Intrusion-generated waves in a linearly stratified fluid

Maurer, Benjamin; Linden, P. F.

doi:10.1017/jfm.2014.316

Cited by 10 publications

(12 citation statements)

References 19 publications

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“…It has been inferred that internal gravity waves do play a significant role in some intrusive flows through stratified fluids, such as in the collapse of a mixed region (Amen & Maxworthy 1980). In other flows, however, such as the lock-release currents of Maurer & Linden (2014), energy loss to gravity waves was observed to have a relatively small effect on the propagation of the intrusion. Ansong & Sutherland (2010) made direct experimental observations of gravity waves generated by the scenario that we consider in this paper, namely a buoyant plume and subsequent radial intrusion into a stratified environment.…”

Section: Shallow-layer Modelmentioning

confidence: 99%

Modelling intrusions through quiescent and moving ambients

et al. 2015

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Volcanic eruptions commonly produce buoyant ash-laden plumes that rise through the stratified atmosphere. On reaching their level of neutral buoyancy, these plumes cease rising and transition to horizontally spreading intrusions. Such intrusions occur widely in density-stratified fluid environments, and in this paper we develop a shallow-layer model that governs their motion. We couple this dynamical model to a model for particle transport and sedimentation, to predict both the time-dependent distribution of ash within volcanic intrusions and the flux of ash that falls towards the ground. In an otherwise quiescent atmosphere, the intrusions spread axisymmetrically. We find that the buoyancy-inertial scalings previously identified for continuously supplied axisymmetric intrusions are not realised by solutions of the governing equations. By calculating asymptotic solutions to our model we show that the flow is not self-similar, but is instead time-dependent only in a narrow region at the front of the intrusion. This non-self-similar behaviour results in the radius of the intrusion growing with time t as t 3/4 , rather than t 2/3 as suggested previously. We also identify a transition to drag-dominated flow, which is described by a similarity solution with radial growth now proportional to t 5/9 . In the presence of an ambient wind, intrusions are not axisymmetric. Instead, they are predominantly advected downstream, while at the same time spreading laterally and thinning vertically due to persistent buoyancy forces. We show that close to the source, this lateral spreading is in a buoyancy-inertial regime, whereas far downwind, the horizontal buoyancy forces that drive the spreading are balanced by drag. Our results emphasise the important role of buoyancy-driven spreading, even at large distances from the source, in the formation of the flowing thin horizontally extensive layers of ash that form in the atmosphere as a result of volcanic eruptions.

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Section: Shallow-layer Modelmentioning

confidence: 99%

Modelling intrusions through quiescent and moving ambients

et al. 2015

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“…Possible forcing mechanisms for near‐bottom internal waves include current‐topography interaction (Aghsaee et al, 2010; Bourgault et al, 2011, 2014; Klymak & Moum, 2003), shear flow instability (Apel, 2003; Thorpe, 1975) and gravity current intrusions (Flynn and Sutherland, 2004; Maurer & Linden, 2014). Although we lack the spatial coverage needed to determine the exact location of the wave forcing, the observations indicate that the solibores form midway through a spring tidal cycle when strong flood currents are flowing through the main channel immediately to the west of Mooring GVRD‐S.…”

Section: Discussionmentioning

confidence: 99%

Gravity Currents Facilitate Formation of High‐Frequency Internal Solitons and Bores at the Base of the Fraser Delta in the Southern Strait of Georgia

Thomson

Spear

2020

JGR Oceans

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We use velocity and acoustic backscatter records from downward looking acoustic Doppler current profilers moored 47 m above the seafloor, 20 km apart in water depths of 100 and 240 m to examine internal wave dynamics near the seabed in the southern Strait of Georgia. The observations reveal that the lower layer of the strait becomes a seasonal (wintertime) waveguide for energetic internal "solibores"-internal bores trailed by large-amplitude internal solitons-that propagate up the flank of the Fraser Delta. The rotor-like bores have shoreward bottom velocities of 0.2 m s −1 and vertical velocities of 0.05 m s −1 ; the trailing wave trains attain amplitudes of tens of meters, periods as short as 10 min and durations of several hours. Establishment of the seabed waveguide required to support the high-frequency internal motions begins in summer, when gravity currents originating from the shallow (~100 m) Skipjack-Java sill in neighboring Boundary Pass, descend deep into the strait at monthly intervals. During the ensuing winter, lower-density gravity currents form over the sill and subsequently override the denser bottom layer in the strait, increasing the buoyancy frequency of the guide. Based on the direction of solibore propagation, we speculate that the winter gravity currents also initiate generation of the internal waves during their descent over the rapidly deepening seafloor leading to the central strait. We further propose that the solibores are contributing to the formation and/or shoreward propagation of the O(10) m-high sediment waves found on the delta foreslope. Plain Language Summary Six-month duration records of currents and other oceanic variables near the base of the Fraser Delta in the southern Strait of Georgia reveal frequent occurrences of intense, near-bottom internal wave events. The events take the form of a lead, rotor-like internal bore followed by a train of high-amplitude internal waves. Termed solibores, the features propagate eastward up the flank of the delta at speeds of up to 20 km hr −1 and cause roughly 10 m vertical displacements of the water column over periods of 1 min to an hour. The motions suspend the bottom sediments and could be responsible for changes in the shoreward advancing bedforms (the "Foreslope Hills") observed near 300-m depth over the shoaling delta front. Establishment of the near-bottom density structure needed to support the motions begins in summer with the intrusion of dense gravity currents into the strait from neighboring Boundary Pass.

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“…A possible aspect inhibiting the excitation of the V1H1 mode is that, although a 12-h periodic onset of the wind exists, its duration is less than one-quarter of the mode period (i.e., ~3h), which is the minimum duration of a wind event leading to the formation of the horizontal baroclinic gradients (Spigel & Imberger, 1980) necessary for the complete excitation of the mode. On the other hand, it has been shown that when internal waves are excited by density currents there is a strong influence of the level of neutral buoyancy of the currents on the relative proportion of energy transferred to the different vertical wave modes (Maurer & Linden, 2014). Although more research is needed to prove it, we suggest that the inhibition of the V1 mode excitation is caused by the currents from the selective withdrawal and the river plume moving between 10 m and 40 m, which is the depth range on what the V1 mode should change the velocity direction (Figure 6d).…”

Section: Discussionmentioning

confidence: 99%

Preliminary characterization of the dominant baroclinic modes of a tropical Andean reservoir during a dry period

Posada-Bedoya

Gómez-Giraldo

Román-Botero

2019

Rev. Acad. Colomb. Cienc. Ex. Fis. Nat.

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A partir de datos de campo y un modelo numérico simple, se investigaron las ondas internas escala de cuenco en un embalse tropical andino durante un período seco. La estructura y el período de las oscilaciones baroclínicas observadas, se infirieron a partir del análisis espectral de las series de temperatura medidas y de los desplazamientos verticales de las isotermas asociadas. El modo de oscilación dominante identificado a partir de los datos de campo constaba de dos capas que oscilaban con velocidades verticales y fases contrarias (modo V2), y con un período de 24 h. Los modos baroclínicos verticales teóricos del embalse se estimaron a partir de un eigenmodelo simple, el cual predijo el período y la estructura vertical del modo dominante identificado a partir de las observaciones, indicando también que las oscilaciones de la velocidad horizontal en cada capa estaban en fase (modo V2H1) y con un período natural cercano a 24 h. El forzamiento del viento mostró una variabilidad periódica con un período dominante cercano a las 24 h, por lo que concluimos que el modo V2H1 fue el dominante durante el período analizado y sometido a excitación por resonancia con el forzamiento diurno del viento. Se modeló el embalse como un sistema lineal masa-resorte, amortiguado y forzado, con el fin de estimar la relación de amortiguamiento de las oscilaciones baroclínicas, y se obtuvieron oscilaciones subamortiguadas con una tasa de amortiguamiento similar a la reportada en otros lagos alrededor del mundo. También se investigó el potencial del modo V2H1 en la generación vertical de turbulencia debido a inestabilidades cortantes usando el número de Richardson del gradiente, y encontramos que la producción de turbulencia era baja. Por último, se discuten aquí la estructura vertical de la fase en las oscilaciones de las ondas internas y las potenciales implicaciones del campo de ondas internas en la ecología del embalse. © 2019. Acad. Colomb. Cienc. Ex. Fis. Nat.

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Intrusion-generated waves in a linearly stratified fluid

Cited by 10 publications

References 19 publications

Modelling intrusions through quiescent and moving ambients

Modelling intrusions through quiescent and moving ambients

Gravity Currents Facilitate Formation of High‐Frequency Internal Solitons and Bores at the Base of the Fraser Delta in the Southern Strait of Georgia

Preliminary characterization of the dominant baroclinic modes of a tropical Andean reservoir during a dry period

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