RT4 coordinated experiments

1. RT4 coordinated experiments Rowan Sutton Centre for Global Atmospheric Modelling Department of Meteorology University of Reading

2. RT4 Coordinated Experiments Basic idea: • Controlled experiments repeated with several different climate models to advance understanding of the factors/processes controlling future climate and related uncertainty in climate forecasts. • A cross cutting activity in RT4, linking WPs 4.1, 4.2, 4.3 and 4.4 From the DoW: • “The experiments will be designed to investigate and understand factors controlling climate at selected time periods (e.g. 1850, 2000, 2050). The experiments will be conducted with both atmospheric and coupled GCMs. Specific experiments will be designed to investigate issues such as the role of specific feedbacks, sensitivity to resolution, and sensitivity to oceanic initial conditions.”

3. Participating Groups • NERSC • Kiel • CGAM • CNRM • CERFACS • INGV More welcome!

4. Broad Aims • Understanding climate, and climate forecast uncertainty (WP4.4), at a mechanistic/process level, particularly in terms of the role of specific feedbacks (WP4.1), the regional patterns of climate change (WP4.2), and the factors governing the frequency and characteristics of extreme events (WP4.3) • Add value to information available from core ENSEMBLES hindcasts, forecasts and scenario integrations • Need a simple core set of experiments so that they can be done by all groups

5. Some Scientific Questions/Issues • Role of specific feedbacks (clouds, land surface, sea ice etc) in response to GHG forcing • What factors govern regional patterns of climate change, e.g. land/sea contrast in “global warming”? • Impact of the “1976 climate shift” on ENSO; Interaction of the tropical oceans • Euro-Atlantic weather regimes & the “non-linear paradigm for climate change” • Changes in the hydrological cycle – role of direct GHG forcing vs SST change • Impact of anomalies in SST (e.g. AMOC related or Indian Ocean), sea ice and snow extent on European climate including extreme events.

6. What determines the land/sea contrast in warming? Multi model ensemble annual mean temperature change for 2071-2100 relative to 1961-1990 under SRES A2 scenario Source: IPCC

7. Uncertainty about the land/sea temperature contrast just as important as uncertainty about climate sensitivity Source: Jonathan Gregory, CGAM

8. An overarching issue for ENSEMBLES (and climate policy) • How can we narrow, or more reliably quantify, uncertainty in climate forecasts/projections for key (high impact) climate variables in key vulnerable regions, e.g.: • S.E. Asia: • Monsoon rainfall; sea level rise • Europe: • Frequency of heat waves; THC change • Drought frequency in semi-arid areas • Essential to understand processes that shape these critical aspects of climate and lead to forecast spread. • In combination with observations, the only basis for weighting forecasts/models.

9. Straw Man • Design a simple core set of experiments relevant to a range of scientific questions • Additional sensitivity experiments performed by some but not all groups • In terms of understanding forecast uncertainty, suggestion here is to focus on understanding model uncertainty (rather than i.c. or scenario uncertainty) – this is where coordinated expts can most obviously add value.

10. Proposed Experiments • CMIP 1% pa increasing CO2 experiment • Time slice AGCM experiments to understand processes in the atmosphere / land system, including the impact of changing SST and sea ice boundary conditions • Core set: 2*10 year integrations using common, time invariant, boundary conditions (SST, SIE, CO2, other?) representative of 1950, 2000, 2050. • Additional sensitivity experiments: to be proposed by groups, e.g. turning off specific feedbacks, impact of MOC change (imposed via SST), impact of sea ice, snow or soil moisture anomalies on extremes etc

11. Possible Process • CGAM would supply boundary conditions for core set of time slice experiments. Individual groups responsible for interpolation onto their model grid (taking care over sea ice) and carrying out integrations. • WP leaders to supply list of diagnostics required. Consolidated list to be distributed by CGAM. • Output to be archived at a common centre (ENSEMBLES or IPCC data centre?) in a common format (netCDF). • System for diagnostic subprojects as in CMIP? • Experimental details and status to be documented on RT4 www site

12. Groups and Models

13. Issues for discussion • Which are the most interesting/important scientific questions that might be addressed by coordinated experiments? • Is the straw man acceptable? Can it be improved? • Details of the core set • What sensitivity experiments are of most interest to individual WPs or groups? • Process issues

14. Core experiments Proposal: 2*10 year integrations using common, time invariant, boundary conditions (SST, SIE, CO2, other?) representative of 1950, 2000, 2050. To discuss: • Duration of integrations/ensemble size • SST/SIE: • Observations averaged over 1945-54 &1995-2004? (what about SIE for early period?) • Projections from one model (e.g. HadGEM1): take average anomalies for 2045-2054 and add to observed climatology? SIE needs care (see PRUDENCE). • Other: GHG, aerosols, land surface variables?

15. Proposed Timetable • Mo 12 (Aug 05) – finalise experimental design (details not just concept) • Mo 24 – complete core experiments • Mo 24+ • analysis of core experiments • additional sensitivity experiments