scholarly journals Spring Circulation Associated with the Thermal Bar in Large Temperate Lakes

1995 ◽  
Vol 26 (4-5) ◽  
pp. 331-358 ◽  
Author(s):  
Joakim Malm
Keyword(s):  
Numerical Model ◽  
Water Exchange ◽  
Bottom Topography ◽  
Practical Importance ◽  
Circulation Pattern ◽  
Maximum Density ◽  
Thermal Bar ◽  
Temperature Of Maximum Density ◽  
Induced Currents ◽  
Bottom Depth

The overall circulation pattern in spring is rather specific as density-induced currents may be of significance. The density-driven circulation perpendicular to the shore can be described as consisting of two circulation cells, with a zone of convergence, referred to as thermal bar, in between. The thermal bar, which coincides with the 4°C isotherm (the temperature of maximum density), inhibits horizontal water exchange, implying its practical importance. In this paper, a hydrodynamic numerical model is used to study the relative influence of wind- and density-driven currents in a large temperate lake during spring. The study shows that the general density-driven circulation is strongly dependent on the bottom topography, with a more pronounced circulation and considerable descending motions in the thermal bar zone in lakes with steep sloping bottoms. In shallow lakes, the wind-driven circulation dominates, and the effect of density-induced currents is marginal, except at locations with a drastic change in bottom depth.

Horizontal exchange across the thermal bar front: laboratory and numerical modelling

2012 ◽  
Vol 47 (3-4) ◽  
pp. 436-450 ◽  
Author(s):  
Natalia Demchenko ◽  
Irina Chubarenko
Keyword(s):  
Numerical Experiments ◽  
Maximum Density ◽  
Water Dynamics ◽  
Scaling Analysis ◽  
Thermal Bar ◽  
Intermediate Layers ◽  
Image Velocimetry ◽  
Temperature Of Maximum Density ◽  
Passive Tracers ◽  
Physical Reasons

Laboratory and numerical experiments have revealed physical reasons for the permeability of the thermal bar to horizontal transport. The thermal bar is understood as a front associated with the temperature of maximum density (Tm = 3.98 °C for fresh water). Laboratory experiments were performed in a 2-metre-long non-rotating channel with a sloping bottom, filled with water with T < Tm and naturally heated from above. Analysis of Particle Image Velocimetry (PIV) images revealed water dynamics in the presence of Tm. It was revealed that the compensating flow in intermediate layers is responsible for the horizontal exchange across the thermal bar front. We applied a 3D non-hydrostatic MIKE3-FlowModel (www.dhi.dk) to investigate the permeability of the spring thermal bar in basins on the scale of lakes and a laboratory flume. We performed an analysis of the concentration distribution of 12 passive tracers released at different locations in the flow domain. Scaling analysis, corroborated by the results of laboratory and numerical experiments, predicts the discharge across the thermal bar as Q = 0.1[g × Δρ/ρ0]1/2h3/2, where h is the depth of the upper thermo-active layer, ρ0 is a maximum density and Δρ is a characteristic horizontal density difference. A combined analysis of data shows that this law is obeyed.

scholarly journals Characteristics of Turbulent Cross and Alongshore Momentum Exchanges During a Thermal Bar Episode in lake Ontario

1998 ◽  
Vol 29 (1) ◽  
pp. 57-72 ◽  
Author(s):  
Messon B. Gbah ◽  
Raj C. Murthy
Keyword(s):  
Kinetic Energy ◽  
Lake Ontario ◽  
Maximum Density ◽  
Small Scale ◽  
Biotic Factors ◽  
Coastal Zones ◽  
Thermal Bar ◽  
Temperature Of Maximum Density ◽  
Turbulent Momentum ◽  
Coastal Flow

Time series flow data obtained during the thermal bar episode of 17 April to 24 May 1990 in Lake Ontario are analyzed to provide a kinematic description of the coastal flow and cross-margin exchange characteristics. A thermal bar is a shore-parallel front which separates descending waters at or near the fresh water temperature of maximum density (4°C) during Spring and Fall seasons. Thermal bars are important because of their influence mixing, cross-shore exchanges, and the variability of biotic factors in coastal zones. The analysis shows that cross-frontal exchange coefficients, Ky, are nearly constant and consistently smaller than along-frontal counterparts, Kx. Moreover, these exchange coefficients are several orders of magnitude smaller than typical coastal and oceanic values in the absence of the bar. The turbulent kinetic energy represents less than 6% of the total kinetic energy in the flow. These results suggest that small-scale horizontal fluctuations and cross-frontal turbulent momentum exchanges are severely inhibited in the spring during the thermal bar.

scholarly journals The Behavior of Jet Currents over a Continental Slope Topography with a Possible Application to the Northern Current

2005 ◽  
Vol 35 (5) ◽  
pp. 790-810 ◽  
Author(s):  
M. M. Flexas ◽  
G. J. F. van Heijst ◽  
R. R. Trieling
Keyword(s):  
High Speed ◽  
Bottom Topography ◽  
Laboratory Model ◽  
Topographic Effects ◽  
Topographic Slope ◽  
Eddy Formation ◽  
Slow Flows ◽  
Bottom Depth ◽  
Source Sink ◽  
Northern Current

Abstract The Northern Current is a slope current in the northwest Mediterranean that shows high mesoscale variability, generally associated with meander and eddy formation. A barotropic laboratory model of this current is used here to study the role of the bottom topography on the current variability. For this purpose, a source–sink setup in a cylindrical tank placed on a rotating table is used to generate an axisymmetric barotropic current. To study inviscid topographic effects, experiments are performed over a topographic slope and also over a constant-depth setup, the latter being used as a reference for the former. With the aim of obtaining a fully comprehensive view of the vorticity balance at play, the flow may be forced in either azimuthal direction, leading to a “westward” prograde current (similar to the Northern Current) or an “eastward” retrograde current. For slow flows, eastward and westward currents showed similar patterns, dominated by Kelvin–Helmholtz-type instabilities. For high-speed flows, eastward and westward currents showed very different behavior. In eastward currents, the variability is observed to concentrate toward the center of the jet and shows strong meandering formation. Westward currents, instead, showed major variability toward the edges of the jet, together with a strong variability over the uppermost slope, which has been associated here with a topographic Rossby wave trapped over the shelf break. The differences between eastward and westward jets are explained through the balance between shear-induced and topographically induced vorticity at play in each case. Moreover, a model of jets over a beta plane is successfully applied here, allowing its extension to any ambient potential vorticity gradient caused either by latitudinal or bottom depth changes.

scholarly journals From solar eruption to transformer saturation: the space weather chain

2013 ◽  
Vol 8 (S300) ◽  
pp. 500-501
Author(s):  
Larisa Trichtchenko
Keyword(s):  
Space Weather ◽  
Geomagnetic Field ◽  
Geomagnetic Storms ◽  
Interplanetary Medium ◽  
Practical Importance ◽  
Power Grids ◽  
Geomagnetically Induced Currents ◽  
Large Space ◽  
Weather Events ◽  
Induced Currents

AbstractCoronal mass ejections (CME) and associated interplanetary-propagated solar wind disturbances are the established causes of the geomagnetic storms which, in turn, create the most hazardous impacts on power grids. These impacts are due to the large geomagnetically induced currents (GIC) associated with variations of geomagnetic field during storms, which, flowing through the transformer windings, cause extra magnetisation. That can lead to transformer saturation and, in extreme cases, can result in power blackouts. Thus, it is of practical importance to study the solar causes of the large space weather events. This paper presents the example of the space weather chain for the event of 5-6 November 2001 and a table providing complete overview of the largest solar events during solar cycle 23 with their subsequent effects on interplanetary medium and on the ground. This compact overview can be used as guidance for investigations of the solar causes and their predictions, which has a practical importance in everyday life.

Dynamics of the Ningaloo Current off Point Cloates, Western Australia

2006 ◽  
Vol 57 (3) ◽  
pp. 291 ◽  
Author(s):  
Mun Woo ◽  
Charitha Pattiaratchi ◽  
William Schroeder
Keyword(s):  
Numerical Model ◽  
Continental Shelf ◽  
Western Australia ◽  
Bottom Topography ◽  
Volume Transport ◽  
Relative Strength ◽  
Cross Sectional ◽  
Leeuwin Current ◽  
The North ◽  
Tropical Water

The Ningaloo Current (NC) is a wind-driven, northward-flowing current present during the summer months along the continental shelf between the latitudes of 22° and 24°S off the coastline of Western Australia. The southward flowing Leeuwin Current is located further offshore and flows along the continental shelf break and slope, transporting warm, relatively fresh, tropical water poleward. A recurrent feature, frequently observed in satellite images (both thermal and ocean colour), is an anti-clockwise circulation located offshore Point Cloates. Here, the seaward extension of the coastal promontory blocks off the broad, gradual southern shelf, leaving only a narrow, extremely steep shelf to the north. The reduction in the cross-sectional area, from the coast to the 50 m contour, between southward and northward of the promontory is ~80%. Here, a numerical model study is undertaken to simulate processes leading to the development of the recirculation feature offshore Point Cloates. The numerical model output reproduced the recirculation feature and indicated that a combination of southerly winds, and coastal and bottom topography, off Point Cloates is responsible for the recirculation. The results also demonstrated that stronger southerly winds generated a higher volume transport in the NC and that the recirculation feature was dependent on the wind speed, with stronger winds decreasing the relative strength of the recirculation.

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