Assessment of the Met Office Global Coupled model version 4 (GC4) configurations.
Authors: Xavier Prince ,   Met Office, Exeter, UK, Willett Martin , Graham Tim , Earnshaw Paul , Copsey Dan , Marzin Charline , Sellar Alistair , Ackerley Duncan , Alves Oscar , Blockley Ed , Bodas-Salcedo Alejandro , Bushell Andrew , Butchart Neal , Calvert Daley , Cho Jeong-A , de Burgh-Day Catherine , Edwards John , Furtado Kalli , Field Paul , Guiavarch Catherine , Hardy Sam , Harris Chris , Heywood Karen , Heming Julian , Hendon Harry , Hewitt Helene , Hyder Pat , Hyun Yu-Kyung , Hyun Seung-Hwon , Ineson Sarah , Jones Richard , Kim Jiyeong , Kim Ki-Young , Klingaman Nick , Levine Richard , Lee Sang-Min , Lekakou Kyriaki , Lock Adrian , Martin Gill , Mathiot Pierre , Megann Alex , Meijers Andrew , Moon Ja-Yeon , Morgenstern Olaf , North Rachel , Nurser George , Park Yeon-Hee , Regayre Leighton , Roberts Malcolm , Rodriguez Jose , Ridley Jeff , Rawlins Rick , Sinha Bablu , Shin Beoncheol , Semple Adrian , Storkey Dave , Stephens David , Shim Taehyoun , Tomassini Lorenzo , Tsushima Yoko , Titley Helen , Tennant Warren , Varma Vidya , Vellinga Michael , Weedon Graham , Williams Keith , Yang Young-Min , Zhao Maggie , Zhou Xiaobing , Zhu Hongyan ,   The Bureau of Meteorology, Melbourne, Australia,   National Institute of Meteorological Sciences, Korea Meteorological Administration, Jeju, Korea,   National Centre for Atmospheric Science (NCAS), University of Reading, Reading, United Kingdom,   NIWA - The National Institute of Water and Atmospheric Research, New Zealand,   Centre for Environmental Modelling and Computation, University of Leeds, Leeds, United Kingdom,   School of Earth and Environment, University of Leeds, Leeds, United Kingdom,   National Oceanography Centre, Southampton, United Kingdom,   Joint Marine Modelling Programme (JMMP)
Published: July 2024

The Global Coupled model version 4 (GC4) is an upgraded configuration of the MetUM system, building upon its predecessor, GC3.0/3.1. It incorporates various improvements and changes in the atmospheric and land components (Global Atmosphere 8 and Global Land 9 - GA8GL9) while keeping the ocean component (Global Ocean 6 - GO6) unchanged, except for minor bug fixes. The GC4 model introduces several enhancements, such as the drag package for land surface and hydrology, improvements in radiation and large-scale precipitation parametrisations, advancements in the boundary layer and convection representation (including the prognostic-based convective entrainment rate - ProgEnt), and updates in aerosol properties. Additionally, the inclusion of a multi-grid solver in the dynamics module aims to improve model stability and reduce computational costs. Key improvements in GC4 include better representation of the diurnal cycle of convection over land, reduced Southern Ocean warm bias, increased rainfall over India during the JJA season, improved distribution of precipitation, enhanced representation of low-medium clouds over Northern Europe, and positive impacts of atmosphere-ocean coupling on NWP scores. However, challenges and areas for further improvement persist, including excessive global precipitation, warm biases over coastal regions of East Asia, wet biases over East Asia, weak cloud forcing over certain regions, hydrological cycle discrepancies, biases in gross primary productivity, persistent Southern Ocean biases, enhanced warming and weakened trade winds in the equatorial east Pacific, excessive surface warming in the North Atlantic, weakening of monsoon low-pressure systems and tropical cyclones, drying over Africa, and excessive thick cloud biases in mid-latitudes. The next version of GC (GC5) will attempt to address some of these biases in the next development and assessment cycle with inputs from relevant evaluation groups and partners.