scholarly journals Evidence for double diffusion in temperate meromictic lakes

2010 ◽  
Vol 14 (4) ◽  
pp. 667-674 ◽  
Author(s):  
C. von Rohden ◽  
B. Boehrer ◽  
J. Ilmberger
Keyword(s):  
Double Diffusion ◽  
Stability Ratio ◽  
Temperature Changes ◽  
Convective Mixing ◽  
Diffusive Convection ◽  
Seasonal Basis ◽  
Mixing Patterns ◽  
Diffusive Effects ◽  
The Stability ◽  
Double Diffusive

Abstract. We present CTD-measurements from two shallow meromictic mining lakes. The lakes, which differ in size and depth, show completely different seasonal mixing patterns in their mixolimnia. However, the measurements document the occurrence of similar seasonal convective mixing in discrete layers within their monimolimnia. This mixing is induced by double diffusion and can be identified by the characteristic step-like structure of the temperature and electrical conductivity profiles. The steps develop in the upper part of the monimolimnion, when in autumn cooling mixolimnion temperatures have dropped below temperatures of the underlying monimolimnion. The density gradient across the chemocline due to solutes overcompensates the destabilizing temperature gradient, and moreover, keeps the vertical transport close to molecular level. In conclusion, preconditions for double diffusive effects are given on a seasonal basis. At in general high local stabilities N2 in the monimolimnia of 10−4–10−2s−2, the stability ratio Rρ was in the range of 1–20. This quantitatively indicates that double diffusion can become visible. Between 1 and 6 sequent steps, with sizes between 1 dm and 1 m, were visually identified in the CTD-profiles. In the lower monimolimnion of the deeper lake, the steps systematically emerge at a time delay of more than half a year, which matches with the progression of the mixolimnetic temperature changes into the monimolimnion. In none of the lakes, the chemocline interface is degraded by these processes. However, double diffusive convection is essential for the redistribution of solutes in the inner parts of the monimolimnion at longer time scales, which is crucial for the assessment of the ecologic development of such lakes.

scholarly journals Double diffusion in meromictic lakes of the temperate climate zone

2009 ◽  
Vol 6 (6) ◽  
pp. 7483-7501 ◽  
Author(s):  
C. von Rohden ◽  
B. Boehrer ◽  
J. Ilmberger
Keyword(s):  
Double Diffusion ◽  
Heat Fluxes ◽  
Temperate Climate ◽  
Seasonal Temperature ◽  
Meromictic Lakes ◽  
Temperature Changes ◽  
Convective Mixing ◽  
Lake Bottom ◽  
Diffusive Processes ◽  
Double Diffusive

Abstract. Meromictic lakes are characterized by strong stable density stratification in and below the chemocline, which separates the oxic mixolimnion from the mostly anoxic monimolimnion. Stable density gradients involve slow vertical exchange, especially in the chemocline, where vertical transport can be as low as molecular. Typically, destabilizing temperature gradients establish in the monimolimnion as a consequence of seasonal changing heat fluxes. At the same time, gradients of solutes extending to the lake bottom stabilize the overall stratification. Double diffusive processes may create local instabilities and subsequently cause convective mixing when the destabilization due to heat gradients exceeds the stabilization by solutes (diffusive regime). This configuration can annually occur in the upper part of the monimolimnion, if seasonal temperature changes in the mixolimnion reach the top of the monimolimnion. We present CTD-measurements from two meromictic mining lakes in Germany, which document the seasonal occurrence of convective mixing in discrete horizontal homogeneous layers within the monimolimnion which can be identified by the characteristic step-like structure. In the deeper layers, the steps emerge with a time delay which is determined by the progression of the mixolimnetic temperature changes into the monimolimnion. Interestingly, the chemocline interface is not degraded by these processes. However, double diffusive convection is essential for the redistribution in the inner parts of the monimolimnion at longer time scales, which is crucial for the assessment of the ecologic development of such lakes.

Numerical Study of the Onset of Double-Diffusive Cellular Convection Due to a Uniform Lateral Heat Flux

1982 ◽  
Vol 104 (4) ◽  
pp. 649-655 ◽  
Author(s):  
S. Takao ◽  
M. Tsuchiya ◽  
U. Narusawa
Keyword(s):  
Heat Flux ◽  
Numerical Study ◽  
Vertical Wall ◽  
Lewis Number ◽  
Double Diffusive Convection ◽  
Physical Experiments ◽  
Diffusive Convection ◽  
The Stability ◽  
Double Diffusive ◽  
Lateral Heat

When a fluid with a vertical solute gradient of (−dS/dy)0 is heated laterally, roll cells start to form at the boundary, developing into a series of convective layers. Numerical experiments were performed to investigate the onset of the abovementioned double-diffusive convection under the application of a uniform lateral heat flux. The paper reports the results and discussion of the following aspects of the stability of double-diffusive convection; (i) the relationship between the critical value, (Ra/Rs)c, above which convection cells form along the vertical wall and the nondimensional slot width, (d/L), (ii) the effect of the Lewis number on (Ra/Rs)c. It was also confirmed that values of (Ra/Rs)c as well as H/L (the nondimensional vertical size of incipient cells) obtained in this numerical experiment for wide slot widths (d/L>∼30), agreed well with those obtained previously by physical experiments.

Hydroclimatic Controls on Salt Fluxes and Halite Deposition in the Dead Sea and the Shaping of “Salt Giants”

2021 ◽  
Author(s):  
Ido Sirota ◽  
Raphael Ouillon ◽  
Ziv Mor ◽  
Eckart Meiburg ◽  
Yehouda Enzel ◽  
...  
Keyword(s):  
Double Diffusion ◽  
Dead Sea ◽  
Salt Flux ◽  
Stability Ratio ◽  
Thermohaline Structure ◽  
Diffusive Convection ◽  
Hypersaline Water ◽  
The Mean ◽  
Salt Deposits ◽  
The Dead

<p>As the only deep hypersaline, halite‐precipitating lake on Earth today, the Dead Sea is the<br>single modern analog for investigating the mechanisms by which large‐scale and thick salt deposits,<br>known as “salt giants”, have accreted in the geological record. We directly measure the hydroclimatic forcing<br>and the physical limnologic processes leading to halite sedimentation, the vertical thermohaline structure,<br>and salt fluxes in the Dead Sea. We demonstrate that changes in these forcing lead to strong seasonal<br>and regional variations in the stratification stability ratio, triggering corresponding spatiotemporal<br>variations in thermohaline staircase formation and diapycnal salt flux, and finally control the thickness of<br>the halite layer deposited. The observed staircase formation is consistent with the mean‐field γ instability,<br>causing layering in double‐diffusive convection. We show that double diffusion and thermohaline staircase<br>formation drive the spatial variability of halite deposition in hypersaline water bodies, underlying and<br>shaping “salt giants” basin architecture.</p>

Experiments on double-diffusive sugar–salt fingers at high stability ratio

1996 ◽  
Vol 321 ◽  
pp. 315-333 ◽  
Author(s):  
John R. Taylor ◽  
George Veronis
Keyword(s):  
High Stability ◽  
Laboratory Experiments ◽  
Stability Ratio ◽  
Finger Length ◽  
A Value ◽  
Initial Stability ◽  
Salt Fingers ◽  
Cast Doubt ◽  
The Stability ◽  
Double Diffusive

In a series of laboratory experiments the growth of double-diffusive salt fingers from an initial configuration of two homogeneous reservoirs with salt in the lower and sugar in the upper layer was investigated. For most of the experiments the stability ratio was between 2.5 and 3, where the latter value is at the upper limit (the ratio of salt to sugar diffusivities) for which fingers can exist. In these experiments long slender fingers are generated at the interface. Essentially all theories or physical bases for models of salt fingers presuppose such a configuration of long fingers. Our measurements show that the length of fingers at high stability ratio increases with time like t1/2, with a coefficient that is consistent with the diffusive spread of the faster diffusing component (salt). When the initial stability ratio is closer to unity, fingers penetrate into the reservoirs very rapidly carrying with them large anomalies of salt and sugar which give rise to convective overturning of the reservoirs. The convection sweeps away the ends of the fingers, and when it is intense enough (as it is when the sugar anomaly is large) it can reduce the finger height to a value less than the width. After this initial phase the finger length grows linearly with time as has been found in previous studies. These results show that salt fingers can evolve in quite different ways depending on the initial stability ratio and must cast doubt on the use of simple similarity arguments to parameterize the heat and salt fluxes produced by fingers.

Two-dimensional instabilities of steady double-diffusive interleaving

1992 ◽  
Vol 242 ◽  
pp. 99-116 ◽  
Author(s):  
Oliver S. Kerr
Keyword(s):  
Density Gradient ◽  
Linear Analysis ◽  
Finite Amplitude ◽  
Two Dimensional ◽  
Density Profiles ◽  
Vertical Density ◽  
Diffusive Effects ◽  
The Stability ◽  
Secondary Instabilities ◽  
Double Diffusive

The stability of finite-amplitude double–diffusive interleaving driven by linear gradients of salinity and temperature is considered. We show that as the sinusoidal interleaving predicted by linear analysis grows to finite amplitude it is subject to instabilities centred along the lines of minimum vertical density gradient and maximum shear. These secondary instabilities could lead to the step-like density profiles observed in experiments. We show that these instabilities can occur for large Richardson numbers and hence are not driven by shear, but are driven, by double-diffusive effects.

Stability of time-dependent double-diffusive convection in an inclined slot

1977 ◽  
Vol 83 (1) ◽  
pp. 83-95 ◽  
Author(s):  
C. F. Chen ◽  
R. D. Sandford
Keyword(s):  
Relative Magnitude ◽  
Initial Distribution ◽  
Flow Reversal ◽  
Time Dependent ◽  
Double Diffusive Convection ◽  
Diffusive Convection ◽  
Dependent Flow ◽  
The Stability ◽  
Experimental Values ◽  
Double Diffusive

The two-dimensional motion of a stably stratified fluid containing two solutes with different molecular diffusivities in an inclined slot has recently been examined by Chen (1975, hereafter referred to as I). The two solutes have continuous opposing gradients with the slower-diffusing one more dense at the bottom. It is found that, in the steady state, there exists a slow upward flow along the slope driven by the slight buoyancy difference near the wall, not unlike the solution found by Wunsch (1970) and Phillips (1970) for a single solute. For the time-dependent flow resulting from switching on the diffusivities at t = 0, there may be a flow reversal near the wall depending on the relative magnitude of λ and τ (where λ is the ratio of the density gradient and τ−1 is the ratio of the diffusivity of the faster-diffusing solute T to that of the slower-diffusing one S). By examining the distributions of S and T across the slot, it becomes apparent that in cases with flow reversal double-diffusive instability is likely to occur.In this paper, we examine the stability of time-dependent double-diffusive convection in an inclined slot both analytically and experimentally. The time-dependent perturbation equations are numerically integrated starting with an initial distribution of small random disturbances in the vorticity. The growth or decay of the kinetic energy of the perturbations serves to indicate whether the flow is unstable or stable. The results show that the flow becomes more unstable (a) with increasing λ at a given angle of inclination with respect to the vertical and (b) with increasing angle of inclination at a given value of λ. Experiments were carried out in a 2[sdot ]54 cm wide slot using sugar and salt solutions at angles of inclination of 30°, 45° and 60°. Results obtained confirm the trends predicted by the analysis. Good agreement was obtained between the predicted and the experimental values of the critical wavelength for the case λ = 0[sdot ]7.

The Onset of Double-Diffusive Convection in a Vertical Cylinder With Vertical Throughflow

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
D. A. Nield ◽  
A. V. Kuznetsov
Keyword(s):  
Vertical Cylinder ◽  
Double Diffusion ◽  
Linear Stability Theory ◽  
Double Diffusive Convection ◽  
Vertical Temperature Gradient ◽  
Vertical Temperature ◽  
Onset Of Convection ◽  
Diffusive Convection ◽  
Vertical Throughflow ◽  
Double Diffusive

The effect of vertical throughflow on the onset of convection, induced by an applied vertical temperature gradient in a vertical cylinder is studied analytically using linear stability theory. This problem is important to hydrologists to investigate under what conditions convection is taking place in a well or borehole. The effect of double diffusion is included. Both nonoscillatory and oscillatory situations are studied.

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