Observations of Diurnal Coastal-Trapped Waves with a Thermocline-Intensified Velocity Field

AbstractUsing 18 days of field observations, we investigate the diurnal (D1) frequency wave dynamics on the Tasmanian eastern continental shelf. At this latitude, the D1 frequency is subinertial and separable from the highly energetic near-inertial motion. We use a linear coastal-trapped wave (CTW) solution with the observed background current, stratification, and shelf bathymetry to determine the modal structure of the first three resonant CTWs. We associate the observed D1 velocity with a superimposed mode-zero and mode-one CTW, with mode one dominating mode zero. Both the observed and mode-one D1 velocity was intensified near the thermocline, with stronger velocities occurring when the thermocline stratification was stronger and/or the thermocline was deeper (up to the shelfbreak depth). The CTW modal structure and amplitude varied with the background stratification and alongshore current, with no spring–neap relationship evident for the observed 18 days. Within the surface and bottom Ekman layers on the shelf, the observed velocity phase changed in the cross-shelf and/or vertical directions, inconsistent with an alongshore propagating CTW. In the near-surface and near-bottom regions, the linear CTW solution also did not match the observed velocity, particularly within the bottom Ekman layer. Boundary layer processes were likely causing this observed inconsistency with linear CTW theory. As linear CTW solutions have an idealized representation of boundary dynamics, they should be cautiously applied on the shelf.

Download Full-text

Stratified Ekman layers evolving under a finite-time stabilizing buoyancy flux

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.58 ◽

2018 ◽

Vol 840 ◽

pp. 266-290 ◽

Cited By ~ 4

Author(s):

S. M. Iman Gohari ◽

Sutanu Sarkar

Keyword(s):

Finite Time ◽

Richardson Number ◽

Ekman Layer ◽

Buoyancy Flux ◽

Late Time ◽

Near Surface ◽

Obukhov Length ◽

Low Level ◽

Ekman Layers ◽

Low Level Jet

Stratified flow in nocturnal boundary layers is studied using direct numerical simulation (DNS) of the Ekman layer, a model problem that is useful to understand atmospheric boundary-layer (ABL) turbulence. A stabilizing buoyancy flux is applied for a finite time to a neutral Ekman layer. Based on previous studies and the simulations conducted here, the choice of $L_{\mathit{cri}}^{+}=Lu_{\ast }/\unicode[STIX]{x1D708}\approx 700$ ($L$ is the Obukhov length scale and $u_{\ast }$ is the friction velocity) provides a cooling flux that is sufficiently strong to cause the initial collapse of turbulence. The turbulent kinetic energy decays over a time scale of $4.06L/u_{\ast }$ during the collapse. The simulations suggest that imposing $L_{\mathit{cri}}^{+}\approx 700$ on the neutral Ekman layer results in turbulence collapse during the initial transient, independent of Reynolds number, $Re_{\ast }$. However, the long-time state of the flow, i.e. turbulent with spatial intermittency or non-turbulent, is found to depend on the initial value of $Re_{\ast }$ since the cooling flux and resultant stratification increase with $Re_{\ast }$ for a given $L^{+}$. The lower-$Re_{\ast }$ cases have sustained turbulence with shear and stratification profiles that evolve in a manner such that the gradient Richardson number, $Ri_{g}$, in the near-surface layer, including the low-level jet, remains subcritical. The highest $Re_{\ast }$ case has supercritical $Ri_{g}$ in the low-level jet and turbulence does not recover. A theoretical discussion is performed to infer that the bulk Richardson number, $Ri_{b}$, is more suitable than $L^{+}$ to determine the fate of stratified Ekman layers at late time. DNS results support the implications of $Ri_{b}$ for the effect of initial $Re_{\ast }$ and $L^{+}$ on the flow.

Download Full-text

At What Time of Day Do Daily Extreme Near-Surface Wind Speeds Occur?

Journal of Climate ◽

10.1175/jcli-d-13-00286.1 ◽

2014 ◽

Vol 27 (11) ◽

pp. 4226-4244 ◽

Cited By ~ 2

Author(s):

Robert Fajber ◽

Adam H. Monahan ◽

William J. Merryfield

Keyword(s):

Extreme Values ◽

Mechanistic Model ◽

Surface Wind ◽

Time Of Day ◽

Near Surface ◽

Wind Speeds ◽

Boundary Layer Processes ◽

Early Afternoon ◽

Extreme Wind ◽

Extreme Wind Speeds

Abstract The timing of daily extreme wind speeds from 10 to 200 m is considered using 11 yr of 10-min averaged data from the 213-m tower at Cabauw, the Netherlands. This analysis is complicated by the tendency of autocorrelated time series to take their extreme values near the beginning or end of a fixed window in time, even when the series is stationary. It is demonstrated that a simple averaging procedure using different base times to define the day effectively suppresses this “edge effect” and enhances the intrinsic nonstationarity associated with diurnal variations in boundary layer processes. It is found that daily extreme wind speeds at 10 m are most likely in the early afternoon, whereas those at 200 m are most likely in between midnight and sunrise. An analysis of the joint distribution of the timing of extremes at these two altitudes indicates the presence of two regimes: one in which the timing is synchronized between these two layers, and the other in which the occurrence of extremes is asynchronous. These results are interpreted physically using an idealized mechanistic model of the surface layer momentum budget.

Download Full-text

Numerical modeling of low-frequency wave dynamics over a fringing coral reef

Coastal Engineering ◽

10.1016/j.coastaleng.2012.11.004 ◽

2013 ◽

Vol 73 ◽

pp. 178-190 ◽

Cited By ~ 90

Author(s):

Ap Van Dongeren ◽

Ryan Lowe ◽

Andrew Pomeroy ◽

Duong Minh Trang ◽

Dano Roelvink ◽

...

Keyword(s):

Numerical Modeling ◽

Coral Reef ◽

Low Frequency ◽

Frequency Wave ◽

Wave Dynamics

Download Full-text

Near-surface dispersion studies at Tonto Forest Seismological Observatory

Bulletin of the Seismological Society of America ◽

10.1785/bssa0570030311 ◽

1967 ◽

Vol 57 (3) ◽

pp. 311-340

Author(s):

A. Frank Linville ◽

Stanley J. Laster

Keyword(s):

Linear Array ◽

Wave Number ◽

Near Surface ◽

Frequency Wave ◽

Local Dispersion ◽

Analysis Techniques ◽

Surface Dispersion ◽

Seismological Observatory ◽

Comparison Of The Results ◽

Correlation Properties

abstract Four local seismic events digitally recorded on a crossed-linear array have been analyzed to evaluate the representation of wave propagation in inhomogeneous earth models in terms of “local” dispersion. The analysis techniques studied are based primarily on the correlation properties of the signals at various receivers. Frequency-wave-number spectra are computed for the events by various methods, and a comparison of the results indicates that the best method is a combination using averaged correlations computed from prewhitened data. The utility of computing dispersion curves from an estimation of the “transfer function” has been evaluated. The studies show the necessity of recording data directly “inline” in areas of rough topography between source and array if “local” dispersion is to be measured. Multichannel prediction studies indicate that the signals are highly nonstationary in space.

Download Full-text

Meanders and Eddies from Topographic Transformation of Coastal-Trapped Waves

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-0224.1 ◽

2014 ◽

Vol 44 (4) ◽

pp. 1133-1150 ◽

Cited By ~ 4

Author(s):

J. T. Rodney ◽

E. R. Johnson

Keyword(s):

Nonlinear Wave ◽

Incident Wave ◽

Short Wave ◽

Wave Transformation ◽

Local Geometry ◽

Long Wave ◽

Coastal Trapped Waves ◽

Wkbj Method ◽

Shelf Slope ◽

Coastal Trapped Wave

Abstract This paper describes how topographic variations can transform a small-amplitude, linear, coastal-trapped wave (CTW) into a nonlinear wave or an eddy train. The dispersion relation for CTWs depends on the slope of the shelf. Provided the cross-shelf slope varies sufficiently slowly along the shelf, the local structure of the CTW adapts to the local geometry and the wave transformation can be analyzed by the Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) method. Two regions of parameter space are straightforward: adiabatic transmission (where, at the incident wave frequency, a long wave exists everywhere along the shelf) and short-wave reflection (where somewhere on the shelf no long wave exists at the incident frequency, but the stratification is sufficiently weak that a short reflected wave can coexist with the incident wave). This paper gives the solutions for these two cases but concentrates on a third parameter regime, which includes all sufficiently strongly stratified flows, where neither of these behaviors is possible and the WKBJ method fails irrespective of how slowly the topography changes. Fully nonlinear integrations of the equation for the advection of the bottom boundary potential vorticity show that the incident wave in this third parameter regime transforms into a nonlinear wave when topographic variations are gradual or into an eddy train when the changes are abrupt.

Download Full-text

Fine wave dynamics in umbral flash sources

Astronomy and Astrophysics ◽

10.1051/0004-6361/201732139 ◽

2018 ◽

Vol 618 ◽

pp. A123 ◽

Cited By ~ 3

Author(s):

R. Sych ◽

M. Wang

Keyword(s):

Spatial Structure ◽

Extreme Ultraviolet ◽

Time Increase ◽

Wave Processes ◽

Wave Fronts ◽

Frequency Wave ◽

Time Dynamics ◽

Wave Dynamics ◽

Temporal Oscillation ◽

Short Time

Context. Umbral flashes (UFs) are most common phenomenon of wave processes in sunspots. Studying the relationship between wave time dynamics and the origin of UFs requires further investigation of their fine spatial and height structure. Aims. We investigated the association between a short time increase in the variations of 3-min extreme ultraviolet (EUV) emission at footpoints of coronal magnetic loops and the UF emergence in sunspot umbra. The spatial structure of magnetic field lines and their inclination determine the cut-off frequency and visibility of sunspot waves. Methods. We applied the pixelized wavelet filtering (PWF) technique to analyse a cube of the images obtained by SDO/AIA at 1600 Å, 304 Å, and 171 Å to study the spatio-temporal oscillation power distribution in individual magnetic loops. Time–distance plots were used to obtain the wave front propagation velocity. Results. For the first time, we obtained the 2D images of the fine wave processes in magnetic structures of different spatial scales related to the UFs in sunspot. We revealed two types of the UFs as background and local. The background UFs associated with random increasing of separate parts of wave fronts. These UFs are seen as weak and diffuse details that ride the wave fronts without stable shapes and localization in space. The local UFs sources mainly localize near to the footpoint of magnetic loops, anchored in an umbra, along which the propagation of 3-min waves was observed. The time dynamics of flash evolution shows an increase in 3-min oscillations before the UFs peak value within one low-frequency wave train. It is shown that the maxima of 3-min oscillation trains coincide with the peak intensity of UFs. During the UFs evolution, a fine wave spatial and temporal dynamics in UFs local sources was observed. Conclusions. The sunspot 3-min wave dynamics showed a relation between the localization of oscillations power peak at the coronal magnetic loop footpoints and the UFs origin. The spatial structure of the UFs sources, their power, and lifetime are determined by the cut-off frequency of the waves for the detected waveguides. We concluded that UFs are a global process of short-time increase of the wave activity.

Download Full-text

Coastal Trapped Wave Propagation along the Southwest African Shelf as Revealed by Moored Observations

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0046.1 ◽

2019 ◽

Vol 49 (3) ◽

pp. 851-866

Author(s):

Tim Junker ◽

Volker Mohrholz ◽

Martin Schmidt ◽

Lydia Siegfried ◽

Anja van der Plas

Keyword(s):

Bottom Topography ◽

Velocity Signal ◽

Remote Forcing ◽

Coastal Trapped Waves ◽

Benguela Upwelling ◽

Typical Time ◽

Coastal Trapped Wave ◽

Comparison Of The Results ◽

Wavelet Methods ◽

Phase Spectra

AbstractCoastal trapped waves (CTWs) that propagate poleward along the southwest African shelf potentially leak energy from lower latitudes into the Benguela Upwelling System (BUS). Thus, in addition to local winds, these waves provide an important remote forcing mechanism for the upwelling region. The present study aims at elucidating the nature of CTWs in the northern BUS. To this end, we make use of multisite velocity observations from the Namibian shelf (18°, 20°, 23°S) and examine the alongshore velocity signal for signatures of CTWs by means of wavelet methods. We found that a substantial amount of energy is concentrated within a submonthly to subseasonal frequency band (10–50 days). Based on the coherence and phase spectra of the alongshelf currents, we provide evidence for a predominantly southward phase propagation and establish typical time and length scales of CTWs in the region. It turns out that their properties differ significantly within a few hundred kilometers along the coast. A comparison of the results with theoretical dispersion curves shows that this difference may be explained by variations in the bottom topography. Finally, we investigate the coupling of the alongshore currents with the coastal and equatorial wind stress and highlight regions of potential wave generation.

Download Full-text

Near-surface scattering effects observed with a high-frequency phased array at Pinyon Flats, California

Bulletin of the Seismological Society of America ◽

10.1785/bssa0880061548 ◽

1998 ◽

Vol 88 (6) ◽

pp. 1548-1560

Author(s):

Frank L. Vernon ◽

Gary L. Pavlis ◽

Tom J. Owens ◽

Dan E. McNamara ◽

Paul N. Anderson

Keyword(s):

Surface Wave ◽

Wave Field ◽

Particle Motion ◽

High Frequency ◽

Body Wave ◽

Three Dimensional ◽

Wave Guide ◽

Surface Scattering ◽

Near Surface ◽

Frequency Wave

Abstract Analysis of data collected by a high-frequency array experiment conducted at Pinyon Flat in southern California provides strong evidence that the high-frequency wave field from local earthquakes at this hard-rock site are strongly distorted by near-surface scattering. The seismic array we deployed consisted of 60, 2-Hz natural frequency, three-component sensors deployed in a three-dimensional array. Two of the sensors were located in boreholes at 150 and 275 m depth. The other 58 sensors were deployed in an areal array above these boreholes. Thirty-six of these were deployed in a 6-by-6 element grid array with a nominal spacing of 7 m centered over the borehole sensors. The remaining 22 seismometers were laid out in two 11-element linear arrays radiating outward from the grid. Coherence calculations reveal a rapid loss of coherence at frequencies over 15 Hz at all but the shortest length scales of this array. Three-dimensional visualization techniques were used to closely examine the spatial stability of particle motions of P and S waves. This reveals systematic variations of particle motion across the array in which the particle motion tracks tilt drastically away from the backazimuth expected for an isotropic medium. These variations, however, are frequency dependent. Below around 8 Hz, the particle motions become virtually identical for all stations. At progressively higher frequencies, the wave-field particle motion becomes increasingly chaotic. Frequency-wave-number analysis of these data provide quantitative measures of the same phenomena. We find that direct wave f-k spectra are bathed in a background of signal-generated noise that varies from 10 to 30 dB down from the direct arrival signal. This signal-generated noise appears to be nearly white in wavenumber indicating the wavelength of this “noise” on the scale of tens of meters and less. Refraction measurements we made on two lines crisscrossing the array reveal that the weathered layer velocities are highly variable and define a very strong wave guide. Measured surface P-wave velocities varied from 400 to 1300 m/sec, and velocities at depth of approximately 15 m varied from 1600 to 2700 m/sec. Previous measurements in the boreholes showed that the intact granite below about 65 m depth has a velocity of approximately 5400 m/sec. These results demonstrate the extreme velocity contrast and degree of velocity heterogeneity of the near surface at this site. We conclude that all the observations we made can be explained by strong scattering of incident body-wave signals into a complex mishmash of body-wave and surface-wave modes in this heterogeneous near-surface wave guide.

Download Full-text