Role of surface latent heat flux in shallow cloud transitions: A mechanism-denial LES study

Abstract

AbstractSurface latent heat flux (LHF) has been considered as the determinant driver of the stratocumulus-to-cumulus transition (SCT). The distinct signature of the LHF in driving the SCT, however, has not been found in observations. This motivates us to ask: how determinant is the LHF to SCT? To answer it, we conduct large-eddy simulations in a Lagrangian setup in which the sea-surface temperature increases over time to mimic a low-level cold air advection. To isolate the role of LHF, we conduct a mechanism-denial experiment in which the LHF adjustment is turned off. The simulations confirm the indispensable roles of LHF in sustaining (although not initiating) the boundary layer decoupling (first stage of SCT) and driving the cloud regime transition (second stage of SCT). However, using theoretical arguments and LES results, we show that decoupling can happen without the need for LHF to increase as long as the capping inversion is weak enough to ensure high entrainment efficiency. The high entrainment efficiency alone cannot sustain the decoupled state without the help of LHF adjustment, leading to the recoupling of the boundary layer that eventually becomes cloud-free. Interestingly, the stratocumulus sheet is sustained longer without LHF adjustment. The mechanisms underlying the findings are explained from the perspectives of cloud-layer budgets of energy (first stage) and liquid water path (second stage).