Gung Ho: A code design for weather and climate prediction on exascale machines

The Met Office's numerical weather prediction and climate model code, the Unified Model (UM) is almost 25 years old. It is a unified model in the sense that the same global model is used to predict both short term weather and longer term climate change. More accurate and timely predictions of extreme weather events and better understanding of the consequences of climate change require bigger computers. Some of the underlying assumptions in the UM will inhibit scaling to the number of processor cores likely to be deployed at Exascale. In particular, the longitude-latitude discretisation presently used in the UM would result in 12 m grid resolution at the poles for a 10 km resolution at mid-latitudes. The Gung Ho project is a research collaboration between the Met Office, NERC funded researchers and STFC Daresbury to design a new dynamical core that will exhibit very large data parallelism. The accompanying software design and development project to implement this new core and enable it to be coupled to physics parameterisation, ocean models and sea ice models, for example is presented and some of the computational science issues discussed.