IP14a

Diurnal Cycling: Stratification and Matter Transport Processes in a Lake Surface Layer

K.A. Korotenko

PP Shirshov Institut of Oceanology, Moscow, Russia

A hybrid model is proposed to describe an influence of solar heating and wind stress changes on the matter transport in the surface layer of the lake. Main parts of the model are as follows: • particle model based on the random walk concept, • Ekman flow model, • turbulence model with the third-order closure.

Data obtained in field experiments on the Wellington Reservoir (Imberger, Limno. Oceanogr. 30(4), 1985) were used as initial and background conditions to describe evolution hydrophysical fields and behaviour of a jet of particles in the surface layer under solar heating, wind mixing and convective cooling. Results of turbulence, temperature and current velocity simulations are in good agreement with the observed data.

To distinguish the influence of each period a particle source was ‘switched on’ for a short time interval (4h) in various characteristic time periods. Results of numerical simulation can be itemized as follows:

1. Intensive heating and slight wind lead to formation of a shallow quasi homogeneous surface layer, in which the particles propagate. The vertical extent of the particle jet can be extremely shortened under intensive solar heating influence, leading to relatively high concentration levels.

2. Strengthening of the wind with sufficiently intense heating leads to a thickening of the turbulent layer and to a deepening of a mixed layer. The region occupied by the particles is also determined by the thickness of the mixed layer.

3. Weakening of the wind leads in the beginning to stabilisation in the position of the mixed layer, and later with the attenuation of the current velocity over the entire thickness of the surface layer, to depression of turbulence in the mixed layer.

4. With the beginning of cooling and for a sufficiently high wind velocity there is a rapid deepening of the surface layer. The additive propagation process into the sea depth at this stage proceeds rather intensively, and the depth of penetration for additives was restricted by the condition of the particle transport experiment.

 

IPI4B

intensive shortperiod internal waves generation in enclosed sea’s frontal zone of coastal upwelling

V.I. Vlasenko, V.A. Ivanov, I.G. Krasing and A.D. Lisichenok

Marine Hydrophysical Institute, Sevastopol, Kapitanskaya Str. Crimea, UKRAINE

The generation mechanism of short-period internal waves with anomalous amplitudes for Black Sea conditions have been studied using measurement data on antenna of distributed temperature sensors on the South Crimea coast’s shelf near Kaciveli.

It is hypothesised that a reason for the generation of intensive internal wave packets with anomalous amplitudes was an interaction of the Main Black Sea Stream’s jet with the shelf edge.

A numerical model of the investigated phenomena has been made. Results of computations of the anomalous internal waves’ generation are presented. Field data are compared with model output and are shown to be qualitatively and quantitatively consistent.

 

 

ip14c

Stability and Meromixis in a Mid-Latitude Pit-Lake

Craig Stevens* and Gregory Lawrence

Environmental Fluid Mechanics, Dept. Civil Engineering, University of British Columbia, Vancouver BC, Canada
*Present address, NIWA, Greta Point, Wellington, New Zealand

A significant proportion of the water contained in lakes throughout the world is at a temperature of around 4ƒC. The low stability associated with this temperature range results in high sensitivity to external forcing. This talk presents observations of stratification in a disused water-filled open-cut mine pit, a system providing unique conditions as the surface area of the water body is relatively small (0.25 km2) compared to the maximum depth (150m). The geometry attenuates a number of paths for energy transfer to mixing. A first look at the data indicated that the basin was probably meromictic as dissolved salts were sufficient to retard the spring and autumn turnover events. However, closer examination suggests that a warm groundwater inflow provided suitable conditions for double diffusion so that diffusion in the hypolimnion was greater than otherwise expected. Thus, the inflow was required to maintain the appearance of meromixis. Additionally, even with ice-cover, there was clear evidence of internal wave activity leading to further increases in hypolimnetic diffusion. These observations illustrate the complexity of mixing regimes in such weakly stratified environments.

 

IP14d

On Optical Properties of Estonian and Finnish Lakes.

T. Kutser, H.Arst. S. M”ekivi, A. Reinart, A. Blanco, A. Herlevi, T. Nžges and P. Nžges

Estonian Marine Institute, Tallinn, Estonia,
Department of Geophysics, University of Helsinki, Finland
Vžrtsj”rv Limnological Station, Rannu, Estonia

Optical properties of water bodies are of great importance for solving many problems: modelling of heat budget, remote sensing, monitoring of water state are just some of them. Four Finnish and seven Estonian lakes were selected as test sites for joint optical experiments during summers 1995/96. Trophic state of these lakes varies from oligotrophic to hypertrophic and optically variation is from blue watered clear lake to bog watered brown lakes.

Spectra of downwelling radiance, downwelling irradiance and upwelling radiance were measured above the water surface. Telespectrometer ‘Pegasus’ that measures photosynthetically active light (400-700 nm) with spectral resolution 6 nm was used. Vertical profiles of upwelling and downwelling spectral irradiance were measured in the water by Li-1800UW spectrophotometer and integral irradiance was measured by Li-Cor underwater quantum sensor. This spectral beam attenuation coefficient was measured in laboratory from water samples by Hitachi-1000 spectrophotometer and in situ water transparency was estimated by Secchi disk. Chlorophyll, yellow substance and suspended matter concentrations were measured from water samples. Primary production and species composition of phytoplankton was estimated in case of Estonian lakes. Vertical profiles of oxygen concentration and water temperature were measured.

Remote sensing algorithms that estimate Secchi depth, beam attenuation coefficient and concentrations of chlorophyll a, yellow substance and suspended matter were found. A simple model that simulates water color spectrum above and beneath the water surface is elaborated to solve the optical inverse problem and estimate concentrations of phytoplankton, yellow substance and suspended matter on the basis of spectral reflectance measured in the water or using remote sensing instruments. Obtained data enables us to test heat budget models and study mutual relationships between optical properties of the lake water and other characteristics of the water bodies.

 

IP14E

validation of a numerical model of vertical velocity in lakes by wind-driven analytical solutions

P.F. Hamblin

Aquatic Ecosystems Restoration Branch, National Water Research Institute, Burlington, Onatario CANADA

The motivation for this study stems from a need to better understand the supply of nutrient laden hypolimnetic water to the photic zone in lakes. It is thought that primary productivity is closely linked to localized upwelling zones in lakes.

The topic of upwelling in lakes remains a largely indirect or theoretical study from the point of view of actual observations. Despite the rapid advances in methods of observations of flow and the possibility of field measurement of the vertical component of flow with acoustic doppler current meters the velocities (order 10-5m/s) are still much too small to directly measure. Approaches in the past have followed isotherm displacements in the vertical direction which are straightforward to determine but this approach assumes no mass exchange between the layers. With the recent development of continuous models in the vertical cross slope vertical overturning circulation ought to be capable of being numerically modelled.

Due to the lack of either field or valid laboratory data with which to test more advanced mathematical upwelling models a new analytical model of upwelling due to a suddenly turned on wind in a rotating and variable topography basin is developed and compared to numerically obtained vertical flows. Because of the need to obtain accurate solutions in the nearshore and in regions influenced by varying bathymetry the finite element method of numerical modelling is adopted. While the model performs well in general the largest errors occur near the shoreline. Numerical experiments show that model errors are sensitive to the vertical resolution of the Ekman layers and that at least four levels are required to resolve the layer. Analytical and modelled particle flux paths are compared in the upwelling energy region. Application of the numerical model to upwelling in large lakes is discussed.

 

IP14f

Geometry and Energetics of a Periodic Internal Wave

Yu. D. Chashechkin and Yu. V. Kistovich

Institute for Problems in Mechanics, RAS, Moscow, RUSSIA

Problems of generation of periodic internal waves, and their propagation in a fluid with arbitrary smooth stratification, reflection off a solid boundary, from critical layer or thin irregularities of buoyancy frequency are studied analytically and experimentally. In a continuously stratified liquid a compact source produces unimodal or bimodal wave beams. Spatial structure of a beam generated by different oscillating bodies, its geometry, geometry of an induced boundary current are studied by optic (schlieren interferometer, Maksutov method, colour rainbow method) and conductivity probe methods.

Asymptotic linearized theory of an infinitesimal internal wave beam dynamics in a viscous fluid with diffusion is constructed. Integral transformation connecting parameters of a beam in a fluid with arbitrary smooth and exponential stratification are obtained. Geometry and energetics of a beam and induced boundary current with split scales of spatial variability for velocity and density near reflecting surface are calculated. Energy loss coefficient near a critical level in a quiescent liquid is also obtained. It is shown that a running internal wave beam produces an internal boundary current with its own intrinsic split scales for velocity and density on both sides of a discontinuity of buoyancy frequency profile (or its higher derivatives).

 

ip14g

GENERATION of Nonlinear Surface Waves in flow past Bottom Inhomogeneities

V.V. Bakhanov, V.I.Kazakov, O.N.Kemarskaya, I.A.Okomel’kova, V.P.Pozdnjakova and I.A.Shereshevsky

Institution of Applied Physics, RAS, N. Novgorod, Russia, Institution for Physics of Microstructure, RAS, N. Novgorod, Russia

The generation of counterstream flow nonlinear quasimonochromatic surface waves produced by an underwater localized obstacle perturbation of a current is investigated theoretically and experimentally. The model equation, describing the variation of surface waves amplitude has been derived:

where - dimensionless perturbation of current. The time evolution of an initially homogeneous wave field is numerically investigated in the framework of this equation for two kinds of underwater obstacles: an underwater mountain range and an underwater solitary mountain. It is shown, that even the weak inhomogeneous currents modifies appreciably the regime of surface waves long-wave instability develop. If underwater obstacle horizontal sizes do not exceed depth of sea , the space dimensions of the domain, where the wave field transformation is essential, greatly exceed . If , the space sizes of surface wave anomaly are close to . With increasing time the value of nonlinear surface wave amplitude change first increase, and then run into constant level, but the surface anomaly structure still continues to grow in complexity. The dependence of surface wave field evolution on the amplitude, wave-length and propagation direction of surface waves is investigated.

The theoretical model has been checked experimentally in a laboratory tank. The inhomogeneous stream was created by flow around a sphere, the surface waves were generated by a wavemaker. The two-dimensional patterns of surface wave amplitude were recorded. Comparison of the experimental data with theoretical results has shown, that the offered model well describes main features of generation of nonlinear surface waves on non-uniform current.

 

ip14I

Internal Waves near Sloping Boundaries

S.A. Thorpe and R. Jiang

Department of Oceanography, The University of Southampton, Southampton Oceanography Centre, Southampton, UK, Laboratoire de Recherches Hydrauliques, Ecole Polytechnique FÈdÈrale de Lausanne, Lausanne, Switzerland

After periods of strong wind forcing during summer and autumn, when lakes are stratified with a seasonal thermocline, internal surges with soliton-like form, are observed. Their fronts are marked by a change in epilimnion depth (an increase when the thickness of the epilimnion is less than that of the hypolimnion), often by 10m or more in a period of 10-20mins. This is followed by a train of 3-8 waves with typical period 10 mins. In narrow lakes of width less than the internal Rossby radius, such as Loch Ness, these waves are known to represent the non-linear form of standing oscillations, sometimes generated in resonance with migrating weather systems. Less is known about the form of the waves in large wide lakes, but they appear to travel along the boundaries predominantly in a cyclonic direction. These are related to internal Kelvin waves which extend to a distance comparable to the Rossby radius from the lake boundaries, although modified by their shape and slope. These 'Kelvin waves' are accompanied by high frequency waves which, as in narrow lakes, may be an integral part of their structure, or perhaps are generated by the movement of water induced by the larger waves over and around the rough topography of the lake sides (Thorpe et al., 1996).

Even after a period of fairly calm weather with winds < 4m s-1 lasting for 2-3 weeks, high frequency internal waves of amplitudes exceeding 1m are observed near the sloping sides of Lake Geneva. It appears that the ambient wave field in the lake is decaying, but that over the sloping boundaries wave amplification and breaking, an internal 'surf zone', may develop. Particularly large waves are observed within 2m of the sloping bed at frequencies close to the critical frequency,sc = N sina, where N is the mean buoyancy frequency and a is the bottom slope. A study has been made of the directional properties of the wave field in this region of the lake and its generation of local regions of small Richardson number. These are used to make estimates of the variation of the vertical diffusion coefficient, Kv and the rate of dissipation of turbulent kinetic energy, e , with wave activity. It is notable that the largest internal waves, the quasi-Kelvin waves mentioned above, are traveling around the lake and therefore not normal to the boundaries, as is usually supposed in laboratory models of internal waves on slopes. The consequences for wave reflection, resonances and mean along-slope drift are considered. A consequence of reflection is the generation of along-slope drift currents, even in the absence of dissipation.

Thorpe, S.A., Keen, J.M., Jiang, R., and Lemmin, U., 1996. High-frequency waves in Lake Geneva. Phil. Trans, R. Soc. Lond. A, 354, 237-257.

 

IP14j

Geometric Focusing of Internal Waves: the Absence of Internal Seiches

Frans-Peter Lam, Leo Maas, Dominique Benielli and Joel Sommeria

Netherlands Institute for Sea Research, Physical Oceanography Department, Texel, Netherlands
Ecole Normale et Superieure de Lyon, Lab. de Physique, Lyon Cedex, France

When a bucket of water is shaken, it responds by vibrating in terms of large-scale standing waves: its eigenmodes. "Shaking" of a lake is done typically by windstress, and the eigenmodes are then called seiches. The belief that enclosed continuous media always possess eigenmodes is deeply rooted. It will be shown here that internal gravity waves in stably-stratified fluids in non-trivially shaped containers or lakes present a counter-example. In this case, regardless of the forcing location and shape, waves propagate towards an attractor determined by the forcing period. At the attractor these waves intensify, leading to mixing, which is of vital importance for marine processes. The appearance of attractors rather than eigenmodes leads to the unexpected mathematical result, that a linear equation can be solved by iteration of a nonlinear map, and possesses solutions with a fractal spatial structure which depends fractally on the parameters involved. The physically most relevant, basin-scale ground mode (internal seiche) exists for a rectangular basin only at a particular resonant frequency, while for a non-rectangular basin large scale motions (associated with the existence of an attractor) exist over a finite range of frequencies. Fundamental differences between a rectangle and more general (2D) topographies will be shown, and some video demonstrations of laboratory experiments and related theoretical models capturing the internal wave attractor will be presented.

 

IPI4K

Mixing processes and length scales in gravity currents: a study in the field and the laboratory

Sherrill Mausshardt

Harris Hydraulics Laboratory, Seattle, WA, USA

At the Chittenden locks in Seattle, Washington, a saltwater gravity current forms every time the navigation locks are used, which separate the fresh water of Lake Washington from the saltwater of Puget Sound. With boats coming from Puget Sound into the lake system, a gravity current of saline water is introduced from the lock chamber. The research presented combines controlled field experiments at the locks with a series of laboratory experiments of a scaled model of the locks. Density profiles were taken at fixed locations for both the laboratory and the field work. Bulk velocity measurements are also taken in the field using an Acoustic Doppler Current Profile (ADCP). Mixing length scales are quantified which change as the current progresses upstream. Reynolds number effects are discussed. The Thorpe scales, Available Potential Energy of the Fluctuation (APEF) and the ‘steepness of the overturns’ are compared.

IP14L

Observations of Density Currents and Internal Bores in the Bottom Waters of Lake Champlain

James H. Saylor, Gerald S. Miller and Brian Nowak

NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, Michigan, USA
Department of Atmospheric and Oceanic Sciences, University of Michigan, Ann Arbor, USA

The shape of lake basins, with their large variety of configurations, plays a strong role in determining their current patterns. Elongated and narrow basins are often found in formerly glaciated areas. Examples are the finger lakes of northern New York State, Loch Ness in Scotland, the sub-alpine lakes of Italy and Switzerland, and Lake Champlain which separates the States of New York and Vermont. Such lakes have special interest because their currents are restricted to nearly one-dimensional flow, simplifying both their observational and theoretical investigation. Deep lakes with these configurations often exhibit extremely large thermocline displacements and oscillations in response to wind forcing during seasons of density stratification.

Time series of currents and water temperature structure in Lake Champlain have been recorded during several summer seasons. The measurements were obtained with arrays of Acoustic Doppler Current Profilers and themistor strings moored along the lake's thalweg that forms the boundary between the two states. The main lake basin is nearly 120 km long, 6.3 km wide, and has a mean depth of about 30 m. The depth decreases rather uniformly toward the north end from a maximum of 120 m in the south. The long axis of the lake is oriented approximately north-south. During seasons of weak stratification, strong winds from the south transport almost all of the less dense water to the north end of the basin. The observed density distribution then approximates that encountered in lock exchange. Density currents and bores propagating northward along the lake floor characterize the recovery from downwelling. Several specific episodes are examined and we conclude that these events are common in the lake basin.

 

IP14m

Study of Geochemical Association of Trace Metal Ions in Chilka Lake - A Statistical Approach

D.M. Mahapatra and U. Panda

Birla Periclase, Visakhapatnam, India
Department of Marine Sciences, Berhampur University, Orissa, India

To establish the natural processes and the geochemical factors responsible for enrichment of trace metal ions (Cu, Co, Ni, Zn and Cr) with respect to textural parameters (sand, silt and clay wt. %) along with physical parameter depth, a multi-variate statistical approach have been made, for about 20 stations of Chilka Lake collected during post monsoon period of 1994. Based on the point of inflextion or minima of cumulative eigenvalues, three common factors for different groups were retained for subsequent varimax rotation and interpretation.

In both fresh and saline water silt associates with Co. In mixed water it associates with Cu. It is therefore concluded that Co enrichment is controlled both by textural parameters as well as by adsorption mechanism. In fresh water and saline water textural parameters and in mixed water adsorption mechanism controls its enrichment. For Ni concentration, in fresh water zone both clay fraction and adsorption are important, whereas in mixed water adsorption phenomena predominate. Towards saline water zone both clay coupled with depth indicates their importance for Ni enrichment. Zn in fresh water is related to clay whereas in mixed water it is in adsorbed state, while in saline water no specific association is observed. Fresh water sediments contain Cu in adsorbed state and in mixed water it is related to depth and Co. In saline water no specific trend is seen for Cu. Chromium does not show any specific association in fresh water, whereas in both mixed and saline water it is associated with clay minerals.

Identical observations were also made with cluster analysis.

IP14n

Eddy formation by dense flows on slopes in a rotating fluid

Gregory F. Lane-Serff and Peter G. Baines

Department of Oceanography, University of Southampton, Southampton Oceanography Centre, Southampton UK.
Division of Atmospheric Research, CSIRO, Victoria AUSTRALIA.

Properties of the flow generated by a continuous source of dense fluid on a slope in a rotating system are investigated by making laboratory experiments. The dense fluid initially flows down the slope but turns (under the influence of rotation) to flow along the slope. However, some of the dense fluid drains downslope in a viscous Ekman layer. In some cases the Ekman layer becomes unstable to growing waves. The initial downslope flow of the dense current takes the ambient upper layer fluid out into deeper water, thus stretching columns of ambient fluid. Stretching is measured by the parameter G = La /D, where L is the Rossby deformation radius, a the bottom slope and D the total depth. Provided that the viscous draining is not too strong, cyclonic vortices form in the upper layer and the dense flow breaks up into a series of domes. The strength of the upper layer cyclones increases linearly with G , reaching approximately 2f in the experiments presented here. The frequency at which the eddy/dome structures are produced also increases with G , while the speed at which the structures propagate along the slope depends on viscous effects. Implications of these processes for flows in lakes are discussed.

 

ip14o

Optic and Acoustic Profiling of a 2D Stratified Wake

Yu. D. Chashechkin, V.V. Mitkin and V.E. Prokhorov

Institute for Problems in Mechanics, RAS, 117526 Moscow, RUSSIA

Traditional Schlieren, interferometer, "rainbow" methods, density markers and Echo Sounder (operating frequency MHz) are used for investigation of internal structure of a stratified wake past a towed cylinder. In given experiments density gradient is uniform or varies on distances of the order of the cylinder diameter. Upstream disturbances, internal waves, downstream wake are studied for a wide range of flow parameters, yielding laminar, vortex and turbulent regimes. The effect of self consistent high gradient interfaces arising in a flow on wake geometry and its fine internal structures as well as on the backscattering of the ultrasound is studied in details.

Due to accumulation of fluid ahead of the body, joining at a different level past the body and formation of the split boundary current, general deformation of the stratification takes place both on large and small scales. In this case one can observe two types of vortex elements – a large scale one (order of a body diameter) and small scale billows located on thin interfaces. Structure of flow and all profiles which are not similar with each other for different variables are changed with time correspondingly. Temporal variation of different profiles occurs with correspondence of intrinsic local buoyancy frequency in an arbitrary smooth stratified liquid.

Classification of flow regimes includes stable large- and small-scale specific vortices and vortex systems and among them a regime of structural turbulence when complicated relief of isopycnals is observed on the background of smooth velocity profiles.