The Met Office Unified Model and JULES Global Atmosphere and Land 9.0 configuration

Abstract. We describe Global Atmosphere and Land 9.0 (GAL9) and GAL9.1: the latest science configurations of the Met Office Unified Model and JULES land surface model developed for seamless use across weather and climate timescales. GAL9 builds upon GA8GL9. GAL9 includes developments to most areas of the science. Some of the key changes include: moving to a single "mid-level" scheme to represent surface driven deep convection and elevated convection that improves the diurnal cycle of convection, an enhancement to the semi-Lagrangian advection that improves local conservation, improved physical realism in the gravity wave drag scheme by accounting for the effects of humidity and latent heating, and a package of enhancements to the land surface which improve soil moisture profiles and remove the need to apply an observation based albedo scaling. The NWP and climate performance of GAL9 is evaluated against the previous configuration, GA8GL9. In NWP tests GAL9 is shown have reduced errors relative to GA8GL9. The mean climate in GAL9 is shown to be improved relative to GA8GL9 with notable improvements in the top of atmosphere outgoing shortwave radiation. NWP tests of GAL9.0 at 10 km showed a degradation relative to GA8GL9. This issue is resolved in the branch configuration GAL9.1 that includes an improved orography. GAL9/GAL9.1 is the atmosphere and land component of GC5/GC5.1. From early 2026, GC5.1 is being be used as the operational global NWP model at the Met Office and GC5 will be a key component of the UK's contribution to CMIP7.