Abstract
We describe the approach taken to develop the United Kingdom's first community Earth system model, UKESM1. This is a joint effort involving the Met Office and the Natural Environment Research Council (NERC), representing the U.K. academic community. We document our model development procedure and the subsequent U.K. submission to CMIP6, based on a traceable hierarchy of coupled physical and Earth system models. UKESM1 builds on the well‐established, world‐leading HadGEM models of the physical climate system and incorporates cutting‐edge new representations of aerosols, atmospheric chemistry, terrestrial carbon, and nitrogen cycles and an advanced model of ocean biogeochemistry. A high‐level metric of overall performance shows that both models, HadGEM3‐GC3.1 and UKESM1, perform better than most other CMIP6 models so far submitted for a broad range of variables. We point to much more extensive evaluation performed in other papers in this special issue. The merits of not using any forced climate change simulations within our model development process are discussed. First results from HadGEM3‐GC3.1 and UKESM1 include the emergent climate sensitivity (5.5 and 5.4 K, respectively) which is high relative to the current range of CMIP5 models. The role of cloud microphysics and cloud‐aerosol interactions in driving the climate sensitivity, and the systematic approach taken to understand this role, is highlighted in other papers in this special issue. We place our findings within the broader modeling landscape indicating how our understanding of key processes driving higher sensitivity in the two U.K. models seems to align with results from a number of other CMIP6 models.
Highlights
The HadGEM models of the climate system developed at the Met Office Hadley Centre (MOHC) and their predecessors are widely recognized as world‐leading contributions to the international ensemble of climate models
The recognition of the increasing commitment of the Natural Environment Research Council (NERC) Research Centres and U.K. universities to the Unified Model (UM) system led to the first discussions on how to jointly develop a second‐generation Earth system (ES) model that was targeted at the sixth‐generation Coupled Model Intercomparison Project (CMIP6, Eyring et al, 2016)
We describe how a jointly funded core team dispersed across many centers, but with a critical mass based at the Met Office, has been able to pull through new representation of ES processes developed at many institutions into UKESM1, while building on parallel developments of the core physical model, HadGEM3‐GC3.1, made through the Met Office seamless Unified Model system
Summary
The HadGEM models of the climate system developed at the Met Office Hadley Centre (MOHC) and their predecessors are widely recognized as world‐leading contributions to the international ensemble of climate models. These models have been part of the Coupled Model Intercomparison Project (CMIP Meehl et al, 2005) from its first incarnation (CMIP1) to CMIP5, and projections from them have been included in every Intergovernmental Panel on Climate Change (IPCC) scientific assessment report. The recognition of the increasing commitment of the NERC Research Centres and U.K. universities to the UM system led to the first discussions on how to jointly develop a second‐generation Earth system (ES) model that was targeted at the sixth‐generation Coupled Model Intercomparison Project (CMIP6, Eyring et al, 2016)
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