Framework for ENSEMBLES RT4 Coordinated Experiments
Proposed scientific focus
- With respect to climate forecast uncertainty, focus on understanding model uncertainty (rather than scenario uncertainty or initial condition uncertainty) since that is where coordinated experiments can most obviously add value.
- A major but not exclusive scientific focus should be understanding the factors that determine the land sea-warming contrast . The ratio of warming over land to warming over ocean varies between about 1.2 and 1.9 in IPCC TAR models. This level of uncertainty is comparable to that in global mean temperatures (i.e. climate sensitivity) and has significant implications for climate impacts. Analysis to date points to the potential importance of land surface and cloud feedbacks but the reasons different models give different ratios are not understood. This topic has clear relevance to all four work packages in RT4. The relevance to WP4.1 and WP4.2 is obvious. For WP4.3 the focus could be understanding how patterns of mean change are related to changes in the frequency of extreme events such as heat waves and droughts. For WP4.4 the focus would be understanding and quantification of model uncertainty.
A. AGCM experiments
- I. Core set: control and 2xCO2 experiments using common time invariant, SST and ice fields as lower boundary conditions, taken from a coupled experiment with the Hadley Centre model. (See boundary conditions).
The purpose of using common lower boundary conditions is to remove some sources of inter-model variance (e.g. sea ice-albedo feedback) in order to better understand others.
- II. Enhanced set: additional experiments to involve perturbations which may influence, e.g., cloud or land-surface feedbacks, effect of SST or sea-ice anomalies etc.
B. AGCM-slab experiments
- I. Selected experiments from the core set to be repeated with the same AGCM coupled to slab ocean model to examine impact of interactive SST. A limitation of AGCM experiments is that prescribed SST allows no interactive response of the ocean to changing climate, and this limitation can sometimes give the wrong answer (e.g. Douville, Climate Dynamics, in press; Sutton and Mathieu, QJRMS, 128, 1259-1275, 2002). Some improvement, or at least understanding of sensitivity, can be achieved by using experiments in which the atmosphere model is coupled to a slab ocean. In this case the ocean heat flux convergence (aka Q-flux) must be specified. The core set of experiments could be repeated with some agreement as to how to specify the Q-flux (and possibly the change in Q-flux) in order to provide maximum insight. (Note that an ensemble of slab model experiments with perturbed model parameters has already been conducted at the Hadley Centre with versions of the HadCM3 model. The ENSEMBLES RT4 coordinated experiments would expand the scope of these experiments to include structural uncertainty.) These experiments could involve use of Q-flux anomalies.
Proposed process and timeline