Planning for a Large Ensemble based on the HadGEM3 climate model

Large ensembles of climate-scale model simulations are key tools for assessing climate risks, separating internal variability from external forcing, and interpreting the observational record. Several modelling centres have produced such ensembles over the past years. Here we present early plans for the development of a new Large Ensemble based on the HadGEM3 (Hadley Centre Global Environment Model version 3) climate model. The initial plan envisages a 40-member ensemble spanning 150 years of historical/scenario climate (1950-2100) at a resolution of N216 (about 60 km) in the atmosphere and ¼° in the ocean.

This initiative is part of the recently started UK NERC multi-centre project CANARI (Climate change in the Arctic-North Atlantic Region and Impacts on the UK). CANARI aims to advance understanding of the impacts on the UK arising from climate variability and change in the Arctic-North Atlantic region, with a focus on extreme weather and the potential for rapid, disruptive change. While we aim for the new Large Ensemble to become a resource for a wide range of applications, it will support addressing the CANARI science questions in particular. These questions are concerned with, for example, the (i) projected Arctic change and potential lower-latitude influences through atmospheric or oceanic pathways, (ii) the projected change in the large-scale (North Atlantic) ocean/atmosphere circulation, its drivers, and interaction with weather systems, and (iii) projected impacts on the UK arising from extreme weather (windstorms and flooding, blocking, heatwaves and droughts).

This poster invites discussion with the community on all aspects of the design of the new Large Ensemble, and particularly seeks input regarding

• the choice/number of experiments to follow (from CMIP6 Scenario MIP),
• the initialisation strategy, and the degree to which slow (10 years and longer) variability, particularly in the ocean, should be sampled, and
• the desired output.