Evaluation of the Met Office Unified Model/JULES third Regional Atmosphere and Land configuration (RAL3)

The Met Office Unified Model/JULES has different physical parametrizations and their settings for its global and regional model simulations. The Met Office, in collaboration with its international partners, have been developing the third Regional Atmosphere and Land (RAL3) science configuration in recent years. Previous RAL configurations used different parametrization schemes in the tropics and mid-latitudes. The RAL3 configuration marks a major advance by unifying these. Thus, creating a single configuration suitable for any simulations in any region. This became possible due to the introduction of a new double-moment microphysics scheme: CASIM. Further changes include the introduction of a new bi-modal cloud scheme and consolidation of the global and regional land settings. The development within the partnership has led to a more complete evaluation across the globe. The overall assessment of RAL3, against the current configuration (RAL2), indicates more realistic precipitation distributions; more realistic representation of clouds; more realistic visibility; a continued trend to more realistic representation of convection (including improvement in the diurnal cycle); an increase in the cold temperature bias; and reduced 10 m windspeeds. We further examine the differences between RAL3 and RAL2 through the consideration of a case study: 23 October 2022. This case study is of a quasi-linear convective system that produced a strong downburst over the UK. This event is used to show some of the feedbacks and changes that the new configuration has produced in terms of the event’s structure and processes via analysis of potential vorticity and microphysical properties, amongst other variables. Overall, it is determined that RAL3 has added value over RAL2 in three broad ways: i) a significant improvement in the science, including the unification of tropical and midlatitude configurations and unification of the global and regional land surface parameters; ii) many physically-based improvements have been made in areas that are identified as model development priorities; and iii) an overall improvement in the verification scores and climatological distributions in the majority of areas assessed.