4D-VAR: OPTIMISATION AND PERFORMANCE ON THE NEC SX-6

The Met Office 4D-Var scheme went operational in October 2004. In order to produce a timely analysis, 6 nodes are used compared to the 1 node that was previously required for 3D-Var. Even with this 6 fold increase in resources, 4D-Var takes 4 times longer to complete: Optimising 4D-Var for the SX-6 has been, and continues to be, a high priority. Porting the code from the Cray T3Es is described, along with optimisations introduced to October 2004, such as techniques for increasing vector lengths and reducing communication overheads. Impacts of optimisations are presented, along with investigations into the scalability of the code on a per routine basis.