Assessment of GA4/GL4/GO4/GSI4 and progress in addressing key model biases

GA4 has consolidated a large number of science developments into the model development trunk. Overall, the performance of GA4 is similar to GA3, in both coupled atmosphere-ocean and atmosphere-only configurations, and across all timescales. Key improvements include reduced lower tropospheric cold temperature biases in the coupled model, reduced warm bias in the stratosphere, and improved cloud radiative effects across most regions. However, the meridional overturning circulation (MOC) in the North Atlantic has deteriorated by reducing in strength, Arctic summer sea-ice extents have become too low and there is poor conservation of freshwater in the atmosphere. A number of coupled model biases are notably improved when the ocean model resolution is increased from ORCA1 to ORCA0.25 including the MOC strength, North Atlantic SST biases, European blocking, and the spatial pattern of ENSO. It is strongly recommended that future coupled model configurations use ORCA0.25 rather than ORCA1 and that this is the primary use of additional computational power in atmosphere/ocean components of future climate modelling systems. A considerable amount of work is ongoing to address key long-standing biases. There is evidence that the convective entrainment rate is important for a number of tropical biases (MJO, strength of tropical cyclones, ENSO, rainfall in African Easterly Waves and Indian monsoon rainfall). High priority model biases have been identified and agreed, and groups set up to investigate these. The groups will report their progress in a year’s time