Improved regional scale processes reflected in projected hydrological changes over large European catchments

by Stefan Hagemann,  H. Göttel, D. Jacob, P. Lorenz and E. Roeckner

 

Outline
For the 4th assessment report of the Intergovernmental Panel on Climate Change (IPCC), the recent version of the coupled atmosphere/ocean general circulation model (GCM) ECHAM5/MPIOM (Roeckner et al. 2003, Jungclaus et al. 2006) of the Max Planck Institute for Meteorology has been used to conduct an ensemble of transient climate simulations These simulations comprise three control simulations for the past century covering the period 1860-2000, and nine simulations for the future climate (2001-2100) using greenhouse gas and aerosol concentrations according to the three IPCC scenarios B1, A1B and A2. For each scenario three simulations were performed. The global simulations were dynamically downscaled over Europe using the regional climate model (RCM) REMO (Jacob 2001) at 0.44° horizontal resolution (about 50 km), whereas the physics packages of the GCM and RCM largely agree. The regional simulations comprise the three control simulations (1950-2000), the three A1B simulations and one simulation for B1 as well as for A2 (2001-2100). In our study we concentrate on the climate change signals in the hydrological cycle and the 2m temperature by comparing the mean projected climate at the end of the 21st century (2071-2100) to a control period representing current climate (1961-1990). The robustness of the climate change signal projected by the GCM and RCM is analysed focussing on the large European catchments of Baltic Sea (land only), Danube and Rhine, which are representing different climate conditions. In this respect, a robust climate change signal was defined as a projected change (2071-2100 compared to 1961-90) that is larger than the spread representing the natural climate variability in these models.

Results
The analysis of the annual mean changes yielded a robust increase in all components of the terrestrial water balance over the Baltic Sea catchment, an overall robust increase in evapotranspiration except for the Danube catchment, and a robust decrease of runoff for Danube and Rhine in the GCM. The latter is much smaller in the RCM and not even robust for the Danube catchment. In addition, pronounced robust seasonal signals were found, even in cases where the projected signal in the annual mean is not robust. The projected future warming at the end of this century is robust in all month over all catchments. Over the Baltic Sea catchment, a general increase of precipitation is projected except for the summer, which is accompanied by a general increase in evapotranspiration and an increase in runoff in the autumn and winter. For the Danube and Rhine catchments, a decrease of summer time precipitation and runoff is projected. For the Danube, the drying of soil moisture leads to reduced evapotranspiration in the summer, while the wetter mean state of the Rhine catchment yields larger buffer capacities of soil water storage that cause a robust evapotranspiration decrease only in the late autumn in the GCM simulations. In addition, robust increases in evapotranspiration are projected in the winter (except for the RCM over the Danube catchment) and spring. The general changes agree well with the large-scale climate change patterns over Europe obtained in previous studies (e.g. within the EU project PRUDENCE; Christensen and Christensen 2007).

Noticeable differences in the robustness of the climate change signals between the GCM and RCM simulations are related to a stronger warming of about 1 K projected by the GCM in the winter over the Baltic Sea catchment and in the summer over the Danube (see Fig. 1) and Rhine catchments. The first is accompanied by a much larger evapotranspiration increase in the GCM, especially in the winter. The latter is associated with a stronger projected summer drying in the two catchments (see Fig. 1 for Danube precipitation changes).

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Fig. 1    Monthly mean temperature (upper panels) and precipitation (lower panels) changes (2071-2100 compared to 1961-90) over the Danube catchment as projected by the GCM ECHAM5/MPIOM (left panels) and the RCM REMO (right panels). Max Std denotes the maximum spread S for the corresponding ensembles.

 

Rhine and Danube
The different behaviour of GCM and RCM is likely to be caused by the fact that the higher resolution of the RCM leads to a better representation of local scale processes including soil moisture feedbacks to the atmosphere. Seneviratne et al. (2006) stated that due to the northward shift of climatic regimes in Europe in response to increasing anthropogenic greenhouse gas concentrations, a new transitional climate zone between dry and wet climates with strong landatmosphere coupling will be created in central and eastern Europe. They also pointed out that landatmosphere coupling is significantly affected by global warming and is itself a key player for climate change, thereby highlighting the importance of soil-moisturetemperature feedbacks (in addition to soil-moistureprecipitation feedbacks) for future climate changes over this region. Van den Hurk et al. (2005) stated that in many cases models overemphasize the positive land-atmosphere feedback that leads to a dry soil, strong evaporation stress and reduced precipitation, which poses severe problems in the interpretation of hydrological aspects of climate change in future greenhouse gas emission scenarios. In this respect, the so called !Hsummer drying problem!I (the too dry and too warm simulation of the summertime climate over central and eastern Europe) is often reported for many GCMs and RCMs. Hagemann et al. (2004) considered this problem over the Danube area in more detail for five different RCMs.

Fig. 2 shows that the coupled GCM ECHAM5/MPIOM has a relatively strong summer drying problem in both catchments, which is consistent with the behaviour of the atmospheric GCM ECHAM5 forced by observed SST, as shown for the Danube by Hagemann et al. (2006). The problem is much less pronounced in the RCM, which even shows some overestimation of summer rainfall over the Rhine catchment. Within PRUDENCE, results of Hagemann and Jacob (2007) indicated that the use of RCMs can overcome problems that a driving GCM might have with the representation of local scale processes or parameterizations. This supports that the RCM has the potential for an improved simulation of soil moisture feedbacks to the atmosphere, which in turn leads to the lower projected summer time warming and drying than projected by the GCM.

 

Fig. 2    Observed and simulated monthly ensemble mean precipitation over the Danube (left panel) and Rhine (right panel) catchment for the control climate 1961-90. Observations are taken from the Global Precipitation Climatology Centre (GPCC; Fuchs et al. 2007) and from the Global Precipitation Climatology Project (GPCP; Huffman et al.1997) at 2.5° resolution.

 

Conclusions
The better description of surface processes, higher resolution and non-linear scale interactions in the RCM gives a better representation of present day climate and hence a more credible climate change projection. This is even along the lines of thoughts provided in the IPCC AR4 global and regional climate change chapters (IPCC 2007). Over the Baltic Sea catchment, the RCM has an improved representation of the land sea contrast, and, hence, improved related moisture transport processes between water and land areas. Over the Danube and Rhine catchments, the better distribution of soil moisture leads to an improved representation of soil moisture feedbacks to the atmosphere.

The described study is published in:

Hagemann, S., H. Göttel, D. Jacob, P. Lorenz and E. Roeckner (2008) Improved regional scale processes reflected in projected hydrological changes over large European catchments. Clim. Dyn., doi: 10.1007/s00382-008-0403-9.

 

References

  • Christensen JH, Christensen OB (2007) A summary of the PRUDENCE model projections of changes in European climate by the end of this century. Climatic Change (Prudence Special Issue) 81, Supplement 1, doi: 10.1007/s10584-006-9210-7: 7-30.
  • Fuchs T, Schneider U, Rudolf B (2007) Global Precipitation Analysis Products of the GPCC. Global Precipitation Climatology Centre (GPCC). Deutscher Wetterdienst, Offenbach, Germany.
  • Hagemann S, Arpe K, Roeckner E (2006) Evaluation of the hydrological cycle in the ECHAM5 model. J Climate 19: 3810-3827.
  • Hagemann S, Jacob D (2007) Gradient in the climate change signal of European discharge predicted by a multi-model ensemble. Climatic Change (Prudence Special Issue) 81, Supplement 1, doi:10.1007/s10584-006-9225-0: 309-327.

 

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