A regional implementation of a mixed finite-element, semi-implicit dynamical core

This paper explores how to adapt a new dynamical core to enable its use in one-way nested regional weather and climate models, where lateral boundary conditions (LBCs) are provided by a lower-resolution driving model. The dynamical core has recently been developed by the Met Office and uses an iterated-semi-implicit time discretisation and mixed finite-element spatial discretisation. The essential part of the adaptation is the addition of the LBCs to the right-hand-side of the linear system which solves for pressure and momentum simultaneously. The impacts on the associated Helmholtz preconditioner and multigrid techniques are also described. The regional version of the dynamical core is validated through big-brother experiments based on idealised dynamical core tests. These experiments demonstrate that the subdomain results are consistent with those from the full domain, confirming the correct application of LBCs. Inconsistencies arise in cases where the LBCs are not perfect, but it is shown that the application of blending can be used to overcome these problems.