Diurnal cycle of the semi–direct effect over marine stratocumulus in
large–eddy simulations
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. The sign of the daily mean semi–direct effect depends on the properties of the aerosol layer, the properties of the boundary layer, and the model setup. Diurnal variations in the cloud response mean that an instantaneous semi–direct effect is unrepresentative of the daily mean, and that observational studies may under– or over–estimate semi–direct effects depending on the observed time of day. The observed role of the distance between the cloud top and the absorbing layer in modulating the strength of the cloud and radiative response is reproduced by the large eddy simulations. Both cloud response and semi–direct effect increase for thinner, denser, layers of absorbing aerosol located nearer the cloud layer. 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. 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.