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 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 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.

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.

scholarly journals The 6 May 2010 Elevated Supercell during VORTEX2

2017 ◽  
Vol 145 (7) ◽  
pp. 2635-2657 ◽  
Author(s):  
Christopher W. MacIntosh ◽  
Matthew D. Parker
Keyword(s):  
Cold Pool ◽  
Surface Winds ◽  
Stable Layer ◽  
Low Level ◽  
Stable Inversion ◽  
Idealized Modeling ◽  
Weak Surface ◽  
Negatively Buoyant

An elevated supercell from the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) on 6 May 2010 is investigated. Observations show that the supercell formed over a stable inversion and was likely decoupled from the surface. Quintessential features of a supercell were present, including a hook echo (albeit bent anticyclonically) and midlevel mesocyclone, and the storm was quasi steady during the observing period. A weak surface cold pool formed, but it was apparently devoid of air originating from midlevels. Idealized modeling using near-storm soundings is employed to clarify the structure and maintenance of this supercell. The simulated storm is decoupled from the surface by the stable layer. Additionally, the reflectivity structure of the simulated supercell is strikingly similar to the observed storm, including its peculiar anticyclonic-curving hook echo. Air parcels above 1 km reached their LFCs as a result of the simulated supercell’s own dynamic lifting, which likely maintained the main updraft throughout its life. In contrast, low-level air in the simulation followed an “up–down” trajectory, being lifted dynamically within the stable layer before becoming strongly negatively buoyant and descending back to the surface. Up–down parcels originating in the lowest 100 m are shown to be a potential driver of severe surface winds. The complementary observations and simulations highlight a range of processes that may act in concert to maintain supercells in environments lacking surface-based CAPE.

Mobile Integrated Profiler System (MIPS) Observations of Low-Level Convergent Boundaries during IHOP

2006 ◽  
Vol 134 (1) ◽  
pp. 92-112 ◽  
Author(s):  
Haldun Karan ◽  
Kevin Knupp
Keyword(s):  
Boundary Layer ◽  
Cold Front ◽  
The Other ◽  
Density Current ◽  
Wind Profiler ◽  
Density Currents ◽  
General Increase ◽  
Convective Boundary ◽  
Convective Initiation ◽  
Slow Moving

Abstract Characteristics of convergent boundary zones (CBZs) sampled by the Mobile Integrated Profiling System (MIPS) during the 2002 International H2O Project (IHOP_2002) are presented. The MIPS sensors (915-MHz wind profiler, 12-channel microwave profiling radiometer, ceilometer, and surface instrumentation) provide very fine temporal kinematic and thermodynamic profiles of the atmospheric boundary layer and CBZ properties, including enhanced 915-MHz backscatter within the CBZ updraft (equivalent to the radar fine line), a general increase in integrated water vapor within the updrafts of the CBZ, an increase in the convective boundary layer (CBL) depth, and changes in ceilometer backscatter that are typically coincident with arrival of cooler, moister air (the case for density current CBZ). Three contrasting CBZs are analyzed. Convective initiation was associated with a slow-moving dryline as it passed over the MIPS on 19 June. Updrafts up to 6 m s−1 were measured, and the CBL attained its greatest depth within the CBZ. The CBZ in the other two cases were quite similar to density currents. The retrograding dryline of 18 June produced an enhancement in preexisting convection within 30 km of the MIPS. On 24 May, a shallow cold front, about 800 m deep, was sampled.

Sub-mesoscale observations of cold pools during FESSTVaL

2020 ◽  
Author(s):  
Bastian Kirsch ◽  
Felix Ament ◽  
Cathy Hohenegger ◽  
Daniel Klocke
Keyword(s):  
Low Cost ◽  
Cold Pool ◽  
Density Currents ◽  
Horizontal Structure ◽  
Cold Pools ◽  
Meteorological Observatory ◽  
First Results ◽  
Spatio Temporal ◽  
Operational Networks ◽  
Measurement Strategies

<p>Cold pools are areas of cool downdraft air that form through evaporation underneath precipitating clouds and spread on the surface as density currents. Their importance for the development and maintenance of convection is long known. Modern Large-Eddy simulations with a grid spacing of 1 km or less are able to explicitly resolve cold pools, however, they lack reference data for an adequate validation. Available point measurements from operational networks are too coarse and, therefore, miss the horizontal structure and dynamics of cold pools.</p><p>The upcoming measurement campaign FESSTVaL (Field Experiment on Sub-mesocale Spatio-Temporal Variability in Lindenberg) aims to test novel measurement strategies for the observation of previously unresolved sub-mesoscale boundary layer structures and phenomena, such as cold pools. The key component of the experiment during this summer will be a dense network of ground-based measurements within 15 km around the Meteorological Observatory Lindenberg near Berlin. The network of 100 low-cost APOLLO (Autonomous cold POoL LOgger) stations allows to record air pressure and temperature with a spatial and temporal resolution of 100 m and 1 s, respectively. We present first results from a test campaign during last summer that successfully demonstrated the ability of the proposed network stations to observe cold pool dynamics on the sub-mesoscale.</p>

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