Next Generation Weather and Climate Prediction: Atmospheric Model Dynamical Core

The next generation of weather and climate models used by the UK Met Office (as well as the community of weather and climate researchers in the UK) will be run on supercomputers with an enormous number of processors operating in parallel. Running a weather model on such a supercomputer has the simple requirement that doubling the number of parallel processors will halve the time required to make a forecast: this is called 'parallel scaling'. It is a great challenge of scientific computing to get a computer model to scale well on massive supercomputers; unfortunately some algorithms used in models make the forecast very accurate, but do not scale well. The aim of the NGWCP project is to identify new algorithms that can scale well on supercomputers without sacrificing forecast skill (the ability of a model to correctly forecast the weather). In this proposal, we choose to concentrate on two aspects, the horizontal grid, and the transport algorithms. In the current Met Office model, a latitude-longitude grid is used in the horizontal. This type of grid has the problem that all the lines of latitude converge on the poles, so they get closer and closer together. It turns out that this is very bad for parallel scaling. In this proposal, we suggest three new types of grids that keep grid lines the same distance apart all over the globe, keeping various favourable properties of the latitude-longitude grid. Transport algorithms in the model are responsible for modelling the movement of quantities (such as temperature or moisture) with the wind. In the Met Office Model, this is done by tracing back the paths of fluid parcels around the globe. It turns out that this method is also bad for parallel scaling. In this proposal, we will investigate some recently developed alternatives that do not require these path calculations. It is essential that these algorithms reflect physical principles at work in the real atmosphere, such as the Law of Conservation of Mass.