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

Evolution of the snow height in the Alps over the 20th century using the regional atmospheric model MAR

peer reviewedL’évolution de l’enneigement dans les Alpes peut fortement affecter le tourisme, mais aussi la disponibilité en eau de la région. Dans cette étude, nous avons reproduit l’évolution du climat des Alpes au cours du 20e siècle à l’aide du modèle atmosphérique régional MAR forcé par trois réanalyses (ERA-20C, NCEP/NCAR et ERA-Interim). Le MAR montre que la hauteur de neige a augmenté depuis le début du 20e siècle, d’abord uniquement en haute altitude, puis également aux altitudes inférieures, avant de connaitre une forte et brusque diminution entre 1985 et 1990. Cette évolution, qui est en accord avec les observations décrites dans la littérature, est directement liée aux fluctuations de la NAO et de l’AO. En effet, les changements de circulation atmosphérique que traduisent la NAO et l’AO entrainent des variations de température et de précipitations qui déterminent directement la hauteur de neige dans les Alpes.The evolution of the snow height over the Alps can strongly impact tourism, but also the water availability of the region. In this study, we have reproduced the evolution of the climate in the Alps over the 20th century with the help of the regional atmospheric model MAR forced by three reanalyses (ERA-20C, NCEP/NCAR, and ERA-Interim). MAR shows that the snow height has increased since the beginning of the 20th century, first only at higher altitudes, then also at lower levels, before knowing a strong and abrupt decrease between 1985 and 1990. This evolution, which is consistent with observations given in the literature, is directly linked with the trends of NAO and AO. In fact, the atmospheric circulation changes highlighted by NAO and AO induce temperature and precipitation changes that directly determine the snow height in the Alps

Estimation of the Sea Level Rise by 2100 Resulting from Changes in the Surface Mass Balance of the Greenland Ice Sheet

Medical imagin

Evaluation of the present and future general circulation over western Europe simulated by the IPCC AR5/CMIP5 GCMs with the help of a circulation type classification

Downscaling methods forced by General Circulation Model (GCM) simulations are not able to correct the biases in the general circulation simulated by the GCMs. Moreover, since the GCMs have a coarse spatial resolution, they have difficulties to simulate reliably ground variables like temperature and precipitation which are affected by topography, land use and local features. So, we can attempt that they simulate better the large-scale atmospheric circulation. That is why it is of special interest to evaluate the GCM simulations of atmospheric circulation for current climate by comparing them with the NCEP-NCAR 1 and the ECMWF reanalysis data over 1961-1990. This analysis is done over western Europe for summer (JJA) and winter (DJF) for the GCMs (available on http://cmippcmdi.llnl.gov/cmip5/) proposed by the IPCC for its upcoming report (AR5). The method used is an automated circulation type classification based on the daily geopotential height at 500 hPa. It is a leader-algorithm correlation based method taking part of the COST733CAT classification catalogue. Unlike the usually used methods based on the monthly mean circulation, this approach allows a precise analysis of each circulation type. So, it gives much more information on the ability of the GCMs to simulate the different circulation types and consequently the climatic variability of a region. In order to allow a direct comparison between the GCM simulations and the reanalysis data, the classification is done first only for the reanalysis dataset over 1961-1990. Then, the main types individualised here are imposed for the classification of the GCM outputs. Since the circulation types are the same, the comparison between the datasets can be made on the basis of the differences of the frequency distribution throughout the classes. Moreover, the mean intraclass repartition of the circulation situations may differ from one dataset to another. So, the study of this mean and its standard deviation gives an idea of the differences between the reanalysis and the GCMs within each class. Firstly, this approach is applied to current climate (1961-1990) for evaluating the ability of the GCMs driven by the historical experiment to simulate the climate of the last decades over western Europe. In fact, if one GCM is not able to reproduce reliably the main characteristics of the current climate, its future projections may be questionable. Then, the best matching GCMs are retained and the same approach is applied to the future simulations driven by RCP concentrations or emissions (2011-2040, 2041-2070 and 2071-2100). So, the evolution of the frequency of the circulation types and maybe the appearance of new types can be analysed under climate change conditions. Moreover, it is interesting to compare the uncertainty of the current climate simulations to the projected changes for future climate. If the uncertainty is of the same order or higher than the projected changes, the reliability of the simulations for future climate may be very questionable

Analyse de la circulation atmosphérique simulée par les GCM au Groenland

The variability of the geopotential height at 500 hPa simulated by General Circulation Models (GCMs) over Greenland is evaluated using an atmospheric circulation type classification. The GCM outputs for the current climate (20C3M) are first compared to reanalysis data over 1961-1990. The comparison shows that most of them simulate well the main circulation types but fail to reproduce their frequencies because of underestimations of circulation variability and biases in the mean geopotential height. GCM-based future projections do not individualise new circulation types but show a general increase of the geopotential height. Based on this approach, the correlation between surface temperature and atmospheric circulation offers a new way for estimating the Greenland ice sheet melt

Evaluation of model “Modèle Atmosphérique Régional” (MAR) capacity to simulate rainfall season in Intertropical Africa

peer reviewedEn Afrique intertropicale, le climat est essentiellement caractérisé par les quantités de précipitations et leur régime annuel. Ces précipitations et leur évolution au cours de la période 1970-1999 ont été modélisées à l’aide du Modèle Atmosphérique Régional (MAR), développé à l’ULg, en le forçant par les réanalyses NCEP1 ainsi que par les sorties de trois modèles globaux (GCM) de la base de données CMIP5. Ces simulations ont ensuite été comparées aux observations maillées du Climate Research Unit (CRU). Il ressort de nos investigations que la simulation du modèle MAR forcé par les réanalyses NCEP1 parvient à mieux reproduire les lames d’eau et leur régime annuel dans les régions semi-arides qu'en régions équatoriales. En revanche, les simulations du MAR forcé par les sorties des GCM sont peu voire très peu satisfaisantes sur l'ensemble du domaine intertropical tant au niveau des quantités que de la saisonnalité des précipitations.In Intertropical Africa, climate is essentially characterized by the amount of precipitation and its annual regime. These precipitations and their evolution during the period 1970-1999 are simulated thanks to the Regional Atmospheric Model (MAR), developed at the ULg, and forced by the NCEP1 reanalyses and by the outputs of three global models (GCM) of the CMIP5 database. These MAR simulations are compared to the gridded data of the Climate Research Unit (CRU). It is clear from our investigations that the simulation of the MAR model forced by the NCEP1 reanalyses is better reproducing the quantities as well as the annual rainfall regime in the semi-arid regions than in equatorial regions. On the other hand, simulations of the MAR forced by the outputs of the GCMs are globally unsatisfactory throughout the intertropical domain in terms of quantities as well as the seasonality of precipitation.AFRIFOR

How uncertain are precipitation and peak flow estimates for the July 2021 flooding event?

The disastrous July 2021 flooding event made us question the ability of current hydrometeorological tools in providing timely and reliable flood forecasts for unprecedented events. This is an urgent concern since extreme events are increasing due to global warming, and existing methods are usually limited to more frequently observed events with the usual flood generation processes. For the July 2021 event, we simulated the hourly streamflows of seven catchments located in western Germany by combining seven partly polarimetric, radar-based quantitative precipitation estimates (QPEs) with two hydrological models: a conceptual lumped model (GR4H) and a physically based, 3D distributed model (ParFlowCLM). GR4H parameters were calibrated with an emphasis on high flows using historical discharge observations, whereas ParFlowCLM parameters were estimated based on landscape and soil properties. The key results are as follows. (1) With no correction of the vertical profiles of radar variables, radar-based QPE products underestimated the total precipitation depth relative to rain gauges due to intense collision–coalescence processes near the surface, i.e., below the height levels monitored by the radars. (2) Correcting the vertical profiles of radar variables led to substantial improvements. (3) The probability of exceeding the highest measured peak flow before July 2021 was highly impacted by the QPE product, and this impact depended on the catchment for both models. (4) The estimation of model parameters had a larger impact than the choice of QPE product, but simulated peak flows of ParFlowCLM agreed with those of GR4H for five of the seven catchments. This study highlights the need for the correction of vertical profiles of reflectivity and other polarimetric variables near the surface to improve radar-based QPEs for extreme flooding events. It also underlines the large uncertainty in peak flow estimates due to model parameter estimation.</p

Detection of past and future atmospheric circulation changes over the North Atlantic region with the help of an automatic circulation type classification

Future projections of the atmospheric circulation over the Northern Hemisphere high latitudes, especially the North Atlantic, have high uncertainties and some of the projected changes are opposed to circulation changes that have been observed since the 2000s. In this thesis, we focus on three particular aspects of the past and projected future summertime atmospheric circulation over the broader North Atlantic region. First, we analyse whether the 2007-2012 summertime anticyclonic anomaly over the Beaufort Sea, the Canadian Arctic Archipelago, and Greenland might rather be due to global warming or to the internal variability of the atmospheric circulation by putting it in perspective with the circulation variability over the last 150 years given by five reanalysis datasets. Then, this analysis is extended for the future circulation projected towards 2100 by CMIP3 and CMIP5 General Circulation Models (GCMs) over Greenland. Finally, we evaluate the impact of the uncertainties of the future atmospheric circulation projections on the mitigating or enhancing influence of the summertime circulation changes on temperature and precipitation over Europe. We use an automatic circulation type classification to analyse in detail the atmospheric circulation changes by grouping similar daily SLP (mean sea level pressure) or Z500 (500 hPa geopotential height) fields into homogeneous circulation types. It appears that the choice of the index, on the basis of which the days are grouped together, strongly influences the characteristics of the circulation types and the kinds of circulation changes that can be detected. In comparison with Euclidean distance and pressure gradient-based indices, correlation-based indices, especially the Spearman rank correlation, are the most suitable indices when focusing on the circulation pattern. Over the Arctic region, four periods with circulation anomalies similar to that of 2007-2012 (i.e. a summertime anticyclonic anomaly over the western Arctic region) have been detected over the last 150 years, despite a higher uncertainty of the circulation given by the reanalyses due to the scarcity of observational data before 1940. Nevertheless, the 2007-2012 anomaly appears to be exceptional and several connexions with other variables, such as the North Atlantic Oscillation index and sea ice loss, suggest that it could be part of a major climatic anomaly extending beyond the Arctic region. However, the occurrence of similar periods in the past and the influence of several external and internal forcings do not allow us to attribute it to global warming. The future summertime atmospheric circulation projected by GCMs over Greenland confirms this conclusion. In fact, no significant circulation pattern changes are simulated towards 2100, besides a generalised Z500 increase caused by the projected warming. Since GCMs are able to simulate atmospheric circulation changes over other regions and since the atmospheric circulation itself is influenced by other variables, such as sea ice or snow extent, which are already impacted by long-term changes, we conclude that the 2007-2012 anomaly could rather be attributed to the internal variability of the climatic system. Finally, we evidence that projected future atmospheric circulation changes impact on the SLP and precipitation changes simulated over Europe towards 2100 for summer. Over north-western Europe, these circulation changes could mitigate the SLP decrease by around 50 % and cancel out the precipitation increase. Nevertheless, high uncertainties among the GCMs on the magnitude and even on the sign of these changes cast doubt on the reliability of these projections. On the other hand, future atmospheric circulation changes are not projected to affect significantly the warming and drying simulated for the next decades over the Mediterranean region and eastern Europe.Les projections futures de la circulation atmosphérique pour les hautes latitudes de l'hémisphère nord, en particulier l'Atlantique Nord, sont entachées d'une grande incertitude. Certains changements projetés sont opposés aux changements de circulation observés depuis les années 2000. Dans cette thèse, nous nous focalisons sur trois aspects de la circulation estivale, à la fois passée et projetée dans le futur, pour l'Atlantique Nord. D'abord, nous tentons de déterminer si l'anomalie anticyclonique estivale de 2007-2012 au-dessus de la mer de Beaufort, de l'archipel arctique canadien et du Groenland pourrait être attribuée au réchauffement climatique ou plutôt à la variabilité interne de la circulation atmosphérique. Pour cela, nous la comparons à la variabilité de la circulation des 150 dernières années donnée par cinq réanalyses. Ensuite, cette analyse est étendue à la circulation future simulée d'ici 2100 par les Modèles de Circulation Générale (GCM) pour le Groenland. Enfin, nous évaluons l'impact des incertitudes des projections futures de la circulation atmosphérique sur l'effet atténuateur ou amplificateur des changements de circulation sur la température et les précipitations en été en Europe. Nous utilisons une classification de types de circulations automatique pour analyser en détail les changements de circulation atmosphérique en groupant les champs journaliers de SLP (pression réduite au niveau de la mer) ou de Z500 (hauteur géopotentielle à 500 hPa) semblables en types de circulations homogènes. Il apparaît que le choix de l'indice, sur base duquel les jours sont regroupés, influence fortement les caractéristiques des types de circulations, ainsi que les différents changements de circulation qui peuvent être détectés. Les indices basés sur la corrélation, en particulier la corrélation des rangs de Spearman, sont les plus appropriés pour étudier la localisation des centres de pression, par rapport aux indices basés sur la distance euclidienne ou sur le gradient de pression. Pour l'Arctique, nous avons détecté, sur les 150 dernières années, quatre périodes présentant des anomalies de circulation semblables à celle de 2007-2012 (à savoir une anomalie anticyclonique sur l'ouest de l'Arctique en été), alors que la circulation représentée par les réanalyses est entachée d'une plus grande incertitude due à la rareté des données d'observations avant 1940. Toutefois, l'anomalie de 2007-2012 se révèle être exceptionnelle. En outre, plusieurs connexions avec d'autres variables, comme l'oscillation nord-atlantique et la diminution de la banquise, permettent de supposer que cette anomalie fait partie d'une anomalie climatique majeure s'étendant au-delà de l'Arctique. Cependant, l'existence de périodes similaires dans le passé et l'influence de nombreux forçages internes et externes ne nous permettent pas d'attribuer l'anomalie de 2007-2012 au réchauffement climatique. Cette conclusion est confirmée par la circulation atmosphérique future projetée par les GCM pour le Groenland en été. En effet, aucun changement significatif de types de circulations n'est simulé d'ici 2100, excepté une hausse généralisée de Z500 due au réchauffement projeté. Vu que les GCM sont capables de simuler des changements de circulation pour d'autres régions et que la circulation atmosphérique est elle-même influencée par des variables qui subissent déjà des changements à long terme, telles que l'étendue de la banquise et de la couverture neigeuse, nous concluons que l'anomalie de 2007-2012 en Arctique pourrait être attribuée à la variabilité interne du système climatique. Finalement, nous montrons que les changements de circulation atmosphérique projetés pour le futur influencent les changements de SLP et de précipitations simulés pour l'Europe en été, d'ici 2100. Au nord-ouest de l'Europe, ces changements de circulation pourraient atténuer la diminution de SLP de 50 % et annuler l'augmentation des précipitations. Néanmoins, de grandes incertitudes entre les GCM sur l'ampleur et même sur le signe de ces changements jettent le doute sur la fiabilité de ces projections. Par ailleurs, les changements de circulation atmosphérique projetés pour le futur ne devraient pas affecter significativement le réchauffement et la diminution des précipitations simulés pour les prochaines décennies pour la région méditerranéenne et l'est de l'Europe

Evaluation of the present and future general circulation over Greenland simulated by the IPCC AR5/CMIP5 GCMs with the help of a circulation type classification

Future projections of the Greenland ice sheet melt are based on General Circulation Model (GCM) simulations. In particular, the reliability of downscaling methods forced by these simulations depends on the quality of the atmospheric circulation simulated by GCMs. Therefore, it is essential to analyse and evaluate the GCMs modelled general circulation for current climate (1961-1990). Atmospheric circulation type classifications offer a very interesting approach for evaluating the GCM-based circulation at a daily time scale compared to the most used methods based only on monthly means. Indeed, the circulation type classification allows a precise and detailed analysis of each circulation type and so, it gives much more information on the ability of GCMs to simulate the different circulation types and consequently the climatic variability of a region. In fact, exceptional circulation events over Greenland, which cannot be taken into account by the monthly mean approach, have much more impact on the melt than the mean atmospheric state. Thus, an automated correlation-based atmospheric circulation type classification (CTC) is used for evaluating the new GCM outputs (available on http://cmip-pcmdi.llnl.gov/cmip5/) computed for the upcoming IPCC report (AR5). The daily geopotential height at 500 hPa simulations of the GCMs for current climate are compared to the NCEP-NCAR 1 and the ECMWF reanalysis data for the summer months (JJA), when melt is the most important. To achieve this, the classification is first done for the reanalysis data over 1961-1990 and afterwards, the types of the reanalysis based CTC are imposed for classifying the GCM datasets over 1961-1990 (from the historical experiment) to allow a direct type per type comparison based on the frequency distribution of each dataset. This approach also gives the opportunity to study the intraclass repartition differences between the reanalysis and the GCMs. After the evaluation of the GCM simulations for current climate, the future projections driven by RCP concentrations or emissions (2011-2040, 2041-2070 and 2071-2100) from the best matching GCMs are analysed in the same way. For current climate, it clearly appears that only a few GCMs are able to reproduce reliably the variability of the atmospheric circulation over Greenland during summer. The differences of frequency between the GCMs and the reanalysis are mainly due to biases of the geopotential height which is systematically over or underestimated by most GCMs and to the underestimation of the variability of the circulation by most GCMs. For future projections, no new circulation types are detected, but rather a general increase of the mean geopotential height regardless of the circulation type. It is also important to note that for many GCMs, the uncertainty of the current climate simulations (given by the differences of the classification results between the GCM simulations for current climate and the reanalysis data for the same time) are of the same order than the projected changes for future climate. Therefore, these projections may be questionable