Large Scale Numerical Simulations of the Climate

The Met Office Hadley Centre for Climate Science and Services, based at the Met Office's Exeter HQ, provides world-class guidance on the science of climate change and is the primary focus in the UK for climate science. The Hadley Centre makes significant contributions to scientific literature and to a variety of climate science reports, including the International Panel on Climate Change (IPCC).In October 2014 the Government confirmed its investment of £97 million in a new high performance computing facility for the Met Office. The new Cray XC40 located at Exeter Science Park is the largest HPC system in the world dedicated to weather and climate research.I will first give an overview of how climate numerical experiments are organised worldwide through the coupled model intercomparison project (CMIP) under the auspices of the world climate research program (WCRP).The numerical results of these simulation campaigns are submitted to intense scrutiny by the scientific community and policymakers. Numerical reproducibility is therefore of paramount importance. I will explain the parameters of what numerical reproducibility means to our community and how we aim to achieve it.I will present the use of different types of floating point arithmetic in the models. Two examples are the use double-double precision for reproducible global sums and research on single precision algorithms for computational efficiency.Finally, I will look at some of the challenges to maintain numerical reproducibility in the exascale era.