Satellite météorologique

Résumé

vignette|Satellite Météosat de première génération. Un satellite météorologique est un satellite artificiel qui a comme mission principale le recueil de données utilisées pour la surveillance du temps et du climat de la Terre. Chaque nouvelle génération de satellite comporte des senseurs plus performants et capables d'effectuer des mesures sur un plus grand nombre de canaux ce qui permet de les utiliser pour différencier les divers phénomènes météorologiques : nuages, précipitations, vents, brouillard, etc. Plusieurs pays lancent et maintiennent des réseaux de satellites météorologiques : les États-Unis, l'Europe par l'Agence spatiale européenne (ESA), l'Inde, la Chine, la Russie et le Japon. Tous ces satellites assurent une couverture globale de l'atmosphère. Historique Le premier satellite météorologique, est le Vanguard 2, lancé le pour mesurer la couverture nuageuse. Mais, lors de sa mise en orbite, son axe de rotation est mal orienté et il ne peut donner que peu d'in

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Generative adversarial networks (GANs) have been recently adopted for super-resolution, an application closely related to what is referred to as "downscaling'' in the atmospheric sciences: improving the spatial resolution of low-resolution images. The ability of conditional GANs to generate an ensemble of solutions for a given input lends itself naturally to stochastic downscaling, but the stochastic nature of GANs is not usually considered in super-resolution applications. Here, we introduce a recurrent, stochastic super-resolution GAN that can generate ensembles of time-evolving high-resolution atmospheric fields for an input consisting of a low-resolution sequence of images of the same field. We test the GAN using two data sets: one consisting of radar-measured precipitation from Switzerland; the other of cloud optical thickness derived from the Geostationary Earth Observing Satellite 16 (GOES-16). We find that the GAN can generate realistic, temporally consistent super-resolution sequences for both data sets. The statistical properties of the generated ensemble are analyzed using rank statistics, a method adapted from ensemble weather forecasting; these analyses indicate that the GAN produces close to the correct amount of variability in its outputs. As the GAN generator is fully convolutional, it can be applied after training to input images larger than the images used to train it. It is also able to generate time series much longer than the training sequences, as demonstrated by applying the generator to a three-month data set of the precipitation radar data. The source code to our GAN is available at https://github.com/jleinonen/downscaling-rnn-gan.

2020

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As in oceans, large-scale coherent circulations such as gyres and eddies are ubiquitous features in large lakes that are subject to the Coriolis force. They play a crucial role in the horizontal and vertical distribution of biological, chemical and physical parameters that can affect water quality. In order to make coherent circulation patterns evident, representative field measurements of near-surface currents have to be taken. This, unfortunately, is difficult due to the high spatial and temporal variability of gyres/eddies. As a result, few complete field observations of coherent circulation in oceans/lakes have been reported. With the advent of high-resolution satellite imagery, the potential to unravel and improve the understanding of mesoscale and submesoscale processes has substantially increased. Features in the satellite images, however, must be verified by field measurements and numerical simulations. In the present study, Sentinel-1 SAR satellite imagery was used to detect gyres/eddies in a large lake (Lake Geneva). Comparing SAR images with realistic high-resolution numerical model results and in situ observations allowed for identification of distinct signatures of mesoscale gyres, which can be revealed through submesoscale current patterns. Under low wind conditions, cyclonic gyres manifest themselves in SAR images either through biogenic slicks that are entrained in submesoscale and mesoscale currents, or by pelagic upwelling that appears as smooth, dark elliptical areas in their centers. This unique combination of simultaneous SAR imagery, three-dimensional numerical simulations and field observations confirmed that SAR imagery can provide valuable insights into the spatial scales of thus far unresolved mesoscale and submesoscale processes in a lake. Understanding these processes is required for developing effective lake management concepts.

2022

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Climate change and its influence on the planning and operation of electric power systems

Raphaël Barben, Edgard Gnansounou

The IPCC Report (Intergovernmental Panel on Climate Change 2007) raises serious concerns about the increase of extreme weather events (storms, rains, snowfalls). Over the past 100 years the average global temperature has risen by around 0.75°C and the sea level increases by 3 millimetres a year. There is a consensus on the impact of this climate change phenomena on the planning and operation of the electric power systems: winds are growing in strength, resulting in a greater frequency of operations interruption (falling trees, avalanches, etc.). Furthermore, due to the increasing incidence of heavy precipitation, lines and substations are often flooded, damaged or destroyed. Repairing the resultant damage is often very time-consuming. And also the temperature of permafrost layers is rising. As a result the bedrock is becoming loose. This in turn increases the risk of landslides, which also damage and destroy lines and substations. There are many cases of extreme weather which have caused enormous damage to electric power systems, e.g. the ice storm in Canada (1998) or the hurricane "Lothar" in Western Europe (1999). Considering the climate change at the planning of electric power systems is therefore highly relevant. In order to plan the power system we need to evaluate the risk of outages due to extreme weather. The vulnerability is assessed, based on historic meteorological data. It results in geographic and seasonal exposure maps. E.g. in Switzerland, alpine regions are more prone to extreme conditions which can vary from one valley to the other. In this paper, the planning methodology uses scenarios to assess the uncertainty related to the vulnerability of the power system under extreme weather conditions. These scenarios are based on weather-related contingencies and reference cases of the Swiss transmission system. The climate change has also to be considered at the operation of electric power systems. At the control centres new operating tools have to be introduced which take into account the impact of the climate change, e.g. an early warning weather system, or a Geographic Information System (GIS) which pinpoints (together with information of protection systems), the location of a damaged line so that the repair staff can be on the spot very quickly, thereby minimising downtime. Furthermore the dispatchers must be trained on how to manage their electric power system during extreme weather conditions. Finally experienced crisis organisations must be established.

2008

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