The extremely hot and dry 2018 summer in central and northern Europe from a multi-faceted weather and climate perspective

The summer of 2018 was an extraordinary season in climatological terms for northern and central Europe, bringing simultaneous, widespread, and concurrent heat and drought extremes in large parts of the continent with extensive impacts on agriculture, forests, water supply, and the socio-economic sector. Here, we present a comprehensive, multi-faceted analysis of the 2018 extreme summer in terms of heat and drought in central and northern Europe, with a particular focus on Germany. The heatwave first affected Scandinavia in mid-July and shifted towards central Europe in late July, while Iberia was primarily affected in early August. The atmospheric circulation was characterized by strongly positive blocking anomalies over Europe, in combination with a positive summer North Atlantic Oscillation and a double jet stream configuration before the initiation of the heatwave. In terms of possible precursors common to previous European heatwaves, the Eurasian double-jet structure and a tripolar sea surface temperature anomaly over the North Atlantic were already identified in spring. While in the early stages over Scandinavia the air masses at mid and upper levels were often of a remote, maritime origin, at later stages over Iberia the air masses primarily had a local-to-regional origin. The drought affected Germany the most, starting with warmer than average conditions in spring, associated with enhanced latent heat release that initiated a severe depletion of soil moisture. During summer, a continued precipitation deficit exacerbated the problem, leading to hydrological and agricultural drought. A probabilistic attribution assessment of the heatwave in Germany showed that such events of prolonged heat have become more likely due to anthropogenic global warming. Regarding future projections, an extreme summer such as that of 2018 is expected to occur every 2 out of 3 years in Europe in a +1.5 ∘C warmer world and virtually every single year in a +2 ∘C warmer world. With such large-scale and impactful extreme events becoming more frequent and intense under anthropogenic climate change, comprehensive and multi-faceted studies like the one presented here quantify the multitude of their effects and provide valuable information as a basis for adaptation and mitigation strategies

The worldwide C3S CORDEX grand ensemble: A major contribution to assess regional climate change in the IPCC AR6 Atlas

peer reviewedAbstract The collaboration between the Coordinated Regional Climate Downscaling Experiment (CORDEX) and the Earth System Grid Federation (ESGF) provides open access to an unprecedented ensemble of Regional Climate Model (RCM) simulations, across the 14 CORDEX continental-scale domains, with global coverage. These simulations have been used as a new line of evidence to assess regional climate projections in the latest contribution of the Working Group I (WGI) to the IPCC Sixth Assessment Report (AR6), particularly in the regional chapters and the Atlas. Here, we present the work done in the framework of the Copernicus Climate Change Service (C3S) to assemble a consistent worldwide CORDEX grand ensemble, aligned with the deadlines and activities of IPCC AR6. This work addressed the uneven and heterogeneous availability of CORDEX ESGF data by supporting publication in CORDEX domains with few archived simulations and performing quality control. It also addressed the lack of comprehensive documentation by compiling information from all contributing regional models, allowing for an informed use of data. In addition to presenting the worldwide CORDEX dataset, we assess here its consistency for precipitation and temperature by comparing climate change signals in regions with overlapping CORDEX domains, obtaining overall coincident regional climate change signals. The C3S CORDEX dataset has been used for the assessment of regional climate change in the IPCC AR6 (and for the interactive Atlas) and is available through the Copernicus Climate Data Store (CDS)

Land Cover Impacts on Land-Atmosphere Coupling Strength in Climate Simulations With WRF Over Europe

Land use and land cover changes are important human forcings to the Earth's climate. This study examines the land-atmosphere coupling strength and the relationship between surface fluxes and clouds and precipitation for three land cover scenarios in the European summer. The WRF model was used to simulate one scenario with extreme afforestation, one with extreme deforestation, and one with realistic land cover for the time period between 1986 and 2015. The simulations were forced with ERA-Interim reanalysis data. The analysis followed a two-step approach. First, the convective triggering potential–low-level humidity index framework was applied to locate potential coupling hot spots, which were then analyzed with regard to their sensitivity toward land use and land cover changes. Second, actual feedbacks between evaporative fraction, cloud cover, and precipitation were analyzed statistically with focus on sign and location of the feedbacks. The results demonstrate that coupling hot spots, exhibiting predominantly positive feedbacks, were identified over parts of Eastern Europe and Scandinavia. In this strongly coupled region, afforestation and deforestation modified the atmospheric humidity and stability by changing the surface flux partitioning. Afforestation is associated with a net drying of the atmosphere due to a disproportionately strong increase in the sensible heat flux. In contrast, deforestation initiated a moistening of the atmosphere. The total precipitation changed only in limited areas significantly, which are mostly located in mountainous regions and the northeast of the domain. In summary, the results indicate a land surface influence on the atmospheric background conditions, and an impact on the potential strength of land surface-precipitation feedbacks

Validation of TERRA-ML with discharge measurements

We evaluate the runoff-simulation performance of a water transport model (routing scheme) coupled to the Land Surface Parameterization module TERRA-ML of the operational COSMO (Consortium for Small-Scale Modelling) weather forecast model. In addition to the successful implemention of the routing scheme, we also included an alternative vertical soil water transport parameterisation in TERRA-ML in order to estimate the uncertainty caused by the component of the LSP central to runoff generation. A combination of two data sets, both operational products by DWD, is used for precipitation input. These are the hourly precipitation data set RADOLAN RW, which is based on radar data and is calibrated by rain gauges, as well as the daily REGNIE data set, which is only based on gauge data. The mesoscale Sieg river catchment located in Western Germany is used as the evaluation testbed. The extended TERRA-ML was run in standalone mode (decoupled from the atmospheric part of the COSMO model) with 1 × 1 km spatial resolution from April to September 2005 based on and provided with spatially more detailed descriptions of topography, land use and soil texture. The model was driven by operational COSMO analysis data and two different sources of observed precipitation (gauge and radar measurements). The results are compared to discharge measurements. They indicate a good representation of the observed discharge by the extended TERRA-ML system. The additionally implemented linear vertical soil water parameterization overestimates total discharge less (6 %) than the default exponential parameterization (20 %) when compared to a gauging station located at the lower reaches of the river Sieg. Suggestions are given on how to further enhance the modelled discharge by improvements in the LSP scheme

Streamflow simulations reveal the impact of the soil parameterization

Land surface models calculate runoff from the land surface. Therefore they must be coupled to a hydrological model to receive the streamflow which then allows for the comparison to measurements at gauging stations. The land surface model TERRA-ML of the weather forecast model COSMO of the German Weather Service is coupled to a river routing model and applied to the Enz watershed upstream of Höfen (Baden-Württemberg, Germany). The comparison of simulated to measured streamflow revealed, deficiencies in the hydraulic conductivity parameterization and in the applied soil texture data set. Small changes in the vertical soil water fluxes and high resolution soil texture data led to a significant improvement in the streamflow simulation. This resulted in different simulations of evapotranspiration and soil moisture, namely in spring and early summer, i.e. during the most pronounced growing season