Modelling of the 1991 Greenland Summer with the coupled atmosphere-snow regional climate model MAR

Peer reviewe

Changement climatique : conséquences en Ardenne

Le GIEC (Groupe d'experts Intergouvernemental sur l'évolution du climat) prévoit pour le futur plus de précipitations hivernales et donc à priori un risque accru d'inondations en Belgique. En Ardenne, la majorité des débordements de rivières, telles que l'Ourthe, l'Amblève ou encore la Vesdre, survient en hiver et près de la moitié d'entre eux est due à la combinaison de fortes pluies à une fonte rapide du manteau neigeux. Une reconstitution de l'évolution des précipitations et de l'enneigement en Belgique à l'aide d'un modèle du climat, développé au Laboratoire de Climatologie de l'Université de Liège, montre cependant que les conditions climatiques favorisant les inondations hivernales ont diminué en Ardenne au cours de ces cinquante dernières années

Decrease in hydroclimatic conditions generating floods in the southeast of Belgium over the last 50 years

As a consequence of climate change, several studies concluded that winter flood occurrence could increase in the future in many rivers of northern and western Europe in response to an increase in extreme precipitation events. This study aims to determine if trends in extreme hydroclimatic events generating floods can already be detected over the last century. In particular, we focus on the Ourthe River (southeast of Belgium) which is one of the main tributaries of the Meuse River with a catchment area of 3500 km² . In this river, most of the floods occur during winter and about 50% of them are due to rainfall events associated with the melting of the snow which covers the Ardennes during winter. In this study, hydroclimatic conditions favourable to floods were reconstructed over the period 1959-2010 using the regional climate model MAR (“Modèle Atmosphérique Régional”) forced by the following reanalyses: the ERA-20C, the ERA-Interim and the NCEP/NCAR-v1. The use of the MAR model allows to compute precipitation, snow depth and run-off resulting from precipitation events and snow melting in any part of the Ourthe river catchment area. Therefore, extreme hydroclimatic events, namely extreme run-off events, which could potentially generate floods, can be reconstructed using the MAR model. As validation, the MAR results were compared to weather station-based data. A trend analysis was then performed in order to study the evolution of conditions favourable to flooding in the Ourthe River catchment. The results show that the MAR model allows the detection of about 90% of the hydroclimatic conditions which effectively generated observed floods in the Ourthe River over the period 1974-2010. Whatever the reanalysis used to force the MAR model, the conditions favourable to floods due to snowpack melting combined with rainfall events present a significant negative trend over the last 50 years as a result of a decrease in snow accumulation. However, regarding the conditions favourable to floods due to rainfall events alone, the signal of the trend depends on the reanalysis used to force the model

Circulations atmosphériques et anomalies de fonte à la surface de la calotte glaciaire du Groenland

peer reviewedWith the aim to study the impact of the 500hPa general circulation on the Greenland ice sheet surface melt simulated by the regional climate model MAR, we developed a new Circulation Type Classification (CTC) based on the 500hPa geopotential height from the ECMWF (re)analysis over the period 1958-2007. This CTC shows that the dominant mode of the regional atmospheric variability around the Greenland is linked to the North Atlantic Oscillation (NAO) and that the surface anomalies are highly correlated to the general circulation. It explains also why a record surface melt was observed during the summer 2007. The 27th August of 2003, where the temperature was 10°C higher than the normal, is the consequence of an almost unique 500 hPa circulation in the 50 last years.Pour étudier les impacts de la circulation atmosphérique à 500hPa sur la fonte estivale à la surface de la calotte du Groenland simulée par le modèle du climat MAR, nous avons mis au point une nouvelle classification automatique des types de circulations atmosphériques appliquée à la période qui s'étend de 1958 à 2007. Cette classification, basée sur la hauteur géopotentielle à 500hPa issue des (ré)analyses du Centre Européen, permet de montrer que le mode dominant de la variabilité atmosphérique au Groenland est lié à l'Oscillation Nord-Atlantique (NAO) et que la fonte à la surface de la calotte est hautement corrélée à la circulation générale. Cette classification explique pourquoi une fonte record fut enregistrée durant l'été 2007. De même, le caractère exceptionnel du 27 août 2003, où la température moyenne au Groenland était près de 10°C supérieure à la moyenne 1958-2007, est clairement une conséquence d'une circulation à 500hPa presque unique durant les 50 dernières années de l'histoire climatique du Groenland

Do global warming-induced circulation pattern changes affect temperature and precipitation over Europe during summer?

Future climate change projections are not limited to a simple warming, but changes in precipitation and sea level pressure (SLP) are also projected. The SLP changes and the associated atmospheric circulation changes could directly mitigate or enhance potential projected changes in temperature and precipitation associated with rising temperatures. With the aim of analysing the projected circulation changes and their possible impacts on temperature and precipitation over Europe in summer [June–July–August (JJA)], we apply an automatic circulation type classification method, based on daily SLP, on general circulation model (GCM) outputs from the Coupled Model Intercomparison Project phase 5 (CMIP5) database over the historical period (1951–2005) and for climate under two future scenarios (2006–2100). We focus on summer as it is the season when changes in temperature and precipitation have the highest impact on human health and agriculture. Over the historical observed reference period (1960–1999), our results show that most of the GCMs have significant biases over Europe when compared to reanalysis data sets, both for simulating the observed circulation types and their frequencies, as well as for reproducing the intraclass means of the studied variables. The future projections suggest a decrease of circulation types favouring a low centred over the British Isles for the benefit of more anticyclonic conditions. These circulation changes mitigate the projected precipitation increase over north-western Europe in summer, but they do not significantly affect the projected temperature increase and the precipitation decrease over the Mediterranean region and eastern Europe. However, the circulation changes and the associated precipitation changes are tarnished by a high uncertainty among the GCM projections

Contribution du bilan de masse de surface antarctique à l’évolution du niveau des mers avec le modèle atmosphérique régional MAR

peer reviewe

ENSO forecast using a wavelet-based mode decomposition

We introduce a new method for forecasting major El Niño/ La Niña events based on a wavelet mode decomposition. This methodology allows us to approximate the ENSO time series with a superposition of three periodic signals corresponding to periods of about 31, 43 and 61 months respectively with time-varying amplitudes. This pseudo-periodic approximation is then extrapolated to give forecasts. While this last one only resolves the large variations in the ENSO time series, three years hindcast as retroactive prediction allows to recover most of the El Niño/ La Niña events of the last 60 years

Influences of the environment of a wind farm on the forecasts of its power generation using models GFS (50km/3h) and WRF (2km/15min) : The case study of Amel wind farm (High Belgium)

peer reviewedThe economic and climate contexts require to use more electricity from wind farms. However this kind of production is intermittent, therefore it is necessary to forecast this resource at least 1 day ahead. Our laboratory has developed a forecasting model of wind-based electricity generation based on a global meteorological model (GFS) with a resolution of 50 km and 3 h. But this model has a resolution too coarse for a wind farm. So we have configured the regional model WRF with resolution of 2 km and 15 min to obtain better forecasts. Finally, the WRF model provides better forecasts, but both must be adjusted to take into account the direct environment of the wind farm

Assessing the future evolution of climate extremes favouring floods using the regional climate model MAR over the CORDEX.be domain

In Belgium, most flooding events occur in winter as a result of intense precipitation events but also to the abrupt melting of the snow that covers the Ardennes summits. These conditions favourable to floods exhibit a decreasing trend over the period 1959–2010 resulting from the reduction in snow accumulation thought extreme precipitation events show a positive but non-significant signal. In this study, we investigate how these trends could evolve in a warmer climate by using future projections performed with the regional climate model MAR (for “Modèle Atmosphérique Régional”) in the framework of CORDEX.be, the Belgian EURO-CORDEX project. These future projections were obtained by nesting MAR into NorESM1-M and MIROC5 under the RCP8.5 scenario. Both these global models were selected from the CMIP5 archive after evaluation of their ability to represent the current (1976-2005) mean climate over Europe. This assessment is based on the skill score methodology. Results show that the period 2071-2100 would be marked by a decrease in snowfall amount, in snow accumulation, and consequently in conditions favourable to floods generated by snowpack melting with respect to 1976-2005. Regarding total PPN amount and extremes, the signal is less clear as both GCMs simulate different patterns and trends

Reconstruction of the 1979-2005 Greenland ice sheet surface mass balance using satellite data and the regional climate model MAR

In order to improve our knowledge on the current state and variability of the Greenland ice sheet surface mass balance (SMB), a 27-year simulation (1979-2005) has been performed with the coupled atmosphere-snow regional model MAR. This simulation reveals an increase in the main factors of the SMB which are, on the one hand, the snowfall (+ 1.6 ± 1.8 km3 yr-1) in winter and on the other hand, the run-off (+ 4.2 ± 1.9 km3 yr-1) in summer. The net effect of these two competing factors leads to a SMB loss rate of – 2.7 ± 3.0 km3 yr-1, which has a significance of 87%. The melt extent derived from the passive microwave satellite data since 1979 also shows this trend. The melt water supply has increased because the Greenland ice sheet has been warming up by + 0.09 ± 0.04 °C yr-1 since 1979. This warming comes from a uniform increase of downward infra-red radiation which can not be explained by the natural variability. These changes result very likely from the global warming induced by human activities. As a result, it seems that: i) increased melting dominates over increased accumulation in a warming scenario, ii) the Greenland ice sheet has been significantly losing mass since the beginning of the 1980's by an increasing melt water run-off as well as by a probable increase of iceberg discharge into the ocean due to the "Zwally effect" (the melt water-induced ice sheet flow acceleration) and iii) the Greenland ice sheet is projected to continue to lose mass in the future. The Greenland ice sheet melting could have an effect on the stability of the thermohaline circulation (THC) and the global sea level rise. On the one hand, increases in the freshwater flux from the Greenland ice sheet (glacier discharge and run-off) could perturb the THC by reducing the density contrast driving it. On the other hand, the melting of the whole Greenland ice sheet would account for a global mean sea level rise of 7.4 m.Afin d'améliorer notre connaissance de l'état actuel du bilan de masse de la calotte du Groenland et de sa variabilité inter-annuelle, nous avons simulé le climat du Groenland de 1979 à 2005 à l'aide du modèle atmosphérique régional MAR couplé à un modèle de neige. Cette simulation révèle, d'une part, une augmentation des chutes de neige (+ 1.6 ± 1.8 km3 an-1) en hiver résultant de l'augmentation de la capacité de stockage d'eau dans l'atmosphère suite au réchauffement de celle-ci et d'autre part, une accélération de la fonte (+ 4.2 ± 1.9 km3 an-1) en été. L'effet net de ces deux augmentations sur le bilan de masse en surface est un taux de perte de – 2.7 ± 3.0 km3 par an avec une significance de 87%. L'étendue de fonte détectée par les satellites micro-onde depuis 1979 affiche aussi une tendance à l'augmentation de la fonte. La fonte en surface a augmenté parce que la calotte s'est réchauffée de + 0.09 ± 0.04 °C an-1 depuis 1979. Ce réchauffement, non expliqué par la variabilité naturelle, est dû à une augmentation uniforme sur le Groenland du rayonnement infra-rouge descendant. Ces changements résultent très probablement du réchauffement global induites par les activités humaines. En conséquence, il semble, i) que la fonte estivale des neiges/glaces serait suffisamment importante pour l'emporter sur l'accumulation nivale en hiver dans un scénario climatique plus chaud, ii) que la calotte du Groenland est entrain de perdre de la masse significativement depuis les années 1980 à cause de l'augmentation de la fonte en surface ainsi que de l'augmentation très probable de la décharge d'iceberg dans l'océan à cause de l'effet Zwally (accélération de la vitesse d'écoulement des glaciers à cause de l'augmentation de l'eau de fonte qui lubrifie l'interface socle rocheux/base du glacier) et iii) que la calotte du Groenland est projetée à continuer à perdre de la masse dans le futur. La fonte, même partielle, de la calotte groenlandaise élèverait le niveau des mers de quelques cm voire de plusieurs dizaines de cm, avec les conséquences que l'on connaît pour des pays comme les Pays-Bas ou même le nôtre. De même, un important apport d'eau de fonte (eau douce) en Mer du Groenland et du Labrador aurait tendance, en stabilisant ces eaux, à freiner voire stopper à long terme, les descendes (appelées convection profonde) d'eau froide et salée (plus dense), que l'on observe dans ces régions; la convection profonde étant véritablement le moteur de la circulation océanique en Atlantique et donc notamment du Gulf Stream qui tempère notre climat