Elena Roget

Turbulent Patch Identification in Microstructure Profiles: A Method Based on Wavelet Denoising and Thorpe Displacement Analysis

Abstract A new method based on wavelet denoising and the analysis of Thorpe displacements dT profiles is presented for turbulent patch identification. Thorpe profiles are computed by comparing the observed density profile ρ(z) and the monotonic density profile ρm(z), which is constructed by reordering ρ(z) to make it gravitationally stable. This method is decomposed in two main algorithms. The first, based on a wavelet denoising procedure, reduces most of the noise present in the measured profiles. This algorithm has been tested from theoretical profiles and has demonstrated a high efficiency in noise reduction, only some limitations were detected in very low-density gradient conditions. The second algorithm is based on a semiquantitative analysis of the Thorpe displacements. By comparing each displacement dT with its potential error EdT, it is possible to classify samples in three possible states: Z (dT = 0), U (dT EdT). This classification makes it possible to compute two statistical...

Bottom currents induced by baroclinic forcing in Lake Banyoles (Spain)

Because of the difference in morphometry and in the underground heating of the two lobes of Lake Banyoles there is a differential cooling that causes a density current — the denser water of the shallower lobe plunging into the deepest lobe forming a bottom current — which redistributes water between the lobes and replaces that of the northern lobe about every 5 days. This current has been studied during the mixing periods of the years 1989–1991 from temperature and current measurements and it has been found that it increases or slows down depending on the direction of the wind. Furthermore, it is observed that it mainly flows along the west shore of the lake, deflecting towards the right as an effect of the bottom morphometry. However, due to the long time scale of the density current the influence of the earths rotation should not be neglected in advance. Finally, a simple numerical model has been used to corroborate the magnitude of this current — whose velocity has been directly measured — which is found to be the most important in the lake during the mixing period and so, it dominates the lake-wide circulation

DAYTIME HEAT BALANCE FOR ESTIMATING NON-RADIATIVE FLUXES OF LAKE BANYOLES, SPAIN

Combining a six-term heat balance equation and a seasonal thermocline model, a new equation to calculate non- radiative fluxes of Lake Banyoles has been determined. Mean daily measurements of global solar radiation, downward longwave radiation, wind speed, air temperature and water surface temperature were used as input data and lake temperature as the calculated output data of the model. To calibrate performance of the new equation, calculated lake temperature was compared with measured lake temperature during both the mixing and the stratified period of the lake. The new coefficients in the wind function to calculate the non-radiative fluxes at the water surface were assumed to depend on the variability of the wind speed, the air temperature of the study area and the surface temperature of the lake. In addition, the results were used to estimate the heat balance of the air–water interface of Lake Banyoles over a period of two years. The processes that have been taken into account are shortwave and longwave radiation, back-radiation emitted by the lake, sensible and latent heat and throughflow.

Intermittency of near‐bottom turbulence in tidal flow on a shallow shelf

U.S. Office of Naval Research [N00014-05-1-0245]; Spanish Ministry of Education and Science [FIS2008-03608]; Major State Program of China for Basic Research [2006CB400602]; Catalan Institute for Water Research (ICRA)

On the presence of aggregates in the basins of Lake Banyoles

Suspension of sediment by subterranean springs is a very common phenomena in the karstic, multibasin Lake Banyoles. The flow of groundwater into the lake introduces a suspension of marly and argilaceous materials. The temperature of this slurry is nearly constant throughout the year, around 19°C. The lake is formed by 6 basins and at the bottom of these basins there are 12 sources where the presence of suspended material signifies entering groundwater (Figure 1). Suspension of bottom lake sediment by groundwater is a feature of Lake Banyoles that has not been described elsewhere. Here we present experimental results which show that the maximum height that the sediment can rise to is affected by the formation of aggregates.

Flow characteristics of a gravity current induced by differential cooling in a small lake

Extensive field data of a density current driven by the differences in the cooling rates between the two lobes of a small lake are presented. These data illustrate the fact that this gravitational motion affects the whole system and dominates its lakewide circulation. Moreover, data are used to evaluate the entrainment into the density current and also to discuss the regime of the flow from a scaling analysis of the horizontal momentum equations. This analysis is specifically applied to the central area of the lake, where the density current flows from one lobe to the other. Results of this study show that in the longitudinal direction there is a balance between the pressure gradient and the turbulent viscous term. Further, it is found that geostrophy correctly describes the transversal pattern of the passage of the gravity current from the northern lobe to the southern, where it flows confined to the western shoreline.

The seasonal cycle of a groundwater dominated lake

Lake Banyoles is a multibasin lake dominated by groundwater derived inflows. It is composed of three major basins, two of them being meromictic. The relatively warm and saline groundwater inflows, together with the basin morphometry, give rise to characteristic thermal structures in each basin, with different thermocline levels and temperature inversions distinguishing the basins. A one-dimensional lake mixing model has been adapted to include the form of mixing associated with the groundwater inflows, and the results are compared with data collected from the lake.

Calculation of the flow into a lake from underground springs using sedimentation rates

The scope of this work is to quantify the upflows through the underground sources in the bottom of Lake Banyoles (NE-Iberia). The method applied uses the mean settling rate and the fraction of solids per unit of volume of the suspensions at the underground sources, and also the horizontal areas covered by these suspensions. The results obtained reveal that about 90% of the total underground inflow enters the lake through one of the sources. This result agrees with the total underground inflow calculated from the difference of the surface in and outflow of the lake. The behaviour of the suspensions at the underground sources was in a accordance with the theory used to developed the measuring method. Finally, the degree of turbulence in the upper part of the suspension and the importance of the transmissivity of the limestone layer next to the lake are discussed and are found to be negligible.

Heat flux through a geothermally heated fluidized bed at the bottom of a lake

Heat fluxes and the underground inflow through a natural fluidized bed within the main sub-basin of Lake Banyoles are studied and parameterized. In the upper part of this fluidized bed, at a depth of about 30 m, the vertical gradients of particle concentration and temperature are very sharply located within an interface a few centimeters thick. Within this interface (lutocline), the depths where the temperature and the concentration gradients are maximum match exactly. On the other hand, the lutocline determines a flat, horizontal surface dividing the water column into a hot, turbid medium at the bottom and clear, colder, bulk water above. Through this interface the flow regime also varies from being laminar just below it, to turbulent due to convective processes developing above it. More precisely, in studied main sub-basin a buoyant plume develops above the lutocline, as a result of the heat flux, and affects the lakes water quality due to particles dragged along by it. In this paper it is proposed to determine the temperature at the depth of maximum gradient within the interface by means of measured temperature profiles, and consider the stationary heat transport equation in the laminar region below it, in order to obtain the water velocity and the heat flux. Heat flux parameterization is given based on a large number of thermal high-resolution profiles, covering six campaigns in different years and seasons. Furthermore, and in consideration of the fact that high-resolution thermal profiles are not always available, some alternative parameterizations for the heat flux are presented based only on the temperature of the fluidized bed and that of the lower hypolimnion.

Particle and turbulence measurements in lakes: application to the rising plume of Lake Banyoles

Use of fine-scale and microstructure instruments provides an additional perspective on topics that have traditionally been studied by limnologists. Furthermore, knowledge of phenomena occurring on smaller scales helps our understanding of what happens on larger scales. In this paper we present the results of a field survey where particle and microstructure measurements were performed in different stations ofLake 8anyoles. Most water (~85%) enters Lake 8anyoles via basin 81 as groundwater (Fig. l) which keeps the sediments in suspension forming a lutocline, i.e. a suspended sediment interface (CASAMITJANA & RoGET 1993). The temperature of the entering groundwater is nearly constant throughout the year (17 -19 °C) and higher than that of the hypolimnetic water and, therefore, the groundwater discharge in 81 behaves as a buoyant jet. Given the conical shape of the basin 81 (Fig. l), the jet loses momentum in the upward direction and eventually becomes a plume. In this paper we discuss particle distributions, the turbulent structure and other physical and chemical pararneters in 81 and compare these results with those obtained in the second basin, 82, which has lower groundwater inputs. From these results we infer the presence of convective mixing in the plume and the formation of intrusions.