The characteristics of the recirculating bulge region in coastal buoyant outflows

A relatively large recirculating gyre, or bulge (anticyclonic buoyant lens) region attached to the source has been observed to occur in some numerical simulations and in some oceanic outflows. The purpose of this paper is to determine the dynamics of such recirculating bulge regions. Laboratory experiments were conducted for the purpose of examining the characteristics, evolution, and impact of a recirculating bulge in a coastal buoyant outflow. All experiments were performed such that the buoyant layer was thin compared to the total water depth, and the width of the bay exit was approximately equal to the internal Rossby deformation radius of the buoyant outflow. The geometry of the bay-exit (radius of curvature and exit angle) was varied in order to produce experiments with recirculating bulge formation, and experiments without. Measurements of the density and velocity fields were undertaken. The dimensions of the recirculating bulge were found to evolve identically (when appropriately scaled) for all experiments in which such a bulge formed, regardless of bay-exit geometry. When recirculating bulges formed, it was determined that the freshwater storage within the bulge was approximately 60 -70% of the source freshwater flux. The impact on the attached coastal current downshelf of the recirculating bulge was found to be significant: the width, length, and velocities of such attached coastal currents were much reduced in comparison to coastal currents that evolved in the absence of a recirculating bulge. Growth rates of the bulge were measured with time: the recirculating bulge was found to grow radially as ~t2/5 and vertically as ~t1/5. Baroclinic instabilities were observed in the recirculating bulge. The instabilities evolved as multiple rotating cores within the larger anticyclonic gyre. The presence of the instabilities in the recirculating bulge may account for its relatively large radial growth but weak vertical growth

The role of outflow geometry in the formation of the recirculating bulge region in coastal buoyant outflows

Density-driven coastal currents are a common feature in the world\u27s coastal oceans. These currents may separate from the coastline due to variations in geometry. Past studies have shown that this separation may produce two distinctly different flow states: a continuation of the coastal current, or a recirculating gyre downshelf of, and attached to, the separation point. Laboratory experiments of coastal buoyant outflows (rotationally dominated, buoyancy driven) were undertaken to examine the role of bay geometry on the evolution of the outflow. Experiments were conducted on a rotating turntable in relatively deep water (such that the buoyant layer depth was much less than the total ocean depth). The geometry of the bay-exit was varied, both in exit angle () and in radius of curvature (rc). The width of the bay was varied such that the bay exit Kelvin number (a ratio between the width of the bay exit and the internal Rossby deformation radius) was order 1 for all experiments. A recirculating bulge (a large, anticyclonic gyre joining the coastal current to the buoyant source) was occasionally observed to form. Results are compared to the Bormans and Garrett (1989) hypothesis: this hypothesis is found to explain a portion of the results only. Geometrical arguments are presented that build upon the Bormans and Garrett hypothesis that parameterizes the magnitude of the flow separation between the buoyant fluid and the exit. A separation ratio, Γ, is defined as a ratio between the inertial turning radius of the flow and the maximum offshore distance between the separated flow and the coast. A recirculating bulge was observed to form for flows with values of Γ \u3e 0.5. The separation ratio, Γ, is shown to be equivalent to the impact angle, Φ, of the buoyant fluid re-encountering the wall. The impact angle governs the upshelf and downshelf volume flux of the impacting fluid: recirculating bulge formation is found to occur when at least 50% of the source volume flux returns to the source region. This is equivalent to an impact angle greater than or equal to 60-degrees

Fast Steering Mirror systems for the U-AVLIS program at LLNL

We have successfully deployed several fast steering mirror systems in the Uranium Atomic Vapor Isotope Separation (U-AVLIS) facility at LLNL. These systems employ 2 mm to 150 mm optics and piezoelectric actuators to achieve microradian pointing accuracy with disturbance rejection bandwidths to a few hundred hertz

Modeling of pulsed laser guide stars for the Thirty Meter Telescope project

The Thirty Meter Telescope (TMT) has been designed to include an adaptive optics system and associated laser guide star (LGS) facility to correct for the image distortion due to Earth's atmospheric turbulence and achieve diffraction-limited imaging. We have calculated the response of mesospheric sodium atoms to a pulsed laser that has been proposed for use in the LGS facility, including modeling of the atomic physics, the light-atom interactions, and the effect of the geomagnetic field and atomic collisions. This particular pulsed laser format is shown to provide comparable photon return to a continuous-wave (cw) laser of the same average power; both the cw and pulsed lasers have the potential to satisfy the TMT design requirements for photon return flux.Comment: 16 pages, 20 figure

Enhanced turbulence due to the superposition of internal gravity waves and a coastal upwelling jet

The upwelling-driven coastal jet off Oregon is in geostrophic balance to first order. The accompanying thermal wind shear is stable to shear instability. Yet enhanced turbulence is observed in the upwelling jet, typically as long, thin patches with horizontal to vertical aspect ratios of 10² to 10³ (median value ~300). These patches are clearly defined by regions of low Richardson number and occur where and when the linear superposition of the three dominant shear constituents (near-inertial, M₂, and thermal wind) interferes constructively. This is most pronounced at the base of the coastal jet, where the thermal wind shear is largest. While the effect of the turbulence stress divergence on the jet is small compared to geostrophy (~1%), it is significant in the second-order force balance governing secondary circulation. The timescale associated with the decay of the thermal wind shear via turbulence stress is O(10) days. We confirm that the vertical salt flux due to mixing is comparable to the net Ekman transport of salt onto the shelf within the bottom boundary layer. Because numerical models of coastal circulation lack turbulence in midwater column, any vertical transport of scalars, including salt and heat, must be achieved inshore of the 40-m isobath. This is inconsistent with the observations presented in this study, in which significant vertical turbulent salt transport is found to exist across the entire shelf.Keywords: turbulence, internal gravity wave

On the response of a buoyant plume to downwelling-favorable wind stress

Author Posting. © American Meteorological Society, 2012. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 42 (2012): 1083–1098, doi:10.1175/JPO-D-11-015.1.Here, the response of a coastally trapped buoyant plume to downwelling-favorable wind forcing is explored using a simplified two-dimensional numerical model and a prognostic theory for the resulting width, depth, and density anomaly and along-shelf transport of the plume. Consistent with the numerical simulations, the analytical model shows that the wind causes mixing of the plume water and that the forced cross-shelf circulation can also generate significant deepening and surface narrowing, as well as increased along-shelf transport. The response is due to a combination of the purely advective process that leads to the steepening of the isopycnals and the entrainment of ambient water into the plume. The advective component depends on the initial plume geometry: plumes that have a large fraction of their total width in contact with the bottom (“bottom trapped”) suffer relatively small depth and width changes compared to plumes that have a large fraction of their total width detached from the bottom (“surface trapped”). Key theoretical parameters are Wγ/Wα, the ratio of the width of the plume detached from the bottom to the width of the plume in contact with it, and the ratio of the wind-generated mixed layer δe to the initial plume depth hp, which determines the amount of water initially entrained into the plume. The model results also show that the cross-shelf circulation can be strongly influenced by the wind-driven response in combination with the geostrophic shear of the plume. The continuous entrainment into the plume, as well as transient events, is also discussed.This work has been supported by FONDECYT Grant 1070501. S. Lentz received support by theNational Science Foundation GrantOCE-0751554. C. Moffat had additional support from the National Science Foundation Office of Polar Programs through U.S. Southern Ocean GLOBEC Grants OPP 99-10092 and 06- 23223.2013-01-0

Surface currents and winds at the Delaware Bay mouth

Knowledge of the circulation of estuaries and adjacent shelf waters has relied on hydrographic measurements, moorings, and local wind observations usually removed from the region of interest. Although these observations are certainly sufficient to identify major characteristics, they lack both spatial resolution and temporal coverage. High resolution synoptic observations are required to identify important coastal processes at smaller scales. Long observation periods are needed to properly sample low-frequency processes that may also be important. The introduction of high-frequency (HF) radar measurements and regional wind models for coastal studies is changing this situation. Here we analyze synoptic, high-resolution surface winds and currents in the Delaware Bay mouth over an eight-month period (October 2007 through May 2008). The surface currents were measured by two high-frequency radars while the surface winds were extracted from a data-assimilating regional wind model. To illustrate the utility of these monitoring tools we focus on two 45-day periods which previously were shown to present contrasting pictures of the circulation. One, the low-outflow period is from 1 October through 14 November 2007; the other is the high-outflow period from 3 March through 16 April 2008. The large-scale characteristics noted by previous workers are clearly corroborated. Specifically the M2 tide dominates the surface currents, and the Delaware Bay outflow plume is clearly evident in the low frequency currents. Several new aspects of the surface circulation were also identified. These include a map of the spatial variability of the M2 tide (validating an earlier model study), persistent low-frequency cross-mouth flow, and a rapid response of the surface currents to a changing wind field. However, strong wind episodes did not persist long enough to set up a sustained Ekman response

Keck adaptive optics: control subsystem

Adaptive optics on the Keck 10 meter telescope will provide an unprecedented level of capability in high resolution ground based astronomical imaging. The system is designed to provide near diffraction limited imaging performance with Strehl {gt} 0.3 n median Keck seeing of r0 = 25 cm, T =10 msec at 500 nm wavelength. The system will be equipped with a 20 watt sodium laser guide star to provide nearly full sky coverage. The wavefront control subsystem is responsible for wavefront sensing and the control of the tip-tilt and deformable mirrors which actively correct atmospheric turbulence. The spatial sampling interval for the wavefront sensor and deformable mirror is de=0.56 m which gives us 349 actuators and 244 subapertures. This paper summarizes the wavefront control system and discusses particular issues in designing a wavefront controller for the Keck telescope

Wideband Fiberoptic Analog Information Link

International Telemetering Conference Proceedings / September 15-17, 1969 / Sheraton Park Hotel, Washington, D.C.A high frequency data transmission system which is unaffected by high energy electromagnetic fields is described. The system utilizes a gallium arsenide (GaAs) infrared light emitting diode as the transmitting source, a glass fiber optic light guide as the transmitting medium, and a photomultiplier tube (PMT) as the optical receiving sensor. The photomultiplier output is displayed on a real-time wideband oscilloscope where it is permanently recorded on film. The overall system concept was chosen and each major component type was evaluated for optimum performance in this application. It was determined during the feasibility phase of the program that cryogenic cooling of the GaAs diode would be necessary to obtain high frequency response and high signal to noise ratio (SNR). Liquid nitrogen (LN₂) was chosen as the cryogen due to its low temperature, low cost, availability, and relatively long holding time. The described system results in a 40-ft fiber optic, analog data link with a frequency response of 80 MHz and a dynamic range of 32 db. The system is not susceptible to electromagnetic fields.International Foundation for TelemeteringProceedings from the International Telemetering Conference are made available by the International Foundation for Telemetering and the University of Arizona Libraries. Visit http://www.telemetry.org/index.php/contact-us if you have questions about items in this collection

Performance of laser guide star adaptive optics at Lick Observatory

A sodium-layer laser guide star adaptive optics system has been developed at Lawrence Livermore National Laboratory (LLNL) for use on the 3-meter Shane telescope at Lick Observatory. The system is based on a 127-actuator continuous-surface deformable mirror, a Hartmann wavefront sensor equipped with a fast-framing low-noise CCD camera, and a pulsed solid-state-pumped dye laser tuned to the atomic sodium resonance line at 589 nm. The adaptive optics system has been tested on the Shane telescope using natural reference stars yielding up to a factor of 12 increase in image peak intensity and a factor of 6.5 reduction in image full width at half maximum (FWHM). The results are consistent with theoretical expectations. The laser guide star system has been installed and operated on the Shane telescope yielding a beam with 22 W average power at 589 nm. Based on experimental data, this laser should generate an 8th magnitude guide star at this site, and the integrated laser guide star adaptive optics system should produce images with Strehl ratios of 0.4 at 2.2 {mu}m in median seeing and 0.7 at 2.2 {mu}m in good seeing