Jane Patricia Mulcahy

We describe Global Atmosphere and Land 9.0 (GAL9) and GAL9.1: the latest science configurations of the Met Office Unified Model and JULES land surface model developed for seamless use across weather and climate timescales. GAL9 builds upon GA8GL9. GAL9 includes developments to most areas of the science. Some of the key changes include: moving to a single "mid-level" scheme to represent surface driven deep convection and elevated convection that improves the diurnal cycle of convection, an enhancement to the semi-Lagrangian advection that improves local conservation, improved physical realism in the gravity wave drag scheme by accounting for the effects of humidity and latent heating, and a package of enhancements to the land surface which improve soil moisture profiles and remove the need to apply an observation based albedo scaling. The NWP and climate performance of GAL9 is evaluated against the previous configuration, GA8GL9. In NWP tests GAL9 is shown have reduced errors relative to GA8GL9. The mean climate in GAL9 is shown to be improved relative to GA8GL9 with notable improvements in the top of atmosphere outgoing shortwave radiation. NWP tests of GAL9.0 at 10 km showed a degradation relative to GA8GL9. This issue is resolved in the branch configuration GAL9.1 that includes an improved orography. GAL9/GAL9.1 is the atmosphere and land component of GC5/GC5.1. From early 2026, GC5.1 is being be used as the operational global NWP model at the Met Office and GC5 will be a key component of the UK's contribution to CMIP7.

The Global Coupled model version 5 (GC5) of the Met Office Unified Model incorporates substantial developments across its model components and shows improved performance for a range of applications. However, it exhibits a very high effective climate sensitivity (EffCS) of 6.7 K and an excessive rise in recent global-mean surface air temperatures (GMSAT), limiting its suitability for some climate applications. This motivated the development of an alternative GC5-based configuration with EffCS constrained to lie within the IPCC Sixth Assessment Report “very likely” range, improved simulation of historical temperatures, and acceptable climatological performance. This new configuration, called UKCM2-LL, will form part of the UK’s submission to CMIP7. We describe a two-stage methodology used to develop UKCM2-LL. First, a 503-member atmosphere-only perturbed parameter ensemble (PPE) of GC5 variants was used to train statistical emulators that predict climatological performance metrics and atmosphere-only feedbacks. These emulators were then used to generate 41 candidate configurations predicted to have substantially reduced EffCS relative to GC5. Second, coupled preindustrial control and abrupt 4×CO₂ experiments were used to evaluate the candidates against large-scale climatological metrics and to diagnose EffCS, progressively narrowing the set of viable configurations through an expert-led evaluation process. Final fine-tuning of a single candidate was performed manually using the coupled experiments and was informed by parameter sensitivity information derived from the PPE. The resulting UKCM2-LL configuration has an EffCS of 3.6 K and exhibits an improved simulation of historical temperatures relative to GC5. However, achieving this lower EffCS required a degradation in climatological performance, reflecting a structural constraint of the GC5 model. This work demonstrates the value of PPE-based approaches for systematically exploring such structural constraints, and for parameter tuning during model development. We discuss potential improvements to the methodology and consider the implications of explicitly constraining climate sensitivity for future model development and multi-model ensemble diversity.

The Global Coupled model version 5 (GC5) of the Met Office Unified Model incorporates substantial developments across its model components and shows improved performance for a range of applications. However, it exhibits a very high effective climate sensitivity (EffCS) of 6.7 K and an excessive rise in recent global-mean surface air temperatures (GMSAT), limiting its suitability for some climate applications. This motivated the development of an alternative GC5-based configuration with EffCS constrained to lie within the IPCC Sixth Assessment Report “very likely” range, improved simulation of historical temperatures, and acceptable climatological performance. This new configuration, called UKCM2-LL, will form part of the UK’s submission to CMIP7. We describe a two-stage methodology used to develop UKCM2-LL. First, a 503-member atmosphere-only perturbed parameter ensemble (PPE) of GC5 variants was used to train statistical emulators that predict climatological performance metrics and atmosphere-only feedbacks. These emulators were then used to generate 41 candidate configurations predicted to have substantially reduced EffCS relative to GC5. Second, coupled preindustrial control and abrupt 4×CO₂ experiments were used to evaluate the candidates against large-scale climatological metrics and to diagnose EffCS, progressively narrowing the set of viable configurations through an expert-led evaluation process. Final fine-tuning of a single candidate was performed manually using the coupled experiments and was informed by parameter sensitivity information derived from the PPE. The resulting UKCM2-LL configuration has an EffCS of 3.6 K and exhibits an improved simulation of historical temperatures relative to GC5. However, achieving this lower EffCS required a degradation in climatological performance, reflecting a structural constraint of the GC5 model. This work demonstrates the value of PPE-based approaches for systematically exploring such structural constraints, and for parameter tuning during model development. We discuss potential improvements to the methodology and consider the implications of explicitly constraining climate sensitivity for future model development and multi-model ensemble diversity.

We describe Global Atmosphere and Land 9.0 (GAL9) and GAL9.1: the latest science configurations of the Met Office Unified Model and JULES land surface model developed for seamless use across weather and climate timescales. GAL9 builds upon GA8GL9. GAL9 includes developments to most areas of the science. Some of the key changes include: moving to a single "mid-level" scheme to represent surface driven deep convection and elevated convection that improves the diurnal cycle of convection, an enhancement to the semi-Lagrangian advection that improves local conservation, improved physical realism in the gravity wave drag scheme by accounting for the effects of humidity and latent heating, and a package of enhancements to the land surface which improve soil moisture profiles and remove the need to apply an observation based albedo scaling. The NWP and climate performance of GAL9 is evaluated against the previous configuration, GA8GL9. In NWP tests GAL9 is shown have reduced errors relative to GA8GL9. The mean climate in GAL9 is shown to be improved relative to GA8GL9 with notable improvements in the top of atmosphere outgoing shortwave radiation. NWP tests of GAL9.0 at 10 km showed a degradation relative to GA8GL9. This issue is resolved in the branch configuration GAL9.1 that includes an improved orography. GAL9/GAL9.1 is the atmosphere and land component of GC5/GC5.1. From early 2026, GC5.1 is being be used as the operational global NWP model at the Met Office and GC5 will be a key component of the UK's contribution to CMIP7.

We have conducted an ensemble of idealised climate overshoot simulations in a state-of-the-art Earth System Model in which global mean temperature is increased at a constant rate to global warming levels (GWLs) from 1.5 to 5 °C by prescribing constant CO₂ emissions, followed by a 50 year period of zero CO₂ emissions started at different GWLs, and then a period of cooling in which CO₂ is removed from the atmosphere. We give an overview of the ice sheet, sea-ice and sea level responses in these simulations highlighting long term responses at different GWLs and discuss the degree to which those responses can be simply reversed as global surface temperature cools. We show a broad divide between the two hemispheres, in which Northern Hemisphere polar processes have larger direct responses to warming which are more simply reversible than those in the Southern Hemisphere. Cessation of CO₂ emissions at most GWLs stabilises surface temperatures at high northern latitudes, although a slow warming trend continues at high southern latitudes. Northern Hemisphere sea-ice extent and Greenland surface mass balance both stabilise under zero CO₂ emissions and appear to return toward preindustrial levels along with surface temperatures when CO₂ is sequestered, but Southern Hemisphere sea-ice continues to decline under zero emissions and does not simply increase again as global temperatures cool. Likewise, the Antarctic circumpolar westerlies exhibit strengthening and shift poleward under global warming, but do not simply return to their preindustrial state when the climate is cooled, with implications for ocean circulation and Antarctic surface mass balance. The thermosteric contributions to global mean sea level from ocean heat uptake and the Greenland ice sheet continue when CO₂ emissions are ceased or reversed, at rates which slowly decline on centennial timescales. The net mass balance of Antarctica and its contribution to sea level do not simply scale with global temperature; they result from a complex interaction between the basal and surface mass balances, the ice-dynamic response to those forcings and significant trends inherent to the initialisation of our ice sheet model state.

The Global Coupled model version 5 (GC5) of the Met Office Unified Model incorporates substantial developments across its model components and shows improved performance for a range of applications. However, it exhibits a very high effective climate sensitivity (EffCS) of 6.7 K and an excessive rise in recent global-mean surface air temperatures (GMSAT), limiting its suitability for some climate applications. This motivated the development of an alternative GC5-based configuration with EffCS constrained to lie within the IPCC Sixth Assessment Report “very likely” range, improved simulation of historical temperatures, and acceptable climatological performance. This new configuration, called UKCM2-LL, will form part of the UK’s submission to CMIP7. We describe a two-stage methodology used to develop UKCM2-LL. First, a 503-member atmosphere-only perturbed parameter ensemble (PPE) of GC5 variants was used to train statistical emulators that predict climatological performance metrics and atmosphere-only feedbacks. These emulators were then used to generate 41 candidate configurations predicted to have substantially reduced EffCS relative to GC5. Second, coupled preindustrial control and abrupt 4×CO₂ experiments were used to evaluate the candidates against large-scale climatological metrics and to diagnose EffCS, progressively narrowing the set of viable configurations through an expert-led evaluation process. Final fine-tuning of a single candidate was performed manually using the coupled experiments and was informed by parameter sensitivity information derived from the PPE. The resulting UKCM2-LL configuration has an EffCS of 3.6 K and exhibits an improved simulation of historical temperatures relative to GC5. However, achieving this lower EffCS required a degradation in climatological performance, reflecting a structural constraint of the GC5 model. This work demonstrates the value of PPE-based approaches for systematically exploring such structural constraints, and for parameter tuning during model development. We discuss potential improvements to the methodology and consider the implications of explicitly constraining climate sensitivity for future model development and multi-model ensemble diversity.

This poster provides an overview of the PROMOTE (Progressing Earth System Modelling for Tipping Point Early Warning Systems) project. One project aim is to develop a version of the UK Earth System Model (UKESM) that is suitable to become the model component of a potential early warning system of Subpolar Gyre or Greenland Ice Sheet tipping. In PROMOTE, we (i) undertake targeted development of UKESM to advance its representation of the Greenland Ice Sheet and North Atlantic Subpolar Gyre, (ii) develop innovative simulation techniques aiming to make the simulation of tipping behaviour in the Subpolar Gyre and Greenland Ice Sheet more controlled and efficient, (iii) use machine-learning and other analysis techniques as well as model simulations to inform the design of observational networks, and (iv) evaluate tipping processes and impacts of tipping in our newly developed model versions. PROMOTE is run by a team of scientists and model developers at 7 UK national research centres and universities during 2025-2030, and is part of the “Forecasting Tipping Points” programme funded by the Advanced Research and Invention Agency (ARIA).

We describe Global Atmosphere 8.0 and Global Land 9.0 (GA8GL9) that are science configurations of the Met Office Unified Model and Joint UK Land Environment Simulator (JULES) land surface model developed for use across weather and climate timescales. GA8GL9 builds upon GA7GL7. It not only consolidates the changes made for the climate branch configuration GA7.1GL7.1 (the atmosphere and land components of the physical model used in HadGEM3-GC3.1, UKESM1 and UKESM1.1 which were all used in the Met Office's CMIP6 submissions) and NWP branch configuration GA7.2GL8.1 (the operational global NWP model at the Met Office between 2019 and 2022), but also includes developments to most areas of the science. Some of the key changes include: prognostic-based entrainment, which adds convective memory and improves precipitation rates and spatial structures; time-smoothed convective increments, which improves the convection-dynamics coupling and greatly reduces the detrimental dynamical effects of convective intermittency; a new riming parametrisation, which increases the amount of supercooled water and hence reduces Southern Ocean biases; and a package of land surface changes, which improves the forecast of near-surface fields and hence removes the need for the aggregate surface tile in NWP applications. Several changes are made that reduce numerical artefacts and improve the numerical stability of the model. The NWP and climate performance of GA8GL9 is evaluated against the previous configuration, GA7GL7. In NWP tests GA8GL9 is shown have reduced errors and improved spatial structure. The mean climate in GA8GL9 is shown to be improved relative to GA7GL7 with notable improvements in the top of atmosphere outgoing shortwave radiation. GA8GL9 is the atmosphere and land component of GC4, and GC4 has been used as the operational global NWP model at the Met Office since May 2022.

We have conducted an ensemble of idealised climate overshoot simulations in a state-of-the-art Earth system model in which global mean temperature is increased at a constant rate to various global warming levels (GWLs) by prescribing constant CO₂ emissions, followed by a period of zero CO₂ emissions started at different GWLs, and then a period of cooling in which CO₂ is removed from the atmosphere. We give an overview of the ice sheet, sea-ice and sea level responses in these simulations highlighting long term responses at different GWLs and discuss the degree to which those responses can be simply reversed as global surface temperature cools. We show a broad divide between the two hemispheres, in which northern hemisphere polar processes have larger direct responses to warming which are more simply reversible than those in the southern hemisphere. Cessation of CO₂ emissions at most GWLs stabilises surface temperatures at high northern latitudes, although a slow warming trend continues at high southern latitudes. Northern hemisphere sea-ice extent and Greenland surface mass balance both stabilise under zero CO₂ emissions and appear to return toward preindustrial levels along with surface temperatures when CO₂ is sequestered, but southern hemisphere sea-ice continues to decline under zero emissions and does not simply increase again as global temperatures cool. Likewise, the Antarctic circumpolar westerlies exhibit strengthening and shift poleward under global warming, but do not simply return to their preindustrial state when the climate is cooled, with implications for ocean circulation and Antarctic surface mass balance. The thermosteric contributions to global mean sea level from ocean heat uptake and the Greenland ice sheet continue when CO₂ emissions are ceased or reversed, at rates which slowly decline on centennial timescales. The net mass balance of Antarctica and its contribution to sea level do not simply scale with global temperature; they result from a complex interaction between the basal and surface mass balances, the ice-dynamic response to those forcings and significant trends inherent to the initialisation of our ice sheet model state.

We have conducted an ensemble of idealised climate overshoot simulations in a state-of-the-art Earth system model in which global mean temperature is increased at a constant rate to various global warming levels (GWLs) by prescribing constant CO₂ emissions, followed by a period of zero CO₂ emissions started at different GWLs, and then a period of cooling in which CO₂ is removed from the atmosphere. We give an overview of the ice sheet, sea-ice and sea level responses in these simulations highlighting long term responses at different GWLs and discuss the degree to which those responses can be simply reversed as global surface temperature cools. We show a broad divide between the two hemispheres, in which northern hemisphere polar processes have larger direct responses to warming which are more simply reversible than those in the southern hemisphere. Cessation of CO₂ emissions at most GWLs stabilises surface temperatures at high northern latitudes, although a slow warming trend continues at high southern latitudes. Northern hemisphere sea-ice extent and Greenland surface mass balance both stabilise under zero CO₂ emissions and appear to return toward preindustrial levels along with surface temperatures when CO₂ is sequestered, but southern hemisphere sea-ice continues to decline under zero emissions and does not simply increase again as global temperatures cool. Likewise, the Antarctic circumpolar westerlies exhibit strengthening and shift poleward under global warming, but do not simply return to their preindustrial state when the climate is cooled, with implications for ocean circulation and Antarctic surface mass balance. The thermosteric contributions to global mean sea level from ocean heat uptake and the Greenland ice sheet continue when CO₂ emissions are ceased or reversed, at rates which slowly decline on centennial timescales. The net mass balance of Antarctica and its contribution to sea level do not simply scale with global temperature; they result from a complex interaction between the basal and surface mass balances, the ice-dynamic response to those forcings and significant trends inherent to the initialisation of our ice sheet model state.