Hiver

Résumé

vignette|Paysage d'un jour d'hiver enneigé au bois de la Cambre à Bruxelles, en 2017. vignette|Paysage d'un jour d'hiver enneigé à Neulingen (Bade-Wurtemberg, Allemagne). thumb|L'hiver en Finlande. Lhiver est l’une des quatre saisons de l’année dans les zones tempérées et polaires de la Terre. L'hiver suit l'automne et précède le printemps. Il existe plusieurs définitions de l'hiver : astronomique (saison comprenant les jours les plus courts de l'année), météorologique (saison comprenant les mois les plus froids de l'année), et calendaire (dont les dates varient selon les pays). Dans l’hémisphère nord, l'hiver se situe entre le dernier trimestre de l'année et le premier de l'année suivante. Dans l'hémisphère sud comme en Australie, Madagascar ou l'île de La Réunion, il se situe entre les deuxième et troisième trimestres de l'année. Quelle que soit la définition, c'est la période du repos des plantes et de l'hibernation des animaux. La distance entre le Soleil et la Terre la plus petit

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https://fr.wikipedia.org/wiki/Hiver

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This master thesis provides an extended study of winter precipitations in Valais (Switzerland). It was rst tried to relate local conditions in Valais, as measured at gauges, to the Hess-Brezowski classication for Europe. It was not possible to clearly relate this classication to local precipitation events in Winter in this region. Self-organizing maps were used to try to distinguish between sub-regions only based on the precipitation regime in Valais. The obtained results present coherence with the existing literature. ERA-Interim (global atmospheric reanalysis) data were then used to develop a new classication to link European meteorological conditions to local events. Three unsupervised clustering techniques were used for this purpose: k-means, k-medoids, hierachical clustering. The optimal number of cluster was determined using a com- bination four commonly used internal clusters indices (Dunn index, Davies-Bouldin index, silhouette index and Calinski-Harabasz index). Two clusters appeared to be optimal compared to the various parameters of the problem. The outputs of the three classication techniques were compared in term of robustness and results. A physical interpretation was then made. It was found that the two classes were associated respectively with small or large precipitation events. The associated synoptic conditions at the European scale reveal that the small precipitation class is associated with a North-East ux over Switzerland whereas the large precipitation class is linked to a South-West ux linked to a stationary front over Switzerland.

2017

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Water quality in lakes is closely linked to hydrodynamics and often dominated by stratification, limiting the exchange between the epi- and hypolimnion. Thus, the vertical redistribution of biogeochemical tracers such as dissolved oxygen and nutrients by convective overturning during winter is an important process in annual lake cycles. In deep, monomictic lakes convective cooling often does not reach the deepest layers. Studies have shown that climate change will further reduce the efficiency of this process, motivating a good understanding of alternative deepwater renewal mechanisms.Combining field observations, three-dimensional (3D) hydrodynamic modeling, and particle tracking, this thesis explores wind-induced transport processes contributing to vertical exchange in Lake Geneva during winter. A first focus is on Ekman-type coastal upwelling at the northern shore. Then, wind-induced interbasin exchange and hypolimnetic upwelling between the Petit Lac (depth 75 m) and Grand Lac (depth 309 m) basins of Lake Geneva are investigated. Finally, the effect of stratification in the Petit Lac on the latter phenomenon is revealed by model-based momentum budget analysis.Coastal upwelling at the northern shore of Lake Geneva was investigated in the winter 2017/2018. Following a strong alongshore wind, nearshore temperatures decreased and remained low for 10 days, with the lowest temperatures corresponding to values found at 200-m depth. Current measurements at 30-m depth showed dominant alongshore currents with episodic upslope transport of cold hypolimnetic water in the lowest 10 m. Model results suggest that upwelled waters spread over ~10% of the Grand Lac's surface area. Furthermore, particle tracking confirmed that upwelled waters originated from far below the thermocline and descended back to these layers after the wind ceased.Exchange and upwelling between the lake's two basins was investigated in the winter 2018/2019. Following a strong along-axis wind, a two-layer flow established, with the downwind surface drift into the Grand Lac being balanced by opposing bottom currents into the Petit Lac and the current reversal occurring around the thermocline depth. Bottom temperatures at the confluence gradually decreased to values found in the deep Grand Lac hypolimnion. Furthermore, particle tracking revealed a current loop, whereby hypolimnetic water from below 150-m depth first upwelled into the Petit Lac, intruding ~10 km, i.e., half its length, and then descended back into the Grand Lac hypolimnion. Model results indicated a temporary doubling of the Petit Lac hypolimnetic volume during the upwelling.The observed interbasin upwelling occurred only during early winter, with the bottom inflow of deep hypolimnetic water driven by baroclinic pressure gradients at the confluence, produced by upwind upwelling at the western end of the Grand Lac. Once the Petit Lac was fully mixed, the wind-induced net volume exchange between the basins was reduced by > 50% and deep hypolimnetic interbasin upwelling was suppressed altogether.This thesis shows that wind-induced coastal upwelling and hypolimnetic interbasin exchange are complex 3D processes, regularly occurring during winter and providing important mechanisms for deepwater renewal in Lake Geneva. They cannot be represented by one-dimensional models frequently employed to predict climate change induced shifts and merit further attention when assessing deepwater renewal in large, deep lakes.

EPFL2021

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Spatial dispersion of photovoltaic panels in Switzerland and effect on electricity production

Sonia Joëlle Dupuis

This semester project looked at the effect of snow cover on productivity of solar panels in alpine areas. Snow is highly reflective and increases the irradiance that reaches PV panels from the ground, notably in winter when demand is high and production is low. To illustrate this effect on a concrete example this study computes the hourly production profile of two very different installation scenarios for PV panels in Switzerland over three years; an urban one and an alpine one. Our results show that in regions with extended periods of snow cover the winter production can be 10% higher without sacrifices on the annual total production. This helps significantly in reducing the energy gap mentioned above; annual required import can be lowered by 10%-20% and forced export due to overproduction during summer months reduces to less than half.

2017

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EPFL2021