We describe Global Atmosphere 7.0 and Global Land 7.0 (GA7.0/GL7.0), the latest science configurations of the Met Office Unified Model (UM) and the Joint UK Land Environment Simulator (JULES) land surface model developed for use across weather and climate timescales. GA7.0 and GL7.0 include incremental developments and targeted improvements that, between them, address four critical errors identified in previous configurations: excessive precipitation biases over India, warm and moist biases in the tropical tropopause layer (TTL), a source of energy non-conservation in the advection scheme and excessive surface radiation biases over the Southern Ocean. They also include two new parametrisations, namely the UK Chemistry and Aerosol (UKCA) GLOMAP-mode (Global Model of Aerosol Processes) aerosol scheme and the JULES multi-layer snow scheme, which improve the fidelity of the simulation and were required for inclusion in the Global Atmosphere/Global Land configurations ahead of the 6th Coupled Model Intercomparison Project (CMIP6). In addition, we describe the GA7.1 branch configuration, which reduces an overly negative anthropogenic aerosol effective radiative forcing (ERF) in GA7.0 whilst maintaining the quality of simulations of the present-day climate. GA7.1/GL7.0 will form the physical atmosphere/land component in the HadGEM3–GC3.1 and UKESM1 climate model submissions to the CMIP6.

We describe Global Atmosphere 6.0 and Global Land 6.0 (GA6.0/GL6.0): the latest science configurations of the Met Office Unified Model and JULES (Joint UK Land Environment Simulator) land surface model developed for use across all timescales. Global Atmosphere 6.0 includes the ENDGame (Even Newer Dynamics for General atmospheric modelling of the environment) dynamical core, which significantly increases mid-latitude variability improving a known model bias. Alongside developments of the model's physical parametrisations, ENDGame also increases variability in the tropics, which leads to an improved representation of tropical cyclones and other tropical phenomena. Further developments of the atmospheric and land surface parametrisations improve other aspects of model performance, including the forecasting of surface weather phenomena. We also describe GA6.1/GL6.1, which includes a small number of long-standing differences from our main trunk configurations that we continue to require for operational global weather prediction. Since July 2014, GA6.1/GL6.1 has been used by the Met Office for operational global numerical weather prediction, whilst GA6.0/GL6.0 was implemented in its remaining global prediction systems over the following year.

In this paper we define the first Regional Atmosphere and Land (RAL) science configuration for kilometre-scale modelling using the Unified Model (UM) as the basis for the atmosphere and the Joint UK Land Environment Simulator (JULES) for the land. RAL1 defines the science configuration of the dynamics and physics schemes of the atmosphere and land. This configuration will provide a model baseline for any future weather or climate model developments to be described against, and it is the intention that from this point forward significant changes to the system will be documented in the literature. This reproduces the process used for global configurations of the UM, which was first documented as a science configuration in 2011. While it is our goal to have a single defined configuration of the model that performs effectively in all regions, this has not yet been possible. Currently we define two sub-releases, one for mid-latitudes (RAL1-M) and one for tropical regions (RAL1-T). The differences between RAL1-M and RAL1-T are documented, and where appropriate we define how the model configuration relates to the corresponding configuration of the global forecasting model.

High‐resolution simulations of flows over South Georgia (South Atlantic) are used to increase understanding of the likely influence of small isolated mountainous islands on the large‐scale flow and to ascertain the extent to which parametrization schemes can account for the missing drag in models where such islands are only partially resolved. Long‐duration (1 month) austral winter forecasts with a horizontal grid spacing of 1.5 km are used to quantify the mountain‐wave momentum fluxes generated by the island and the low‐level drag associated with flow‐blocking dynamics. The characteristics of the drag, such as the occurrence of high and low drag states, its dependence on wind direction and the spectral contributions to the mountain‐wave momentum flux, are considered. Flow splitting and low‐level wave breaking are shown to be responsible for wake regions that extend for hundreds of kilometres from the island. Regions of deceleration are also evident in the stratosphere, due to mountain‐wave dissipation. The extent to which an orographic drag parametrization scheme can reproduce the drag is investigated by comparison with coarse‐resolution (15 km grid spacing) simulations in which the orography is poorly resolved and a large proportion of the drag is parametrized. It is demonstrated that the total of the resolved plus parametrized drag in these simulations closely resembles that at high resolution, although it is underpredicted during instances of high drag. Simple modifications to the scheme, which enhance the drag when the low‐level flow is approximately normal to the major axis of the subgrid orography, are shown to rectify this. The study demonstrates that, at least for relatively simple isolated mountain ranges, the drag and mountain‐wave momentum fluxes can be predicted in a deterministic sense by a well‐tuned parametrization scheme of suitable complexity, although inclusion of stochastic effects might lead to yet further improvements.