Grand Bassin des États-Unis | EPFL Graph Search

Le Grand Bassin des États-Unis (Great Basin en anglais) est un désert de l'ouest des États-Unis. C'est une région de montagnes, de bassins sédimentaires et de hauts plateaux. La région offre une grande variété de paysages : ceux du parc national du Grand Bassin sont protégés. Faune et flore sont présentes, bien que discrètes, surtout autour des lacs dont la salinité généralement assez élevée est due à l'endoréisme. Couvrant quelque 10 % des États-Unis, le Grand Bassin est la région la plus sèche du pays. Géographie La région du Grand Bassin se trouve à l'est de la sierra Nevada et à l'ouest du plateau du Colorado et de la chaîne Wasatch. Elle est bordée au nord par le plateau du Columbia. Le Grand Bassin est constitué de nombreux petits bassins encadrés de rides montagneuses d'altitude généralement comprise entre mais pouvant dépasser . Les bassins sont globalement orientés nord-sud ; leur altitude dépasse souvent . La région est un bassin endoréique. Les précipitations

Dynamics of Turbulence in Low-Speed Oscillating Bottom-Boundary Layers of Stratified Basins

Tobias Jonas, Alfred Johny Wüest

This paper focuses on the impact of an oscillating low-speed current on the structure and dynamics of the bottom-boundary layer (BBL) in a small stratified basin. A set of high-resolution current profile measurements in combination with temperature-microstructure measurements were collected during a complete cycle of the internal oscillation ('seiching') in the BBL of Lake Alpnach, Switzerland. It was found that even a relatively long seiching period of 24 hours significantly changed the form of the near-bottom current profiles as well as the dynamics of the turbulent dissipation rate compared to the steady-state law-of-the-wall. A logarithmic fit to the measured current profiles starting at a distance of 0.5 m above the sediment led to inconsistent estimates of both friction velocity and roughness length. Moreover, a phase lag between the current and the turbulent dissipation of 1.5 hours and a persistent maximum in the current profile at a height of 2.5 to 3 m above the sediment were observed. The experimental findings were compared to the results of a k-epsilon turbulence model and showed good agreement in general. Specifically, the inconsistent logarithmic fitting results and the observed phase lag were reproduced well by the model.

Springer2002

Surface heat flux variability of a large lake: Lake Geneva, Switzerland

David Andrew Barry, Damien Bouffard, Abolfazl Irani Rahaghi, Ulrich Lemmin

The heat budget of a lake is a fundamental component of physical limnology, and is strongly dependent on the surface heat flux. However, the surface energy exchange depends on several factors, making it difficult to estimate. In this study we employed several bulk formulas to estimate Lake Geneva’s surface heat flux. Combination of different surface heat flux terms leads to a surface heat exchange model which requires various data. Different data sources were used in the heat flux estimates. Meteorological data were taken from an operational numerical weather prediction model, namely COSMO-2 (run by the Swiss meteorological service), while satellite imagery was used for the lake surface water temperature (LSWT). In order to find the best combination of the bulk formulas and to calibrate the model, the temporal evolution of the heat budget was estimated using long-term time series of vertical temperature profiles. Vertical temperature profiles at two points (one in the Lake Geneva’s large basin and one in its small basin) were used. A sensitivity analysis was performed to find the key parameters, and more significantly the optimal combination of different heat flux terms. Finally, the spatio-temporal surface heat flux variation was calculated according to the proposed model. In addition, the relationship between variability of the surface heat flux and meteorological forcing was assessed. The different models, which are of differing complexity, gave reasonably consistent results, with differences attributed to simplifications inherent in them. The modeling results revealed that the LSWT and wind forcing are dominant factors underlying Lake Geneva surface heat flux spatial variation, while its temporal variability is mainly due to the global radiation and air temperature changes. In conclusion, the bulk heat balance approach is a useful tool to estimate various heat flux terms as well as their monthly or seasonally contributions. But, in large lakes where the LSWT is highly variable, the variable surface heat flux would be unavoidable.

2014

Surface thermal patterns of Lake Geneva, Switzerland, from 2008 to 2012

David Andrew Barry, Damien Bouffard, Abolfazl Irani Rahaghi, Ulrich Lemmin

In this study we characterize the spatial and temporal variability of the lake surface water temperature (LSWT), lake surface heat fluxes as well as the heat content of Lake Geneva from March 2008 to December 2012. This was accomplished using Advanced Very High Resolution Radiometer (AVHRR) data for the LSWT and an operational numerical weather prediction model, namely COSMO-2, for the meteorological data. Available bulk models for different components of the surface heat flux were cataloged and then combined (using all possible combinations). Each of the assembled models was calibrated to produce the best overall model for the surface heat flux. Calibration was based on the temporal evolution of the heat budget, which was estimated using two long-term time series of vertical temperature profiles (one in the Lake Geneva’s large basin and one in its small basin). Empirical Orthogonal Function (EOF) analysis was used to assess the relationship between the variability of the LSWT and meteorological forcing. The dominant EOF mode, which explains 74% of the observed variance for wind speed, 78% for evaporative heat flux and more than 90% for other parameters, shows uniform patterns associated with the annual cycle. Their temporal amplitude reveal a time lag between total surface heat flux and LSWT variation. On the other hand, some zones are detectable in the spatial patterns of the second and third modes. This analysis indicates a good correlation between the variation of wind forcing and evaporative heat flux in the first three modes.

2015

Surface heat flux variability of a large lake: Lake Geneva, Switzerland

David Andrew Barry, Damien Bouffard, Abolfazl Irani Rahaghi, Ulrich Lemmin

2014