Added Value of High-resolution Regional Climate Simulations for Regional Impact Studies

We present a comparison of results from the regional climate model COSMO-CLM at a horizontal resolution of 2.8 km with observations and assess the added value of such higher resolution compared to a coarser resolution of 7 km. Such an added value is expected to result from a better resolution of orography and land use as well as from direct simulation of deep convection.The simulations are driven by ERA40 reanalyses for the years 1971 to 2000 and cover southwestern Germany and parts of eastern France. We show that 2.8 km horizontal resolution simulations yield in many, but not all, cases a better agreement of temperature, precipitation, humidity, and global radiation with observation data than simulations with 7 km resolution, especially during the summer half year. At 2.8 km resolution, the model also is well able to capture the mechanisms generating small-scale features, e.g. wind systems. However, the added value is highly dependent on region and altitude. In general, we conclude that high-resolution climate modeling allows studying the impact of climatological parameters on regional scales. It produces encouraging results and has a high potential for applications and direct use in regional and local impact models and impact studies

CLM-Assembly 2018 Conference proceedings

From September 18 to 21, the 13th General Assembly of the CLM community (https://www.clm-community.eu/) took place at Campus South of the Karlsruhe Institute of Technology. Nearly 60 international participants learned over these four days about the latest results and developments of the COSMO-CLM and ICON model systems in 23 plenary lectures and 21 posters. The premises in building 10.81 (“altes Ingenieursgebäude”) also offered the opportunity to engage in parallel sessions in in-depth discussions in the individual working groups of the CLM community. The present conference proceedings hold all the abstracts of the oral and poster presentations during the assembly and gives a good insight in the broad work and applications of the CLM Community. Herewith, the organizing team would like to sincerely thank • the participants of the conference, • the CLM working group leaders, • the scientific advisory board, • the catering service, • the janitors of building 10.81, • the student assistants, and • all others involved in organizing the assembly

Optical detection of mechanical motion of graphene by light-matter interactions

[ANGLÈS] In this thesis, we study both experimentally and theoretically the optical detection of the motion of graphene membranes based on fluorescence quenching of the emitters close to graphene. Fluorescence quenching occurs due to distance-dependent resonant energy transfer from the emitter dipoles to electron-hole dipoles in the graphene, and may be detected by lifetime measurement of the emitters. Nanoresonators consisting of graphene suspended over hole and trench structures coated with quantum dots are fabricated and their surface and emission properties characterised. The membranes are actuated electrostatically, and their movement is detected both mechanically by atomic force microscopy and also by lifetime measurements of the quantum dots.[CASTELLÀ] En esta tesis, estudiamos experimental y teóricamente la detección óptica del movimiento de membranas de grafeno mediante la extinción de la fluorescencia de emisores ópticos cerca del grafeno. La extinción de fluorescencia es debida a la transferencia de energía resonante a partir de los dipolos de los emisores hacia dipolos electrón-hueco en el grafeno. Este efecto, que depende de la distancia entre los emisores y el grafeno, puede ser detectado por medidas de tiempo de vida de los emisores. Estudiamos igualmente las propiedades de superficie y de emisión de nano-resonadores, hechos a partir de membranas de grafeno suspendidas sobre estructuras que contienen huecos y trincheras cubiertos de cajas quánticas. Las membranas se accionan de manera electroestática y su movimiento se detecta mecánicamente con un microscopio de fuerza atómica y ópticamente gracias a las medidas de tiempo de vida de las cajas quánticas.[CATALÀ] En aquesta tesi, estudiem experimentalment i teòrica la detecció òptica del moviment del grafè mitjançant l'extinció de la fluorescència d'emissors òptics propers al grafè. L'extinció és deguda a la distància de la transferència d'energia ressonant entre els dipols de l'emissor i els dipols electró-forat del grafè, i pot ser detectada mesurant la vida mitjana dels emissors. S'han fabricat nanoressonadors formats per grafè suspès sobre estructures amb forats o franges recobertes amb punts quàntics, i se n'han caracteritzat la superfície i l'emissió. Les membranes s'han desplaçat electroestàticament i el seu moviment s'ha detectat mecànicament mitjançant el microscopi de força atòmica (AFM) i també amb mesures de vida mitjana dels punts quàntics

Compound extremes in a changing climate - a Markov chain approach

Studies using climate models and observed trends indicate that extreme weather has changed and may continue to change in the future. The potential impact of extreme events such as heat waves or droughts depends not only on their number of occurrences but also on "how these extremes occur", i.e., the interplay and succession of the events. These quantities are quite unexplored, for past changes as well as for future changes and call for sophisticated methods of analysis. To address this issue, we use Markov chains for the analysis of the dynamics and succession of multivariate or compound extreme events. We apply the method to observational data (1951–2010) and an ensemble of regional climate simulations for central Europe (1971–2000, 2021–2050) for two types of compound extremes, heavy precipitation and cold in winter and hot and dry days in summer. We identify three regions in Europe, which turned out to be likely susceptible to a future change in the succession of heavy precipitation and cold in winter, including a region in southwestern France, northern Germany and in Russia around Moscow. A change in the succession of hot and dry days in summer can be expected for regions in Spain and Bulgaria. The susceptibility to a dynamic change of hot and dry extremes in the Russian region will probably decrease

UTCI climatology and its future change in Germany – an RCM ensemble approach

n the present study, the quantity, duration and intensity of heat stress events in Germany as well as their future change and relation with weather types were investigated. A small ensemble of regional climate simulations with the regional climate model (RCM) COSMO-CLM driven by four general circulation models (GCMs) was used to calculate the Universal Thermal Climate Index (UTCI); the UTCI is a well-accepted thermal comfort index which we use here to quantify thermal stress. The variables entering the UTCI were bias corrected with a method that preserves their interdependencies. The projected climate changes cause a significant increase of both the mean UTCI and the number, duration and intensity of heat stress events between the control period (1981–2000) and the projection period (2031–2050). The projected future hourly frequency distribution of the UTCI at a location can be described by a shift to higher UTCI values with an almost constant shape of distribution. The investigations of the projected changes in weather types show no significant changes between the periods covered, with a few exceptions. An exception concerning heat stress events is the increase of summer anticyclonic weather types. Although more anticyclonic weather types in summer lead to an increase in heat stress events, they are not the primary cause of the projected increases. Rather, it turns out that the characteristics of the air masses associated with the weather types change towards warmer and more humid conditions

Soil moisture impacts on convective indices and precipitation over complex terrain

The impact of soil moisture on convective precipitation, convective indices, surface energy balance components, and near-surface meteorological variables is analysed for seven intensive observation periods of the Convective and Orographically induced Precipitation Study (COPS) conducted in summer 2007 using a non-hydrostatic limited-area atmospheric prediction model. The control runs are compared to sensitivity experiments under dry (-25 %) and wet (+25 %) initial soil moisture conditions. In the wet experiment, surface fluxes produce moister and cooler boundary layers with increased equivalent potential temperatures. Furthermore, the lifting condensation level and the level of free convection are lowered for all analysed regions, even under different synoptic controls. The comparison of boundary-layer and mid-tropospheric forcing regimes reveal that the impact of soil moisture on the atmosphere is not systematically higher for boundary-layer forcing. Whereas the Bowen ratio exhibits a clear dependence on soil moisture conditions, the impact on precipitation is complex and strongly depends on convective inhibition. A considerable, but non-systematic dependence of convective precipitation on soil moisture exists in the analysed complex orography. The results demonstrate the high sensitivity of numerical weather prediction to initial soil moisture fields

Applying an isotope-enabled regional climate model over the Greenland ice sheet: effect of spatial resolution on model bias

In order to investigate the impact of spatial resolution on the discrepancy between simulated δ18O and observed δ18O in Greenland ice cores, regional climate simulations are performed with the isotope-enabled regional climate model (RCM) COSMO_iso. For this purpose, isotope-enabled general circulation model (GCM) simulations with the ECHAM5-wiso general circulation model (GCM) under present-day conditions and the MPI-ESM-wiso GCM under mid-Holocene conditions are dynamically downscaled with COSMO_iso for the Arctic region. The capability of COSMO_iso to reproduce observed isotopic ratios in Greenland ice cores for these two periods is investigated by comparing the simulation results to measured δ18O ratios from snow pit samples, Global Network of Isotopes in Precipitation (GNIP) stations and ice cores. To our knowledge, this is the first time that a mid-Holocene isotope-enabled RCM simulation is performed for the Arctic region. Under present-day conditions, a dynamical downscaling of ECHAM5-wiso (1.1∘×1.1∘ ) with COSMO_iso to a spatial resolution of 50 km improves the agreement with the measured δ18O ratios for 14 of 19 observational data sets. A further increase in the spatial resolution to 7 km does not yield substantial improvements except for the coastal areas with its complex terrain. For the mid-Holocene, a fully coupled MPI-ESM-wiso time slice simulation is downscaled with COSMO_iso to a spatial resolution of 50 km. In the mid-Holocene, MPI-ESM-wiso already agrees well with observations in Greenland and a downscaling with COSMO_iso does not further improve the model–data agreement. Despite this lack of improvement in model biases, the study shows that in both periods, observed δ18O values at measurement sites constitute isotope ratios which are mainly within the subgrid-scale variability of the global ECHAM5-wiso and MPI-ESM-wiso simulation results. The correct δ18O ratios are consequently not resolved in the GCM simulation results and need to be extracted by a refinement with an RCM. In this context, the RCM simulations provide a spatial δ18O distribution by which the effects of local uncertainties can be taken into account in the comparison between point measurements and model outputs. Thus, an isotope-enabled GCM–RCM model chain with realistically implemented fractionating processes constitutes a useful supplement to reconstruct regional paleo-climate conditions during the mid-Holocene in Greenland. Such model chains might also be applied to reveal the full potential of GCMs in other regions and climate periods, in which large deviations relative to observed isotope ratios are simulated

Impact of soil-vegetation-atmosphere interactions on the spatial rainfall distribution in the Central Sahel

In a Regional Climate Model (RCM) the interactions between the land surface and the atmosphere are described by a Soil-Vegetation-Atmosphere-Transfer Model (SVAT). In the presented study two SVATs of different complexity (TERRA-ML and VEG3D) are coupled to the RCM COSMO-CLM (CCLM) to investigate the impact of different representations of soil-vegetation-atmosphere interactions on the West African Monsoon (WAM) system. In contrast to TERRA-ML, VEG3D comprises a more detailed description of the land-atmosphere coupling by including a vegetation layer in its structural design, changing the treatment of radiation and turbulent fluxes. With these two different model systems (CCLM-TERRA-ML and CCLM-VEG3D) climate simulations are performed for West Africa and analyzed. The study reveals that the simulated spatial distribution of rainfall in the Sahel region is substantially affected by the chosen SVAT. Compared to CCLM-TERRA-ML, the application of CCLM-VEG3D results in higher near surface temperatures in the Sahel region during the rainy season. This implies a southward expansion of the Saharian heat-low. Consequently, the mean position of the African Easterly Jet (AEJ) is also shifted to the south, leading to a southward displacement of tracks for Mesoscale Convective Systems (MCS), developing in connection with the AEJ. As a result, less precipitation is produced in the Sahel region, increasing the agreement with observations. These analyses indicate that soil-vegetation-atmosphere interactions impact the West African Monsoon system and highlight the benefit of using a more complex SVAT to simulate its dynamic

Applying an isotope-enabled regional climate model over the Greenland ice sheet: effect of spatial resolution on model bias

In order to investigate the impact of spatial resolution on the discrepancy between simulated δ$^{18}$O and observed δ$^{18}$O in Greenland ice cores, regional climate simulations are performed with the isotope-enabled regional climate model (RCM) COSMO_iso. For this purpose, isotope-enabled general circulation model (GCM) simulations with the ECHAM5-wiso general circulation model (GCM) under present-day conditions and the MPI-ESM-wiso GCM under mid-Holocene conditions are dynamically downscaled with COSMO_iso for the Arctic region. The capability of COSMO_iso to reproduce observed isotopic ratios in Greenland ice cores for these two periods is investigated by comparing the simulation results to measured δ$^{18}$O ratios from snow pit samples, Global Network of Isotopes in Precipitation (GNIP) stations and ice cores. To our knowledge, this is the first time that a mid-Holocene isotope-enabled RCM simulation is performed for the Arctic region. Under present-day conditions, a dynamical downscaling of ECHAM5-wiso (1.1∘×1.1∘) with COSMO_iso to a spatial resolution of 50 km improves the agreement with the measured δ$^{18}$O ratios for 14 of 19 observational data sets. A further increase in the spatial resolution to 7 km does not yield substantial improvements except for the coastal areas with its complex terrain. For the mid-Holocene, a fully coupled MPI-ESM-wiso time slice simulation is downscaled with COSMO_iso to a spatial resolution of 50 km. In the mid-Holocene, MPI-ESM-wiso already agrees well with observations in Greenland and a downscaling with COSMO_iso does not further improve the model–data agreement. Despite this lack of improvement in model biases, the study shows that in both periods, observed δ$^{18}$O values at measurement sites constitute isotope ratios which are mainly within the subgrid-scale variability of the global ECHAM5-wiso and MPI-ESM-wiso simulation results. The correct δ$^{18}$O ratios are consequently not resolved in the GCM simulation results and need to be extracted by a refinement with an RCM. In this context, the RCM simulations provide a spatial δ$^{18}$O distribution by which the effects of local uncertainties can be taken into account in the comparison between point measurements and model outputs. Thus, an isotope-enabled GCM–RCM model chain with realistically implemented fractionating processes constitutes a useful supplement to reconstruct regional paleo-climate conditions during the mid-Holocene in Greenland. Such model chains might also be applied to reveal the full potential of GCMs in other regions and climate periods, in which large deviations relative to observed isotope ratios are simulated