The second Met Office Unified Model–JULES Regional Atmosphere and Land configuration, RAL2

• Published in: Geoscientific Model Development 2023-03, Vol.16 (6), p.1713-1734
• Main Authors: Bush, Mike, Boutle, Ian, Edwards, John, Finnenkoetter, Anke, Franklin, Charmaine, Hanley, Kirsty, Jayakumar, Aravindakshan, Lewis, Huw, Lock, Adrian, Mittermaier, Marion, Mohandas, Saji, North, Rachel, Porson, Aurore, Roux, Belinda, Webster, Stuart, Weeks, Mark
• Format: Article
• Language: English
• Subjects: Atmosphere; Case studies; Cloud amount; Clouds; Configurations; Diurnal cycle; Diurnal variations; Humidity; Ice; Low clouds; Mathematical models; Modelling; Performance evaluation; Physics; Radiation; Regions; Science; Simulators; Tropical environment; Tropical environments
• ISSN: 1991-9603; 1991-959X; 1991-962X; 1991-9603; 1991-962X
• DOI: 10.5194/gmd-16-1713-2023

In this paper we define RAL2 – the second Regional Atmosphere and Land (RAL) science configuration for regional modelling. RAL2 uses the Unified Model (UM) as the basis for the atmosphere and the Joint UK Land Environment Simulator (JULES) for the land. RAL2 defines the science configuration of the dynamics and physics schemes of the atmosphere and land and builds on the baseline of RAL1. There are two RAL2 sub-releases, one for mid-latitudes (RAL2-M) and one for tropical regions (RAL2-T). We document the differences between them and where appropriate discuss how RAL2 relates to RAL1 and the corresponding configuration of the global forecasting model. Our results show an increase in medium and low cloud amounts in the mid-latitudes leading to improved cloud forecasts. The increase in cloud amount leads to a reduced diurnal cycle of screen temperature. There is also a reduction in the frequency of heavier precipitation rates. RAL2 is expected to be the last RAL science configuration with two sub-releases as research effort is focused on producing a single defined configuration of the model that performs effectively in all regions of the world.