circulation and characteristics, particularly in environments such as a western boundary current, lead to complex interaction between ocean circulation and tropical cyclones. Not only are ocean dynamics in the open ocean affected by tropical cyclones, but the impacts can extend to coasts outside of the predicted storm impact area, leading to unprepared coastal communities due to these poorly understood interactions. This can improve parameterizations of variables such as mixing and fluxes in tropical cyclone forecasting models. An additional computational fluid dynamics model has been developed that predicts and characterizes small to mesoscale ocean circulation and dynamics in a western boundary current. This body of work aims to further understand ocean circulation in the surface layer in western boundary currents and complex microphysics at the air-sea interface during tropical cyclones including spray and spume generation, evaporation, and related fluxes, air-sea gas exchange, and the effects of factors such as surfactants. The multitude of ocean dynamics and air-sea interaction processes to be studied in this work converge to strive for a more complete understanding of the ocean water column and the air-sea interface under tropical cyclones that could ideally be implemented into tropical cyclone prediction models to improve intensity forecasting

Effects of Surfactants on the Generation of Sea Spray During Tropical Cyclones

Despite significant improvement in computational and observational capabilities, predicting intensity and intensification of major tropical cyclones remains a challenge. In 2017 Hurricane Maria intensified to a Category 5 storm within 24 hours, devastating Puerto Rico. In 2019 Hurricane Dorian, predicted to remain tropical storm, unexpectedly intensified into a Category 5 storm and destroyed the Bahamas. The official forecast and computer models were unable to predict rapid intensification of these storms. One possible reason for this is that key physics, including microscale processes at the air-sea interface, are poorly understood and parameterized in existing forecast models. Under tropical cyclones, the air-sea interface becomes a multiphase environment involving bubbles, foam, and spray. The presence of surface-active materials (surfactants) alters these microscale processes in an unknown way that may affect tropical cyclone intensity. The current understanding of the relationship between surfactants, wind speed, and sea spray generation remains limited. Here we show that surfactants significantly affect the generation of sea spray, which provides some of the fuel for tropical cyclones and their intensification. A computational fluid dynamics (CFD) model was used to simulate spray radii distributions starting from a 100 micrometer radius as observed in laboratory experiments at the University of Miami Rosenstiel School of Marine and Atmospheric Sciences SUSTAIN facility. Results of the model were verified with laboratory experiments and demonstrate that surfactants increase spray generation by 34% under Category 1 tropical cyclone conditions (~40 m s-1 wind). In the model, we simulated Category 1 (4 Nm-2 wind stress), 3 (10 Nm-2 wind stress), and 5 (20 Nm-2 wind stress) conditions and found that surfactants increased spray generation by 20-34%. The global distribution of bio-surfactants on the earth is virtually unknown at this point. Satellite oceanography may be a useful tool to identify the presence of surfactants in the ocean in relation to tropical cyclones. Color satellite imagery of chlorophyll concentration, which is a proxy for surfactants, may assist in identifying surfactant areas that tropical cyclones may pass over. Synthetic aperture radar imagery also may assist in tropical cyclone prediction in areas of oil spills, dispersants, or surfactant slicks. We anticipate that bio-surfactants affect heat, energy, and momentum exchange through altered size distribution and concentration of sea spray, with consequences for tropical cyclone intensification or decline, particularly in areas of algal blooms and near coral reefs, as well as in areas affected by oil spills and dispersants

Wave breaking at high wind speeds and its effects on air-sea gas transfer

Gravity waves are ubiquitous at the surface of the ocean and play a key role in the coupled ocean-atmosphere system. These wind generated waves, for which gravity provides the restoring force, influence the kinematics and dynamics of the upper ocean and lower atmosphere. Their breaking injects turbulence into the upper ocean, generates bubble plumes and sea-spray thus transferring energy, momentum, heat and mass between the atmosphere and the ocean. In the anthropocene, with CO2 driving the warming trend and the ocean acting as the main carbon sink, it is imperative to understand the complex physical controls of air-sea gas transfer. Large uncertainties still remain under high wind speed conditions where wave breaking processes are dominant. This dissertation seeks to shed light onto the dependence of wave breaking and air-sea gas transfer on environmental parameters. It further explores process based models of air-sea gas transfer that explicitly account for the breaking related processes. Air entraining breaking waves are easily detectable as bright features on the ocean surface composed of foam and subsurface bubble plumes. These features, termed whitecaps, arise at wind speed as as low as 3 m s−1 . The whitecap coverage (W) has been recognized as a useful proxy for quantifying wave breaking related processes. It can be determined from shipboard, air-borne and satellite remote sensing. W is most commonly parameterized as a function of wind speed, but previous parameterizations display over three orders of magnitude scatter. Concurrent wave field and flux measurements acquired during the Southern Ocean Gas Exchange (SO GasEx) and the High Wind Gas exchange Study (HiWinGS) projects permitted evaluation of the dependence of W on wind speed, wave age, wave steepness, mean square slope, as well as on wave-wind and breaking Reynolds numbers. W was determined from over 600 high frequency visible imagery recordings of 20 minutes each. Wave statistics were computed from in situ and remotely sensed data as well as from a WAVEWATCH-III® hind cast. The first ship-borne estimates of W under sustained wind speeds (U10N ) of 25 m s−1 were obtained during HiWinGS. These measurements suggest that W levels off at high wind speed, not exceeding 10% when averaged over 20 minutes. Combining wind speed and wave height in the form of the wave-wind Reynolds number resulted in closely agreeing models for both datasets, individually and combined. These are also in good agreement with two previous studies. When expressing W in terms of wave field statistics only or wave age, larger scatter is observed and/or there is little agreement between SO GasEx, HiWinGS, and previously published data. The wind–speed-only parameterizations deduced from the SO GasEx and HiWinGS datasets agree closely and capture more of the observed W variability than Reynolds number parameterizations. However, these wind-speed-only models do not agree as well with previous studies than the wind-wave Reynolds numbers. The ability to quantify air-sea gas transfer hinges on parameterizations of the gas transfer velocity k. k represents physical mass transfer mechanisms and is usually parameterized as a non-linear function of wind forcing. Previous eddy-covariance measurements and models based on the global radio carbon inventory led to diverging parameterizations with both cubic and quadratic wind speed dependence. At wind speeds above 10 m s−1 these parameterizations differ considerably and measurements display large scatter. In an attempt to reduce uncertainties in k, explored empirical parameterizations that incorporate both wind speed and sea state dependence via breaking and wave-wind Reynolds numbers, were explored. Analysis of concurrent eddy covariance gas transfer and measured wave field statistics supplemented by wave model hindcasts shows for the first time that wave-related Reynold numbers collapse four open ocean datasets that have a wind speed dependence of CO2 transfer velocity ranging from lower than quadratic to cubic. Wave-related Reynolds number and wind speed show comparable performance for parametrizing DMS which, because of its higher solubility, is less affected by bubble-mediated exchange associated with wave breaking. While single parameter models may be readily used in climate studies, their application is gas specific and may be limited to select environments. Physically based parameterizations that incorporate multiple forcing factors allow to model the gas transfer of gases with differing solubility for a wide range of environmental conditions. Existing mechanistic models were tested and a novel framework to model gas transfer in the open ocean in the presence of breaking waves is put forward. This analysis allowed to update NOAA’s Coupled OceanAtmosphere Response Experiment Gas transfer algorithm (COAREG) and exposed limitation of other existing physically based parameterizations. The newly proposed mechanistic model incorporates both the turbulence and bubble mediated transfer. It is based on various statistics determined from the breaking crest length distribution (Λ(c)). Λ(c) was obtained by tracking the advancing front of breaking waves in the high frequency videos taken during HiWinGS. Testing the mechanistic model with the HiWinGS dataset shows promising results for both CO2 and DMS, though it does not perform better than COAREG. Uncertainties remain in the quantification of bubble cloud which are at the core of the formulation of the bubble mediated transfer and additional field measurements are necessary to characterize bubble plume properties in the open ocean

Direct Numerical Simulations of Interfacial Turbulence at Low Froude and Weber Numbers

Sea surface temperature accessible through use of remote sensing techniques (IR imaging, etc.) suggests abundant flow and thermal field information at the ocean surface that is closely related to subsurface turbulent activities. The suggested information includes wind stress, surface dissipation, underneath velocity and vorticity, and heat and gas transportation. Due to the constantly outgoing interfacial latent and sensible heat flux, the very surface of the ocean is often cooler than the bulk. This so called ‘cool skin layer’ below the very surface is greatly involved in the underlying interfacial turbulence and is the primary support of using sea surface temperature imaging to detect the subsurface activities. In addition, studies have shown that for this detection method the effects of ubiquitous surfactants (surface free agents) to the subsurface turbulence should also be considered. In the case when the wind stress at the surface is far less significant than the buoyancy force in the water phase, the cool skin layer accumulates and triggers free convection. A series of numerical simulations is conducted to reproduce such a free convection flow to obtain detailed statistics and structural features in order to investigate the correlation between the surface temperature and the subsurface activities of the flow. The simulations are also aimed at the quantitative evaluation of the surfactant effects on the flow. The results of the simulations demonstrate that the surface temperature is statistically and structurally correlated to the subsurface activities in various patterns, and that surfactant has a certain influence to the subsurface turbulence with an overall effect of reducing the average surface temperature. Based upon the framework of the controlled flux method, a novel approach to actively determine the interfacial gas transfer velocity at the free convection surface is proposed and numerically investigated. The proposed and simulated approach employs a temporal volumetric heating source to suppress the free convection. The heating source is defined and parameterized with respect to the physical properties of radiation absorption in water phase. Observation and interpretation of the surface temperature evolution and the flow features during and after the heating suggest the effective suppression of the free convection, the onset of the Rayleigh instability and the re-establishment of the free convection. Based on that, an analytical conduction model is formulated to obtain the heat transfer velocity at the free surface from the surface temperature. The gas transfer velocity is then inferred through similarity

Free-surface turbulence and air-water gas exchange

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2000This thesis investigates the physical mechanisms of air-water gas transfer through direct measurements of turbulence at the air--water interface. To enable this study, a new approach to the particle image velocimetry (PIV) technique is developed in order to quantify free-surface flows. Two aspects of this work are innovative. First, the use of a three-dimensional laser light cone and optical filtering of the camera allow for the motion of fluorescent flow tracers at the water surface to be isolated and measured. Validation experiments indicate that this measurement reflects the fluid motion within the upper few hundred microns. A key benefit to this approach is the ability to deal with deforming surfaces, provided the amplitudes are not prohibitively large. This feature was used in this thesis to explore the surface flow induced by mechanically generated waves. Second, a new hybrid PIV image processing algorithm was developed that provides high accuracy velocity estimation with improved computational efficiency. This algorithm combines the concepts of dynamic Fourier-domain cross-correlation with a localized direct multiplicative correlation. In order to explore relationships between free-surface hydrodynamics and air-water gas transfer, an oscillating grid-stirred tank was constructed. By its design, this tank can be managed for chemical cleanliness, offers an unobstructed free surface, and is suited for turbulent mixing and air--water gas-exchange studies. A series of acoustic Doppler velocimeter, PIV, and infrared imaging experiments are presented that characterize the flow for the grid forcing conditions studied. Results indicate that the flows are stationary and reasonably repeatable. In addition, the flows exhibit near-isotropic turbulence and are quasi-homogeneous in horizontal planes. Secondary circulations are revealed and investigated. Finally, PIV measurements of free-surface turbulence are performed with concurrent measurements of gas transfer in the grid tank for a range of turbulent mixing and surface conditions. Surface turbulence, vorticity, and divergence are all affected by the presence of a surface film, with significant effects realized for relatively small surface pressures. Results show that while a relationship between surface turbulence and the gas-transfer velocity is an obvious improvement over that found using an estimate of the bulk flow turbulence, this relationship is dependent on the flow regime. This is revealed through additional surface wave studies. However, the data from both the wave experiments and the grid turbulence experiments can be reconciled by a single relationship between the gas-transfer velocity and the 1/2-power of the surface divergence, which agrees with previous conceptual models. These results (1) further our understanding of interfacial transport processes, (2) demonstrate the important role of surface divergence in air-water gas exchange, and (3) relate, in a physically meaningful way, the interactions between surface renewal, surfactants, and gas transfer.I was supported as an Office of Naval Research Graduate Fellow. This assistance was the impetus for my pursuit of a doctoral degree and is gratefully acknowledged. Very special thanks also to the WHOI Ocean Ventures Fund Program, Mr. F. Thomas Westcott, and the WHOI Education Department for generous financial support over the past several years