the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jul 2026
Assessment of the Met Office Global Coupled model version 5 (GC5) configurations
Abstract. GC5 is the latest global coupled configuration of the Met Office Unified Model (MetUM), which is a combination of Global Atmosphere and Land (GAL9) and Global Ocean and Sea Ice (GOSI9). GAL9 includes more than 26 science tickets that cover changes in almost all areas of global model science, including several enhancements to the 6A convection scheme, latent heating in the gravity wave drag scheme, and the ’fountain buster’ scheme, which adds improvements to the advection conservation under conditions of near-grid-scale convergence, bimodal cloud initiation, and dust-dependent ice-nucleation temperature. GOSI9 is based on the NEMO 4.0.4 ocean model and a new sea ice (SI) model SI3. This includes a new equation of state (TEOS-10), 4th-order tracer advection, adaptive-implicit vertical advection and Southern Ocean tuning. GC5 is evaluated against Global Coupled Model version 4 (GC4) and ERA5 reanalysis and observations in terms of the global mean state, variability and NWP performance.
Important model improvements in global temperature and winds, subtropical jet, monsoon processes over India and the Maritime continent, and their diurnal cycle in convection were noted. Most aspects of processes over Africa were also notably improved. Southern Ocean biases continue to decrease in GC5 as a result of the Southern Ocean package including the Antarctic Sea ice area. On NWP (Numerical Weather Prediction) scales, significant improvements in the tropical temperature at 850 hPa and precipitation SEEPS (Stable Equitable Error in Probability Space) score at all lead times were achieved. In addition, the tropical cyclone track and intensity are significantly better in GC5 compared to GC4. There are aspects where the model has slightly degraded such as the MJO, Indian Ocean SSTs and winds, biases over northeastern parts of South America, western Pacific high in northern hemisphere summer and European blocking in northern hemisphere summer. These degradations require more detailed investigations. Degradations in NWP performance at higher resolution also need to be investigated before operational implementation, but will require minimal adjustments.
Although this paper documents the performance of the GC5 model at N216 resolution (6~0 km at mid-latitudes) as the physical model configuration for various applications such as seasonal forecasting, more work is being done in the climate science community to address the cold bias in global mean temperatures in the GC5 N96ORCA1 (1~30 km atmospheric and 1 degree ocean resolution) configuration. This configuration will form the basis for the UK Earth System Model (UKESM). The resulting tuned GC5 configurations for a wider range of climate applications will be documented separately.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4281', Anonymous Referee #1, 07 Sep 2026
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RC2: 'Comment on egusphere-2026-4281', Anonymous Referee #2, 11 Sep 2026
General comments
This paper provides a useful description of the Met Office Global Coupled model version 5 (GC5), including the recent physics upgrades, configuration changes, and an assessment of model performance. It is an important contribution and will serve as a valuable GC5 reference. My major concern, however, is that the manuscript is difficult to follow in its current form. I outline several required edits below. A careful and thorough revision is recommended before the manuscript can be considered for publication.Specific comments
- A careful revision of the manuscript is needed. For example, acronyms should be spelled out at first use (e.g., “GC5” in line 2, “CMIP6” in line 34, “GWD” in line 58, “SL” in line 63, “GPP” in line 120). Readers should not be left to infer meanings. There is also repetition of identical text (line 18)
- Line 10: Please indicate the intended forecast period length.
- Line 86: It would be helpful to include a brief description of the new sea‑ice model and the rationale for replacing the previous one.
- Figure 1 caption: The caption is unclear. Please define “observed range.” What does it imply when a dot falls outside this range? Additionally, at least two green dots lie to the right of the x = 1 line, indicating larger RMSE than GC4 (i.e., degradation), yet the caption states these values are improved. Similarly, several red dots lie to the left of x = 1 (smaller RMSE than GC4), which would indicate improvement rather than degradation. This needs clarification.
- Please specify which figures and panels are being discussed in the text. For example, lines 99–106 on precipitation and lines 107–114 on 1.5 m temperature should reference the relevant figure panels explicitly.
- Lines 122–123: The statement “Globally GPP is far lower than expected (around 85 GT C yr⁻¹ in GC5 compared to ~120 GT yr⁻¹ in the observations)” is unclear. Please define the unit “GT C yr⁻¹” and provide a brief explanation of GPP for readers unfamiliar with the term.
- Figure 3 caption: I suggest replacing “temperature anomalies” with “temperature biases,” which more accurately reflects the content.
- Lines 124–129: I recommend quantifying the comparison using average RMS differences or absolute differences for GC4 and GC5, both globally and for key regions.
- Lines 130–133: It is unclear which figures these lines refer to. If the discussion pertains to Figs. 2c–2f, note that vector differences alone do not necessarily indicate increases or decreases in wind speed.
- The manuscript uses both “Fig.” and “Figure” inconsistently. Please standardize the format throughout.
- I would suggest a brief description at the beginning of section 2 about what to be analyzed and why, like the beginning of Section 3. The authors discussed “Middle Atmosphere”, “Hydrological cycle”, and “High-resolution NWP trials”. These subsections appear to be randomly plugged in.
- Figure 21: (1) The color bars and the black graphics do not match; (2) Please indicate in the figure x-axis is longitude; (3) I would suggest calculating a statistical quantity to quantify the comparison in addition to a visual comparison.
Citation: https://doi.org/10.5194/egusphere-2026-4281-RC2
Data sets
Data, analysis codes and supporting information for the paper "Assessment of the Met Office Global Coupled model version 5 (GC5) configurations" to be cited as Xavier et al., 2026 Prince Xavier et al. https://doi.org/10.5281/zenodo.21353929
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Configuration-documentation papers of this kind are useful and GMD is the right venue. The GC5 development represents a large body of work, the breadth of the assessment is impressive, and several of the individual results are interesting in their own right, particularly the improved African easterly jet and associated MCS propagation, the improved Maritime Continent precipitation diurnal cycle, the Antarctic sea ice improvement, and the recovery of tropical cyclone intensity. The paper deserves to be published.
See the uploaded PDF for further comments.