145 research outputs found
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Forcing a three-dimensional, hydrostatic, primitive-equation model for application in the surf zone : 1. formulation
An Eulerian analysis for wave forcing of three-dimensional (3D) wave-averaged mean
circulation in the surf zone is presented. The objective is to develop a dynamically
consistent formulation for applications in a 3D primitive equation model. The analysis is
carried out for the case of shallow-water linear waves interacting with wave-averaged
depth-independent horizontal currents that vary on larger space scales and timescales.
Variations in wave properties are governed by a wave action equation that includes wavecurrent
interactions and dissipation representative of wave breaking. Wave forcing of the
mean currents consists of a surface stress and a body force. The surface stress is
proportional to the wave energy dissipation. The body force includes one term that is
related to gradients of part of the radiation stress tensor and a second term that is related to
the vortex force and is proportional to a product of the mean wave momentum and the
vertical component of the mean vorticity vector. In addition, there is a nonzero normal
velocity at the mean surface that arises from the divergence of the mean Eulerian
wave mass flux. This velocity results in an additional momentum flux forcing of the mean
flow. Applications of this formulation to the DUCK94 field experiment are presented
in part 2
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Forcing a three-dimensional, hydrostatic, primitive-equation model for application in the surf zone : 2. application to DUCK94
A threeâdimensional primitiveâequation model for application to the nearshore surf zone has been developed. This model, an extension of the Princeton Ocean Model (POM), predicts the waveâaveraged circulation forced by breaking waves. All of the features of the original POM are retained in the extended model so that applications can be made to regions where breaking waves, stratification, rotation, and wind stress make significant contributions to the flow behavior. In this study we examine the effects of breaking waves and wind stress. The nearshore POM circulation model is embedded within the NearCom community model and is coupled with a wave model. This combined modeling system is applied to the nearshore surf zone off Duck, North Carolina, during the DUCK94 field experiment of October 1994. Model results are compared to observations from this experiment, and the effects of parameter choices are examined. A process study examining the effects of tidal depth variation on depthâdependent waveâaveraged currents is carried out. With identical offshore wave conditions and model parameters, the strength and spatial structure of the undertow and of the alongshore current vary systematically with water depth. Some threeâdimensional solutions show the development of shear instabilities of the alongshore current. Inclusion of waveâcurrent interactions makes an appreciable difference in the characteristics of the instability
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An inertial subrange in microstructure spectra
An inertial subrange was found in spectra calculated from vertical profiles of temperature gradient recorded in the bottom boundary layer of the Oregon shelf. Spectra were calculated for 53-cm vertical segments. An ensemble average of those spectra that were fully resolved and had high Cox number was compared to the universal form. Good agreement was found with the Batchelor form. The high wave number end of the inertial range was resolved. A relationship between the Kolmogorov constant for temperature,Ă, and the Batchelor constant,q, was established, Ăq-2=/ 30 .172( +0.012).If Ă = 0.5, as determined from atmospheric data, then q = 4.95 (4.28< q < 6.65) and the transition from the inertial to the viscous-convective range occurs at a wave number k = 0.035k s:( 0.021< k/ks: < 0.043)where kk is the Kolmogorov wave number
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On Symmetric Instabilities in Oceanic Bottom Boundary Layers
Model studies of two-dimensional, time-dependent, wind-forced, stratified downwelling circulation on the continental shelf have shown that the near-bottom offshore flow can develop time- and space-dependent fluctuations involving spatially periodic separation and reattachment of the bottom boundary layer and accompanying recirculation cells. Based primarily on the observation that the potential vorticity Î , initially less than zero everywhere, is positive in the region of the fluctuations, this behavior was identified as finite amplitude slantwise convection resulting from a symmetric instability. To further support that identification, a direct stability analysis of the forced, time-dependent, downwelling circulation would be useful, but is difficult because the instabilities develop as an integral part of the evolving flow field. The objectives of the present study are 1) to examine the linear stability of a near-bottom oceanic flow over sloping topography with conditions dynamically similar to those in the downwelling circulation and 2) to establish a link between the instabilities observed in the wind-forced downwelling problem and the results of recent theoretical studies of bottom boundary layer behavior in stratified oceanic flows over sloping topography. These objectives are addressed by investigating the two-dimensional linear stability and the nonlinear behavior of the steady, inviscid, âarrested Ekman layerâ solution produced by transient downwelling in one-dimensional models of stratified flow adjustment over a sloping bottom. A linear stability analysis shows that this solution is unstable to symmetric instabilities and confirms that a necessary condition for instability is Î > 0 in the bottom layer. Numerical experiments show that the unstable, time-dependent, nonlinear behavior in the boundary layer involves the formation of slantwise circulation cells with characteristics similar to those found in the wind-forced downwelling circulation and the development of weak stable stratifiction close to that corresponding to marginally stable conditions with Î = 0
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Modeling study of turbulent mixing over the continential shelf : comparison of turbulent closure schemes
The sensitivity of model-produced time-dependent wind-driven circulation on the
continental shelf to the turbulent closure scheme employed is studied with a twodimensional
approximation (variations across-shelf and in depth) using the Princeton
Ocean Model. The level 2.5 Mellor-Yamada closure (MY), k-Δ closure, and K-Profile
Parameterization schemes are used to evaluate the mesoscale fields and the spatial and
temporal variability of mixing. All three submodels produce similar features in the
mesoscale circulation. They produce qualitatively similar eddy diffusivities and eddy
viscosities, although the turbulent structure and the mixing intensities can differ
quantitatively. The k-Δ length scale follows the buoyancy length scale when stratification
is important. In contrast, the length scale produced by the qÂČl equation in the MY
scheme deviates substantially from the buoyancy scale unless a stratification-dependent
limitation is imposed. During upwelling-favorable winds, the majority of turbulent mixing
occurs in the top and the bottom boundary layers and in the vicinity of the vertically and
horizontally sheared coastal jet. Turbulent mixing in the coastal jet is primarily driven
by shear-production. The near-surface flow on the inner shelf becomes convectively
unstable as wind stress forces the upwelled water to flow offshore in the surface layer.
During downwelling-favorable winds, the strongest mixing occurs in the vicinity of the
downwelling front. The largest turbulent kinetic energy and dissipation are found near the
bottom of the front. Turbulence in the bottom boundary layer offshore of the front is
concentrated between recirculation cells which are generated as a result of symmetric
instabilities in the boundary layer flow.Keywords: turbulent closure, shelf circulation, turbulent mixing over the shelf, numerical modelin
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Nearshore sandbar migration predicted by an eddy-diffusive boundary layer model
We simulated the erosion and accretion of a natural beach using a wave-resolving
eddy-diffusive model of water and suspended sediment motion in the bottom boundary
layer. Nonlinear advection was included in this one-dimensional (vertical profile) model
by assuming that waves propagated almost without change of form. Flows were forced by
fluctuating pressure gradients chosen to reproduce the velocity time series measured
during the Duck94 field experiment. The cross-shore flux of suspended sediment beneath
each field-deployed current meter was estimated, and beach erosion (accretion) was
calculated from the divergence (convergence) of this flux. Horizontal pressure forces on
sediment particles were neglected. The model successfully predicted two bar migration
events (one shoreward bar migration and one seaward) but failed to predict a third
(seaward migration) event. Simulated seaward sediment transport was due to seaward
mean currents. Simulated shoreward sediment transport was due to covariance between
wave-frequency fluctuations in velocity and sediment concentration and was mostly
confined to the wave boundary layer. Predicted seaward (shoreward) bar migration was
driven by a maximum in the current-generated (wave-generated) flux over the sandbar. A
wave-generated downward flux of shoreward momentum into the wave boundary layer
contributed to shoreward sediment transport and often had a local maximum over the bar
crest. Second-order nonlinear advection of sediment, mostly representing shoreward
advection by the Stokes drift, also often had a local maximum over the bar crest. Together,
wave-generated momentum fluxes and the Stokes drift substantially increased shoreward
transport and were essential to predictions of shoreward bar migration
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Nonlinear shear instabilities of alongshore currents over barred beaches
The nonlinear dynamics of finite amplitude shear instabilities of
alongshore currents in the nearshore surf zone over barred beach topography are
studied using numerical experiments. These experiments extend the recent study
of Allen et al. [1996], which utilized plane beach (constant slope) topography by
including shore-parallel sandbars. The model involves finite-difference solutions to
the nonlinear shallow water equations for forced, dissipative, initial-value problems
and employs periodic boundary conditions in the alongshore direction. Effects
of dissipation are modeled by linear bottom friction. Forcing for the alongshore
currents is specified using a model formulated by Thornton and Guza [1986] (T-G).
Distinct classes of flows develop depending on the dimensionless parameter Q, the
ratio of an advective to a frictional timescale. For Q greater than a critical value
Qc the flows are linearly stable. For âQ = Qc - Q > 0 the flow is unstable. For
small values of âQ, equilibrated shear waves develop that propagate alongshore at
phase speeds and wavelengths that are in agreement with predictions from linear
theory for the most unstable mode. At intermediate values of âQ, unsteady vortices
form and exhibit nonlinear interactions as they propagate alongshore, occasionally
merging, pairing, or being shed seaward of the sandbar. At the largest values of
âQ examined, the resulting flow field resembles a turbulent shear flow. A net
effect of the instabilities at large âQ is to distribute the time-averaged alongshore
momentum from local maxima of the T-G forcing, located over the sandbar and
near the shore, into the region of the trough. The across-shore structure of the
time-averaged alongshore current is in substantially better qualitative agreement
with observations than that given by a steady frictional balance with T-G forcing.
The results point to the possible existence in the nearshore surf zone of an energetic
eddy field associated with instabilities of the alongshore current.Copyrighted by American Geophysical Union
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The thinness of oceanic temperature gradients
A test of the scaling of the extent of the thinnest vertical temperature gradients, in the near-bottom
boundary layer on the Oregon shelf, shows that the Batchelor wave number determines the cutoff wave
number in vertical temperature gradient spectra. In combination with previous results, in other words,
this test shows that the smallest scale at which significant temperature variance due to turbulence exists at
any given point in the ocean is determined by the Batchelor scale, (vDÂČ/Δ)^(1/4), v being the kinematic viscosity, D
the thermal diffusivity, and Δ the kinetic energy dissipation per unit mass. Stress measurements
in the viscous sublayer provide estimates of Δ
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The scaling of vertical temperature gradient spectra
Tests of a formula derived for the cutoff wave number of vertical temperature gradient spectra, using
data taken in the upper layers of the North Pacific, show encouraging results. To derive this formula, the
cutoff wave number is assumed to be the Batchelor wave number, with kinetic energy dissipation calculated
by combining a form used in the atmosphere for calculating the vertical eddy diffusivity in terms of
the dissipation with the Osborn-Cox formula for calculating eddy diffusivity from the variance of the
temperature gradient spectrum. Kinetic energy dissipation in the water column can be determined in this
way; a vertical profile of dissipation shows values of the order of 10^(-3) cmÂČ s^(-3) at the base of a stormtossed
mixing layer. In the thermocline below, dissipation occurs in patches
Long-term greenhouse gas measurements from aircraft
In March 2009 the NOAA/ESRL/GMD Carbon Cycle and Greenhouse Gases Group collaborated with the US Coast Guard (USCG) to establish the Alaska Coast Guard (ACG) sampling site, a unique addition to NOAA's atmospheric monitoring network. This collaboration takes advantage of USCG bi-weekly Arctic Domain Awareness (ADA) flights, conducted with Hercules C-130 aircraft from March to November each year. Flights typically last 8 h and cover a large area, traveling from Kodiak up to Barrow, Alaska, with altitude profiles near the coast and in the interior. NOAA instrumentation on each flight includes a flask sampling system, a continuous cavity ring-down spectroscopy (CRDS) carbon dioxide (CO2)/methane (CH4)/carbon monoxide (CO)/water vapor (H2O) analyzer, a continuous ozone analyzer, and an ambient temperature and humidity sensor. Air samples collected in flight are analyzed at NOAA/ESRL for the major greenhouse gases and a variety of halocarbons and hydrocarbons that influence climate, stratospheric ozone, and air quality. We describe the overall system for making accurate greenhouse gas measurements using a CRDS analyzer on an aircraft with minimal operator interaction and present an assessment of analyzer performance over a three-year period. Overall analytical uncertainty of CRDS measurements in 2011 is estimated to be 0.15 ppm, 1.4 ppb, and 5 ppb for CO2, CH4, and CO, respectively, considering short-term precision, calibration uncertainties, and water vapor correction uncertainty. The stability of the CRDS analyzer over a seven-month deployment period is better than 0.15 ppm, 2 ppb, and 4 ppb for CO2, CH4, and CO, respectively, based on differences of on-board reference tank measurements from a laboratory calibration performed prior to deployment. This stability is not affected by variation in pressure or temperature during flight. We conclude that the uncertainty reported for our measurements would not be significantly affected if the measurements were made without in-flight calibrations, provided ground calibrations and testing were performed regularly. Comparisons between in situ CRDS measurements and flask measurements are consistent with expected measurement uncertainties for CH4 and CO, but differences are larger than expected for CO2. Biases and standard deviations of comparisons with flask samples suggest that atmospheric variability, flask-to-flask variability, and possible flask sampling biases may be driving the observed flask versus in situ CO2 differences rather than the CRDS measurements
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