Assessment of the Met Office Global Coupled model version 5 (GC5) configurations

Abstract. GC5 is the latest global coupled configuration of the Met Office Unified Model (MetUM), which is a combination of Global Atmosphere and Land (GAL9) and Global Ocean and Sea Ice (GOSI9). GAL9 includes more than 26 science tickets that cover changes in almost all areas of global model science, including several enhancements to the 6A convection scheme, latent heating in the gravity wave drag scheme, and the ’fountain buster’ scheme, which adds improvements to the advection conservation under conditions of near-grid-scale convergence, bimodal cloud initiation, and dust-dependent ice-nucleation temperature. GOSI9 is based on the NEMO 4.0.4 ocean model and a new sea ice (SI) model SI³. This includes a new equation of state (TEOS-10), 4th-order tracer advection, adaptive-implicit vertical advection and Southern Ocean tuning. GC5 is evaluated against Global Coupled Model version 4 (GC4) and ERA5 reanalysis and observations in terms of the global mean state, variability and NWP performance.

Important model improvements in global temperature and winds, subtropical jet, monsoon processes over India and the Maritime continent, and their diurnal cycle in convection were noted. Most aspects of processes over Africa were also notably improved. Southern Ocean biases continue to decrease in GC5 as a result of the Southern Ocean package including the Antarctic Sea ice area. On NWP (Numerical Weather Prediction) scales, significant improvements in the tropical temperature at 850 hPa and precipitation SEEPS (Stable Equitable Error in Probability Space) score at all lead times were achieved. In addition, the tropical cyclone track and intensity are significantly better in GC5 compared to GC4. There are aspects where the model has slightly degraded such as the MJO, Indian Ocean SSTs and winds, biases over northeastern parts of South America, western Pacific high in northern hemisphere summer and European blocking in northern hemisphere summer. These degradations require more detailed investigations. Degradations in NWP performance at higher resolution also need to be investigated before operational implementation, but will require minimal adjustments.

Although this paper documents the performance of the GC5 model at N216 resolution (6^~0 km at mid-latitudes) as the physical model configuration for various applications such as seasonal forecasting, more work is being done in the climate science community to address the cold bias in global mean temperatures in the GC5 N96ORCA1 (1^~30 km atmospheric and 1 degree ocean resolution) configuration. This configuration will form the basis for the UK Earth System Model (UKESM). The resulting tuned GC5 configurations for a wider range of climate applications will be documented separately.