Abstract
Abstract. The rapid adjustment, or semi-direct effect, of marine stratocumulus clouds to elevated layers of absorbing aerosols may enhance or dampen the radiative effect of aerosol–radiation interactions. Here we use large-eddy simulations to investigate the sensitivity of stratocumulus clouds to the properties of an absorbing aerosol layer located above the inversion layer, with a focus on the location, timing, and strength of the radiative heat perturbation. The sign of the daily mean semi-direct effect depends on the properties and duration of the aerosol layer, the properties of the boundary layer, and the model setup. Our results suggest that the daily mean semi-direct effect is more elusive than previously assessed. We find that the daily mean semi-direct effect is dominated by the distance between the cloud and absorbing aerosol layer. Within the first 24 h the semi-direct effect is positive but remains under 2 W m−2 unless the aerosol layer is directly above the cloud. For longer durations, the daily mean semi-direct effect is consistently negative but weakens by 30 %, 60 %, and 95 % when the distance between the cloud and aerosol layer is 100, 250, and 500 m, respectively. Both the cloud response and semi-direct effect increase for thinner and denser layers of absorbing aerosol. Considerable diurnal variations in the cloud response mean that an instantaneous semi-direct effect is unrepresentative of the daily mean and that observational studies may underestimate or overestimate semi-direct effects depending on the observed time of day. The cloud response is particularly sensitive to the mixing state of the boundary layer: well-mixed boundary layers generally result in a negative daily mean semi-direct effect, and poorly mixed boundary layers result in a positive daily mean semi-direct effect. The properties of the boundary layer and model setup, particularly the sea surface temperature, precipitation, and properties of the air entrained from the free troposphere, also impact the magnitude of the semi-direct effect and the timescale of adjustment. These results suggest that the semi-direct effect simulated by coarse-resolution models may be erroneous because the cloud response is sensitive to small-scale processes, especially the sources and sinks of buoyancy.
Highlights
Semi-permanent decks of marine stratocumulus clouds represent an important negative radiative effect within the Earth’s energy budget (Hartmann et al, 1992; Hartmann and Short, 1980; Wood, 2012)
In this study the UK Met Office Large Eddy Model (LEM) is used to investigate and quantify the impact that the properties of a persistent elevated absorbing aerosol layer have on the cloud and radiative response of marine stratocumulus, with a focus on the role that the location, timing, and strength of the heat perturbation has in the underlying cloud and boundary layer
The Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) and Cloud–Aerosol Transport System (CATS) analysis (Fig. 1) suggests that elevated aerosol layers predominantly exist within 1500 m of the cloud top, with a common occurrence of layers in close proximity to the cloud. In line with this we focus on layers of absorbing aerosol that range from directly above the cloud layer ( z = 0 m) to elevated layers at z = 500 m, and we examine the role of the aerosol layer depth, which, for a given aerosol optical depth (AOD), will impact the vertical distribution and strength of the localized heat perturbation
Summary
Semi-permanent decks of marine stratocumulus clouds represent an important negative radiative effect within the Earth’s energy budget (Hartmann et al, 1992; Hartmann and Short, 1980; Wood, 2012). Observational and modelling studies suggest that elevated absorbing layers result in thicker clouds and a negative SDE (Adebiyi and Zuidema, 2018; Johnson et al, 2004; Wilcox, 2010), which may impact the stratocumulus-to-cumulus transition process (Yamaguchi et al, 2015; Zhou et al, 2017). In this study the UK Met Office Large Eddy Model (LEM) is used to investigate and quantify the impact that the properties of a persistent elevated absorbing aerosol layer have on the cloud and radiative response of marine stratocumulus, with a focus on the role that the location, timing, and strength of the heat perturbation has in the underlying cloud and boundary layer.
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