Air-Sea Interface in Hurricane Conditions

Improving hurricane prediction models requires better understanding of complex processes taking place at the air-sea interface at high wind speeds. The change of the air-sea interaction regime in hurricane conditions has been linked to the mechanism of direct disruption of the air-sea interface by pressure fluctuations working against the surface tension force. This can be achieved through the Kelvin-Helmholtz type instability. In order to investigate this mechanism, we have conducted a series of 3D numerical experiments using a volume of fluid multiphase model. The experiments were initialized with either a flat interface or short wavelets and wind stress applied at the upper boundary of the air layer. The direct disruption of the air-water interface and formation of two-phase transition layer were observed in the numerical model under hurricane force wind. The vertical profiles of density and velocity in the transition layer were consistent with the regime of marginal stability, which permitted estimation of the lower limit on the drag coefficient under hurricane conditions. This limit was appreciably lower than the wave resistance law; though, it was gradually increasing with wind speed. The numerical experiments with imposed short wavelets demonstrated the tearing of wave crests, formation of water sheets and spume ejected into the air, smoothing of the water surface, as well as quasiperiodic structures on the top of wave crests resembling the Tollmien-Schlichting instability. This study can help in developing a framework for combining the effects of the two-phase environment with the contribution to the drag from waves

Effects of Optical Turbulence and Density Gradients On Particle Image Velocimetry

Particle image velocimetry (PIV) is a well-established tool to collect high-resolution velocity and turbulence data in the laboratory, in both air and water. Laboratory experiments are often performed under conditions of constant temperature or salinity or in flows with only small gradients of these properties. At larger temperature or salinity variations, the changes in the index of refraction of water or air due to turbulent microstructure can lead to so-called optical turbulence. We observed a marked influence of optical turbulence on particle imaging in PIV. The effect of index of refraction variations on PIV has been described in air for high Mach number flows, but in such cases the distortion is directional. No such effect has previously been reported for conditions of isotropic optical turbulence in water. We investigated the effect of optical turbulence on PIV imaging in a large Rayleigh-Bénard tank for various path lengths and turbulence strengths. The results show that optical turbulence can significantly affect PIV measurements. Depending on the strength of the optical turbulence and path length, the impact can be mitigated in post-processing, which may reduce noise and recover the mean velocity signal, but leads to the loss of the high-frequency turbulence signal

Modification of Turbulence at the Air-Sea Interface Due to the Presence of Surfactants and Implications for Gas Exchange. Part II: Numerical Simulations

We conducted high-resolution non-hydrostatic numerical simulations to study the effect of surfactants on near-surface turbulence. Laboratory experiments at the UM RSMAS ASIST facility presented in a companion paper report a reduction of turbulence below the air-sea interface and an increase of the surface drift velocity in the presence of surfactants. We implement the effect of surfactants as a rheological, viscoelastic boundary condition at the surface. Our numerical experiments are consistent with the results of the laboratory experiments. We also simulated the effect of surfactants on the temperature difference across the thermal molecular sublayer (cool skin) and on gas transfer velocity. The numerical simulations demonstrate an increase in the temperature difference across the cool skin and reduction of the gas transfer velocity in the presence of surfactant. The results also reveal the effect of surfactants on the different types of molecular sublayers (viscous, thermal and diffusion), which is important for the development of proper parameterization of the interfacial component of air-sea gas exchange under low and moderate wind speed conditions.https://nsuworks.nova.edu/occ_facbooks/1052/thumbnail.jp

Fine-Scale Features on the Sea Surface in SAR Satellite Imagery - Part 2: Numerical Modeling

With the advent of the new generation of synthetic aperture radar (SAR) satellites, it has become possible to resolve fine-scale features on the sea surface on the scale of meters. The proper identification of sea surface signatures in SAR imagery can be challenging, since some features may be due to atmospheric distortions (gravity waves, squall lines) or anthropogenic influences (slicks), and may not be related to dynamic processes in the upper ocean. In order to improve our understanding of the nature of fine-scale features on the sea surface and their signature in SAR, we have conducted high-resolution numerical simulations combining a three-dimensional non-hydrostatic computational fluid dynamics model with a radar imaging model. The surface velocity field from the hydrodynamic model is used as input to the radar imaging model. The combined approach reproduces the sea surface signatures in SAR of ship wakes, low-density plumes, and internal waves in a stratified environment. The numerical results are consistent with observations reported in a companion paper on in situ measurements during SAR satellite overpasses. Ocean surface and internal waves are also known to produce a measurable signal in the ocean magnetic field. This paper explores the use of computational fluid dynamics to investigate the magnetic signatures of oceanic processes. This potentially provides a link between SAR signatures of transient ocean dynamics and magnetic field fluctuations in the ocean. We suggest that combining SAR imagery with data from ocean magnetometers may be useful as an additional maritime sensing method. The new approach presented in this work can be extended to other dynamic processes in the upper ocean, including fronts and eddies, and can be a valuable tool for the interpretation of SAR images of the ocean surface

Model for the structure function constant for index of refraction fluctuations in Rayleigh-Benard turbulence

A model for the structure function constant associated with index of refraction fluctuations in Rayleigh-Benard turbulence is developed. The model is based upon the following assumptions: (1) the turbulence is homogeneous and isotropic at or near the mid-plane, (2) the rate of production is in balance with the rate of dissipation, (3) an inertial region exists, and (4) estimates for the rate of dissipation of temperature fluctuations and of turbulent kinetic energy can be made by assuming that the large-scale turbulence is dissipated in one eddy turnover time. From these assumptions, the dependence of the structure function on the geometry, heat flux, and the properties of the fluid is obtained. The model predicts that the normalized structure function constant is independent of the Rayleigh number. To verify the model, numerical simulations of Rayleigh-Benard turbulence were performed using two different approaches: an in-house code based on a pseudo-spectral method, and a finite volume code which employs a model for the smallest scales of the turbulence. The model was found to agree with the results of the simulations, thereby lending support for the assumptions underlying the theory.Comment: 25 pages, 10 figures, 1 tabl

Using SamplePoint to Determine Vegetation Percent Cover in a Sagebrush Steppe Ecosystem

Multi-temporal satellite imagery can be used to map species level vegetation across large areas. This is due to the fact that plants have unique spectral signatures in the electromagnetic spectrum and satellite imagery collects data from specific areas of the electromagnetic spectrum in different wavelengths (or bands) and over different time periods. However, in order to use satellite imagery to map vegetation using spectral signatures, vegetation information from the ground is needed to “train and validate” the satellite imagery. One of the ways of collecting vegetation information is using signature plots. Signature plots are high resolution local images collected with a digital camera of ground vegetation in a specific environment. These signature plots can then be analyzed using a computer software called SamplePoint in order to produce a percent vegetation cover for different vegetation species for the area which the camera covers. The percent vegetation cover information can then be used to train and validate the satellite imagery. SamplePoint offers a unique way to expand small physical observations to large landscapes

Neoclassical transport in strong gradient regions of large aspect ratio tokamaks

We present a new neoclassical transport model for large aspect ratio tokamaks where the gradient scale lengths are of the size of the poloidal gyroradius. Previous work on neoclassical transport across transport barriers assumed large density and potential gradients but a small temperature gradient, or neglected the gradient of the mean parallel flow. Using large aspect ratio and low collisionality expansions, we relax these restrictive assumptions. We define a new set of variables based on conserved quantities, which simplifies the drift kinetic equation whilst keeping strong gradients, and derive equations describing the transport of particles, parallel momentum and energy by ions in the banana regime. The poloidally varying parts of density and electric potential are included. Studying contributions from both passing and trapped particles, we show that the resulting transport is dominated by trapped particles. We find that a non-zero neoclassical particle flux requires parallel momentum input which could be provided through interaction with turbulence or impurities. We derive upper and lower bounds for the energy flux across a transport barrier in both temperature and density and present example profiles and fluxes.Comment: 52 pages, 13 figures, submitted to Journal of Plasma Physic

Getting a Grip on My Depression: How Latina Adolescents Experience, Self-Manage, and Seek Treatment for Depressive Symptoms

Latina (female) adolescents are more likely to experience depressive symptoms and less likely to receive mental health services than their non-Latina White peers. We aimed to develop a framework that explains how Latina adolescents experience, self-manage, and seek treatment for depressive symptoms. Latina young women (n = 25, M age = 16.8 years) who experienced depressive symptoms during adolescence were recruited from clinical and community settings and interviewed about experiences with depressive symptoms. The framework was developed using constructivist grounded theory methods. Participants experienced a psychosocial problem that we labeled being overburdened and becoming depressed. They responded to this problem through a five-phase psychosocial process that we labeled Getting a Grip on My Depression. Family members, peer groups, and mainstream authorities were influential in how participants experienced these phases. Future research should further develop this framework in diverse samples of Latino/a youth. Clinicians can use this framework in discussions with Latina adolescents about depressive symptoms

Surfactant-Associated Bacteria in the Near-Surface Layer of the Ocean

Certain marine bacteria found in the near-surface layer of the ocean are expected to play important roles in the production and decay of surface active materials; however, the details of these processes are still unclear. Here we provide evidence supporting connection between the presence of surfactant-associated bacteria in the near-surface layer of the ocean, slicks on the sea surface, and a distinctive feature in the synthetic aperture radar (SAR) imagery of the sea surface. From DNA analyses of the in situ samples using pyrosequencing technology, we found the highest abundance of surfactant-associated bacterial taxa in the near-surface layer below the slick. Our study suggests that production of surfactants by marine bacteria takes place in the organic-rich areas of the water column. Produced surfactants can then be transported to the sea surface and form slicks when certain physical conditions are met. This finding has potential applications in monitoring organic materials in the water column using remote sensing techniques. Identifying a connection between marine bacteria and production of natural surfactants may provide a better understanding of the global picture of biophysical processes at the boundary between the ocean and atmosphere, air-sea exchange of greenhouse gases, and production of climate-active marine aerosols