The performance of the CoMorph‐A convection package in global simulations with the Met Office Unified Model

• Published in: Quarterly journal of the Royal Meteorological Society 2024-07, Vol.150 (763), p.3527-3543
• Main Authors: Lock, A. P., Whitall, M., Stirling, A. J., Williams, K. D., Lavender, S. L., Morcrette, C., Matsubayashi, K., Field, P. R., Martin, G., Willett, M., Heming, J.
• Format: Article
• Language: English
• Subjects: Convection; Data assimilation; Data collection; Detrainment; Diurnal cycle; evaluation; Extratropical cyclones; global; Madden-Julian oscillation; Mass flux; Microphysics; parametrization; Plumes; Tropical environments; unified model; Weather forecasting
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.4781

The impact on global simulations of a new package of physical parametrizations in the Met Office Unified Model is documented. The main component of the package is an entirely new convection scheme, CoMorph. This has a mass‐flux structure that allows initiation of buoyant ascent from any level and the ability for plumes of differing originating levels to coexist in a grid box. It has a different form of closure, where the mass flux of initiation is dependent on local instability, and an implicit numerical solution for detrainment that yields smooth timestep behaviour. The scheme is coupled more consistently to the cloud, microphysics, and boundary‐layer parametrizations and, as a result, significant changes to these have also been made. The package, called CoMorph‐A, has been tested in a variety of single‐column and idealized regimes. Here we test it in global configurations and evaluate it against observations using a range of standard metrics. Overall it is found to perform well against the control. Biases in the climatologies of the radiative fluxes are significantly reduced across the Tropics and subtropics, tropical and extratropical cyclone statistics are improved, and the Madden–Julian oscillation and other propagating tropical waves are strengthened. It also improves overall scores in numerical weather prediction trials, without revisions to the data assimilation. There is still work to do to improve the diurnal cycle of precipitation over land, where the peak remains too close to the middle of the day. The new CoMorph‐A package of physical parametrizations in the Met Office Unified Model is documented and shown to perform well in global configurations. The main component is an entirely new convection scheme, CoMorph, but it also includes significant changes to the cloud, microphysics, and boundary‐layer parametrizations. Biases in radiative flux climatologies are significantly reduced (top panel for the control annual mean longwave cloud radiative forcing, bottom for CoMorph‐A), tropical and extratropical cyclone statistics are improved, the Madden–Julian oscillation and other tropical waves are strengthened, and it also improves overall scores in numerical weather prediction trials.