Beyond the cold air advection: how do stratocumulus-surface coupling state respond to horizontal temperature advection?

Abstract:

Transitions of marine boundary layer cloud regimes are often associated with changes in boundary layer coupling state. Most prior studies on decoupling of a stratocumulus-topped boundary layer (STBL) are focused on subtropics where cold air advection with moderate strength is dominant. This study expands across a wider spectrum of temperature advection spanning from moderately strong warm air advection to extremely strong cold air advection. A STBL undergoing warm advection is found to be more mixed than a STBL undergoing cold advection. This finding is consistent with the cold advection facilitating turbulent mixing in the boundary layer. When cold advection becomes sufficiently strong (< -5 K/day), the STBL becomes more stably stratified again because of emergence of the cumulus-coupled STBL regime induced by the “deepening-warming” mechanism. Such a “deepening-warming” induced STBL decoupling, however, is still much weaker than that caused by warm advection flows (even weak ones), suggesting that the direction and strength of temperature advection must be considered for any STBL decoupling studies.