Strong control of the stratocumulus-to-cumulus transition time by aerosol: analysis of the joint roles of several cloud-controlling factors using Gaussian process emulation

Abstract

Stratocumulus-to-cumulus transitions are driven primarily by increasing sea-surface temperatures, with additional contributions from numerous interacting cloud-controlling factors. Understanding these interactions is important for improving the accuracy of cloud responses to changes in climate and other environmental factors in global climate models. Many studies have found lower-tropospheric stability dictates the transition time, while aerosol-focused studies found that aerosol concentration plays a key role via the drizzle-depletion mechanism. We consider the role of aerosol together with several other cloud-controlling factors representing a selection of the wider environmental conditions that affect drizzle in a clean to moderately polluted environment. A 34-member perturbed parameter ensemble of idealised large-eddy simulations with 2-moment cloud microphysics is used to train Gaussian process emulators (statistical representations) of the relationships between the factors and two properties of the transition: transition temporal length and average rain water path. We base the ensemble around a composite of trajectories in the Northeastern Pacific during summer. Using these emulators, parameter space can be densely sampled to visualise the joint and individual effects of the factors on the transition properties. We find that in the low-aerosol regime (< 200 cm−3) the transition time is most strongly affected by the aerosol concentration out of the factors considered here. Fast transitions, under 40 h, occur in this regime with high mean rain water path, which is consistent with a drizzle-depletion effect. In the high-aerosol regime, the inversion strength becomes more important than the aerosol concentration through the inversion's effect on entrainment and the deepening-warming decoupling mechanism.