Abstract/Summary

GC5 is the latest global coupled configuration which is a combination of Global Atmosphere and Land (GAL9) and Global Ocean and Sea Ice (GOSI9). GAL9 includes over 26 science tickets that cover changes to almost all areas of global model science, including several enhancements to the 6A convection scheme, latent heating in the gravity wave drag scheme, ‘fountain buster’ scheme which adds improvements to advection conservation in conditions of near-grid-scale convergence, bimodal cloud initiation and dust dependent ice-nucleation temperature. GOSI9 is built on the NEMO 4.0.4 ocean model and a new sea ice model SI3. This includes a new equation of state (TEOS-10), 4th order tracer advection, adaptive-implicit vertical advection and Southern Ocean tuning. Important model improvements in global temperature and winds, subtropical jet, monsoon processes over India and 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 reduce in GC5 as a result of the Southern Ocean package including the Antarctic Sea ice area. At NWP scales, significant improvements in the tropical temperature at 850hPa and precipitation SEEPs 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 NE South America, western Pacific high in northern hemisphere summer and European blocking in northern hemisphere summer. These degradations require more detailed investigations. Degradations to NWP performance at higher resolution also need to be investigated before operational implementation but will require minimal adjustments. While the N216 GC5 model is acceptable for release as the physical model configuration, further work is being undertaken in the climate science community to address the cold bias in GC5 N96ORCA1 configuration. The resulting GC5 configurations for a wider range of climate applications will be documented in a separate paper. Unlike previous configurations, with the aim of getting an early understanding of the impact of GC5, tests were carried out on the UK convective-scale ensemble system, MOGREPS-UK. The results assessed here show degradation in the skills of 10m-wind and clouds in Boreal summer, but improvements in the spatial skill of the precipitation forecasts. The results also show a decrease in the ensemble spread with GC5 along with a slight increase in error and this is being investigated.