Land-atmosphere interactions in multiscale regional climate change simulations over Europe

Interactions between the heterogenous land surface and the atmosphere play a fundamental role in the weather and climate system through their influence on the energy and water cycles. Global climate models (GCMs) currently have coarse horizontal grid resolutions in the order of 100 km. With their higher resolution regional climate models (RCMs) better resolve mesoscale processes in the atmosphere and better represent the heterogenous land surface properties. Thus, RCMs are able to provide more detailed characteristics of regional to local climate. This thesis conducts regional climate simulations in multiple resolutions for the European domain of the Coordinated Regional Climate Downscaling Experiment (EURO-CORDEX) and a central European domain (3kmME) with the RCM WRF downscaling both ERA-Interim reanalysis and GCM MPI-ESM-LR (RCP4.5) climate change scenario data. The analysis focusses on land-atmosphere interactions to gain a better understanding of the regional water cycle components, the involved multi-scale processes, their sensitivities and variabilities both under present-day climate and future climate change conditions. Furthermore, the added value of the convection-permitting 3kmME simulations, being one of the first sets of decade-long convection-permitting regional climate simulations over Central Europe, is investigated. A comparison of summertime land-atmosphere coupling strength is carried out for a subset of the ERA-Interim-driven EURO-CORDEX model ensemble (1989 to 2008). The coupling strength is quantified by the correlation between the surface sensible and the latent heat flux, and by the correlation between the latent heat flux and 2m temperature and compared to European FLUXNET observations and gridded observational Global Land Evaporation Amsterdam Model (GLEAM) data, respectively. The RCM simulations agree with both observational datasets in the large-scale pattern characterized by strong coupling in southern Europe and weak coupling in northern Europe. However, in the transition zone from strong to weak coupling covering large parts of central Europe the majority of the RCMs tend to overestimate the coupling strength in comparison to both observations. The RCM ensemble spread is caused by the different land surface models applied, and by the model-specific weather conditions resulting from different atmospheric parameterizations. Investigation of land-atmosphere coupling strength in ERA-Interim driven WRF simulations in both 3 km and 12 km resolution for central Europe reveals large year-to-year variability related to the individual soil moisture conditions. Coupling strength largely differs for individual land use types. Forest compared to crop type reacts slower to drought conditions. Coupling is overall slightly stronger in the 3 km simulation, attributed to overall drier soils due to less precipitation. The projected climate change based on a WRF 0.44° simulation downscaling GCM MPI-ESM-LR (RCP4.5) data alters the European land-atmosphere coupling regimes in summer. Due to increasingly drier soils, stronger coupling is simulated for large parts of western, central and southern eastern Europe for the period 2071-2100 compared to 1971-2000. Areas of strongest future increase of extreme temperature coincide with strong coupling areas. In order to analyse the added value of convection-permitting 3 km climate simulations, nine years of ERA-Interim driven simulations with the WRF RCM at 12 km and 3 km grid resolution over central Europe are evaluated against observations with a focus on sub-daily precipitation statistics and the relation between extreme precipitation and air temperature. A clear added value of the higher resolution simulation is found especially in the reproduction of the diurnal cycle and the hourly intensity distribution of precipitation. Too much light precipitation in the 12 km simulation results in a positive precipitation bias. Largest differences between both resolutions occur in mountainous regions and during the summer months with high convective activity. Moreover, the observed increase of the temperature–extreme precipitation scaling from the Clausius-Clapeyron (C-C) scaling rate of ~7% K-1 to a super-adiabatic scaling rate is reproduced only by the 3 km simulation. The effect of land surface heterogeneity on the differences between 3 km and 12 km simulations is analysed based on five WRF simulations for JJA 2003, each with the same atmospheric setup in 3 km resolution but different combinations of 12 km resolution land use and soil type, initial soil moisture and orography. A coarser resolved orography significantly alters the flow over and around extensive mountain ridges like the Alps and impact the large-scale flow pattern. The smoothed mountain ridges result in weaker Föhn effects and in enhanced locally generated convective precipitation pattern peaking earlier in the afternoon. The effect of a coarser-resolved land use distribution is overall smaller and mainly related to changes in overall percentages of different land use types, rather than to the loss of heterogeneity in the surface pattern on the scale analysed here. Even small changes in soil moisture have a higher potential to affect the overall simulation results. WRF climate simulations downscaling the MPI-ESM-LR data at 12 km and 3 km resolution for central Europe are analysed for three 12-year periods: a control, a mid-of-century and an end-of-century projection to quantify future changes in precipitation statistics based on both convection-permitting and convection-parameterized simulations. For both future scenarios both simulations suggest a slight decrease in mean summer precipitation and an increase in hourly heavy and extreme precipitation in large parts of central Europe. This increase is stronger in the 3 km runs. Temperature–extreme precipitation scaling curves in the future climate are projected to shift along the 7% K-1 trajectory to higher peak extreme precipitation values at higher temperatures while keeping their typical shape. The results of this thesis clearly confirm the added value of convection-permitting climate simulations, provide further insights into land-atmosphere interaction processes and highlight the relevance of the RCMs ability to properly simulate coupling strength

Regional climate hindcast simulations within EURO-CORDEX: evaluation of a WRF multi-physics ensemble

In the current work we present six hindcast WRF (Weather Research and Forecasting model) simulations for the EURO-CORDEX (European Coordinated Regional Climate Downscaling Experiment) domain with different configurations in microphysics, convection and radiation for the time period 1990?2008. All regional model simulations are forced by the ERA-Interim reanalysis and have the same spatial resolution (0.44°). These simulations are evaluated for surface temperature, precipitation, short- and longwave downward radiation at the surface and total cloud cover. The analysis of the WRF ensemble indicates systematic temperature and precipitation biases, which are linked to different physical mechanisms in the summer and winter seasons. Overestimation of total cloud cover and underestimation of downward shortwave radiation at the surface, mostly linked to the Grell?Devenyi convection and CAM (Community Atmosphere Model) radiation schemes, intensifies the negative bias in summer temperatures over northern Europe (max ?2.5 °C). Conversely, a strong positive bias in downward shortwave radiation in summer over central (40?60%) and southern Europe mitigates the systematic cold bias over these regions, signifying a typical case of error compensation. Maximum winter cold biases are over northeastern Europe (?2.8 °C); this location suggests that land?atmosphere rather than cloud?radiation interactions are to blame. Precipitation is overestimated in summer by all model configurations, especially the higher quantiles which are associated with summertime deep cumulus convection. The largest precipitation biases are produced by the Kain?Fritsch convection scheme over the Mediterranean. Precipitation biases in winter are lower than those for summer in all model configurations (15?30%). The results of this study indicate the importance of evaluating not only the basic climatic parameters of interest for climate change applications (temperature and precipitation), but also other components of the energy and water cycle, in order to identify the sources of systematic biases, possible compensatory or masking mechanisms and suggest pathways for model improvement.The contribution from Universidad de Cantabria was funded by the Spanish R&D programme through projects CORWES (CGL2010-22158-C02-01) and WRF4G (CGL2011-28864), co-funded by the European Regional Development Fund. M. García-Díez acknowledges financial support from the EXTREMBLES (CGL2010-21869) project

Added value and land-atmosphere coupling in convection-permitting WRF climate simulations over a Middle European domain

High-resolution regional climate models with a more detailed representation of heterogeneous land surface properties, as well as an explicit treatment of deep convection can lead to an improved simulation of meteorological processes and the climate system at the meso-gamma scale. In this study, results from 10 years of convection-permitting WRF evaluation simulations at 3 km spatial resolution for a central European model domain are analyzed. The 3 km domain is nested into the pan-European Coordinated Regional Downscaling Experiment (CORDEX) EUR-11 (12 km) model grid, driven by ERA-Interim reanalysis data. The simulated time spans (1992-1995, 2002-2003, 2010-2013) cover much of the variability of central European weather conditions. In our analysis, we focus on two aspects: The first focus is on the validation of precipitation. Results from both resolutions are compared with each other and evaluated against high-resolution reanalysis data and gridded observations. Hourly precipitation data over three regions with a very moderate, low mountain and high mountain topography are compared. Added value in the 3 km simulation is found especially at the sub-daily scale in the reproduction of intensity, diurnal cycle and spatial extent of precipitation. A positive precipitation bias found in both resolutions is more dominant in the 12 km simulation, where too much light precipitation is generated. For different seasons precipitation exhibits clear differences between the simulations whereby largest differences occur in mountainous regions and during the summer months with high convective activity. The second focus is on the comparison of land-atmosphere coupling strength whereby different metrics focusing on the soil moisture-temperature coupling are used. In both resolutions a clear interannual variability in coupling strength, consistent with the individual climate conditions, is seen. The 3 km simulation generally shows a slightly stronger coupling strength in summer

Land-atmosphere interactions in multiscale regional climate change simulations over Europe

Interactions between the heterogenous land surface and the atmosphere play a fundamental role in the weather and climate system through their influence on the energy and water cycles. Global climate models (GCMs) currently have coarse horizontal grid resolutions in the order of 100 km. With their higher resolution regional climate models (RCMs) better resolve mesoscale processes in the atmosphere and better represent the heterogenous land surface properties. Thus, RCMs are able to provide more detailed characteristics of regional to local climate. This thesis conducts regional climate simulations in multiple resolutions for the European domain of the Coordinated Regional Climate Downscaling Experiment (EURO-CORDEX) and a central European domain (3kmME) with the RCM WRF downscaling both ERA-Interim reanalysis and GCM MPI-ESM-LR (RCP4.5) climate change scenario data. The analysis focusses on land-atmosphere interactions to gain a better understanding of the regional water cycle components, the involved multi-scale processes, their sensitivities and variabilities both under present-day climate and future climate change conditions. Furthermore, the added value of the convection-permitting 3kmME simulations, being one of the first sets of decade-long convection-permitting regional climate simulations over Central Europe, is investigated. A comparison of summertime land-atmosphere coupling strength is carried out for a subset of the ERA-Interim-driven EURO-CORDEX model ensemble (1989 to 2008). The coupling strength is quantified by the correlation between the surface sensible and the latent heat flux, and by the correlation between the latent heat flux and 2m temperature and compared to European FLUXNET observations and gridded observational Global Land Evaporation Amsterdam Model (GLEAM) data, respectively. The RCM simulations agree with both observational datasets in the large-scale pattern characterized by strong coupling in southern Europe and weak coupling in northern Europe. However, in the transition zone from strong to weak coupling covering large parts of central Europe the majority of the RCMs tend to overestimate the coupling strength in comparison to both observations. The RCM ensemble spread is caused by the different land surface models applied, and by the model-specific weather conditions resulting from different atmospheric parameterizations. Investigation of land-atmosphere coupling strength in ERA-Interim driven WRF simulations in both 3 km and 12 km resolution for central Europe reveals large year-to-year variability related to the individual soil moisture conditions. Coupling strength largely differs for individual land use types. Forest compared to crop type reacts slower to drought conditions. Coupling is overall slightly stronger in the 3 km simulation, attributed to overall drier soils due to less precipitation. The projected climate change based on a WRF 0.44° simulation downscaling GCM MPI-ESM-LR (RCP4.5) data alters the European land-atmosphere coupling regimes in summer. Due to increasingly drier soils, stronger coupling is simulated for large parts of western, central and southern eastern Europe for the period 2071-2100 compared to 1971-2000. Areas of strongest future increase of extreme temperature coincide with strong coupling areas. In order to analyse the added value of convection-permitting 3 km climate simulations, nine years of ERA-Interim driven simulations with the WRF RCM at 12 km and 3 km grid resolution over central Europe are evaluated against observations with a focus on sub-daily precipitation statistics and the relation between extreme precipitation and air temperature. A clear added value of the higher resolution simulation is found especially in the reproduction of the diurnal cycle and the hourly intensity distribution of precipitation. Too much light precipitation in the 12 km simulation results in a positive precipitation bias. Largest differences between both resolutions occur in mountainous regions and during the summer months with high convective activity. Moreover, the observed increase of the temperature–extreme precipitation scaling from the Clausius-Clapeyron (C-C) scaling rate of ~7% K-1 to a super-adiabatic scaling rate is reproduced only by the 3 km simulation. The effect of land surface heterogeneity on the differences between 3 km and 12 km simulations is analysed based on five WRF simulations for JJA 2003, each with the same atmospheric setup in 3 km resolution but different combinations of 12 km resolution land use and soil type, initial soil moisture and orography. A coarser resolved orography significantly alters the flow over and around extensive mountain ridges like the Alps and impact the large-scale flow pattern. The smoothed mountain ridges result in weaker Föhn effects and in enhanced locally generated convective precipitation pattern peaking earlier in the afternoon. The effect of a coarser-resolved land use distribution is overall smaller and mainly related to changes in overall percentages of different land use types, rather than to the loss of heterogeneity in the surface pattern on the scale analysed here. Even small changes in soil moisture have a higher potential to affect the overall simulation results. WRF climate simulations downscaling the MPI-ESM-LR data at 12 km and 3 km resolution for central Europe are analysed for three 12-year periods: a control, a mid-of-century and an end-of-century projection to quantify future changes in precipitation statistics based on both convection-permitting and convection-parameterized simulations. For both future scenarios both simulations suggest a slight decrease in mean summer precipitation and an increase in hourly heavy and extreme precipitation in large parts of central Europe. This increase is stronger in the 3 km runs. Temperature–extreme precipitation scaling curves in the future climate are projected to shift along the 7% K-1 trajectory to higher peak extreme precipitation values at higher temperatures while keeping their typical shape. The results of this thesis clearly confirm the added value of convection-permitting climate simulations, provide further insights into land-atmosphere interaction processes and highlight the relevance of the RCMs ability to properly simulate coupling strength

Land atmosphere coupling in EURO-CORDEX evaluation experiments

Interactions between the land surface and the atmosphere play a fundamental role in the weather and climate system through their influence on the energy and water cycles. Here we present results of summertime land-atmosphere coupling strength in a subset of the ERA-Interim driven EURO-CORDEX model ensemble (1989-2008) including an evaluation of the coupling related variables soil moisture and surface energy fluxes of latent heat (LE) and sensible heat (H). Most of the regional climate models (RCMs) reproduce soil moisture and surface fluxes for the different European climate zones reasonably well. However, for some regions and models differences are identified, also compared to FLUXNET surface flux measurements used as observational reference. To quantify the coupling strength the H-LE-correlation method has been confirmed as useful and valid for the comparison of different RCMs. An important advantage of the method is that it can be applied to standard RCM output variables (H, LE) as well as to observations, although long time series of high quality flux measurement data are needed, which are available only for a few locations across Europe. For the full 20-year period of summer seasons the EURO-CORDEX simulations agree in the large-scale patterns, with strong coupling in Southern Europe and weak coupling for Northern Europe, also in agreement with the FLUXNET observations. For large parts of Central Europe, however, the model ensemble diverges between strong and weak coupling strength. Compared to the FLUXNET measurements more models tend to overestimate than underestimate the coupling strength. Higher model resolution leads to more small-scale heterogeneity but not necessarily to a change in the large-scale patterns. Diversity in the ensemble can be both explained by the different characteristics of the individual land surface models (LSMs) as well as different climate conditions in the models

Evaluation and projected changes of precipitation statistics in convection-permitting WRF climate simulations over Central Europe

We perform simulations with the WRF regional climate model at 12 and 3 km grid resolution for the current and future climates over Central Europe and evaluate their added value with a focus on the daily cycle and frequency distribution of rainfall and the relation between extreme precipitation and air temperature. First, a 9 year period of ERA-Interim driven simulations is evaluated against observations; then global climate model runs (MPI-ESM-LR RCP4.5 scenario) are downscaled and analyzed for three 12-year periods: a control, a mid-of-century and an end-of-century projection. The higher resolution simulations reproduce both the diurnal cycle and the hourly intensity distribution of precipitation more realistically compared to the 12 km simulation. Moreover, the observed increase of the temperature–extreme precipitation scaling from the Clausius–Clapeyron (C–C) scaling rate of ~ 7% K−1 to a super-adiabatic scaling rate for temperatures above 11 °C is reproduced only by the 3 km simulation. The drop of the scaling rates at high temperatures under moisture limited conditions differs between sub-regions. For both future scenario time spans both simulations suggest a slight decrease in mean summer precipitation and an increase in hourly heavy and extreme precipitation. This increase is stronger in the 3 km runs. Temperature–extreme precipitation scaling curves in the future climate are projected to shift along the 7% K−1 trajectory to higher peak extreme precipitation values at higher temperatures. The curves keep their typical shape of C–C scaling followed by super-adiabatic scaling and a drop-off at higher temperatures due to moisture limitation