Semi-Implicit Semi-Lagrangian (SISL) discretisations are employed in many operational Numerical Weather Prediction (NWP) models. These schemes allow stable and accurate advection of meteorological fields as well as accurate depiction of important atmospheric waves. This is achieved without loss of stability due to inaccuracy in resolving high frequency waves (e.g. acoustic waves).

In this talk, I introduce SISL discretisations and their application in the Met Office's Unified Model (UM). I will discuss a toy example, followed by the formulation of the discretisation for the general case. This provides the dynamics of the UM, but is only one key component of the entire model. I will give an overview of other necessary components before presenting experimental findings from my work to extend the UM into the high atmosphere.