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climate modelling


believes that the critical element here is scalability because, unless the system is able to scale to use tens of thousands of cores in parallel, complex models cannot be run within their operational windows. ‘All of the major modelling groups around the world have efforts underway to address scalability from an algorithmic perspective as they understand that the world is moving towards millions of cores, and that the algorithms need to keep pace,’ he said. So far the results are very encouraging and there have been demonstrations of models scaling up into hundreds of thousands of cores. In the UK, there is an experiment looking at


automated code generation that Pier Luigi Vidale believes will improve portability, by enabling scientists to write their algorithms at a very high level. Essentially, the machine code would be generated by a tool, rather than the scientist, and then submitted to the computational platform. Tis represents a fundamental change as it would isolate the scientist from the underlying hardware. However, it would have the benefit of eliminating the need to readapt and rewrite code every time a new computer becomes available. In terms of the hardware, one challenge within


the climate community is the broad adoption of accelerators. ‘Most weather centres use regular x86 processors, but there is a lot of discussion surrounding GPGPU (general-purpose


computation on GPU) and Intel Xeon Phi,’ said Nyberg. Te question, he explained, of what a processor will look like five years from now is an open one, and the modelling community must factor this uncertainty when investing expertise in code development. He believes that the best option is to try to focus less on particular instantiations of technology – especially from a processor perspective – and more on general parallelisation at all levels, as that will provide a safeguard for algorithmic optimisations. ‘Te relationship [between the HPC and


numerical weather prediction communities] is very much a symbiotic one,’ added Nyberg. ‘Weather centres push us in HPC to improve our technologies, while our advancements enable them to test their modelling and simulation limits.’ At the end of 2013, Archer – the next


generation of a national HPC facility in the UK, will become available – impacting the types of modelling that researchers like Pier Luigi Vidale and Kirsty Pringle are able to conduct. According to Vidale, Archer (Advanced Research Computing High End Resource) promises to be at least twice as fast as the Cray XE6 system, HECToR, currently available to the UK academic community – and equivalent to Hermit, installed at HLRS Stuttgart in Germany, which he used during the Upscale project in


2012. Archer, also a Cray machine, will enable Vidale’s group to set up a coupled model that includes both the atmosphere and ocean at very high resolutions. ‘Te opportunity we had in Germany meant that we were using observed sea surface temperatures, because running a project like that would have taken longer than the single year allocation would have allowed,’ he explained. ‘A coupled model is very important for long-term climate prediction and projection. Our access to Archer will allow us to run two coupled simulations concurrently and continuously for at least two years.’ Pringle agrees that, in the future, more


modelling groups will take the approach of either using multiple models to perform experiments, or perform experiments with multiple different setups of a single model: ‘In this way we can be sure that the results are not strongly sensitive to uncertainties within the model and hopefully this will improve the robustness of our research. Tere continues to be much public concern about the reliance on climate models for climate science; I hope that, as we really probe and test our models, we can better understand how confident (or not) we are in the results and this will help us communicate this to the public.’


PL Vidale holds the ‘Willis Chair of Climate System Science and Climate Hazards’


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