Session T16: Climate Physics / Instabilities and Turbulence

Simultaneous measurement of sphericity and scattering phase functions from single atmospheric aerosol particles in Las Cruces, NM

We report upon the collection of elastic light scattering patterns with high angular resolution and large angular coverage from single atmospheric aerosol particles in Las Cruces, NM. Radiative forcing due to aerosols is a primary source of uncertainty in climate models. Characterization of tropospheric aerosols is carried out by inversion of optical measurements made remotely by land-based instruments and satellites. An integral part of the retrieval procedure is accounting for particle shape (i.e. nonsphericity). In-situ and laboratory measurements of aerosol particles play a critical role in validating and constraining the inversion procedure used in climate models. In this work, we utilize high angular resolution and large angular coverage scattering patterns to simultaneously calculate particle sphericity and the scattering phase of individual atmospheric particles. We examine the relationship between a particle's sphericity and its phase function. In addition, we explore the differences in phase function between nonspherical particles that have high sphericity (i.e. complex particles with overall round shape) and spherical particles. We conclude by commenting on the possible impacts of our findings on inversion procedures used in aerosol characterization.   

Measurement of aerosol optical properties by integrating cavity ring-down spectroscopy and nephelometery

Accurate measurement of optical properties of aerosols is crucial for quantifying the influence of aerosols on climate. Aerosols that scatter and absorb radiation can have a cooling or warming effect depending on the magnitude of the respective scattering and absorption terms. One example is black carbon known for its strong absorption. The reported refractive indices for black carbon particles range from 1.2$+$0i to 2.75$+$1.44i. Our work attempts to measure extinction coefficient, and scattering coefficient of black carbon particles at different incident beam wavelengths using a cavity ring-down spectrometer and a Nephelometer and compare to Mie theory predictions. We report calibration results using polystyrene latex spheres and preliminary results on using commercial black carbon particles.   

Non-Condensable Gas Absorption by Capillary Waves

Oceans and atmosphere are constantly exchanging heat and mass; this has a direct consequence on the climate. While these exchanges are inherently multi-scales, in non-breaking waves the smallest scales strongly govern the transfer rates at the ocean-atmosphere interface. The present experimental study aims at characterizing and quantifying the exchanges of non-condensable gas at a sub-millimeter scale, in the presence of capillary waves. In oceans, capillaries are generated by high winds and are also present on the forward face of short gravity waves. Capillary waves are thus present over a large fraction of the ocean surface, but their effect on interphase phenomena is little known. In the experiment, 2D capillary waves are generated by the relaxation of a shear layer at the surface of a laminar water slab jet. Wave profile is measured with Planar Laser Induced Fluorescence (PLIF) and 2D velocity field of the water below the surface is resolved with Particle Image Velocimetry (PIV). Special optical arrangements coupled with high speed imaging allow 0.1 mm- and 0.1 ms- resolution. These data reveal the interaction of vorticity and free surface in the formation and evolution of capillaries. The effect of the capillaries on the transfer of oxygen from the ambient air to anoxic water is measured with another PLIF system. In this diagnostic, dissolved oxygen concentration field is indirectly measured using fluorescence quenching of Pyrenebutyric Acid (PBA). The three measurements performed simultaneously -surface profile, velocity field, and oxygen concentration- give deep physical insights into oxygen transfer mechanisms under capillary waves.   

The relation between the statistics of open ocean currents and the temporal correlations of the wind-stress

We study the statistics of wind-driven open ocean currents. Using the Ekman layer model for the integrated currents, we investigate, analytically and numerically, the relation between the wind-stress distribution and its temporal correlations and the statistics of the open ocean currents. We find that temporally long-range correlated wind results in currents whose statistics is proportional to the wind-stress statistics. On the other hand, short-range correlated wind leads to Gaussian distributions of the current components, regardless of the stationary distribution of the winds, and therefore, to a Rayleigh distribution of the current amplitude, if the wind-stress is isotropic. We find that the second moment of the current speed exhibits a maximum as a function of the correlation time of the wind-stress for a non-zero Coriolis parameter. The results were validated using an oceanic general circulation model.   

Stochastic Parameterization of Ocean Mesoscale Eddies

Processes smaller than the model resolution or faster than the model time step are parameterized in climate simulations using deterministic closure schemes. Yet, several subgrid-scale processes are turbulent and potentially best represented by stochastic closures. The goal of our study is to construct a stochastic parameterization of mesoscale eddies in ocean models. The output of a quasi-geostrophic model in a double-gyre configuration with horizontal resolution of 7.5 km (eddy-resolving resolution) is used as the ``truth''. A coarse-graining methodology is employed on this output to compute ``eddy fluxes'' tendencies appropriate to the grid scale of a coarse resolution model. The tendencies are binned into different ranges of mean flow and mean shear strength related to the eddy life cycle in order to obtain probability distribution functions (PDFs). The PDFs for the coarse-grained tendencies show that the temporal and spatial eddy fluxes cannot be captured by current downgradient deterministic parameterizations. We rely on the PDFs to implement a novel stochastic parameterization into a coarse resolution model. We show and discuss the impact of this new parameterization on the mean flow and its fluctuations.   

Nonlinear Scale Interactions and Energy Pathways in the Ocean

Large-scale currents and eddies pervade the ocean and play a prime role in the general circulation and climate. The coupling between scales ranging from $O(10^4)$ km down to $O(1)$ mm presents a major difficulty in understanding, modeling, and predicting oceanic circulation and mixing, where the energy budget is uncertain within a factor possibly as large as ten. Identifying the energy sources and sinks at various scales can reduce such uncertainty and yield insight into new parameterizations. To this end, we refine a novel coarse-graining framework to directly analyze the coupling between scales. The approach is very general, allows for probing the dynamics simultaneously in scale and in space, and is not restricted by usual assumptions of homogeneity or isotropy. We apply these tools to study the energy pathways from high-resolution ocean simulations using LANL's Parallel Ocean Program. We examine the extent to which the traditional paradigm for such pathways is valid at various locations such as in western boundary currents, near the equator, and in the deep ocean. We investigate the contribution of various nonlinear mechanisms to the transfer of energy across scales such as baroclinic and barotropic instabilities, barotropization, and Rossby wave generation.   

Suppressing Rayleigh-Taylor Instability with rotation

The stabilizing effects of rotation upon many instabilities are well known. We demonstrate how the Rayleigh-Taylor instability (RTI) in a two-layer fluid may be stabilized by rotating the fluid, and present a critical rotation rate for such stabilization. We show that, in contrast to non-rotating RTI, there is a fundamental difference between placing heavy fluid above a light fluid (unstable arrangement) and simply accelerating a stable arrangement (light above heavy) at a rate greater than gravity vertically downwards. We propose to show novel experiments, conducted using high-powered superconducting magnets (18.7\,T), supporting the theoretical predictions. We believe these to be the first experiments to investigate the effects of rotation upon RTI and they exploit the use of the magnetic field that removes the need for a physical barrier when initializing the experiment. Potential applications for the research lie not only in fundamental fluid mechanics, but also in astrophysical applications where RTI is observed (e.g.~Crab Nebula) and other strategic applications.   

Inverse Energy cascade in 3D Navier-Stokes eqs

We study the statistical properties of homogeneous and isotropic three-dimensional (3D) turbulent flows. We show that all 3D flows in nature possess a subset of possible non-linear evolution leading to a reverse energy transfer: from small to large scales. Up to now, such inverse cascade was only observed in flows under strong rotation and in quasi two-dimensional geometries under strong confinement. We show here that energy flux is always reversed when mirror symmetry is broken leading to a distribution of helicity in the system with a well defined sign at all wavenumbers. Our findings broaden the range of flows where inverse energy cascade may be detected and rationalize the role played by helicity in the energy transfer process showing that both 2D and 3D properties naturally coexist in all flows in nature.   

Rotation rate of tracer and long rods in turbulence

We study the rotational dynamics of single rod-like particles ranging from tracer rods to long rods and quantify the effects of length of rod on its rotation rate in turbulent flow. The orientation and position of rods are measured experimentally using Lagrangian particle tracking with images from multiple cameras in a flow between two oscillating grids. Rods rotate due to the velocity gradient of the flow and also develop alignment with the directions of the velocity gradient tensor as they are carried by the flow. Small tracer rods rotate due to the velocity gradient of the smallest eddies that produce the largest shear rate while longer rods average over length-scales smaller than their size to eddies order of their own length-scales. The rotation rate variance gets smaller as the length of the rod increases.   

Intermittency in 2D Turbulence

The existence of intermittency in three-dimensional turbulence is generally accepted, although with a variety of interpretations. However, the issue of intermittency in two- dimensional turbulence is unresolved. By measuring the velocity in a flowing soap film, we show that there is significant intermittency in both the enstrophy and energy cascades. The intermittency is characterized by the scaling exponents of velocity structure functions $S_n(r)$ as well as the flatness $F$ of velocity derivatives. Both show a strong Reynolds number dependence. However, unlike turbulence in three dimensions, the intermittency decreases with increasing Reynolds number. This work is supported by NSF grant No. 1044105, a Mellon fellowship, and the Okinawa Institute of Science and Technology.   

From a Desingularized Vortex Sheet Model to a Turbulent Mixing Layer

The temporal mixing layer is studied using the model of a slightly perturbed vortex sheet which is unstable and tends to roll-up in a spiral. The flow is inviscid and incompressible. A point vortex model tends to evolve into a chaotic cloud of point vortices instead of a smooth double branched spiral. The vortex sheet model is derived (in closed form) from the basic equations of vortex dynamics. The problem of finite time singularity is handled by a technique that invokes longitudinal circulation density diffusion along the sheet at singular points. The present model uses linear segments to interpolate the sheet. Although it is computationally involved compared to point vortices, the vortex sheet does not get distorted and rolls-up into a smooth double branched spiral. The accuracy of such simulations can be independently verified by using the laws of vortex dynamics and conserved quantities. We observe the growth of the two-dimensional shear layer with time and the merger of vortex like structures. The dependence of the mixing layer on the initial conditions is studied in detail and tries to answer the question whether the vortex sheet model yields a turbulent mixing layer.   

Return to isotropy in high Reynolds number turbulent shear flow

Given that turbulence decays from large scales to smaller scales, and that large scales are anisotropic and the smallest scales are isotropic, can the results of return to isotropy experiments be applied to the cascade of turbulence from large scales to small scales? If energy is added to the system only at larger scales, then probably yes. For atmospheric flow over relatively open and flat terrain (Kansas), the 'decay' of turbulence progresses from fairly anisotropic at the large scales (maximum turbulent kinetic energy) toward pure isotropy at smaller scales via pancake-like axisymmetry. The smallest scale resolvable by the instrumentation is on the order of 1m, so dissipation scales are not evaluated. The flows with cigar-like axisymmetry occur inside an urban canyon. In these cases it is not clear if turbulence is generated at only the maximum turbulent kinetic energy scale. The turbulence at larger scales possesses a strong cigar-like axisymmetry, but can often progress to pancake-like axisymmetry at smaller scales.   

Information Content of Turbulence

This work is one of the few attempts to treat turbulence as an information source that can be controlled experimentally. As the Reynolds number $Re$ is increased, more degrees of freedom are excited and participate in the turbulent cascade. One might therefore expect that on raising $Re$, the system becomes more random, thereby increasing the Shannon entropy $H$. However, because the excited modes are correlated, $H$ is a {\em decreasing} function of $Re$, as is experimentally shown in a study of turbulence in a flowing soap film. A parallel analysis was made of the logistic map, where $H$ is calculated as a function of the control parameter $r$ in the equation $x_{n+1} =r x_n(1 - x_n)$. There, as expected, $H$ is an increasing function of $r$. This work is supported by NSF grant No. 1044105, a Mellon fellowship, and the Okinawa Institute of Science and Technology.   

A hypothesis on nanodust as a source of energy for extreme weather events and climate changes

There are many phenomena that attract energy, the source of which cannot be unerringly identified. Among those are: excess heat alleged to nuclear processes, sonoluminescence, wire fragmentation under high voltage pulses, diverse biophysical experiences, and some atmospheric effects, like ball lightning and terrestrial gamma rays. Destructive atmospheric events associated with intense air movements, such as hurricanes and tornadoes, expend huge amounts of energy equivalent to very many nuclear bombs. Our paper [1] indicates a possibility for a new source of energy due to the so-called ``hot-clocking'' effect related to the holographic mechanism of the Universe that establishes the exclusive property of nonlocality. This may uncover energy in various unusual appearances, particularly, in the suspected trend of global warming as a direct contribution to the extreme weather events. The surmised clocking impacts from holographic reference beam can reveal themselves through gaseous aerosols and suspended contaminants that may have been increased with human technogenesis. According to recent EPRI report nanopowder for Ni-Pd alloys in the size range of 5--10 nm was found~to cause small amounts of excess power, about 4 watt per gram. So, using a minimal norm of contamination (20 micrograms per cubic meter) as an approximate guide, we could estimate that the whole atmosphere would thus generate dozens of terawatts, a contribution comparable to that of the Sun. [1] S.Berkovich, ``Generation of clean energy by applying parametric resonance to quantum nonlocality clocking'', \textbf{Nanotech,} 2011 Vol. 1, pp.771-774