Cloud-Surface Coupling Alters the Morning Transition From Stable to Unstable Boundary Layer

Due to surface heating, the morning boundary layer transits from stable to neutral or convective conditions, exerting critical influences on low tropospheric thermodynamics. Low clouds closely interact with the boundary layer development, yet their interactions bear considerable uncertainties. Our study reveals that cloud-surface coupling alters the morning transition from stable to unstable boundary layer and thus notably affects the diurnal variation of the boundary layer. Specifically, due to the reduction in surface fluxes, decoupled clouds can delay the process of eroding nocturnal inversion by 0.8-hr and even prevent the transition of the boundary layer from happening for 12% of decoupled cases, keeping the boundary layer in a stable state during the noontime. On the other hand, when clouds are coupled with the surface, cloud-top radiative cooling can directly cool the upper boundary layer to facilitate sub-cloud convection, leading to an unstable boundary layer in the earlier morning.

Automated summary

Due to surface heating, the morning boundary layer undergoes a transition from stable to neutral or convective conditions, which has a significant impact on low tropospheric thermodynamics. The interactions between low clouds and the boundary layer development are complex and uncertain. Our study reveals that cloud-surface coupling can influence the morning transition process, with decoupled clouds causing delays and even preventing the transition in some cases, while coupled clouds facilitate sub-cloud convection and lead to an unstable boundary layer in the earlier morning.