scholarly journals Observations of Shoaling Density Current Regime Changes in Internal Wave Interactions

2020 ◽  
Vol 50 (6) ◽  
pp. 1733-1751
Author(s):  
Aviv Solodoch ◽  
Jeroen M. Molemaker ◽  
Kaushik Srinivasan ◽  
Maristella Berta ◽  
Louis Marie ◽  
...  
Keyword(s):  
Propagation Speed ◽  
Finite Amplitude ◽  
State Density ◽  
Critical Regime ◽  
Density Current ◽  
Coastal Current ◽  
Density Currents ◽  
Ambient Conditions ◽  
Plume Front ◽  
Small Width

AbstractWe present in situ and remote observations of a Mississippi plume front in the Louisiana Bight. The plume propagated freely across the bight, rather than as a coastal current. The observed cross-front circulation pattern is typical of density currents, as are the small width (≈100 m) of the plume front and the presence of surface frontal convergence. A comparison of observations with stratified density current theory is conducted. Additionally, subcritical to supercritical transitions of frontal propagation speed relative to internal gravity wave (IGW) speed are demonstrated to occur. That is in part due to IGW speed reduction with decrease in seabed depth during the frontal propagation toward the shore. Theoretical steady-state density current propagation speed is in good agreement with the observations in the critical and supercritical regimes but not in the inherently unsteady subcritical regime. The latter may be due to interaction of IGW with the front, an effect previously demonstrated only in laboratory and numerical experiments. In the critical regime, finite-amplitude IGWs form and remain locked to the front. A critical to supercritical transition eventually occurs as the ambient conditions change during frontal propagation, after which IGWs are not supported at the front. The subcritical (critical) to critical (supercritical) transition is related to Froude number ahead (under) the front, consistently with theory. Finally, we find that the front-locked IGW (critical) regime is itself dependent on significant nonlinear speed enhancement of the IGW by their growth to finite amplitude at the front.

scholarly journals The Influence of Wind Forcing on the Chesapeake Bay Buoyant Coastal Current*

2006 ◽  
Vol 36 (7) ◽  
pp. 1305-1316 ◽  
Author(s):  
Steven J. Lentz ◽  
John Largier
Keyword(s):  
Chesapeake Bay ◽  
Wind Stress ◽  
Vertical Mixing ◽  
Propagation Speed ◽  
Wind Forcing ◽  
Coastal Current ◽  
Plume Front ◽  
Shelf Flow ◽  
Weak Wind

Abstract Observations of the buoyant coastal current that flows southward from Chesapeake Bay are used to describe how the thickness, width, and propagation speed vary in response to changes in the along-shelf wind stress. Three basic regimes were observed depending on the strength of the wind. For weak wind stresses (from −0.02 to 0.02 Pa), the buoyant coastal current was relatively thin, the front slope was not steep, and the width was variable (1–20 km). For moderate downwelling (southward) wind stresses (0.02–0.07 Pa), wind-driven cross-shelf advection steepened the front, causing the plume to narrow and thicken. For stronger downwelling wind stresses (greater than 0.07 Pa), vertical mixing dominated, bulk Richardson numbers were approximately 0.25, isopycnals were nearly vertical, and the plume front widened but the plume width did not change. Plume thickness and width were normalized by the theoretical plume scales in the absence of wind forcing. Normalized plume thickness increased linearly from 1 to 2 as downwelling wind stresses increased from 0 to 0.2 Pa. Normalized plume widths were approximately 1 for downwelling wind stresses from 0.02 to 0.2 Pa. The observed along-shelf propagation speed of the plume was roughly equal to the sum of the theoretical propagation speed and the wind-driven along-shelf flow.

Density currents or density wedges: boundary-layer influence and control methods

1987 ◽  
Vol 177 ◽  
pp. 187-206 ◽  
Author(s):  
Gerhard H. Jirka ◽  
Masamitsu Arita
Keyword(s):  
Boundary Layer ◽  
State Density ◽  
Density Current ◽  
Local Form ◽  
Density Currents ◽  
Viscous Effects ◽  
Long Distance ◽  
Ambient Flow ◽  
Wake Zone ◽  
And Control

Density currents and density wedges are two observed manifestations of interactions between an ambient flow and a horizontal buoyant intrusion. In a density current the buoyant pressure force is primarily balanced by the local form drag of the current head which has a blunt shape and abrupt depth change. In a density wedge a distributed interfacial drag is the primary balancing force, leading to a stretched-out shape and long-distance intrusions. A perturbation analysis of the approach flow to the inclined front of a density current shows that slight momentum changes caused by viscous effects in the ambient flow determine which of these two flow types is established. In a uniform ambient channel flow, any momentum deficit relative to the inviscid case will lead to a local flattening of the front and ultimate breakdown into a density wedge. On the other hand, a momentum surplus will support a steady-state density current. Several exploratory experiments on control of the ambient boundary layer through local non-uniformities were performed with the objective of achieving stable density-current forms with limited intrusion lengths. These methods include a small step, a barrier and suction and are applied for intrusions at either the bottom or surface of an ambient water flow. In all cases, good agreement is found with the force balances predicted by Benjamin's (1968) theory and its extension by Britter & Simpson (1978) which accounts for entrainment in the wake zone of the head.

scholarly journals Near-Surface Density Currents Observed in the Southeast Pacific Stratocumulus-Topped Marine Boundary Layer*

2015 ◽  
Vol 143 (9) ◽  
pp. 3532-3555 ◽  
Author(s):  
Matt C. Wilbanks ◽  
Sandra E. Yuter ◽  
Simon P. de Szoeke ◽  
W. Alan Brewer ◽  
Matthew A. Miller ◽  
...  
Keyword(s):  
Marine Boundary Layer ◽  
Propagation Speed ◽  
Sensor Data ◽  
Density Current ◽  
Density Currents ◽  
Near Surface ◽  
Cold Pools ◽  
Southeast Pacific ◽  
The Mean ◽  
Rain Rates

Abstract Density currents (i.e., cold pools or outflows) beneath marine stratocumulus clouds are characterized using 30 days of ship-based observations obtained during the 2008 Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx) in the southeast Pacific. An air density increase criterion applied to the Improved Meteorological (IMET) sensor data identified 71 density current front, core (peak density), and tail (dissipating) zones. The similarity in speeds of the mean density current propagation speed (1.8 m s−1) and the mean cloud-level advection relative to the surface layer wind (1.9 m s−1) allowed drizzle cells to deposit elongated density currents in their wakes. Scanning Doppler lidar captured prefrontal updrafts with a mean intensity of 0.91 m s−1 and an average vertical extent of 800 m. Updrafts were often surmounted by low-lying shelf clouds not connected to the overlying stratocumulus cloud. The observed density currents were 5–10 times thinner and weaker than typical continental thunderstorm cold pools. Nearly 90% of density currents were identified when C-band radar estimated areal average rain rates exceeded 1 mm day−1 over a 30-km diameter. Rather than peaking when rain rates were highest overnight, density current occurrence peaks between 0600 and 0800 local solar time when enhanced local drizzle co-occurred with shallow subcloud dry and stable layers. The dry layers may have contributed to density current formation by enhancing subcloud evaporation of drizzle. Density currents preferentially occurred in a large region of predominantly open cells but also occurred in regions of closed cells.

Simulated Density Currents beneath Embedded Stratified Layers

2012 ◽  
Vol 69 (7) ◽  
pp. 2192-2200 ◽  
Author(s):  
Robert B. Seigel ◽  
Susan C. van den Heever
Keyword(s):  
Propagation Speed ◽  
Cold Pool ◽  
Density Current ◽  
Density Currents ◽  
Stable Layer ◽  
Environmental Regimes ◽  
Weather Events ◽  
Horizontal Pressure ◽  
Numerical Model Experiments ◽  
Neutrally Stratified

Abstract The goal of this research is to investigate the impacts of a stably stratified layer embedded within a neutrally stratified environment on the behavior of density currents in an effort to extend the environmental regimes examined by Liu and Moncrieff. Such environments frequently support severe weather events. To accomplish this goal, nonhydrostatic numerical model experiments are performed in which the strength and height of the embedded stably stratified layer within a neutrally stratified environment are varied. The 1-km-deep stable layer base is varied between 1, 2, and 3 km AGL. Additionally, the strength of the stable layer is systematically varied between Brunt–Väisälä frequencies of 0.006, 0.012, and 0.018 s−1, following the methodology of Liu and Moncrieff. The model and grid setup are also similar to that of Liu and Moncrieff, utilizing the Arakawa C grid, leapfrog advection, a Robert–Asselin filter, and grid spacing of 100 and 50 m in the horizontal and vertical directions, respectively. Results show that the height of the density current decreases and the propagation speed increases with stronger and lower stable layers, provided that the stable layer is sufficiently thin so as to not act as a gravity wave ducting layer. As the strength of the stable layer increases and the height of this layer decreases, the horizontal pressure gradient driving the density current increases, resulting in faster propagation speeds. Such results have implications for cold pool propagation into more stable environments.

scholarly journals Cold smoke: smoke-induced density currents cause unexpected smoke transport near large wildfires

2015 ◽  
Vol 15 (13) ◽  
pp. 17945-17966
Author(s):  
N. P. Lareau ◽  
C. B. Clements
Keyword(s):  
Boundary Layer ◽  
Propagation Speed ◽  
Current Speed ◽  
Solar Heating ◽  
Density Current ◽  
Doppler Lidar ◽  
Density Currents ◽  
Smoke Transport ◽  
Smoke Layer

Abstract. First observations of smoke-induced density currents originating from large wildfires are presented. Using a novel mobile Doppler LiDAR and additional in situ measurements we document a deep (~ 2 km) smoke-filled density current that propagates more than 25 km at speeds up to 4.5 m s−1 near a large forest fire in northern California. Based on these observations we show that the dynamics governing the spread of the smoke layer result from differential solar heating between the smoke-filled and smoke-free portions of the atmospheric boundary layer. A calculation of the theoretical density current speed agrees well with the observed propagation speed. Additional LiDAR and photographic documentation of other smoke-filled density currents demonstrate that these previously unknown phenomena are relatively common near large wildfires and can cause severe and unexpected smoke inundation of populated areas.

scholarly journals Cold Smoke: smoke-induced density currents cause unexpected smoke transport near large wildfires

2015 ◽  
Vol 15 (20) ◽  
pp. 11513-11520 ◽  
Author(s):  
N. P. Lareau ◽  
C. B. Clements
Keyword(s):  
Boundary Layer ◽  
Propagation Speed ◽  
Current Speed ◽  
Solar Heating ◽  
Density Current ◽  
Doppler Lidar ◽  
Density Currents ◽  
Smoke Transport ◽  
Smoke Layer

Abstract. The first observations of smoke-induced density currents originating from large wildfires are presented. Using a novel mobile Doppler lidar and additional in situ measurements, we document a deep (~ 2 km) smoke-filled density current that propagates more than 25 km at speeds up to 4.5 m s−1 near a large forest fire in northern California. Based on these observations we show that the dynamics governing the spread of the smoke layer result from differential solar heating between the smoke-filled and smoke-free portions of the atmospheric boundary layer. A calculation of the theoretical density current speed agrees well with the observed propagation speed. Additional lidar and photographic documentation of other smoke-filled density currents demonstrate that these previously unknown phenomena are relatively common near large wildfires and can cause severe and unexpected smoke inundation of populated areas.

scholarly journals Development of a Nationwide, Low-Level Wind Shear Mosaic in France

2013 ◽  
Vol 28 (5) ◽  
pp. 1241-1260 ◽  
Author(s):  
Clotilde Augros ◽  
Pierre Tabary ◽  
Adrien Anquez ◽  
Jean-Marc Moisselin ◽  
Pascal Brovelli ◽  
...  
Keyword(s):  
Wind Shear ◽  
Propagation Speed ◽  
Horizontal Wind ◽  
Small Scale ◽  
Wind Gust ◽  
Density Currents ◽  
Use Of Data ◽  
Low Levels ◽  
Cell Propagation ◽  
Convective Cells

Abstract An algorithm for the detection of horizontal wind shear at low levels was developed. The algorithm makes use of data collected by all radars from the Application Radar à la Météorologie Infra-Synoptique (ARAMIS) operational network, in order to build a complete mosaic of wind shear over metropolitan France. The product provides an estimation of the maximum horizontal wind shear detected in the low levels, between 0 and 2 km AGL. Examination of the wind shear mosaic for different cases shows that the product is able to retrieve small-scale wind shear signatures that can be linked to either convergence lines ahead of convective cells, which are indicative of gust fronts, or strong convergence areas inside intense cells. A statistical evaluation of the wind shear mosaic was performed, by comparing horizontal wind shear observed inside the area defined by convective objects with wind gusts recorded along their trajectory by weather stations. A link between those different observations was clearly established. Therefore, the use of wind shear for wind gust prediction was tested in combination with other parameters: an estimation of the energetic potential of density currents, the cell surface with reflectivity over 51 dBZ, relative helicity, and cell propagation speed. Different wind gust warning rules were tested on 468 convection nowcasting objects (CONOs). The results clearly highlighted the benefits of using wind shear for wind gust estimation, and also demonstrated the improvement in forecasting skill when combining different parameters. The wind shear mosaic will be produced operationally before the end of 2013 and will be used to improve wind gust warnings provided to end users.

scholarly journals Effects of morphology in controlling propagation of density currents in a reservoir using uncalibrated three-dimensional hydrodynamic modeling

2020 ◽  
Author(s):  
Behnam Zamani ◽  
Manfred Koch ◽  
Ben R. Hodges
Keyword(s):  
Hydrodynamic Model ◽  
Large Scale ◽  
Three Dimensional ◽  
Density Current ◽  
Density Currents ◽  
Large Reservoir ◽  
Bottom Water Temperature ◽  
The Difference ◽  
Basin Morphology ◽  
Southwest Iran

In this study, effects of basin morphology are shown to affect density current hydrodynamics of a large reservoir using a three-dimensional (3D) hydrodynamic model that is validated (but not calibrated) with in situ observational data. The AEM3D hydrodynamic model was applied for 5-month simulations during winter and spring flooding for the Maroon reservoir in southwest Iran, where available observations indicated that large-scale density currents had previously occurred. The model results were validated with near-bottom water temperature measurements that were previously collected at five locations in the reservoir. The Maroon reservoir consists of upper and lower basins that are connected by a deep and narrow canyon. Analyses of simulations show that the canyon strongly affects density current propagation and the resulting differing limnological characteristics of the two basins. The evolution of the Wedderburn Number, Lake Number, and Schmidt stability number are shown to be different in the two basins, and the difference is attributable to the morphological separation by the canyon. Investigation of the background potential energy (BPE) changes along the length of the canyon indicated that a density front passes through the upper section of the canyon but is smoothed into simple filling of the lower basin. The separable dynamics of the basins has implications for the complexity of models needed for representing both water quality and sedimentation.

Three-Dimensional Structures in Experimental Density Currents

2007 ◽  
Author(s):  
B. Firoozabadi ◽  
H. Afshin ◽  
E. Safaaee
Keyword(s):  
Salt Solution ◽  
Chemical Elements ◽  
Three Dimensional ◽  
Lateral Spreading ◽  
Current Data ◽  
Density Current ◽  
Density Currents ◽  
Ambient Water ◽  
Data Set ◽  
Bed Slope

Density currents are continuous currents which move down-slope due to the fact that their density is greater than that of ambient water. The density difference is caused by temperature differences, chemical elements, dissolved materials, or suspended sediment. Many researchers have studied the density current structures, their complexities and uncertainties. However, there is not a detailed 3-D turbulent density current data set perfectly. In this work, the structure of 3-dimensional salt solution density currents is investigated. A laboratory channel was used to study the flow resulting from the release of salt solution into freshwater over an inclined bed. The experiments were conducted with different bottom slopes, inlet concentrations and flow rates. In these tests, the instantaneous velocities are measured by an ADV apparatus (Acoustic Doppler Velocimeter). Results show that by increasing the bed-slope and inlet concentrations, the height of the current decreases. As the density current moves downward the channel or by increasing the discharge, the height of the density current increases. Finally, the effects of different variables such as the bed slope, concentration and flow rate of entering fluid on the velocity profile in different distances from the entrance is studied. The entrainment coefficient, lateral spreading and drag coefficient of the bed and shear layer between salt solution and ambient water is discussed.

Three-Dimensional Modeling of Density Current in Confined and Unconfined Channels

2006 ◽  
Author(s):  
Ehsan Aram ◽  
Bahar Firoozabadi
Keyword(s):  
Experimental Data ◽  
Fine Particles ◽  
Experimental Studies ◽  
Three Dimensional ◽  
Velocity Profiles ◽  
Density Current ◽  
Three Dimensional Modeling ◽  
Small Width ◽  
Dimensional Modeling ◽  
Large Width

Dense underflows are continuous currents which move down-slope due to the fact that their density is heavier than that ambient water. In this work, 2-D and 3-D density current in a channel were investigated by a set of experimental studies and the data were used to simulate the density current. The velocity components were measured using Acoustic Doppler Velocimetry (ADV). The height of density current (current's depth) was also measured. In this study, the density current with a uniform velocity and concentration enters the channel via a sluice gate into a lighter ambient fluid and moves forward down-slope. A low-Reynolds number turbulent model (Launder and Sharma, 1974) has been applied to simulate the structure of 3-D density current in the confined (small width three dimensional density current) and unconfined (large width three dimensional density current) channels. The computed velocity profiles in unconfined channel were compared with the 3-D experimental data for verification. The height and velocity profiles of the confined current were also compared with 2-D experimental data. It was shown that by decreasing in width of the channel, the height of the current and the magnitude of maximum and average velocity increase and the confined current behaves as 2-D current after a distance. These factors prepare the conditions for minimizing sediment deposition and sedimentation rates can be greatly reduced. Although the k - ε Launder and Sharma model is applied here to a conservative density current, it seems that the analysis can be valid in general for turbidity current laden with fine particles.

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