Publications de Ulrich Lemmin | EPFL Graph Search

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Convective winter cooling is an important process in the annual cycle of a lake ecosystem, because it brings oxygen to the deeper layers. However, in deep lakes, convective processes are often not strong enough to extend the mixing down to the deepest layers. This may become even more problematic in the future considering climate change and potentially milder winters. In large deep lakes, two other processes originating in the coastal zone can significantly contribute to winter deep water renewal: cold wate

Lake Geneva, the largest freshwater lake in Western Europe, is subject to important environmental pressures from its densely populated shores and watershed. To maintain and improve water quality in this lake, as well as in other enclosed or semi‐enclosed b ...

[1] During the winters of 1998 and 1999, observations were made of the cascading of cold water from the nearshore, shallow "shelf'' zones and down the sloping sides of Lake Geneva. Cascading starts on the average 10 hours after the onset of surface cooling. The draining cold water descends like a gravity current, and the downslope speed of the head of these slugs of cold water, U, has a mean value of 5.2 cm s(-1), with slugs persisting, on the average, for 8 hours. When the Monin-Obukov length scale at the

Wintertime deepwater renewal, which is important for heat-oxygen-nutrient exchange in lakes, is traditionally considered to be mainly driven by 1D vertical convective cooling. However, differential cooling between shallow and deep waters can produce densit ...

Lake Geneva (Lac Léman) is the largest freshwater body in Western Europe. It is a deep peri-alpine lake whose importance stems from being not only an essential freshwater source in the region, but also a major tourist destination, a fishery and a waterway. Its dynamics have been the subject of long-term monitoring and study, and its response patterns to wind forcing (the major forcing) are relatively well understood (e.g. Lemmin et al. 2005). On the other hand, the large-scale organisation of the water circ

The dissipation rate of turbulent kinetic energy per unit mass, epsilon, vertical eddy diffusivity, K-z, and the rate of dissipation of temperature variance, X, are estimated over the sloping sides of Lake Geneva both far from and near the bed, using temperature data from horizontal tracks of a submarine during periods of winter cooling. The estimated values are about one order of magnitude greater near the slope than those distant from it. The mean dissipation rate per unit mass averaged over the epilimnio

During calm cooling periods, differential cooling can induce winter cascading which is an important process for littoral-pelagic exchange and deep water renewal in large, deep lakes (Fer et al., 2001; Peeters et al., 2003). Generated in the shallow near-shore regions, such cold-water density currents travel down the sloping lakebed until they reach their depth of neutral buoyancy. The latter is strongly dependent on the entrainment of warmer ambient water, often expressed by the entrainment coefficient (i.e

Density currents generated by differential cooling in lakes with shallow shore regions or side basins are associated with increased vertical mixing, deep water renewal and cross-shore transport of nutrients and other substances. Previous field experiments in Lake Geneva, the largest lake in Western Europe, showed that such cold-water density currents are a common phenomenon during winter, frequently reaching the thermocline at O(100 m) depth. As part of a project investigating large-scale interbasin exchang

In many deep lakes, global warming is weakening wintertime convective cooling, thus reducing the occurrence of complete vertical overturning. At the same time, our understanding of the deepwater dynamics in deep lakes remains elusive, mainly because spatiotemporally resolved in situ measurements are lacking. We address this knowledge gap by exploring the dynamics in the deep hypolimnion of Lake Geneva (max. depth 309 m) by means of extensive field observations. Due to its great depth and the mild central Eu

Coastal regions accumulate particulate matter, including pollutants, that are brought into the lake from surrounding watersheds. In order to assess dispersion of these substances into the lake interior, it is important to understand the near-shore hydrodynamics and resuspension processes. In winter, cascading of near-shore cold water, caused by differential cooling, induces relatively intense density currents down the sloping bottom lake boundary. Previous field measurements in Lake Geneva have shown signs

Lake Geneva (length 74 km on the long east-west axis, surface area 562 km2, volume 89 km3) is a freshwater lake bordered by Switzerland and France. The lake’s hydrodynamics are forced principally by wind and seasonality, with inflows and the Coriolis effect playing relatively minor roles. Of the major forcings, wind is highly variable due to the rapid changes in topography around the lake, with mountains in the east and relatively gentle landscapes in the west. Numerous field investigations have revealed th

Randomly distributed patches of smooth or rough/rippled surfaces are readily observed on most water bodies. Smooth surface patches are called natural slicks and typically form under low wind conditions (< 6 m s-1) when biogenic surfactants in the surface microlayer accumulate above a certain threshold. They may have spatial scales from tens of meters to kilometers. Slicks suppress the formation of wind-induced Gravity-Capillary Waves (GCW), leading to altered surface reflectance of light and microwaves and

In order to simulate fine sediment dynamics over an armored bed in a tidal river, unsteady accelerating, then steady open-channel flow over a movable (but not moving) coarse gravel bed (D-50=5.5 mm) was studied. A layer of fine sediment (D-50=120 mu m) was ...

An improved understanding of surface transport processes is necessary to predict sediment, pollutant and phytoplankton patterns in large lakes. Lake surface water temperature (LSWT) which may spatially and temporarily vary over the lake surface reflects meteorological and climatological forcing more than any other physical lake parameter. There are different data sources for LSWT mapping, including remote sensing and in situ measurements. The available satellite data might be suitable for detecting the larg

We examine near-shore and pelagic current variability in Lake Geneva, a large and deep lake in western Europe, using observations from several measurement locations and a three-dimensional numerical model for the period 2014-2016. Linear internal seiche mo ...

Turbidity currents represent a distinctive type of subaqueous density currents, characterized by a density excess that is due to the sediment load. Turbidity currents are important in lakes, reservoirs and oceans and have implications on hazard management, reservoir sedimentation and water quality. The existence of turbidity currents has been inferred in the 19th century from the existence of canyons on lake bottoms and in the 20th century from successive cable breaks of telecommunication cables on the ocea

A two-platform measurement system for realizing airborne thermography of the Lake Surface Water Temperature (LSWT) with ~0.8 m pixel resolution (sub-pixel satellite scale) is presented. It consists of a tethered Balloon Launched Imaging and Monitoring Plat ...

In lentic water bodies, such as lakes, the water temperature near the surface typically increases during the day, and decreases during the night as a consequence of the diurnal radiative forcing (solar and infrared radiation). These temperature variations ...

Hyperpycnal (negatively buoyant) river inflows into lakes or reservoirs plunge upon entry, generating gravity-driven underflows near the bed. When the density excess of these underflows is primarily due to high sediment concentrations, they are referred to as turbidity currents. If these underflows encounter a layer of equal density, they will detach from the bed and intrude into the water column, forming an interflow. In turbidity currents, such underflow-interflow transitions often happen through a proces

Submesoscale fronts often become visible when the accumulation of biosurfactants in the water surface microlayer causes smooth surfaces, called frontal slicks, to develop. Based on in situ and remotely-sensed data, a frontal slick was documented for the fi ...

Ulrich Lemmin

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Deep-water coastal upwelling events during wintertime in a large lake (Lake Geneva)

Tracking Lagrangian transport in Lake Geneva: A 3D numerical modeling investigation

Winter cascading of cold water in Lake Geneva

Deepwater Renewal in a Large, Deep Lake (Lake Geneva): Identifying and Quantifying Winter Cooling Processes Using Heat Budget Decomposition

Transport and mixing in Lac Léman

Observations of mixing near the sides of a deep lake in winter

A moored profiling platform to study turbulent mixing in density currents in a large lake

Differential cooling and near-shore dynamics in a large lake (Lake Geneva)

Deepwater dynamics and spatial heterogeneity observed in the deep hypolimnion of a large lake (Lake Geneva)

Sediment resuspension and transport from near-shore zones towards the deep interior of Lake Geneva caused by winter cascading

Stochastic Generation of Wind Patterns over Lake Geneva

The effect of natural surfactants on air-water momentum exchange under light wind conditions in Lake Geneva

Fine sediment dynamics in unsteady open-channel flow studied with acoustic and optical systems

A Multiscale Surface Water Temperature Data Acquisition Platform: Tests on Lake Geneva, Switzerland

Nonlinear dynamics of the near-shore boundary layer of a large lake (Lake Geneva)

Characteristics of turbidity current events in Lake Geneva, Switzerland

Achieving high-resolution thermal imagery in low-contrast lake surface waters by aerial remote sensing and image registration

Evolution of superficial lake water temperature profile under diurnal radiative forcing

Hydro-sedimentary processes of a lofting turbidity current revealed by gridded ADCP measurements in the field

A Persistent Submesoscale Frontal Slick: A Novel Marker of the Mesoscale Flow Field in a Large Lake (Lake Geneva)