Glace de mer

Résumé

thumb|upright=1.25|Fragments épars de glace de mer recouverts de neige. thumb|upright=1.25|Lame fine de glace de mer, observée sous une lumière à polarisation croisée. Tous les cristaux (que l'on distingue grâce aux couleurs d'interférence différentes) contiennent des inclusions de saumure (solution saline) et d'air ; inclusions situées dans le plan cristallographique. La glace de mer est constituée d'eau de mer gelée, et parfois aussi de neige tassée ou d'eau douce gelée (l'eau de mer gèle à une température plus basse - environ -2°C - que l'eau douce). C'est un composite complexe principalement formé de glace pure dans divers états de cristallisation, de bulles d'air et de poches de saumure incluses. Étant moins dense que l'eau, elle flotte à la surface de l'océan (tout comme la glace d'eau douce, qui a une densité encore plus faible). Selon son âge et son épaisseur, elle sera plus ou moins dense, et perdra plus ou moins de sel. Enjeux Comprendre ses processus de formati

Source officielle

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

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Modelling pumped-storage effects on thermal structure, ice cover and water quality of lakes and reservoirs in a changing climate

Ulrike Kobler

New renewable electricity is nowadays often generated by photovoltaics and wind. Yet, their intermittent nature calls for energy storage, which is today still provided to ~95% by pumped-storage (PS) hydropower plants. However, PS is known to affect abiotic and biotic characteristics of the two connected water bodies. Thus, a two-dimensional laterally-averaged hydrodynamic and water quality model was set up to assess the impacts on water quality and temperature in a first step. Then, this analysis was extended to evaluate the additional effect of climate change, which also modifies abiotic and biotic characteristics. For this purpose, 150-years long synthetic time series of meteorological conditions for current (1998-2012) and future climate (2078-2092) were generated with a weather generator. To assess the robustness of projected impacts on the ice cover of the upper reservoir (Sihlsee) three one-dimensional, vertical hydrodynamic models were additionally set up. To attribute effects to either the PS flows or to water withdrawal from the hypolimnion, two reference scenarios were defined: one with deep-water withdrawal (NoPS) and another one with surface outflow (QNat). While the hypolimnetic temperature differs by

EPFL2019

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With the Arctic rapidly changing, the needs to observe, understand, and model the changes are essential. To support these needs, an annual cycle of observations of atmospheric properties, processes, and interactions were made while drifting with the sea ice across the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition from October 2019 to September 2020. An international team designed and implemented the comprehensive program to document and characterize all aspects of the Arctic atmospheric system in unprecedented detail, using a variety of approaches, and across multiple scales. These measurements were coordinated with other observational teams to explore cross-cutting and coupled interactions with the Arctic Ocean, sea ice, and ecosystem through a variety of physical and biogeochemical processes. This overview outlines the breadth and complexity of the atmospheric research program, which was organized into 4 subgroups: atmospheric state, clouds and precipitation, gases and aerosols, and energy budgets. Atmospheric variability over the annual cycle revealed important influences from a persistent large-scale winter circulation pattern, leading to some storms with pressure and winds that were outside the interquartile range of past conditions suggested by long-term reanalysis. Similarly, the MOSAiC location was warmer and wetter in summer than the reanalysis climatology, in part due to its close proximity to the sea ice edge. The comprehensiveness of the observational program for characterizing and analyzing atmospheric phenomena is demonstrated via a winter case study examining air mass transitions and a summer case study examining vertical atmospheric evolution. Overall, the MOSAiC atmospheric program successfully met its objectives and was the most comprehensive atmospheric measurement program to date conducted over the Arctic sea ice. The obtained data will support a broad range of coupled-system scientific research and provide an important foundation for advancing multiscale modeling capabilities in the Arctic.

2022

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Ensemble modelling of ice cover for a reservoir affected by pumped-storage operation and climate change

Ulrike Kobler, Martin Schmid

Ensemble modelling was used to assess the robustness of projected impacts of pumped-storage (PS) operation and climate change on reservoir ice cover. To this end, three one-dimensional and a two-dimensional laterally averaged hydrodynamic model were set up. For the latter, the strength of the impacts with increasing distance from the dam was also investigated. Climate change effects were simulated by forcing the models with 150 years of synthetic meteorological time series created with a weather generator based on available air temperature scenarios for Switzerland. Future climate by the end of the 21(st) century was projected to shorten the ice-covered period by similar to 2 months and decrease ice thicknesses by similar to 13 cm. Under current climate conditions, the ice cover would already be affected by extended PS operation. For example, the average probability of ice coverage on a specific day was projected to decrease by similar to 13% for current climate and could further be reduced from similar to 45% to similar to 10% for future climate. Overall, the results of all models were consistent. Although the number of winters without ice cover was projected to increase for all one-dimensional models, studying individual segments of the two-dimensional model showed that the impact was pronounced for segments close to the PS intake/outlet. In summary, the reservoir's ice cover is expected to partially vanish with higher probability of open water conditions closer to the PS intake/outlet.

WILEY2019

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2022