Surface-Atmosphere Decoupling Prolongs Cloud Lifetime Under Warm Advection Due To Reduced Entrainment Drying

An initially well-mixed stratocumulus deck can remain overcast for several tens of hours under warm-advection conditions, although moisture supply is cut off from the ocean due to surface-atmosphere decoupling (stabilization of the surface-atmosphere interface). In this study, a set of idealized large-eddy simulations were performed to investigate the physical mechanism of how warm-air advection impacts the evolution of a pre-existing stratocumulus deck. To mimic warm-air advection, we decrease the sea surface temperature linearly over time in a doubly periodic domain. Given the same initial conditions, the stratocumulus deck is more persistent when experiencing warm-air advection than cold-air advection. This persistence is caused by reduced cloud-top entrainment drying due to decoupling, a process more influential than the decoupling-induced cutoff of moisture supply. This mechanism is more notable when the free troposphere becomes more humid. The relevance of the mechanism to previous observations of less low-level cloudiness under warm-advection conditions is discussed.