The CORDEX.be initiative as a foundation for climate services in Belgium

The CORDEX.be project created the foundations for Belgian climate services by producing high-resolution Belgian climate information that (a) incorporates the expertise of the different Belgian climate modeling groups and that (b) is consistent with the outcomes of the international CORDEX ("COordinated Regional Climate Downscaling Experiment") project. The key practical tasks for the project were the coordination of activities among different Belgian climate groups, fostering the links to specific international initiatives and the creation of a stakeholder dialogue. Scientifically, the CORDEX.be project contributed to the EURO-CORDEX project, created a small ensemble of High-Resolution (H-Res) future projections over Belgium at convection-permitting resolutions and coupled these to seven Local Impact Models. Several impact studies have been carried out. The project also addressed some aspects of climate change uncertainties. The interactions and feedback from the stakeholder dialogue led to different practical applications at the Belgian national level

Evaluation framework for sub-daily rainfall extremes simulated by regional climate models

peer reviewedSub-daily precipitation extremes are high-impact events that can result in flash floods, sewer system overload, or landslides. Several studies have reported an intensification of projected short-duration extreme rainfall in a warmer future climate. Traditionally, regional climate models (RCMs) are run at a coarse resolution using deep-convection parameterization for these extreme events. As computational resources are continuously ramping up, these models are run at convection-permitting resolution, thereby partly resolving the small-scale precipitation events explicitly. To date, a comprehensive evaluation of convection-permitting models is still missing. We propose an evaluation strategy for simulated sub-daily rainfall extremes that summarizes the overall RCM performance. More specifically, the following metrics are addressed: the seasonal/diurnal cycle, temperature and humidity dependency, temporal scaling and spatio-temporal clustering. The aim of this paper is: (i) to provide a statistical modeling framework for some of the metrics, based on extreme value analysis, (ii) to apply the evaluation metrics to a micro-ensemble of convection-permitting RCM simulations over Belgium, against high-frequency observations, and (iii) to investigate the added value of convection-permitting scales with respect to coarser 12-km resolution. We find that convection-permitting models improved precipitation extremes on shorter time scales (i.e, hourly or two-hourly), but not on 6h-24h time scales. Some metrics such as the diurnal cycle or the Clausius-Clapeyron rate are improved by convection-permitting models, whereas the seasonal cycle appears robust across spatial scales. On the other hand, the spatial dependence is poorly represented at both convection-permitting scales and coarser scales. Our framework provides perspectives for improving high-resolution atmospheric numerical modeling and datasets for hydrological applications

Heat stress increase under climate change twice as large in cities as in rural areas : a study for a densely populated midlatitude maritime region

Urban areas are usually warmer than their surrounding natural areas, an effect known as the urban heat island effect. As such, they are particularly vulnerable to global warming and associated increases in extreme temperatures. Yet ensemble climate-model projections are generally performed on a scale that is too coarse to represent the evolution of temperatures in cities. Here, for the first time, we combine unprecedented long-term (35years) urban climate model integrations at the convection-permitting scale (2.8km resolution) with information from an ensemble of general circulation models to assess temperature-based heat stress for Belgium, a densely populated midlatitude maritime region. We discover that the heat stress increase toward the mid-21st century is twice as large in cities compared to their surrounding rural areas. The exacerbation is driven by the urban heat island itself, its concurrence with heat waves, and urban expansion. Cities experience a heat stress multiplication by a factor 1.4 and 15 depending on the scenario. Remarkably, the future heat stress surpasses everywhere the urban hot spots of today. Our results demonstrate the need to combine information from climate models, acting on different scales, for climate change risk assessment in heterogeneous regions. Moreover, these results highlight the necessity for adaptation to increasing heat stress, especially in urban areas

The added value of convection-permitting regional climate modeling for land-use change impact assessment and greenhouse gas induced climate change

An increasingly higher proportion of regional climate model simulations are being performed at convection permitting scales (CPS), i.e., a horizontal grid scale below 4 km. At this scale, deep convection becomes a grid-scale process and is resolved explicitly, rather than parameterized. Several recent studies have investigated the added value of CPS simulations, focusing mainly on the benefits for the representation of summertime convective precipitation. However, some research gaps or areas of disagreement continue to exist. For one, the added value of CPS in studies that use regional climate models to quantify the biogeophysical impact of land-use change (LUC) on climate has not yet been investigated. Given the fact that more and more LUC impact studies that use regional climate models are opting for CPS simulations, such an investigation is timely. Second, considerable disagreement exists in existing literature on the added value of CPS for extreme precipitation on the sub-daily timescale. Specifically, studies that compare the future change signal in extreme precipitation on the hourly timescale of CPS to conventional, non-convection permitting scale (nCPS) simulations have produced mixed results, with some reporting that CPS simulations project a higher increase in extreme hourly accumulations, while others do not. Further research into this topic is therefore needed, especially given the considerable societal impact of short-duration extreme precipitation. Therefore, the goal of this dissertation is to investigate the added value of regional climate model simulations at CPS in the areas of a) LUC impact assessment and b) greenhouse gas emission induced future changes in extreme precipitation. We start our analysis by presenting a new methodology to evaluate the impact of LUC in a regional climate model. The goal in developing this methodology is to address several deficiencies in current LUC impact evaluations. The methodology uses site pair observations from across Europe, and is applied to the regional climate model COSMO-CLM in nCPS configuration (0.22°, ~25 km horizontal resolution). Also, we choose to limit the investigation to one specific LUC, namely deforestation. The observational dataset shows that a considerable seasonal and diurnal cycle exists in the temperature difference between forest and nearby open land sites. COSMO-CLM simulations performed at nCPS resolution are able to reproduce the sign and magnitude of this temperature signal during winter daytime only. An analysis of the difference in the various surface energy budget components shows that the main summer daytime model process related bias is an unrealistic decrease in low and mid-level cloud cover and associated increase in incoming shortwave radiation (δSWin) over open land, triggering additional biases in the latent heat flux and ground heat flux. Additionally, the model’s inability to reproduce the observed nighttime cooling is attributed mainly to a bias in the difference in incoming longwave radiation (δLWin). We then apply our evaluation methodology to both nCPS (0.22° or ~25 km horizontal resolution) and CPS (0.025° or ~2.8 km horizontal resolution) COSMO-CLM simulations. These runs are evaluated against one high quality forest/open land site pair located in eastern Germany. For summer daytime, results show that CPS simulations improve upon nCPS simulations in several aspects. First, δSWin is significantly reduced compared to nCPS simulations, an improvement that can be linked to a better representation of afternoon convective clouds. Second, improvements are seen in the simulated difference in sensible heat flux (δH) and ground flux (δG), which can be linked to a better representation of surface roughness, as well as surface energy budget and atmospheric feedbacks resulting from the improvements in both δSWin and roughness. For nighttime, (smaller) improvements are seen as well. Most notably, the bias in nighttime δLWin is reduced, which could linked to the increase in vertical resolution, and an associated improvement in the difference in lower boundary layer vertical mixing. Thus, we conclude that, overall, CPS simulations improve considerably upon nCPS simulations in simulating the local biogeophysical climate impact of deforestation. Next, we investigate the added value of CPS in simulating both present day extreme precipitation and the future change in extreme precipitation driven by greenhouse gas emissions. Six multi-decadal COSMO-CLM regional climate simulations are performed, namely one set of evaluation, control and future simulations at nCPS (0.11° or ~12 km horizontal resolution), and another set of simulations at CPS (0.025° or ~2.8 km horizontal resolution). The study area is Belgium and surroundings. Present day evaluation results show that CPS simulations are vastly superior in simulating the frequency of extreme precipitation on the hourly timescale, but not on the daily timescale. CPS biases on the daily timescale are linked to small convective cells still being under-resolved at 0.025° resolution. Finally, the difference in the future precipitation change signal is analyzed. When comparing the projected future change in extreme precipitation of nCPS to CPS simulations, considerable differences exist. On the daily timescale, both nCPS and CPS simulations project that extreme precipitation will increase. However, the projected increase is significantly higher at CPS, especially for the most extreme daily precipitation events. With regards to the spatial pattern of the future increase on the daily timescale, no clear link to topography or any other geographic feature can be discerned for either nCPS or CPS. On the hourly timescale, CPS simulations project large increases in extreme precipitation for both Flanders and Ardennes sub-regions, with the highest increases seen over the latter region. nCPS simulations are able to match this increase in the hilly Ardennes region, but not in the relatively flat Flanders regions, where they project a decrease in extreme hourly precipitation for most of the intensity range. We can therefore conclude that on the hourly timescale, the added value of CPS depends on topography. This result explains the discrepancy in available literature, as the studies reporting little difference in the future increase of hourly extreme precipitation events between nCPS and CPS have all been performed for hilly or mountainous terrain.status: publishe

Do convection-permitting models improve the representation of the impact of LUC?

In this study we assess the added value of convection permitting scale (CPS) simulations in studies using regional climate models to quantify the bio-geophysical climate impact of land-use change (LUC). To accomplish this, a comprehensive model evaluation methodology is applied to both non-CPS and CPS simulations. The main characteristics of the evaluation methodology are (1) the use of paired eddy-covariance site observations (forest vs open land) and (2) a simultaneous evaluation of all surface energy budget components. Results show that although generally satisfactory, non-CPS simulations fall short of completely reproducing the observed LUC signal because of three key biases. CPS scale simulations succeed at significantly reducing two of these biases, namely, those in daytime shortwave radiation and daytime sensible heat flux. Also, CPS slightly reduces a third bias in nighttime incoming longwave radiation. The daytime improvements can be attributed partially to the switch from parameterized to explicit convection, the associated improvement in the simulation of afternoon convective clouds, and resulting surface energy budget and atmospheric feedbacks. Also responsible for the improvements during daytime is a better representation of surface heterogeneity and thus, surface roughness. Meanwhile, the modest nighttime longwave improvement can be attributed to increased vertical atmospheric resolution. However, the model still fails at reproducing the magnitude of the observed nighttime longwave difference. One possible explanation for this persistent bias is the nighttime radiative effect of biogenic volatile organic compound emissions over the forest site. A correlation between estimated emission rates and the observed nighttime longwave difference, as well as the persistence of the longwave bias provide support for this hypothesis. However, more research is needed to conclusively determine if the effect indeed exists.status: publishe

Het GDI netwerk in Vlaanderen: Situatie anno 2011 en evolutie

Voor overheidsorganisaties is geodata alomtegenwoordig. Ze worden er dagelijks mee geconfronteerd en steunen er in toenemende mate op tijdens de beleidsvorming. De voorbije jaren werden bovendien verscheidene initiatieven ondernomen om de uitwisseling van geodata binnen de publieke sector in Vlaanderen vlotter te doen verlopen. Naar het geheel van deze initiatieven wordt vaak verwezen met het concept ‘Geografische Data Infrastructuur’ (GDI). Een GDI kan in die zin gedefinieerd worden als het geheel van technologische en niet-technologische maatregelen die worden genomen om het gebruik en de uitwisseling van geodata te bevorderen. Het multidisciplinaire onderzoeksproject SPATIALIST werd in 2007 opgestart met als doel na te gaan wat de vereisten zijn voor de verdere ontwikkeling van een succesvolle GDI in Vlaanderen. In het kader van dit onderzoek organiseerde het team in het voorjaar van 2011 een online bevraging met als doel het GDI in Vlaanderen anno 2011 in kaart te brengen. De resultaten van deze bevraging worden besproken in dit rapport, waarbij zowel de organisatorische aspecten van geodata als de stromen van geodata tussen de organisaties aan bod komen. Een aantal van deze aspecten worden bovendien vanuit een longitudinale invalshoek bekeken, waarbij we de situatie van vandaag vergelijken met die van drie jaar geleden, toen een soortgelijke bevraging werd afgenomen. Ten slotte wordt nagegaan welke aspecten van de GDI de afgelopen jaren in positieve zin zijn geëvolueerd, en waar de knelpunten zitten.nrpages: 131status: publishe

Geo-informatie gevraagd : Verhalen uit Vlaanderen Geoland

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Influence of convection-permitting modeling on future projections of extreme precipitation for a mid-latitude region with mixed topography

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Can model simulations using CCLM2 correctly predict the biophysical impact of forestation on the European climate?

In recent years, afforestation has been proposed as a promising strategy for climate change mitigation. However, when discussing the potential place of forestry in climate change mitigation policy, the biophysical impact of forestation, which could potentially offset or enhance biochemical cooling at the regional scale, is often overlooked. Several studies have therefore tried to quantify the biophysical effects of forestation for Europe, often resulting in conflicting conclusions. For example, while simulations with a GCM show that forestation leads to a mean temperature warming in all mid-latitude climates, some regional simulations for Europe indicate the opposite, and suggest that increased forestation cools the surface by increasing evapotranspiration and precipitation. The goal of this study is to contribute to the understanding of the biophysical impact of forestation in Europe on climate. The first two research questions are as follows: (1) Are there significant differences in surface temperature between open land and forested land, based on direct observations? (2) Can a regional climate model accurately simulate these differences? To answer these research questions, we use, first, eddy-covariance flux tower measurements from locations where towers situated in forest and grassland are found in close proximity. In total, 5 such sites were selected from the Fluxnet network of eddy-covariance towers. Second, model simulations were performed using the regional climate model COSMO-CLM2 (COSMO-CLM coupled to the Community Land Model). Two simulations were performed for the time period 2004/06 - 2005/11. Land use for selected pixels was set to 100% grassland in one run, 100% forest in the other. Observations show that a diurnal asymmetry exists in the biophysical effect of forestation. During daytime, increased mixing due to higher roughness appears to offset the warming associated with lower forest albedo, leading to cooler 2m air temperatures over forests. At night on the other hand, surface air temperatures over forests are generally higher than over grassland. The model is unable to replicate this land use pattern. Compared to observations, the modeled sensitivity of 2m air temperature to land use type is small, and of opposite sign. This can probably be related to a bias in modeled surface energy fluxes. E.g., we hypothesize that observed nighttime temperatures are 0,52 to 2,84 higher for forests because of a stronger turbulent coupling with the upper atmosphere compared to grassland, evident in a relatively strong negative sensible heat flux. The model, however, greatly underestimates the difference in nighttime sensible heat flux between forest and open land.status: publishe

The influence of convection-permitting regional climate modeling on future projections of extreme precipitation : dependency on topography and timescale

Owing to computational advances, an ever growing percentage of regional climate simulations are being performed at convection-permitting scale (CPS, or a horizontal grid scale below 4km). One particular area where CPS could be of added value is in future projections of extreme precipitation, particularly for short timescales (e.g. hourly). However, recent studies that compare the sensitivity of extreme hourly precipitation at CPS and non-convection-permitting scale (nCPS) have produced mixed results, with some reporting a significantly higher future increase of extremes at CPS, while others do not. However, the domains used in these studies differ significantly in orographic complexity, and include both mountain ranges as well as lowlands with minimal topographical features. Therefore, the goal of this study is to investigate if and how the difference between nCPS and CPS future extreme precipitation projections might depend on topographic complexity and timescale. The study area is Belgium and surroundings, and is comprised of lowland in the north (Flanders) and a low mountain range in the south (Ardennes). These two distinct topographical regions are separated in the analysis. We perform and analyze three sets of 30year climate simulations (hindcast, control and end-of-century RCP 8.5) at both nCPS (12km resolution) and CPS (2.5km resolution), using the regional climate model COSMO-CLM. Results show that for our study area, the difference between nCPS and CPS future extreme precipitation depends on both timescale and topography. Despite a background of general summer drying in our region caused by changes in large-scale circulation, the CPS simulations predict a significant increase in the frequency of daily and hourly extreme precipitation events, for both the lowland and mountain areas. The nCPS simulations are able to reproduce this increase for hourly extremes in mountain areas, but significantly underestimate the increase in hourly extremes in lowlands, as well as the increase in the most extreme daily precipitation events in both the lowland and mountain areas