Thermal microstructure on a lake slope 1

Caldwell, Douglas R.; Brubaker, J. M.; Neal, Victor T.

doi:10.4319/lo.1978.23.2.0372

Cited by 29 publications

(13 citation statements)

References 4 publications

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“…These vortices apparently mix boundary fluid which is presumably expelled along the isopycnal corresponding to the new density of the mixed fluid. As suggested by Caldwell et al (1978), these intrusive layers could also explained the presence of a highly "stepped" temperature profile as the steep slope is approached at Lake Tahoe, California.…”

Section: Laboratory and Numerical Experimentsmentioning

confidence: 84%

Near-critical reflection of internal waves

1999

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Using a matched asymptotic expansion we analyze the two-dimensional, near-critical reflection of a weakly nonlinear, internal gravity wave from a sloping boundary in a uniformly stratified fluid. Taking a distinguished limit in which the amplitude of the incident wave, the dissipation, and the departure from criticality are all small, we obtain a reduced description of the dynamics. This simplification shows how either dissipation or transience heals the singularity which is presented by the solution of Phillips (The Dynamics of the Upper Ocean, 1966) in the precisely critical case. In the inviscid critical case, an explicit solution of the initial value problem shows that the buoyancy perturbation and the along-slope velocity both grow linearly with time, while the scale of the reflected disturbance is reduced as 1/t. During the course of this scale reduction, the stratification is 'overturned' and the Miles-Howard condition for stratified shear flow stability is violated. However, for all slope angles, the 'overturning' occurs before the Miles-Howard stability condition is violated and so we argue that the first instability is convective.Solutions of the simplified dynamics resemble certain experimental visualizations of the reflection process. In particular, the buoyancy field computed from the analytic solution is in good agreement with visualizations reported by Thorpe & Haines (1987) J. Fluid Mech. 178, 299-302. One curious aspect of the weakly nonlinear theory is that the final reduced description is a linear equation (at the solvability order in the expansion all of the apparently resonant nonlinear contributions cancel amongst themselves). However the reconstructed fields do contain nonlinearly driven second harmonics which are responsible for an important symmetry breaking in which alternate vortices differ in strength and size from their immediate neighbours.

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Section: Laboratory and Numerical Experimentsmentioning

confidence: 84%

Near-critical reflection of internal waves

1999

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“…These regions might be transient, as, for example, the mixing produced by a storm at sea, or they might be associated with permanent currents or coastal features. Indeed, layered features thought to result from increased local mixing have been noted and discussed by Wunsch [1972] and Hogg et al [1978] in connection with the Bermuda slope and by Caldwell et al [1978] in the nearshore region of Lake Tahoe. The increased turbulence levels in the active region could be produced by a variety of causes, including boundary layer turbulent mixing, internal wave breaking, convectively driven turbulent mixing produced by bottom heating in shallow water, or by a combination of all three.…”

Section: Estimatin•t the Behal:ior Of Full-scale Frontsmentioning

confidence: 99%

The behavior of a turbulent front in a stratified fluid: Experiments with an oscillating grid

Browand¹,

Guyomar²,

Yoon³

1987

J. Geophys. Res.

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A simple laboratory model utilizes an oscillating grid to produce a turbulent front in a linearly stratified fluid. The motion of the front is studied visually using laser fluorescence and neutrally buoyant tracer particles. When the turbulence is vigorous near the grid, stratification has no effect. As the front propagates away, vertical scales become limited by stratification, as postulated by Ozmidov. The breakup of the front can be identified at the nondimensional time Nt = 10. Here a series of layers are formed which intrude into the surrounding nonturbulent fluid. The vertical scale of these layers is also predicted using Ozmidov's postulate. It is approximately 7 times the estimated Ozmidov scale at Nt = 10. The advance of individual intrusions comprising the front is characterized by a local Froude number which decreases as the intrusions lengthen. The mechanism of advance is intimately connected with wave motions produced at the turbulent end of the intrusions.

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“…While it is possible that the long near-horizontal layer shown in Fig. 3b was an intrusive layer (Caldwell et al 1978), it may also have been caused by internal waves. Mixed regions resulting from the propagation and breaking of groups of internal waves are nearly horizontal, provided that the square of the frequency of the waves is much less than f 2 (Thorpe 1988(Thorpe , 1999, and this feature may be a scar left by a breaking internal wave group that passed through the region.…”

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confidence: 99%

Observations of the thermal structure of a lake using a submarine

Thorpe

1999

Limnology & Oceanography

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This note describes how a submarine, the F.A. Forel, carrying a vertical array of high‐resolution temperature sensors, was used along with conventional measurements from a lowered conductivity‐temperature‐depth probe (CTD) to make novel measurements of the temperature field in Lake Geneva during summertime conditions of stable stratification and during winter convection. The submarine speed was about 0.5 m s−1. In addition to the temperatures, the pressure, orientation, and tilt were recorded at frequencies of at least 10 Hz. Observations were made on a vertical scale of 0.1 to 2.5 m and on a horizontal scale from 0.5 m to 1 km. Examples of the data are presented. During the summer, evidence was found of internal waves and of extensive layers of low vertical temperature gradient, with vertical and horizontal scales of 0.5 m and 0.5 km, respectively; within this gradient, the temperature changed monotonically in the horizontal. During periods favoring convection, in the winter, when air temperatures were about 7°C below the surface‐water temperature, convectively unstable regions, typically of 5‐m horizontal scale, were observed in the mixed layer. These appeared to be convective plumes. These winter measurements also included observations of a layer of cold water that was adjacent to the sloping boundary of the lake. This was identified as being a plume of dense cold water with thickness on the order of 10 m, which was driven by surface cooling, and consequent more rapid temperature decrease, in the shallow nearshore water. On meeting the thermocline at a depth of about 100 m, this plume spread horizontally and formed an intrusion some 30 m thick.

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Thermal microstructure on a lake slope 1

Abstract: As shallow water is approached via a steep lake‐bottom slope, increased mixing of heat is indicated by the presence of a highly “stepped” temperature profile. This mixing activity operates over a wide range of vertical scales.

Cited by 29 publications

References 4 publications

Near-critical reflection of internal waves

Near-critical reflection of internal waves

The behavior of a turbulent front in a stratified fluid: Experiments with an oscillating grid

Observations of the thermal structure of a lake using a submarine

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