Abstract
Abstract. Previous study of shallow convection has generally suffered from having to balance domain size with resolution, resulting in high-resolution studies which do not capture large-scale behaviour of the cloud fields. In this work we hope to go some way towards addressing this by carrying out cloud-resolving simulations on large domains. Simulations of trade wind cumulus are carried out using the Met Office Unified Model (UM), based on a case study from the Rain In Cumulus over the Ocean (RICO) field campaign. The UM is run with a nested domain of 500 km with 500 m resolution, in order to capture the large-scale behaviour of the cloud field, and with a double-moment interactive microphysics scheme. Simulations are run using baseline aerosol profiles based on observations from RICO, which are then perturbed. We find that the aerosol perturbations result in changes to the convective behaviour of the cloud field, with higher aerosol leading to an increase (decrease) in the number of deeper (shallower) clouds. However, despite this deepening, there is little increase in the frequency of higher rain rates. This is in contrast to the findings of previous work making use of idealised simulation setups. In further contrast, we find that increasing aerosol results in a persistent increase in domain mean liquid water path and decrease in precipitation, with little impact on cloud fraction.
Highlights
Shallow cumuli are the most common cloud type on Earth (Rossow and Schiffer, 1999; Sassen and Wang, 2008); they are ubiquitous throughout the trade winds, yet their behaviour is still poorly understood
Average profiles of liquid water potential temperature and specific humidity are shown in Fig. 3, which compare well to those used as initial profiles used in the GEWEX Cloud System Study (GCSS) Rain In Cumulus over the Ocean (RICO) model intercomparison study, as well as those shown in Nuijens et al (2009) and those obtained from simulations such as in Seifert and Heus (2013)
Cloud AeroSol Interactive Microphysics (CASIM) is run with a sub-grid cloud fraction scheme based on that of Smith (1990), which parameterise the sub-grid variability in relative humidity
Summary
Shallow cumuli are the most common cloud type on Earth (Rossow and Schiffer, 1999; Sassen and Wang, 2008); they are ubiquitous throughout the trade winds, yet their behaviour is still poorly understood. These small, warm, shallow convective clouds have an important part in regulating the thermodynamics and dynamics of their environment; warming the cloud layer through condensation, transporting moisture to the inversion layer above, and cooling both the inversion and the sub-cloud layer through the evaporation of detraining cloud droplets and precipitation (Hartmann et al, 1992; Zhu and Bretherton, 2004; Neggers et al, 2007). Low-cloud feedbacks are responsible for most of the uncertainty in climate sensitivity (Bony et al, 2004; Bony and Dufresne, 2005; Medeiros et al, 2008, 2015; Vial et al, 2013; Boucher et al, 2013)
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