Search Results

You are looking at 1 - 8 of 8 items for

  • Author or Editor: Johannes Becherer x
  • All content x
Clear All Modify Search
Johannes Becherer and James N. Moum

Abstract

A scheme for significantly reducing data sampled on turbulence devices (χpods) deployed on remote oceanographic moorings is proposed. Each χpod is equipped with a pitot-static tube, two fast-response thermistors, a three-axis linear accelerometer, and a compass. In preprocessing, voltage means, variances, and amplitude of the subrange (inertial-convective subrange of the turbulence) of the voltage spectrum representing the temperature gradient are computed. Postprocessing converts voltages to engineering units, in particular mean flow speed (and velocity), temperature, temperature gradient, and the rate of destruction of the temperature variance χ from which other turbulence quantities, such as heat flux, are derived. On 10-min averages, this scheme reduces the data by a factor of roughly 24 000 with a small (5%) low bias compared to complete estimates using inertial-convective subrange scaling of calibrated temperature gradient spectra.

Full access
Johannes Becherer, Mark T. Stacey, Lars Umlauf, and Hans Burchard

Abstract

Cross-channel transect measurements of microstructure and velocity in a well-mixed and curved tidal inlet in the German Wadden Sea show the occurrence of significant late flood stratification. This stratification is found to be a result of lateral straining. This study observes a strong single-cell lateral circulation, which is strongly pronounced at late flood and absent during most of ebb. This tidal asymmetry is caused by a systematic interplay between centrifugal forcing and the lateral baroclinic pressure gradient. During flood a positive feedback between the terms generates strong lateral circulation, whereas during ebb a negative feedback leads to a suppression of the cross-channel exchange. A theoretical framework based on vorticity is developed, which allows lateral and longitudinal circulation to be studied in a consistent way. With this framework it is possible to show that the tidal asymmetry of the lateral flow is a major driver of residual longitudinal estuarine circulation, here identified with the tidally averaged across-channel vorticity component.

Full access
Johannes Becherer, James N. Moum, John A. Colosi, James A. Lerczak, and Jacqueline M. McSweeney

Abstract

The inner shelf is a region inshore of that part of the shelf that roughly obeys Ekman dynamics and offshore of the surf zone. Importantly, this is where surface and bottom boundary layers are in close proximity, overlap, and interact. The internal tide carries a substantial amount of energy into the inner shelf region were it eventually dissipates and contributes to mixing. A part of this energy transformation is due to a complex interaction with the bottom, where distinctions between nonlinear internal waves of depression and elevation are blurred, indeed, where polarity reversals of incoming waves take place. From an intensive set of measurements over the inner shelf off central California, we identify salient differences between onshore pulses from waves with properties of elevation waves and offshore pulses from shallowing depression waves. While the velocity structures and amplitudes of on/offshore pulses 1 m above the seafloor are not detectably different, onshore pulses are both more energetically turbulent and carry more sediments than offshore pulses. Their turbulence is also oppositely skewed: onshore pulses slightly to the leading edges, offshore pulses to the trailing edges of the pulses. We consider in turn three independent mechanisms that may contribute to the observed asymmetry: propagation in adverse pressure gradients and the resultant inflection point instability, residence time of a fluid parcel in the pulse, and turbulence suppression by stratification. The first mechanism may largely explain higher turbulence in the trailing edge of offshore pulses. The extended residence time may be responsible for the high and more uniform turbulence distribution across onshore compared to offshore pulses. Stratification does not play a leading role in turbulence modification inside of the pulses 1 m above the bed.

Restricted access
Knut Klingbeil, Johannes Becherer, Elisabeth Schulz, Huib E. de Swart, Henk M. Schuttelaars, Arnoldo Valle-Levinson, and Hans Burchard

Abstract

This paper presents thickness-weighted averaging (TWA) in generalized vertical coordinates as a unified framework for a variety of existing tidal-averaging concepts in seas and estuaries. Vertical profiles of resulting residual quantities depend on the specific vertical coordinate, which is held fixed during the averaging process. This dependence is demonstrated through the application to one-dimensional analytical tidal flow with sediment transport, to field observations from a tidal channel, and to model results from a two-dimensional estuary. The use of different coordinate systems provides complementary views on the residual dynamics and stresses the importance of a correct interpretation of residual quantities obtained by tidal averaging.

Full access
Johannes Becherer, James N. Moum, Joseph Calantoni, John A. Colosi, John A. Barth, James A. Lerczak, Jacqueline M. McSweeney, Jennifer A. MacKinnon, and Amy F. Waterhouse

Abstract

Broadly-distributed measurements of velocity, density and turbulence spanning the inner shelf off central California indicate that (i) the average shoreward-directed internal tide energy flux (〈FE〉) decreases to near 0 at the 25 m isobath; (ii) the vertically-integrated turbulence dissipation rate (〈D〉) is approximately equal to the flux divergence of internal tide energy (xFE〉); (iii) the ratio of turbulence energy dissipation in the interior relative to the bottom boundary layer (BBL) decreases toward shallow waters; (iv) going inshore, 〈FE〉 becomes decorrelated with the incoming internal wave energy flux; and (v) 〈FE〉 becomes increasingly correlated with stratification toward shallower water.

Restricted access
Jacqueline M. McSweeney, James A. Lerczak, John A. Barth, Johannes Becherer, John A. Colosi, Jennifer A. MacKinnon, Jamie H. MacMahan, James N. Moum, Stephen D. Pierce, and Amy F. Waterhouse

Abstract

We present observations of shoaling nonlinear internal bores off the coast of central California. The dataset includes 15 moorings deployed during September–October 2017 and cross-shore shipboard surveys. We describe the cross-shore structure and evolution of large-amplitude internal bores as they transit from 9 km (100-m depth) to 1 km offshore (10 m). We observe that two bores arrive each semidiurnal period, both propagating from the southwest; of the total, 72% are tracked to the 10-m isobath. The bore speeds are subtidally modulated, but there is additional bore-to-bore speed variability that is unexplained by the upstream stratification. We quantify temporal and cross-shore variability of the waveguide (the background conditions through which bores propagate) by calculating the linear longwave nonrotating phase speed c o and using the nonlinearity coefficient of the Korteweg–de Vries equation α as a metric for stratification. Bore fronts are generally steeper when α is positive and are more rarefied when α is negative, and we observe the bore’s leading edge to rarefy from a steep front when α is positive offshore and negative inshore. High-frequency α fluctuations, such as those nearshore driven by wind relaxations, contribute to bore-to-bore variability of the cross-shore evolution during similar subtidal waveguide conditions. We compare observed bore speeds with c o and the rotating group velocities c g, concluding that observed speeds are always faster than c g and are slower than c o at depths greater than 32 m and faster than c o at depths of less than 32 m. The bores maintain a steady speed while transiting into shallower water, contrary to linear estimates that predict bores to slow.

Free access
Jacqueline M. McSweeney, James A. Lerczak, John A. Barth, Johannes Becherer, Jennifer A. MacKinnon, Amy F. Waterhouse, John A. Colosi, Jamie H. MacMahan, Falk Feddersen, Joseph Calantoni, Alexandra Simpson, Sean Celona, Merrick C. Haller, and Eric Terrill

Abstract

Temperature and velocity measurements from 42 moorings were used to investigate the alongshore variability of nonlinear internal bores as they propagated across the central California inner shelf. Moorings were deployed September–October 2017 offshore of the Point Sal headland. Regional coverage was ~30 km alongshore and ~15 km across shore, spanning 9–100-m water depths. In addition to subtidal processes modulating regional stratification, internal bores generated complex spatiotemporal patterns of stratification variability. Internal bores were alongshore continuous on the order of tens of kilometers at the 50-m isobath, but the length scales of frontal continuity decreased to O(1 km) at the 25-m isobath. The depth-averaged, bandpass-filtered (from 3 min to 16 h) internal bore kinetic energy (KEIB¯) was found to be nonuniform along a bore front, even in the case of an alongshore-continuous bore. The pattern of along-bore KEIB¯ variability varied for each bore, but a 2-week average indicated that KEIB¯ was generally strongest around Point Sal. The stratification ahead of a bore influenced both the bore’s amplitude and cross-shore evolution. The data suggest that alongshore stratification gradients can cause a bore to evolve differently at various alongshore locations. Three potential bore fates were observed: 1) bores transiting intact to the 9-m isobath, 2) bores being overrun by faster, subsequent bores, leading to bore-merging events, and 3) bores disappearing when the upstream pycnocline was near or below middepth. Maps of hourly stratification at each mooring and the estimated position of sequential bores demonstrated that an individual internal bore can significantly impact the waveguide of the subsequent bore.

Restricted access
Nirnimesh Kumar, James A. Lerczak, Tongtong Xu, Amy F. Waterhouse, Jim Thomson, Eric J. Terrill, Christy Swann, Sutara H. Suanda, Matthew S. Spydell, Pieter B. Smit, Alexandra Simpson, Roland Romeiser, Stephen D. Pierce, Tony de Paolo, André Palóczy, Annika O’Dea, Lisa Nyman, James N. Moum, Melissa Moulton, Andrew M. Moore, Arthur J. Miller, Ryan S. Mieras, Sophia T. Merrifield, Kendall Melville, Jacqueline M. McSweeney, Jamie MacMahan, Jennifer A. MacKinnon, Björn Lund, Emanuele Di Lorenzo, Luc Lenain, Michael Kovatch, Tim T. Janssen, Sean Haney, Merrick C. Haller, Kevin Haas, Derek J. Grimes, Hans C. Graber, Matt K. Gough, David A. Fertitta, Falk Feddersen, Christopher A. Edwards, William Crawford, John Colosi, C. Chris Chickadel, Sean Celona, Joseph Calantoni, Edward F. Braithwaite III, Johannes Becherer, John A. Barth, and Seongho Ahn

Abstract

The inner shelf, the transition zone between the surf zone and the mid shelf, is a dynamically complex region with the evolution of circulation and stratification driven by multiple physical processes. Cross-shelf exchange through the inner shelf has important implications for coastal water quality, ecological connectivity, and lateral movement of sediment and heat. The Inner-Shelf Dynamics Experiment (ISDE) was an intensive, coordinated, multi-institution field experiment from Sep.-Oct. 2017, conducted from the mid shelf, through the inner shelf and into the surf zone near Point Sal, CA. Satellite, airborne, shore- and ship-based remote sensing, in-water moorings and ship-based sampling, and numerical ocean circulation models forced by winds, waves and tides were used to investigate the dynamics governing the circulation and transport in the inner shelf and the role of coastline variability on regional circulation dynamics. Here, the following physical processes are highlighted: internal wave dynamics from the mid shelf to the inner shelf; flow separation and eddy shedding off Point Sal; offshore ejection of surfzone waters from rip currents; and wind-driven subtidal circulation dynamics. The extensive dataset from ISDE allows for unprecedented investigations into the role of physical processes in creating spatial heterogeneity, and nonlinear interactions between various inner-shelf physical processes. Overall, the highly spatially and temporally resolved oceanographic measurements and numerical simulations of ISDE provide a central framework for studies exploring this complex and fascinating region of the ocean.

Full access