Institute for Atmospheric and Climate Science

NCCR climate project (P2.1): Intensification of the water cycle: Scenarios, processes and extremes (HyClim)

PI: Martin Wild, Co-PI: Christoph Schär

Institute for Atmospheric and Climate Science, ETH Zurich

1. Three research questions of the project

2. Research Summary

There is increasing evidence from theory, observations and climate models that climate change will lead to an intensification of the hydrological cycle. On the regional scale, the magnitude of the changes in precipitation and evapotranspiration will strongly depend upon the season and affect the characteristics and frequency of extremes (heavy precipitation events, floods, droughts, and heatwaves). The main goals of the proposed project are:

(1) Global and regional climate change scenarios: New extended integrations using global (ECHAM5-HAM) and regional climate models (CLM/COSMO) will be conducted. They will cover the time period 1850-2100. In comparison with previous simulations (e.g. Schär et al. 2004, Bättig et al. 2006), the new integrations will make use of updated GCM and RCM model versions, will adopt improved histories of greenhouse gas and aerosol emissions, will include improved representations of aerosol effects, and will employ higher computational resolution (GCM: T106, RCM: 25 km). In addition, the simulations will be extended further back in time, considering also the climate evolution in the 19th century. These extended simulations allow addressing a number of open issues associated with hydrological features at the end of the little ice age. For example, the summers in Central Europe in this period frequently experienced extraordinary amounts of precipitation, leading to devastating flooding in various instances. These aspects of will be addressed in a PhD project based on the abovementioned global and regional climate simulations.

(2) Anthropogenic and natural radiative forcings and their impacts on water and energy cycles: The primary anthropogenic forcing upon our climate occurs through a perturbation of the Earth radiation balance (“radiative forcing“) in response to anthropogenic changes in atmospheric greenhouse gas and aerosol concentrations. Recent evidence suggests that significant decadal variations occur not only in the thermal (greenhouse) radiation but also in the amount solar radiation reaching the Earth’s surface (Wild et al. 2005). A substantial reduction of surface solar radiation was observed between the 1950s and 1980s (“global dimming”), with a more recent partial recovery (“brightening”). Recent studies suggest that global dimming and brightening has a major impact on various elements of the climate systems, not only on global warming, glacier and snow cover retreat (Wild, 2007, Wild et al. 2007), but particularly also on the strength of the water cycle (Wild et al. 2008). We intend to investigate the link between these anthropogenic and natural perturbations of the radiation balance and the intensity of the hydrological cycle using the global and regional climate models and comprehensive observational databases.

(3) Impacts of intensified water cycle upon extremes: The main thrust of the work funded by the current proposal will be related to the analysis of our own regional climate simulations in comparison with simulations conducted within the EU-project ENSEMBLES (Hewitt and Griggs 2004). In methodological terms, the work will extend on recent work conducted in relation to heat waves (Schär et al. 2004, Vidale et al. 2007, Fischer et al. 2007, Fischer and Schär 2008) and heavy precipitation events (Frei et al. 2006, Fowler et al. 2007). Simulations will be analysed for changes in characteristics (e.g. frequency, intensity) of the hydrological extremes with a focus on the European to Alpine region. The principal tool used will be statistical extreme value theory, applied to indices representing key aspects of climatic extremes on both seasonal and annual timescales (e.g. multi-day maxima; upper quantile values; return periods), and this builds upon recent such studies (e.g. Frei et al 2006; Fowler et al 2007). The physical nature of extreme events will also be examined in the context of known inter-model deficiencies, such as dry biases in southern European summer (e.g. Giorgi 2006) and the sensitivity of simulations with respect to convection parameterization (e.g. Brockhaus et al. 2008). Simulations will be validated against the latest high-resolution observational datasets (e.g. Haylock et al 2008).

The research on extremes will also be linked to work on cloud-resolving climate simulations (Hohenegger et al. 2008). Recent studies using this approach show that the representation of moist convection is a key uncertainty and can decisively influence climate-relevant feedback processes such as the soil-moisture precipitation feedback (Hohenegger et al. 2009). This approach is currently still too expensive for climate scenario simulations, but will become feasible within a few years.

3. Data and methods

To address the abovementioned scientific goals, we intend to perform extended simulations with the global climate model ECHAM5-HAM and the regional model CLM/COSMO. ECHAM5 HAM is a special version of global climate model ECHAM developed at the Max Planck Institute for Meteroology, Hamburg, which includes in addition a sophisticated aerosol module (Hamburg Aerosol Model - HAM, Stier et al. 2005) and detailed cloud microphysics processes (Lohmann et al. 2007, Lohmann 2008). This allows an improved representation of aerosol direct and indirect radiative effects. Since these effects are considered as major contributors to the observed decadal trends of dimming and brightening (Wild et al., 2005, Norris and Wild, 2007), this model version is particularly well suited to address the abovementioned research questions. The cloud microphysics scheme will undergo continuous development in project 2.2 during Phase 3, and we will employ the latest version provided by this project for the proposed simulations. The close interaction with project 2.2 will ensure the access to cutting edge modelling tools in our project. Decadal variations in surface radiative forcings and associated impacts on the climate system are currently unsatisfactory simulated in most climate models as e.g. used in the 4th IPCC assessment report (Wild 2008). We will thoroughly test the impact of the newly developed model physics on the model’s ability to reproduce the observed changes in the radiative forcing and related impacts, and provide feedback to the ongoing model development process in project 2.2. The comprehensive in house observational radiation datasets, updated to near present during Phase 2, will be used for this testing purpose.

The regional climate model experiments will be carried out using the CCLM numerical model (see, formerly referred to as COSMO or CLM or LM). Within NCCR phase II, we have switched to this model and developed both conventional (Dx=25 km) and cloud-resolving resolution (Dx=2 km) modeling systems. Sine the beginning of work with the CLM model in 2005, considerable experience has been established (cf. Brockhaus et al. 2008, Hohenegger and Schär 2007, Hohenegger et al. 2008, 2009, Jaeger et al. 2008).

4. Milestones and deliverables

After 18 months:

After 36 months:

5. Contribution to the WP1 and collaboration with other NCCR projects and 3rd parties

The results of the model integrations will be made available to P2.3 and P4.2 that rely on global and/or regional climate change scenarios. We rely on P2.2 regarding expertise in cloud and aerosol microphysics and provide feedbacks to P2.2 on the performance of the new scheme in extended simulations. We will further interact with P3.3 regarding land-surface parameterizations. Aerosol information obtained from ice core analyses in P1.2 (and related EU-FP6 projects) will be used in the interpretation of detected variations in surface radiation. Model intercomparison will be conducted with P1.1 using either a control period or a past period. The project will also be linked to the newly founded Center for Climate Systems Modeling (C2SM, see


Baettig, M., Wild, M., and Imboden, D., 2007: A climate change index – where climate change may be most prominent in the 21st century. Geophys. Res. Lett., 34, L01705, doi:10.1029/2006GL028159

Brockhaus, P., D. Lüthi and C. Schär, 2008. Aspects of the Diurnal Cycle in a Regional Climate Model. Meteorol. Z., 17 (4), 433-443

Fischer E.M., S.I. Seneviratne, D. Lüthi and C. Schär, 2007a: Contribution of land–atmosphere coupling to recent European summer heatwaves. Geophys. Res. Letters, 34, Art. No. L06707.

Fischer E.M., S.I. Seneviratne, P.L. Vidale, D. Lüthi and C. Schär, 2007b: Soil moisture - atmosphere interactions during the 2003 European summer heatwave. J. Climate, 20, 5081-5099

Fischer, E.M. and C. Schär, 2008: Future changes in daily summer temperature variability: driving processes and role for temperature extremes. Clim. Dyn., in press, DOI 10.1007/s00382-008-0473-8

Frei, C., R. Schöll, S. Fukutome, J. Schmidli and P.L. Vidale, 2006: Future change of precipitation extremes in Europe: Intercomparison of scenarios from regional climate models. J. Geophys. Res, 111, Art. No. D06105.

Fowler, H.J., M. Ekstrom, S. Blenkinsop and A.P. Smith, 2007: Estimating change in extreme European precipitation using a multimodel ensemble. J. Geophys. Res., 112, Art. No. D18104.

Giorgi, F. Regional climate modeling: Status and perspectives, 2006: J. De Physique IV, 139, 101-118.

Haylock, M.R., N. Hofstra, A.M.G. Klein Tank, E.J. Klok, P.D. Jones and M. New, 2008: A European daily high-resolution gridded dataset of surface temperature and precipitation. J. Geophys. Res., 113, Art. No. D20119.

Hewitt, C. D. and D. J. Griggs, 2004: Ensembles-based Predictions of Climate Changes and their Impacts. Eos, 85, p566.

Hohenegger, C. and C. Schär, 2007: Atmospheric predictability at synoptic versus cloud-resolving scales. Bulletin American Meteorol. Soc., 88 (11), 1783-1793.

Hohenegger, C., P. Brockhaus and C. Schär, 2008: Towards climate simulations at cloud-resolving scales. Meteorol. Z., 17 (4), 383-394

Hohenegger, C., P. Brockhaus, C.S. Bretherton and C. Schär, 2009: The soil moisture-precipitation feedback in simulations with explicit and parameterized convection. J. Clim., cond. accepted

Jaeger, E.B., I. Anders, D. Lüthi, B. Rockel, C. Schär, S. I. Seneviratne, 2008: Analysis of ERA40-driven CLM simulations for Europe. Meteorol. Z., 17 (4), 349-368

Norris, J. R., and M. Wild (2007), Trends in aerosol radiative effects over Europe inferred from observed cloud cover, solar ‘‘dimming,’’ and solar ‘‘brightening,’’ J. Geophys. Res., 112, D08214.

Schär, C., P.L. Vidale, D. Lüthi, C. Frei, C. Häberli, M.A. Liniger and C. Appenzeller, 2004: The role of increasing temperature variability for European summer heat waves. Nature, 427, 332-336.

Schmidli, J., C.M. Goodess, C. Frei, M.R. Haylock, Y. Hundecha, J. Ribalaygua and T. Schmith, 2007: Statistical and dynamical downscaling of precipitation: An evaluation and comparison of scenarios of the European Alps. J. Geophys. Res. – Atmos., 112, Art. No. D04105

Vidale P.L., D. Lüthi, R. Wegmann, C. Schär, 2007: European summer climate variability in a heterogeneous multi-model ensemble. Clim. Change, 81, 209–232

Wild M., and Co-authors, 2005: From dimming to brightening: Decadal changes in solar radiation at Earth's surface. Science, 308, 847-850.

Wild, M., 2007: Decadal changes in surface radiative fluxes and their importance in the context of global climate change, in, Advances in Global Change Research, 33, Editors S. Brönnimann et al.

Wild, M., A. Ohmura, and K. Makowski, 2007: Impact of global dimming and brightening on global warming. Geophys. Res. Lett., 34, L04702, doi:10.1029/2006GL028031

Wild, M., J. Grieser, and C. Schär, 2008, Combined surface solar brightening and increasing greenhouse effect support recent intensification of the global land-based hydrological cycle, Geophys. Res. Lett., 35, L17706, doi:10.1029/2008GL034842.

Wild, M. (2008), How do the IPCC AR4/CMIP3 models capture the effect of global dimming and brightening on 20th century warming?, submitted.


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