Sensitivity of cloud-radiative effects to cloud fraction parametrizations in tropical, midlatitude, and arctic kilometre-scale simulations

• Published in: Quarterly Journal of the Royal Meteorological Society
• Main Authors: Van Weverberg, Kwinten, Morcrette, Cyril J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2022
• Subjects: Earth and Environmental Sciences; OFFICE UNIFIED MODEL; SIZE DISTRIBUTION; CONVECTION; RAINFALL; PARAMETERIZATION; SCHEME; CONFIGURATION; ATMOSPHERE; FORECASTS; IMPACT; cloud fraction parametrization; clouds; convection-permitting; evaluation; radiation; Arctic
• Online Access: https://biblio.ugent.be/publication/8772873; http://hdl.handle.net/1854/LU-8772873; https://doi.org/10.1002/qj.4325; https://biblio.ugent.be/publication/8772873/file/8772934

The regional atmosphere (RA) configuration of the Met Office Unified Model currently requires different cloud fraction parametrizations (CFPs) for tropical and midlatitude simulations. To explore the scope for unification of these two RA configurations, this article presents a detailed evaluation of simulations over tropical, midlatitude, and arctic domains, with two different diagnostic CFPs: a prognostic CFP, and no CFP at all. Furthermore, a novel, hybrid approach was used that treats liquid cloud diagnostically and ice cloud prognostically. Using observations from three US Department of Energy Atmospheric Radiation Measurement supersites, it is shown that none of these CFPs stands out as superior over all domains. Over the frequently overcast Arctic, the all-or-nothing approach best captures the cloud radiative properties. Conversely, CFPs are of benefit in regions with frequent partial cloudiness, such as the midlatitudes and the Tropics. However, their improved cloud radiative properties often hide an error compensation. All models underestimate overcast, low-base cloud with small water paths in convective environments. In addition, midlatitude overcast, low-base, optically thick clouds in the morning, possibly associated with overnight convection, are frequently too broken. Diagnostic schemes compensate for these errors by producing spurious, scattered afternoon cloud, which could be due to a correct cloud response to too eager convective initiation. Winter clouds over the midlatitudes are improved when liquid cloud is represented diagnostically with a bimodal saturation-departure probability density function, without error compensation. Although it is difficult to unify the RA across the globe around a single CFP scheme, the newly proposed hybrid scheme performs reasonably well for cloud cover across all regions. It also exhibits short-wave biases that are smaller than most other configurations and is less affected by excessive liquid water paths and compensating errors than fully diagnostic schemes are. ...