Publications

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2017
Jagannathan, A, Winters KB, Armi L.  2017.  Stability of stratified downslope flows with an overlying stagnant isolating layer. Journal of Fluid Mechanics. 810:392-411.   10.1017/jfm.2016.683   AbstractWebsite

We investigate the dynamic stability of stratified flow configurations characteristic of hydraulically controlled downslope flow over topography. Extraction of the correct 'base state' for stability analysis from spatially and temporally evolving flows that exhibit instability is not easy since the observed flow in most cases has already been modified by nonlinear interactions between the instability modes and the mean flow. Analytical studies, however, can yield steady solutions under idealized conditions which can then be analysed for stability. Following the latter approach, we study flow profiles whose essential character is determined by recently obtained solutions of Winters & Armi (J. Fluid Mech., vol. 753, 2014, pp. 80-103) for topographically controlled stratified flows. Their condition of optimal control necessitates a streamline bifurcation which then naturally produces a stagnant isolating layer overlying an accelerating stratified jet in the lee of the topography. We show that the inclusion of the isolating layer is an essential component of the stability analysis and further clarify the nature and mechanism of the instability in light of the wave-interaction theory. The spatial stability problem is also briefly examined in order to estimate the downstream location where finite-amplitude features might be manifested in streamwise slowly varying flows over topography.

2015
Armi, L, Mayr GJ.  2015.  Virtual and real topography for flows across mountain ranges. Journal of Applied Meteorology and Climatology. 54:723-731.   10.1175/jamc-d-14-0231.1   AbstractWebsite

A combination of real and virtual topography is shown to be crucial to describe the essentials of stratified flow over mountain ranges and leeside valleys. On 14 March 2006 [Intensive Observation Period 4 of the Terrain-Induced Rotor Experiment (T-REX)], a nearly neutral cloud-filled layer, capped by a strong density step, overflowed the Sierra Nevada and separated from the lee slope upon encountering a cooler valley air mass. The flow in this lowest layer was asymmetric across and hydraulically controlled at the crest with subcritical flow upstream and supercritical flow downstream. The density step at the top of this flowing layer formed a virtual topography, which descended 1.9 km and determined the horizontal scale and shape of the flow response aloft reaching into the stratosphere. A comparison shows that the 11 January 1972 Boulder, Colorado, windstorm case was similar: hydraulically controlled at the crest with the same strength and descent of the virtual topography. In the 18 February 1970 Boulder case, however, the layer beneath the stronger virtual topography was subcritical everywhere with a symmetric dip across the Continental Divide of only 0.5 km. In all three cases, the response and strength of the flow aloft depend on the virtual topography. The layer up to the next strong density step at or near the tropopause was hydraulically supercritical for the 18 February case, subcritical for the T-REX case, and critically controlled for the 11 January case, for which a weak density step and isolating layer aloft made possible the strong response aloft for which it is famous.

2014
Winters, KB, Armi L.  2014.  Topographic control of stratified flows: upstream jets, blocking and isolating layers. Journal of Fluid Mechanics. 753:80-103.   10.1017/jfm.2014.363   AbstractWebsite

Optimal solutions to the nonlinear, hydrostatic, Boussinesq equations are developed for steady, density-stratified, topographically controlled flows characterized by blocking and upstream influence. These flows are jet-like upstream of an isolated obstacle and are contained within an asymmetric, thinning stream tube that is accelerated as it passes over the crest. A stagnant, nearly uniform-density isolating layer, surrounded by a bifurcated uppermost streamline, separates the accelerated flow from an uncoupled flow above. The flows are optimal in the sense that, for a given stratification, the solutions maximize the topographic rise above the blocking level required for hydraulic control while minimizing the total energy of the flow. Hydraulic control is defined mathematically by the asymmetry of the accelerated flow as it passes the crest. A subsequent analysis of the Taylor-Goldstein equation shows that these sheared, non-uniformly stratified flows are indeed subcritical upstream, critical at the crest, and supercritical downstream with respect to gravest-mode, long internal waves. The flows obtained are relevant to arrested wedge flows, selective withdrawal, stratified towing experiments, tidal flow over topography and atmospheric flows over mountains.

2013
Winters, KB, Armi L.  2013.  The response of a continuously stratified fluid to an oscillating flow past an obstacle. Journal of Fluid Mechanics. 727:83-118.   10.1017/jfm.2013.247   AbstractWebsite

An oscillating continuously stratified flow past an isolated obstacle is investigated using scaling arguments and two-dimensional non-hydrostatic numerical experiments. A new dynamic scaling is introduced that incorporates the blocking of fluid with insufficient energy to overcome the background stratification and crest the obstacle. This clarifies the distinction between linear and nonlinear flow regimes near the crest of the obstacle. The flow is decomposed into propagating and non-propagating components. In the linear limit, the non-propagating component is related to the unstratified potential flow past the obstacle and the radiating component exhibits narrow wave beams that are tangent to the obstacle at critical points. When the flow is nonlinear, the near crest flow oscillates between states that include asymmetric, crest-controlled flows. Thin, fast, supercritical layers plunge in the lee, separate from the obstacle and undergo shear instability in the fluid interior. These flow features are localized to the neighbourhood of the crest where the flow transitions from subcriticality to supercriticality and are non-propagating. The nonlinear excitation of energetic non-propagating components reduces the efficiency of topographic radiation in comparison with linear dynamics.

2012
Hildebrand, JA, Armi L, Henkart PC.  2012.  Seismic imaging of the water-column deep layer associated with the Deepwater Horizon oil spill. Geophysics. 77:EN11-EN16.   10.1190/geo2011-0347.1   Abstract

The submerged hydrocarbon layer at a depth of approximately 1100-1200 m in the water column near the Deepwater Horizon well site was associated with a temperature anomaly that was imaged by seismic reflection data collected two days before and one day after the well shut-in. Conventional multichannel seismic reflection profiling data were processed to enhance water-column reflections by filtering out low-frequency energy and applying higher gain than typically used for imaging subsurface geology. The deep layer reflectivity, within a few km of the well site, has an impedance less than seawater by about a factor of approximately 3.6 x 10(-4), or equivalently, a negative sound-velocity step of about 0.5 m/s. Water-column profile data collected at the same time as the seismic data show a comparable sound velocity and temperature decrease associated with the deep layer. Mixing of less dense hydrocarbons entrains cold seawater, resulting in a temperature anomaly within the deep layer at the level of neutral buoyancy. This temperature contrast explains the negative sound-velocity anomaly associated with the deep reflector. Because of the small hydrocarbon concentration in the deep layer, their contribution to the seismic velocity anomaly is too small to be significant. The high horizontal resolution (3 m) of the seismic image reveals a layer with patchiness of several hundred meters lateral extent.

Winters, KB, Armi L.  2012.  Hydraulic control of continuously stratified flow over an obstacle. Journal of Fluid Mechanics. 700:502-513.   10.1017/jfm.2012.157   AbstractWebsite

Motivated by the laboratory experiments of Browand & Winant (Geophys. Fluid Dyn., vol. 4, 1972, pp. 29-53), a series of two-dimensional numerical simulations of flow past a cylinder of diameter d are run for different values of the approach Froude number Fr-0 = U/Nd between 0.02 and 0.2 at Re = O(100). The observed flow is characterized by blocking and upstream influence in front of the cylinder and by relatively thin, fast jets over the top and bottom of the cylinder. This continuously stratified flow can be understood in terms of an inviscid non-diffusive integral inertia-buoyancy balance reminiscent of reduced-gravity single-layer hydraulics, but one where the reduced gravity is coupled to the thickness of the jets. The proposed theoretical framework describes the flow upstream of the obstacle and at its crest. The most important elements of the theory are the inclusion of upstream influence in the form of blocked flow within an energetically constrained depth range and the recognition that the flow well above and well below the active, accelerated layers is dynamically uncoupled. These constraints determine, through continuity, the transport in the accelerated layers. Combining these results with the observation that the flow is asymmetric around the cylinder, i.e. hydraulically controlled, allows us to determine the active layer thicknesses, the effective reduced gravity and thus all of the integral flow properties of the fast layers in good agreement with the numerically computed flows.

2011
Armi, L, Mayr GJ.  2011.  The Descending Stratified Flow and Internal Hydraulic Jump in the Lee of the Sierras. Journal of Applied Meteorology and Climatology. 50:1995-2011.   10.1175/jamc-d-10-05005.1   AbstractWebsite

Cross-barrier density differences and westerly flow established a descending stratified flow across the Sierra Nevada (United States) on 9-10 April 2006. Downslope flow and an internal hydraulic jump occurred only when the potential temperature of the westerly descending flow was at least as cold as the existing upvalley-flowing valley air mass. The onset was observed in sequences of visible satellite images and with weather stations. The University of Wyoming King Air flew through the stratified flow and imaged the structure of the internal hydraulic jump with its cloud radar. Shear-layer instabilities, which first developed near the jump face, grew and paired downstream, mixing the internal hydraulic jump layer. A single wave response to the downslope flow and internal hydraulic jump was observed aloft, but only after the downslope flow had become established.

2010
Mayr, GJ, Armi L.  2010.  The Influence of Downstream Diurnal Heating on the Descent of Flow across the Sierras. Journal of Applied Meteorology and Climatology. 49:1906-1912.   10.1175/2010jamc2516.1   AbstractWebsite

The potential for a stably stratified air mass upstream of the Sierra Nevada (California) to descend as foehn into the nearly 3-km-deep Owens Valley was studied for the 2 March 2006 case with observations from sondes, weather stations, and two aircraft flights. While upstream conditions remained almost unchanged throughout the day, strong diurnal heating on the downstream side warmed the valley air mass sufficiently to permit flow through the passes to descend to the valley floor only in the late afternoon. Potential temperatures of air crossing the crest were too warm to descend past a virtual floor formed by the strong potential temperature step at the top of the valley air mass, the height of which changed throughout the day primarily due to diurnal heating in the valley. The descending stably stratified flow and its rebound with vertical velocities as high as 8 m s(-1) were shaped by the underlying topography and the virtual valley floor.

Biescas, B, Armi L, Sallares V, Gracia E.  2010.  Seismic imaging of staircase layers below the Mediterranean Undercurrent. Deep-Sea Research Part I-Oceanographic Research Papers. 57:1345-1353.   10.1016/j.dsr.2010.07.001   AbstractWebsite

Seismic images of staircase layers at the bottom of the Mediterranean Undercurrent with a lateral coherence up to 50 km and a horizontal resolution of 10 m are presented. The images show clearly the interaction between these staircase layers and other flow structures such as meddies, seamounts and internal waves. The staircase layers were imaged during two different surveys that used different sound sources. Comparison between seismic images and historical oceanographic observations illustrates the importance of using a seismic source adapted to the vertical scale of the oceanographic target to be imaged. Wavelengths larger than the size of the staircase structure distort the image in the vertical. For optimal imaging, deconvolution of the data is required. (C) 2010 Elsevier Ltd. All rights reserved.

Cummins, PF, Armi L.  2010.  Upstream Internal Jumps in Stratified Sill Flow: Observations of Formation, Evolution, and Release. Journal of Physical Oceanography. 40:1419-1426.   10.1175/2010jpo4435.1   AbstractWebsite

The time-dependent response of upstream undular bores and internal hydraulic jumps from initial formation to eventual release is documented. Two events, characterized by qualitatively different responses, are discussed. In the first case, an undular bore develops upstream of the sill crest. This disturbance remains upstream through the ebb tidal flow but is transformed to a hydraulic jump as its amplitude increases. Toward the end of ebb tide, it is released and subsequently disperses into a group of solitary-like waves. During the second event, an upstream jump also develops at an early stage of the tide. However, it is subsequently swept downstream by the tidal flow such that the upstream region then appears featureless. Approaching slack tide, as an exchange flow becomes established, a large bore or gravity current is emitted. The different responses seen in these two events are interpreted in terms of the Froude number associated with the near-surface stratification.

Ilicak, M, Armi L.  2010.  Comparison between a non-hydrostatic numerical model and analytic theory for the two-layer exchange flows. Ocean Modelling. 35:264-269.   10.1016/j.ocemod.2010.05.002   AbstractWebsite

Recently, Armi and Riemenschneider (2008) improved the two-layer hydraulic theory and applied it to the co-located sill and contraction. The main goal in this study is to compare the new theory with a non-hydrostatic numerical model. To this end, the numerical model is employed for two different geometries. The first geometry is steep topography with co-located contraction and the second geometry is a much gentler slope so that the non-hydrostatic forces are reduced. It is found that the model captures the two control points, a topographic control at the crest and a virtual control on the dense reservoir side described by the theory. The flow is subcritical in between these control points. There is agreement between the modeled and analytical interface heights in the gentler topography. (C) 2010 Elsevier Ltd. All rights reserved.

2008
Armi, L, Riemenschneider U.  2008.  Two-layer hydraulics for a co-located crest and narrows. Journal of Fluid Mechanics. 615:169-184.   10.1017/s0022112008003546   AbstractWebsite

The theory of two-layer hydraulics is extended to topography with co-varying width and height. When these variations of the non-dimensional width and total depth have a power law relationship, the solutions can still be presented in the Froude-number plane for both unidirectional and exchange flows. These differ from previous solutions, which were limited to treating width and height variations separately.

Mayr, GJ, Armi L.  2008.  Fohn as a response to changing upstream and downstream air masses. Quarterly Journal of the Royal Meteorological Society. 134:1357-1369.   10.1002/qj.295   AbstractWebsite

Observations of fohn from the field phase of the Mesoscale Alpine Programme (MAP) are used to study how differences between the air masses upstream and downstream of the central Alpine crest determine whether the flow can descend to the lee as either shallow fohn, when it passes through passes in the mountains, or deep fohn, when it overflows the Alpine crest. First, the fohn case of 30 October 1999 is examined using ECMWF analyses and radiosonde data at various upstream and downstream locations. Additional measurements from aircraft, dropsondes, an instrumented car and automatic weather stations are then used for a detailed study of the fohn flow across the Brenner Pass. Advection of cold air around the eastern edges of the Alps and warm air around the western edge of the Alps ahead of a synoptic ridge set up a reservoir of colder air on the south side of the Alps and a reservoir of warmer air to the north. The depth to where the air was colder on the southern side was sufficient for a shallow fohn to flow through the pass. After the passage of the ridge axis, synoptic cold air advection provided another source of colder air, this time from the southwest, growing deeper with time and having a synoptically imposed cross-barrier flow component. The maximum depth to where the air upstream was colder than downstream extended just above the peaks of the highest mountains. An analysis of the detailed measurements across the Brenner Pass showed that this depth was also the top of the layer that descended and accelerated down the lee slopes of the Wipp Valley. Upstream, air above the fohn layer had an even stronger cross-barrier component yet did not descend because it did not have lower potential temperatures than the downstream side at that level. Deep fohn never developed. An examination of other well-documented MAP fohn cases confirmed the conclusion from the 30 October event that shallow and deep fohns - at least for the central Alps - are mostly a response to differences in air masses between the upstream and downstream side. A cross-barrier component of the flow was only a modification but in itself not sufficient to cause the flow to both descend and accelerate down the lee slope, unless potential temperatures on the upstream side were lower in this layer than on the downstream side. Copyright (C) 2008 Royal Meteorological Society.

2007
Moum, JN, Farmer DM, Shroyer EL, Smyth WD, Armi L.  2007.  Dissipative losses in nonlinear internal waves propagating across the continental shelf. Journal of Physical Oceanography. 37:1989-1995.   10.1175/jpo3091.1   AbstractWebsite

A single nonlinear internal wave tracked more than 100 wavelengths across Oregon's continental shelf over a 12-h period exhibited nearly constant wave speed, c = 0.75 m s(-1), and amplitude, a = 15 m. The wavelength L gradually decreased from 220 m in 170-m water depth to 60 m in 70-m water depth. As the water shallowed beyond 50 m, the wave became unrecognizable as such. The total energy decreased from 1.1 to 0.5 MJ m(-1). The rate at which wave energy was lost, -dE/dt = 14 [ 7, 22] W m(-1), was approximately equal to the energy lost to turbulence dissipation, rho epsilon = 10 [ 7, 14] W m(-1), as inferred from turbulence measurements in the wave cores plus estimates in the wave-induced bottom boundary layer. The approximate balance, dE/dt = -rho epsilon, differs from the solibore model of Henyey and Hoering in which the potential energy across the wave balances rho epsilon. However, other evidence suggests that the wave evolved from a solibore-like state to a dissipative solitary wavelike state over the observed propagation path.

Armi, L, Mayr GJ.  2007.  Continuously stratified flows across an Alpine crest with a pass: Shallow and deep fohn. Quarterly Journal of the Royal Meteorological Society. 133:459-477.   10.1002/qj.22   AbstractWebsite

Shallow and deep Alpine fohn flows are analyzed within the framework of continuously stratified hydraulics. A distinguishing upstream feature of these flows is the presence of stagnant, nearly well-mixed layers above and beneath the moving stratified fluid. These layers isolate the moving stratified fluid, which has a self-similar parabolic-like velocity distribution, from the stratified fluid above and beneath. The shallow and deep realizations of these flows were found on 20 and 21 October 1999 during the Mesoscale Alpine Programme. The temporal evolution from shallow to deep fohn was captured with a sequence of radiosondes, automatic weather stations, an instrumented car, and research aircraft flights. Copyright (c) 2007 Royal Meteorological Society.

Mayr, GJ, Armi L, Gohm A, Zaengl G, Durran DR, Flamant C, Gaberek S, Mobbs S, Ross A, Weissmann M.  2007.  Gap flows: Results from the Mesoscale Alpine Programme. Quarterly Journal of the Royal Meteorological Society. 133:881-896.: Royal Meteorological Society, 104 Oxford Rd. Reading Berks RG1 7LL UK, [URL:http://www.royal-met-soc.org.uk/]   10.1002/qj.66   AbstractWebsite

An overview of advances in the observation, modelling, forecasting, and understanding of flows through gaps achieved in the Mesoscale Alpine Programme is given. Gaps are lateral constrictions of topography (level gaps) often combined with vertical terrain changes (passes). Of the possible flow configurations, only an asymmetric one (relatively deep and slow upstream, accelerating and thinning downstream), which connects two different [lsquo]reservoirs[rsquo] on each side of the gap, is examined. The flow is strongly nonlinear, making hydraulics (reduced-gravity shallow-water theory) rather than linear theory the simplest conceptual model to describe gap flow. Results from idealized topographical and flow conditions are presented, together with gap flows through a pass in the central Alpine Wipp Valley. For a given depth of the upstream reservoir, the gap controls the mass flux through it and marks the transition from a subcritical flow state upstream to a supercritical one downstream, which eventually adjusts to the downstream [lsquo]reservoir[rsquo] in a hydraulic jump. Three gap flow prototypes were found: a classical layer one with neutral stratification and a capping inversion and two with a continuous stratification, for which a special analytical self- similar hydraulic solution exists. In all three cases, a deepening wedge of nearly mixed and stagnant air forms on top of the gap flow plunging down from the pass. The descent causes a warming and (relative) drying of the air, making gap flows a special case of foehn. Topographical variations smaller than the gap scale cause additional hydraulic jumps, flow separation, vorticity banners, gravity waves, and interactions with cold pools. Turbulent friction cannot be neglected. The climatological frequency of gap flows depends on the establishment of two different reservoirs and reaches 20% for the Wipp Valley. Copyright 2007 Royal Meteorological Society

2006
Cummins, PF, Armi L, Vagle S.  2006.  Upstream internal hydraulic jumps. Journal of Physical Oceanography. 36:753-769.   10.1175/jpo2894.1   AbstractWebsite

In stratified tidal flow over a sill, the character of the upstream response is determined by a Froude number F-s based on the stratification near the surface. This is distinguished from the Froude number governing the response in the neighborhood of the sill crest, which is based on the weak density step associated with a flow bifurcation. For moderate values of F-s, the upstream response consists of nonlinear waves or a weak undular bore. For larger values of F-s, a strong, quasi-stationary, internal hydraulic jump dominates the upstream response. At sufficiently large values of F-s, the upstream bore is swept downstream and lost. Acoustic backscatter and velocity data are presented for the case of a strong internal bore or gravity current in a tidally modulated sill flow. Numerical simulations with varying near- surface stratification are presented to illustrate the upstream responses at different values of F-s. The theory of two- layer hydraulic flows is invoked to account for the development of the upstream jump.

Adachi, Y, Kawamura K, Armi L, Keeling RF.  2006.  Diffusive Separation of the Lower Atmosphere. Science (Washington). 311:1429-1429.: American Association for the Advancement of Science, 1200 New York Avenue, NW Washington DC 20005 USA, [mailto:membership@aaas.org], [URL:http://www.aaas.org]   10.1126/science.1121312   AbstractWebsite

The separation of atmospheric constituents by gravity has been proposed theoretically for almost two centuries. However, turbulent mixing has prevented the detection of this phenomenon in the lower atmosphere. By using precise measurements of the Ar/N2 ratio of air samples taken under strong nocturnal inversions, we have detected such separation in near-surface layers. The effect is shown to be consistent with combined influence of thermal and gravimetric separation, with the thermal contribution being more important.

2005
Siedler, G, Armi L, Muller TJ.  2005.  Meddies and decadal changes at the Azores Front from 1980 to 2000. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 52:583-604.   10.1016/j.dsr2.2004.12.010   AbstractWebsite

Twenty years of time series observations from the deep-sea mooring KIEL276 are used to obtain information on the frequency and propagation of meddies (Mediterranean Water eddies), on long-term changes in flow properties, and on a possible relation to the North Atlantic Oscillation. The mooring was set at the nominal position 33° N, 22° W at a water depth of more than 5200 m in the northern Canary Basin. It is located near the southern boundary of the Azores Current (AC), which is part of the North Atlantic subtropical gyre, and also in the large-scale Mediterranean Water (MW) tongue originating from the Strait of Gibraltar. The amplitudes of time-varying vertical quasi-geostrophic modes and the correlation of current and temperature changes at levels in the MW and the North Atlantic Central Water above are used to identify meddies. A total of 10 meddies passed the mooring during the period 1980-2000. Half of the events can be related to earlier observations. Directional changes in meddy-related velocities are used to estimate speeds and directions of meddy propagation. Directions of propagation are very homogeneous, with all the 10 meddies observed propagating with a southward velocity component within a sector of 90°, and typical speeds are 2-3 cm/s. Meddy occurrence was uneven in time, with six meddies found during the first four years and only four meddies during the remaining 16 years. Decadal changes show the annual-mean and the fluctuating kinetic energy levels at the site changing from lower values in the 1980s to high values in the 1990s. This change appears to be correlated with variations in the North Atlantic Oscillation (NAO) index, with a delay in oceanic response of about 3 years. A conceptual model of AC meanders is used to identify meander signals in the upper-layer time series. The AC axis appears to be closer to the site during the 1990s than during the preceding decade and indicates a southward or southwestward displacement of the AC with increasingly positive values of the NAO index. Meddy frequency is lower when the AC gets closer from the north. A reduction in meddy occurrence in the region just south of the AC is possibly caused by the shear-induced blocking of some meddies crossing the front from the north. © 2005 Elsevier Ltd. All rights reserved.

2004
Eastwood, CD, Armi L, Lasheras JC.  2004.  The breakup of immiscible fluids in turbulent flows. Journal of Fluid Mechanics. 502:309-333.   10.1017/s0022112003007730   AbstractWebsite

The breakup of immiscible fluid particles in a prototypical turbulent flow has been investigated. Dispersed fluids of varying density, viscosity and interfacial tension with water were injected continuously on the centreline in the fully developed region of a turbulent water jet. Digital image-processing techniques were used to track the particle size distributions as the initial globules of the dispersed fluid were broken into smaller particles and convected downstream in the jet. Particle breakup frequencies were calculated from the evolution of the measured particle size distributions using a simplified version of the Boltzmann equation. The results of these calculations indicate that the breakup frequency of fluid particles at low Weber numbers scales with the passage frequency of the large-scale turbulent features of the flow, approximated as u'/L, where u' is the r.m.s. value of turbulent velocity fluctuations and L is the local integral length scale. High-speed video images corroborate this result. Prior to breakup, dispersed fluid particles with initial diameters within the inertial subrange of the background flow stretch to lengths comparable to the local integral scale. These elongated particles subsequently break owing to capillary effects resulting from differences in the radius of curvature along their length. The breakup time of these particles scales with the capillary time t(d) = mu(d)D/sigma, where mu(d) is the dispersed fluid viscosity, D is the undeformed particle diameter, and a is the interfacial tension between the dispersed fluid and water. These results are analogous to the breakup mechanisms observed by several investigators in low-Reynolds-number flows; however, they contradict the classical theory for turbulent particle breakup, which suggests that fragmentation results from isolated interactions with turbulent velocity fluctuations over distances comparable to or smaller than the undeformed dispersed particle diameter.

Mayr, GJ, Armi L, Arnold S, Banta RM, Darby LS, Durran DD, Flamant C, Gabersek S, Gohm A, Mayr R, Mobbs S, Nance LB, Vergeiner I, Vergeiner J, Whiteman CD.  2004.  Gap flow measurements during the Mesoscale Alpine Programme. Meteorology and Atmospheric Physics. 86:99-119.   10.1007/s00703-003-0022-2   AbstractWebsite

The lowest pass through the Alpine crest, the Brenner Pass, was heavily instrumented with ground-based and air-borne in-situ and remote sensors during the Special Observation Period (SOP) of the Mesoscale Alpine Programme (MAP) in the fall of 1999 to study gap flow. The main objectives were to study the combined effects of changes of terrain height and changes of width in altering the flow characteristics, to investigate the coupling of the gap flow to the flow aloft, and to provide high-density measurements in the along- and cross-gap directions. Gap flows occurred during one third of the 70-day SOP, a frequency above the long term average. Gap flows took place with and without accompanying cross-barrier flow and with and without a capping inversion. A case study demonstrates the hydraulic jump-like features that occurred in gap flow on 30 October 1999 and illustrates the types of data available for further analyses.

2003
Moum, JN, Farmer DM, Smyth WD, Armi L, Vagle S.  2003.  Structure and generation of turbulence at interfaces strained by internal solitary waves propagating shoreward over the continental shelf. Journal of Physical Oceanography. 33:2093-2112.   10.1175/1520-0485(2003)033<2093:sagota>2.0.co;2   AbstractWebsite

Detailed observations of the structure within internal solitary waves propagating shoreward over Oregon's continental shelf reveal the evolving nature of interfaces as they become unstable and break, creating turbulent flow. A persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake. This is demonstrated to be due to enhanced density microstructure. Increased small-scale strain ahead of the wave trough compresses select density interfaces, thereby locally increasing stratification. This is followed by a sequence of overturning, high-density microstructure, and turbulence at the interface, which is coincident with the high acoustic backscatter. The Richardson number estimated from observations is larger than 1/4, indicating that the interface is stable. However, density profiles reveal these preturbulent interfaces to be O(10 cm) thick, much thinner than can be resolved with shipboard velocity measurements. By assuming that streamlines parallel isopycnals ahead of the wave trough, a velocity profile is inferred in which the shear is sufficiently high to create explosively growing, small wavelength shear instabilities. It is argued that this is the generation mechanism for the observed turbulence and hence the persistent structure of high acoustic backscatter in these internal solitary waves.

Cummins, PF, Vagle S, Armi L, Farmer DM.  2003.  Stratified flow over topography: upstream influence and generation of nonlinear internal waves. Proceedings of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences. 459:1467-1487.   10.1098/rspa.2002.1077   AbstractWebsite

Results are presented from a recent experiment in Knight Inlet, British Columbia, focusing on the generation and propagation of nonlinear internal waves near the sill during ebb tide. High-quality echo-sounder and velocity measurements were obtained in a novel fashion, using instrumentation carried aboard a small inflatable boat. Complementing these data, a set of photographic images of the surface signature of the internal waves was acquired from both a fixed mountainside location and a helicopter. These photographs situate the observations within the larger-scale structure of the internal response found in the inlet. The observations reveal the development of strongly nonlinear internal waves on a bore occurring upstream of the sill crest. As the tidal flow relaxes, these waves propagate upstream, at first slowly, and later rapidly against the gradually waning flow. A simulation of wave generation with a fully nonlinear numerical model is discussed. The simulation shows that upstream influence, associated with the rapidly increasing tidal forcing, leads to formation of an undular bore upstream of the sill crest. As the tidal flow relaxes, the bore subsequently evolves into a group of upstream-propagating solitary-like internal waves. Taken together, the observations and numerical simulation describe a process for the generation of nonlinear internal waves involving upstream influence, which likely has application in other environments for stratified flow over topography.

2002
Armi, L, Farmer D.  2002.  Stratified flow over topography: bifurcation fronts and transition to the uncontrolled state. Proceedings of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences. 458:513-538.   10.1098/rspa.2001.0887   AbstractWebsite

A distinguishing feature of controlled stratified flows over topography is the formation of a wedge of partly mixed fluid downstream of a bifurcation or plunge point. We describe observations acquired over a sill in a coastal inlet under progressively increased tidal forcing. This wedge of partly mixed fluid is displaced downstream as the flow undergoes a continuous transition from control over the obstacle crest to an uncontrolled state. The effects of changing barotropic forcing and relative density difference between the plunging flow and partly mixed layer above combine to determine the fluid dynamical response. The relative density difference in turn is determined by the prior history of the flow as well as small-scale mixing. In general it decreases with time as denser fluid is entrained into the intermediate layer, thus increasing the effective forcing. For sufficiently strong tidal velocities and small relative density difference. the wedge of partly mixed fluid is displaced downstream of the crest and topographic control is lost. Such flows occur naturally in the ocean over sills and ridges. and in the atmosphere as severe downslope winds.

2001
Farmer, DM, Armi L.  2001.  Stratified flow over topography: models versus observations. Proceedings of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences. 457:2827-2830. AbstractWebsite

Essential mechanisms identified in the authors' observations of flow establishment over topography are restated, with emphasis on those which are at variance with numerical simulations. Specifically, small-scale instabilities were observed to lead to upwards transport of fluid from the primary flow so as to form a nearly stationary intermediate layer, and boundary-layer separation significantly delays establishment of the downslope flow. Simulations that force the streamlines to follow the topography generate a large-amplitude breaking wave, which we did not observe. Our observations show that small-scale instability and separation act in concert to determine the time-dependent evolution of the stratified flow response.