UKESM1 ready to use and in production for CMIP6

By: Till Kuhlbrodt

Development of the UK Earth System Model (UKESM1) has reached a major milestone. After six years of work on the model (see my earlier blog post here) the UKESM core group, and other scientists, are now running simulations for CMIP6.

Scientific configuration and couplings

The foundation of UKESM1 is the physical climate model HadGEM3-GC3.1 N96ORCA1 (Kuhlbrodt et al. 2018). Earth system processes (meaning here: processes involving chemistry and/or biology) are added by including:

1. An interactive stratosphere-troposphere chemistry coupled to the GLOMAP-mode aerosol scheme (Mulcahy et al. 2018).
2. A global carbon cycle, including terrestrial carbon processes with nitrogen limitation on carbon uptake and dynamic vegetation. Marine carbon cycle processes are represented by the MEDUSA2 model, within NEMO-ORCA1.

A further configuration of UKESM1 (UKESM1-IS) that includes interactive treatment of the Greenland and Antarctic ice sheets is under development.

UKESM1 includes a range of couplings between physical and Earth system components, as well as across domains of the coupled model (that is, between the land, ocean and atmosphere). These couplings increase the realism (and degrees of freedom) of the model, enabling an investigation of potential Earth system feedbacks arising from future anthropogenic CO₂ emissions. The primary cross-domain model coupling is CO₂, exchanged between the atmosphere, ocean and land and allowing UKESM1 to run either with prescribed atmospheric CO₂ concentrations or with anthropogenic CO₂ emissions. Other important couplings include:

• Dust emissions that depend on predicted vegetation cover and climate, influencing aerosols and radiation processes in the atmosphere, and providing a source of soluble iron for the ocean.
• Biogenic Volatile Organic Compounds (BVOCs), emitted by vegetation and influencing model cloud-aerosol formation.
• Marine dimethyl sulfide (DMS) and Primary Marine Organic Aerosol (PMOA) emissions, coupled to MEDUSA-predicted seawater DMS and chlorophyll and acting as cloud condensation nuclei in the model atmosphere.
• Concentrations of O₃, CH₄ and N₂O as simulated by the UKESM1 chemistry scheme (UKCA) being active in the model radiation parameterization.

We believe that this number of couplings makes UKESM1 the most comprehensive Earth system model in CMIP6.

Simulations for CMIP6

Many simulations will be run for the various model intercomparisons in CMIP6. So far, we have completed the pre-industrial control run, a number of idealized warming simulations, and nine historical simulations (1850-2014) with varying initial conditions. Projections for the remainder of the 21^st century based on different scenarios for global carbon emissions (scenarioMIP) are currently under way, with some ensemble members having already finished.

As an example from the historical simulations, we show the model’s ability to simulate the Antarctic ozone hole, an important performance metric for UKESM1. Fig.1 displays the temporal evolution of total column ozone at the South Pole, simulated in two UKESM1 historical runs and from observations, the latter beginning in 1964. Monthly mean column ozone is shown for September, October, January and February through the entire historical simulation period. From the early 1970s both the model and observations depict a decrease in column ozone at the South Pole (indicative of the onset of the stratospheric ozone hole). This decrease reaches a minimum, both in the model and observations, around 2005. The observed annual cycle of the ozone hole shows a rapid decrease through September and October (the Antarctic spring) and subsequent dissipation, as the polar vortex breaks up in the following January and February (the Antarctic summer). Both the annual cycle of the growth and decay of the ozone hole and its overall development, from initiation in the early 1970s to minimum values around 2005, are well simulated. The overall magnitude of the ozone decrease also appears well captured by the model. For example, October column ozone decreases from ~310 Dobson units (DU) in the mid-1960s to a minimum of ~125 DU by ~2005, in line with observations.

Figure 1. Total column ozone at the South Pole in two UKESM1 CMIP6 historical simulations (green and blue lines) and observed at the South Pole (1964 to present-day, black crosses). Time scale in years, total column ozone in Dobson units (DU). Monthly mean ozone values are shown for September, October, January and February.

Release to the UK academic community

In latest news, UKESM1 is now ready for use by anyone in the UK academic community. UKESM1 can be run on the high-performance computing (HPC) facilities MONSooN, NEXCS and ARCHER. Instructions to do so are provided here: http://cms.ncas.ac.uk/wiki/UM/Configurations#UKESM1. Two model configurations are available for use following the CMIP6 pre-industrial and historical (1850-2014) experiment protocols.

Ample information about UKESM1 and a regular newsletter are available on the UKESM website https://ukesm.ac.uk/.

References:

Kuhlbrodt, T., Jones, C. G., Sellar, A., Storkey, D., Blockley, E., Stringer, M., et al. (2018). The low-resolution version of HadGEM3 GC3.1: Development and evaluation for global climate. Journal of Advances in Modeling Earth Systems, 10, 2865–2888. https://doi.org/10.1029/2018MS001370

Mulcahy, J. P., Jones, C., Sellar, A., Johnson, B., Boutle, I. A., Jones, A., et al. (2018). Improved aerosol processes and effective radiative forcing in HadGEM3 and UKESM1. Journal of Advances in Modeling Earth Systems, 10, 2786–2805. https://doi.org/10.1029/2018MS001464