Informing the Unification of a Single Cloud Fraction Scheme in the Met Office’s Unified Model

The choice of cloud fraction parametrization scheme in weather and climate models significantly influences model performance. Currently in the Met Office’s Unified Model (UM), two different approaches are used to represent sub-grid clouds: a prognostic scheme in the global atmosphere and land (GAL) configuration, and a diagnostic scheme in the regional atmosphere and land (RAL) configuration.  Historically, prognostic schemes have performed better at climate resolutions where memory is important, whilst diagnostic schemes have been sufficient for higher resolution numerical weather prediction (NWP). Due to recent increases in computational power, both climate simulations and NWP are being run at higher resolutions. This blurs the boundary between the two configurations, and it would therefore be beneficial to unify a single large-scale cloud fraction scheme which works seamlessly across all resolutions. 

A framework for testing candidate cloud fraction schemes has been developed, using high resolution (300m grid spacing) simulations. This grid spacing was chosen as previous comparisons of the UM with observational data show a cloud fraction scheme is required, however most deep convection will be resolved at this resolution and so there is no need for a convection scheme.  

We investigate four different cloud fraction schemes: Smith (diagnostic), Bi-Modal (diagnostic), PC2 (prognostic), and a new hybrid cloud scheme combining PC2 for ice and Bi-Modal for liquid. We also look at two cloud microphysics schemes: Wilson & Ballard (single moment), and Cloud AeroSol Interacting Microphysics (CASIM; double moment).  

Simulations of shallow cumulus and stratocumulus cloud regimes have been performed over a south UK domain for several case study dates. Through comparisons of rainfall rates and storm cell sizes against 1 km radar observations, it’s been demonstrated that all model configurations overpredict the number of small cells even at this high resolution, particularly GAL9 which also hugely overpredicts rainfall rates. Further comparisons against 3D radar composites provide information on timing and morphology errors. In addition, comparisons against the observations from the Wessex UK Summertime Convection Experiment (WesCon) provide further constraints for single-site model output for parameters including liquid water path and cloud-base height. Together, these comparisons will help to identify the configuration that best represents observed cloud at high resolutions, thereby informing the development of a unified physics configuration.