Lateral boundary conditions for limited area models

Limited area models (LAMs) are widely used in numerical weather prediction and regional climate modelling to obtain high-resolution results that would be too expensive computationally to be obtained with a global model. To run LAMs requires lateral boundary conditions (LBCs) which are normally obtained from a global model or another (lower-resolution) LAM containing the domain of the smaller LAM. It is widely thought that the LBCs are a source of significant errors in LAMs but in the Meteorological Office Unified Model (UM) LBC errors are shown to be a tiny part of the overall error. The source of LBC errors is explained and the LBC procedure employed in the UM is described. The UM LBC procedure is effectively transparent for a nested LAM with the same resolution and grid points as a LAM providing its LBCs.