16,388 research outputs found
Modeling the Effect of Oceanic Internal Waves on the Accuracy of Multibeam Echosounders
When ray bending corrections are applied to multibeam echosounder (MBES) data, it is assumed that the varying layers of sound speed lie along horizontally stratified planes. In many areas internal waves occur at the interface where the water’s density changes abruptly (a pycnocline), this density gradient is often associated with a strong gradient in sound speed (a velocline). The internal wave introduces uncertainty into the echo soundings through two mechanisms: (1) tilting of the velocline, and (2) vertical oscillation of the velocline’s depth. A model has been constructed in order to examine how these effects degrade the accuracy of MBES measurements. The model numerically simulates the 3D ray paths of MBES soundings for a synthetic flat seafloor, as though the soundings have been collected through a user-defined internal wave. Along with sound speed information, the ray paths are used to estimate travel times which are then utilized as inputs for a conventional 2D ray trace. The discrepancy between the 3D and 2D ray traced solutions serve as an estimate of uncertainty. The same software can be extended to model the expected anomalies associated with tidal fronts and other phenomena that result in significant tilting or oscillation of the velocline. A case study was undertaken using observed internal wave parameters on the Scotian Shelf. The case study examines how survey design parameters such as line spacing, direction of survey lines, and water column sampling density can influence the uncertainty introduced by internal waves. In particular, an examination is undertaken in which 2D ray tracing models are augmented with MBES water column imaging of the velocline. The investigation shows that internal waves have the potential to cause vertical uncertainties exceeding IHO standards and that the uncertainty can potentially be mitigated through appropriate survey design. Results from the case study also indicate that acoustic tracking of the velocline has the potential to counteract the effects of internal waves through augmentation of 2D ray tracing models. This technique is promising, however, much more research and field testing is required to ascertain the practicality, reliability and repeatability of such an approach
Zonal Flows and Long-Lived Axisymmetric Pressure Bumps in Magnetorotational Turbulence
We study the behavior of magnetorotational turbulence in shearing box
simulations with a radial and azimuthal extent up to ten scale heights. Maxwell
and Reynolds stresses are found to increase by more than a factor two when
increasing the box size beyond two scale heights in the radial direction.
Further increase of the box size has little or no effect on the statistical
properties of the turbulence. An inverse cascade excites magnetic field
structures at the largest scales of the box. The corresponding 10% variation in
the Maxwell stress launches a zonal flow of alternating sub- and
super-Keplerian velocity. This in turn generates a banded density structure in
geostrophic balance between pressure and Coriolis forces. We present a
simplified model for the appearance of zonal flows, in which stochastic forcing
by the magnetic tension on short time-scales creates zonal flow structures with
life-times of several tens of orbits. We experiment with various improved
shearing box algorithms to reduce the numerical diffusivity introduced by the
supersonic shear flow. While a standard finite difference advection scheme
shows signs of a suppression of turbulent activity near the edges of the box,
this problem is eliminated by a new method where the Keplerian shear advection
is advanced in time by interpolation in Fourier space.Comment: Accepted for publication in Ap
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