Abstract
AbstractShallow clouds remain greatly significant in improving our understanding of the atmosphere. Using the Met Office Unified Model, we compare highly idealised simulations of shallow cumuli with those using more realistic domains, with open lateral boundaries and varying large‐scale forcing. We find that the realistic simulations are more capable of representing the cloud field on large spatial scales, and appear to limit the aerosol perturbations leading to impacts on the thermodynamic conditions. Aerosol perturbations lead to changes in the cloud vertical structure, and thermodynamic evolution of the idealised simulations; a central feature of behavior seen previously in idealised simulations. Modelling approaches with open boundaries and time‐varying forcing may allow for improved representation of shallow clouds in the atmosphere, and greater understanding of how they may respond to perturbations.
Highlights
Shallow cumuli play a number of important roles in trade wind regions, affecting both their local environment, and the climate as a whole
We find that the realistic simulations are more capable of representing the cloud field on large spatial scales, and appear to limit the aerosol perturbations leading to impacts on the thermodynamic conditions
We show that certain modelling choices allow for an improved representation of shallow cloud fields on large scales, and show a different response to aerosol perturbations, with implications for future development of estimations of the effects of aerosol on shallow clouds
Summary
Shallow cumuli play a number of important roles in trade wind regions, affecting both their local environment, and the climate as a whole. Low cloud feedbacks are responsible for much of the uncertainty in estimates of climate sensitivity (Bony & Dufresne, 2005; Bony et al, 2004; Boucher et al, 2013; Medeiros et al, 2008, 2015; Vial et al, 2013). One intensely studied aspect of shallow cumuli is how they are affected by changes in atmospheric aerosol, which facilitate the formation of cloud droplets by acting as cloud condensation nuclei (CCN) (Köhler, 1936). Higher concentrations of CCN lead to a greater number of smaller droplets, for a given liquid water content (Twomey, 1977). The greater droplet surface area increases scattered shortwave radiation, and cloud albedo. Smaller droplets due to increased aerosol may inhibit precipitation (Albrecht, 1989), and lead to longer cloud lifetimes. Mechanisms have been proposed for aerosol causing shorter lifetimes due to evaporation and entrainment feedbacks (Small et al, 2009)
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