Abstract
The most comprehensive studies of the spatial and temporal scales of diffusivity rely on internal wave parameterizations that require knowledge of finescale shear and strain. Studies lacking ...either shear or strain measurements have to assume a constant ratio between shear and strain (
R
ω
). Data from 14 moorings collected during five field programs are examined to determine the spatial and temporal patterns in
R
ω
and the influence of these patterns on parameterized diffusivity. Time-mean
R
ω
ranges from 1 to 10, with changes of order 10 observed over a broad range of scales. Temporal variability in
R
ω
is observed at daily, weekly, and monthly scales. Observed changes in
R
ω
could produce a 2–3 times change in parameterized diffusivity. Vertical profiles of
R
ω
,
E
shear
, and
E
strain
(shear or strain variance relative to Garret–Munk) reveal that both local topographic properties and wind variability impact the internal wave field. Time series of
R
ω
from each mooring have strong correlations to either shear or strain, often only at a specific range of vertical wavenumbers. Sites fall into two categories, in which
R
ω
variability is dominated by either shear or strain. Linear fits to the dominant property (i.e., shear or strain) can be used to estimate a time series of
R
ω
that has an RMS error that is 30% less than the RMS error from assuming
R
ω
= 3. Shear and strain level vary in concert, as predicted by the Garret–Munk model, at high
E
shear
values. However, at
E
shear
< 5, strain variations are 3 times weaker than shear.
Internal waves contain a significant fraction of the kinetic energy in the ocean and are important intermediaries between the forcing (by wind and tide) and interior diapycnal mixing. We report here ...on measurements from Mindoro Strait in the Philippines (connecting the South China Sea to the Sulu Sea) of an internal wave field with a number of surprising properties that point to previously‐unrecognized processes at work in the region. Continuum spectral levels are very close to typical “background” values found in the open ocean, but internal tide energy in both the diurnal and semidiurnal frequency bands is significantly elevated—and higher at the northern mooring (MP1) than the southern (MP2). Two particularly energetic depth ranges stand out at MP1: an upper layer centered near 300 m, and one at the bottom of the water column, near 1800 m. The upper layer contains both internal tides and a near‐inertial band with upward and downward propagating waves and an apparent spring‐neap cycle. The combination is suggestive of Parametric Subharmonic Instability as the forcing for the near‐inertial band—a conclusion supported by bicoherence estimates. Mixing, estimated from density overturns, is weak over much of the water column but enhanced by about an order of magnitude in the deep layer and closely tied to the internal tide—both diurnal and semidiurnal. Near‐inertial currents in this deep layer are dominantly rectilinear and not well‐correlated with the mixing. Bulk mixing rates at the two sites are less than required to produce property changes seen in hydrography, suggesting additional enhancement elsewhere in the archipelago.
Key Points
Low background levels of mixing and internal wave energy in Mindoro Strait
Fortnightly modulation of near‐inertial motions suggests generation by PSI
Mixing and internal tide energy elevated by an order of magnitude near bottom
Internal wave measurements from moorings and profiling floats throughout the Philippine Archipelago, collected as part of the Office of Naval Research Philippine Straits Dynamics Experiment, reveal a ...wealth of subsurface processes, some of which have not been observed previously (in the Philippines or elsewhere). Complex bathymetry and spatially varying tide and wind forcing produce distinct internal wave environments within the network of seas and channels, ranging from quiescent interior basins to remotely forced straits. Internal tides in both the diurnal and semidiurnal bands dominate much of the velocity structure and are likely the dominant source of energy for mixing in the region. In addition, the transfer of energy from the internal tide directly to near-inertial motions through parametric subharmonic instability appears to be active and, rather than wind forcing, is the dominant source of near-inertial band energy.
Acoustically tracked float data from 16 experiments carried out in the North Atlantic are used to evaluate the feasibility of estimating eddy heat fluxes from floats. Daily float observations were ...bin averaged in 2' by 2' by 200-db-deep geographic bins, and eddy heat fluxes were estimated for each bin. Results suggest that eddy heat fluxes can be highly variable, with substantial outliers that mean that fluxes do not converge quickly. If 100 statistically independent observations are available in each bin (corresponding to 500-1000 float days of data), then results predict that 80% of bins will have eddy heat fluxes that are statistically different from zero. Pop-up floats, such as Autonomous Lagrangian Circulation Explorer (ALACE) and Argo floats, do not provide daily sampling and therefore underestimate eddy heat flux. The fraction of eddy heat flux resolved using pop-up float sampling patterns decreases linearly with increasing intervals between float mapping and can be modeled analytically. This implies that flux estimates from pop-up floats may be correctable to represent true eddy heat flux. PUBLICATION ABSTRACT
Data from an archive of McLane Moored Profiler (MP) deployments are used to investigate the role of internal waves, and in general physical processes occupying this frequency range, on the ...dissipation of kinetic energy and to study how these processes work to shape the spectral characteristics of the wavefield. The data used come from 5 separate field programs, and by analyzing them as a set, provides the opportunity to not only compare and contrast an unprecedented range of different forcing regimes, but also supports the discovery of unique observations of physical processes at individual sites. This dissertation is composed of four separate chapters that address different aspects of this idea. First, two records, one to the North (MP1) and one to the South (MP2), of Mindoro strait, which lies between the South China Sea and the Sulu Sea, were used to study the processes driving mixing in the Strait. The records are dominated by near inertial variability and internal tides at both the diurnal and semidiurnal frequencies. Analysis of the velocity records from MP1 reveals suggestive evidence for the presence of parametric subharmonic instability of the diurnal tide as well as curious rectilinear bottom trapped near inertial waves. Diffusivities estimated via Thorpe scale analysis at MP1 reveal weak mixing over much of the water column that increases nearly an order of magnitude near the bottom and is strongly tied to the tides. Average mixing rates are found to be insufficient to produce observed changes in water mass properties suggesting additional processes are at play. Second, variability in the shear to strain ratio (Rω) is investigated with the goal of understanding the factors that influence Rω variability, quantifying its time and space scales, and determining how variability impacts estimates of parameterized diffusivity. Time mean Rω from each of the sites spans a range from 1 to 10 and within each record, temporal variability around the mean is sufficient to produce a factor of 2-3 change in parameterized diffusivity. Vertical structure of Rω appears strongly tied to topographic properties and wind forcing. In general, sites can be categorized as having Rω controlled by shear or by strain. Finally, we find that with sufficient characterization of a site, a predictive model of Rω can be made that reduces error in Rω values relative to assuming Rω = 3 by over 30\%. Third, aspects of the the wavenumber frequency spectrum are explored. We find that while a separable representation of the spectrum can be made that has the same level of variance, the observed structure cannot be modeled with a single form in vertical wavenumber. Narrow band peaks attenuate to match the background continuum by wavenumbers of approximately k = 10-2 cpm. At frequencies ω < 5 cpd and wavenumbers k > 10-2 cpm, we find that the spectrum is nearly white in frequency and that nearly all frequency bands have collapsed to this same structure. Background continuum spectral levels of velocity vary as a linear function of N . The background continuum spectrum, with peaks removed, displays significant deviations from GM at low wavenumbers. Addition of the narrow band peaks improves the consistency of the total spectrum with GM at low wavenumbers implying that the often observed GM spectral shape is dependent on the presence of internal tides. Fourth, the impact of the inherent sawtooth sampling pattern in time and depth space of profiling instruments on resolved variance is examined through the use of virtual instruments profiling through an idealized wavefield. By creating a wavefield that has an exactly GM velocity spectrum, the variance resolved by various profiling schemes can be determined. We find that variance resolved, as computed from a spectrum from gridded profile data, can be represented as a linear function of mean time between profile and latitude. A functional dependence is defined such that one can estimate a priori the impact of their sampling patterns. Additionally, the spectral response and bandwidth of the half inertial differencing operation is examined. It is found that the while half inertial differencing does eliminate even multiples of the inertial frequency and alias in odd multiples, the pass band of the filter is quite broad meaning there is potential for significant aliasing of non inertial signals into the inertial band estimates from the half inertial difference filter.
We present the concept of a new Fabry-Perot instrument called BTFI-2, which is based on the design of another Brazilian instrument for the SOAR Telescope, the Brazilian Tunable Filter Imager (BTFI). ...BTFI-2 is designed to be mounted on the visitor port of the SOAR Adaptive Module (SAM) facility, on the SOAR telescope, at Cerro Pachón, Chile. This optical Fabry-Perot instrument will have a field of view of 3 x 3 arcmin, with 0.12 arcsec per pixel and spectral resolutions of 4500 and 12000, at H-alpha, dictated by the two ICOS Fabry-Perot devices available. The instrument will be unique for the study of centers of normal, interacting and active galaxies and the intergalactic medium, whenever spatial resolution over a large area is required. BTFI-2 will combine the best features of two previous instruments, SAM-FP and BTFI: it will use an Electron Multiplication detector for low and fast scanning, it will be built with the possibility of using a new Fabry-Perot etalon which provides a range of resolutions and it will be light enough to work attached to SAM, and hence the output data cubes will be GLAO-corrected.
This work describes the mathematical modelling of motorcycle collisions with deformable structures. It has been developed over a number of years to the current stage where the intention is to ...investigate actual accident situations rather than pre-described tests. A section of this reconstruction task concerns the development and validation of the computer simulation of a motorcycle crash test. The purpose of this computer modelling is to identify the applied loads to the head by using a motorcycle model and dummy rider impacting an obstacle and to apply them afterwards to a FE model of the human head. The stress distribution obtained with the FE model will show how the brain is affected during this type of impact so that we can have an idea of the injury mechanisms developed. TRL has impact-tested many motorcycles and a recent test of a Norton Commander travelling at 50kdh impacting at 90° into the side of a stationary Ford Mondeo was chosen for the development purposes of a multi-body model. The multibody modelling approach has been done with the MADYMO package and the finite element analysis with PAMCRASH explicit code.
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