Alan J. Hewitt

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 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 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.

Airborne microplastics are a recently identified atmospheric aerosol species with potential air quality and climate impacts, yet they are not currently represented in global climate models. Here, we describe the addition of microplastics to the aerosol scheme of the UK Earth System Model (UKESM1.1): the Global Model of Aerosol Processes (GLOMAP). Microplastics are included as both fragments and fibres across a range of aerosol size modes, enabling interaction with existing aerosol processes such as ageing and wet and dry deposition. Simulated microplastics have higher concentrations over land, but can be transported into remote regions including Antarctica despite no assumed emissions from these regions. Lifetimes range between ∼17 d to ∼1 h, with smaller, hydrophilic microplastics having longer lifetimes. Microplastics are present throughout the troposphere, and the smallest particles are simulated to reach the lower stratosphere in small numbers. Dry deposition is the dominant microplastic removal pathway, but greater wet deposition occurs for smaller hydrophilic microplastic, due to interactions with clouds. Although microplastics currently contribute a minor fraction of the total aerosol burden, their concentration is expected to increase in future if plastic production continues to increase, and as existing plastic waste in the environment degrades to form new microplastic. Incorporating microplastics into UKESM1.1 is a key step toward quantifying their current atmospheric impact and offers a framework for simulating future emission scenarios for an assessment of their long term impacts on air quality and climate.

Ammonium nitrate is a major aerosol constituent over many land regions and contributes to air pollution episodes, ecosystem destruction, regional haze, and aerosol-induced climate forcing. Many climate models that represent ammonium nitrate assume that the ammonium–sulfate–nitrate chemistry reaches thermodynamic equilibrium instantaneously without considering kinetic limitations on condensation rates. The Met Office's Unified Model (UM) is employed to investigate the sensitivity of ammonium nitrate concentrations to the nitric acid uptake coefficient (γ) in a newly developed nitrate scheme in which first-order condensation theory is utilised to limit the rate at which thermodynamic equilibrium is attained. Two values of γ representing fast (γ=0.193) and slow (γ=0.001) uptake rates are tested in 20-year global UM integrations. The global burden of nitrate associated with ammonium in the “fast” simulation (0.11 Tg[N]) is twice as great as in the “slow” simulation (0.05 Tg[N]), while the top-of-the-atmosphere radiative impact of representing nitrate is −0.19 W m⁻² in the fast simulation and −0.07 W m⁻² in the slow simulation. In general, the fast simulation exhibits better spatial correlation with observed nitrate concentrations, while the slow simulation better resolves the magnitude of concentrations. Local near-surface nitrate concentrations are found to be highly correlated with seasonal ammonia emissions, suggesting that ammonia is the predominant limiting factor controlling nitrate prevalence. This study highlights the high sensitivity of ammonium nitrate concentrations to nitric acid uptake rates and provides a novel mechanism for reducing nitrate concentration biases in climate model simulations. The new UM nitrate scheme represents a step change in aerosol modelling capability in the UK across weather and climate timescales.

Ammonium nitrate is a major aerosol constituent over many land regions and contributes to air pollution episodes, ecosystem destruction, regional haze, and aerosol-induced climate forcing. Many climate models that represent ammonium nitrate assume that the ammonium-sulphate-nitrate chemistry reaches thermodynamic equilibrium instantaneously without considering kinetic limitations on condensation rates. The Met Office's Unified Model (UM) is employed to investigate the sensitivity of ammonium nitrate concentrations to the nitric acid uptake coefficient (γ) in a newly-developed nitrate scheme in which first order condensation theory is utilised to limit the rate at which thermodynamic equilibrium is attained. Two values of γ representing fast (γ = 0.193) and slow (γ = 0.001) uptake rates are tested in 20-year global UM integrations. The global burden of nitrate associated with ammonium in the “fast” simulation (0.11 Tg[N]) is twice as great as in the “slow” simulation (0.05 Tg[N]), while the top-of-the-atmosphere radiative impact of representing nitrate is −0.19 Wm⁻² in the “fast” simulation and −0.07 Wm⁻² in the “slow” simulation. In general, the “fast” simulation exhibits better spatial correlation with observed nitrate concentrations while the “slow” simulation better resolves the magnitude of concentrations. Local near-surface nitrate concentrations are found to be highly correlated with seasonal ammonia emissions suggesting that ammonia is the predominant limiting factor controlling nitrate prevalence. This study highlights the high sensitivity of ammonium nitrate concentrations to nitric acid uptake rates and provides a mechanism for reducing nitrate concentration biases in climate model simulations. The new UM nitrate scheme represents a step-change in aerosol modelling capability in the UK across weather and climate timescales.

Ammonium nitrate is a major aerosol constituent over many land regions and contributes to air pollution episodes, ecosystem destruction, regional haze, and aerosol-induced climate forcing. Many climate models that represent ammonium nitrate assume that the ammonium-sulphate-nitrate chemistry reaches thermodynamic equilibrium instantaneously without considering kinetic limitations on condensation rates. The Met Office's Unified Model (UM) is employed to investigate the sensitivity of ammonium nitrate concentrations to the nitric acid uptake coefficient (γ) in a newly-developed nitrate scheme in which first order condensation theory is utilised to limit the rate at which thermodynamic equilibrium is attained. Two values of γ representing fast (γ = 0.193) and slow (γ = 0.001) uptake rates are tested in 20-year global UM integrations. The global burden of nitrate associated with ammonium in the “fast” simulation (0.11 Tg[N]) is twice as great as in the “slow” simulation (0.05 Tg[N]), while the top-of-the-atmosphere radiative impact of representing nitrate is −0.19 Wm⁻² in the “fast” simulation and −0.07 Wm⁻² in the “slow” simulation. In general, the “fast” simulation exhibits better spatial correlation with observed nitrate concentrations while the “slow” simulation better resolves the magnitude of concentrations. Local near-surface nitrate concentrations are found to be highly correlated with seasonal ammonia emissions suggesting that ammonia is the predominant limiting factor controlling nitrate prevalence. This study highlights the high sensitivity of ammonium nitrate concentrations to nitric acid uptake rates and provides a mechanism for reducing nitrate concentration biases in climate model simulations. The new UM nitrate scheme represents a step-change in aerosol modelling capability in the UK across weather and climate timescales.

Ammonium nitrate is a major aerosol constituent over many land regions and contributes to air pollution episodes, ecosystem destruction, regional haze, and aerosol-induced climate forcing. Many climate models that represent ammonium nitrate assume that the ammonium-sulphate-nitrate chemistry reaches thermodynamic equilibrium instantaneously without considering kinetic limitations on condensation rates. The Met Office's Unified Model (UM) is employed to investigate the sensitivity of ammonium nitrate concentrations to the nitric acid uptake coefficient (γ) in a newly-developed nitrate scheme in which first order condensation theory is utilised to limit the rate at which thermodynamic equilibrium is attained. Two values of γ representing fast (γ = 0.193) and slow (γ = 0.001) uptake rates are tested in 20-year global UM integrations. The global burden of nitrate associated with ammonium in the “fast” simulation (0.11 Tg[N]) is twice as great as in the “slow” simulation (0.05 Tg[N]), while the top-of-the-atmosphere radiative impact of representing nitrate is −0.19 Wm⁻² in the “fast” simulation and −0.07 Wm⁻² in the “slow” simulation. In general, the “fast” simulation exhibits better spatial correlation with observed nitrate concentrations while the “slow” simulation better resolves the magnitude of concentrations. Local near-surface nitrate concentrations are found to be highly correlated with seasonal ammonia emissions suggesting that ammonia is the predominant limiting factor controlling nitrate prevalence. This study highlights the high sensitivity of ammonium nitrate concentrations to nitric acid uptake rates and provides a mechanism for reducing nitrate concentration biases in climate model simulations. The new UM nitrate scheme represents a step-change in aerosol modelling capability in the UK across weather and climate timescales.

Ammonium nitrate is a major aerosol constituent over many land regions and contributes to air pollution episodes, ecosystem destruction, regional haze, and aerosol-induced climate forcing. Many climate models that represent ammonium nitrate assume that the ammonium-sulphate-nitrate chemistry reaches thermodynamic equilibrium instantaneously without considering kinetic limitations on condensation rates. The Met Office's Unified Model (UM) is employed to investigate the sensitivity of ammonium nitrate concentrations to the nitric acid uptake coefficient (γ) in a newly-developed nitrate scheme in which first order condensation theory is utilised to limit the rate at which thermodynamic equilibrium is attained. Two values of γ representing fast (γ = 0.193) and slow (γ = 0.001) uptake rates are tested in 20-year global UM integrations. The global burden of nitrate associated with ammonium in the “fast” simulation (0.11 Tg[N]) is twice as great as in the “slow” simulation (0.05 Tg[N]), while the top-of-the-atmosphere radiative impact of representing nitrate is −0.19 Wm⁻² in the “fast” simulation and −0.07 Wm⁻² in the “slow” simulation. In general, the “fast” simulation exhibits better spatial correlation with observed nitrate concentrations while the “slow” simulation better resolves the magnitude of concentrations. Local near-surface nitrate concentrations are found to be highly correlated with seasonal ammonia emissions suggesting that ammonia is the predominant limiting factor controlling nitrate prevalence. This study highlights the high sensitivity of ammonium nitrate concentrations to nitric acid uptake rates and provides a mechanism for reducing nitrate concentration biases in climate model simulations. The new UM nitrate scheme represents a step-change in aerosol modelling capability in the UK across weather and climate timescales.