A more general paradigm for understanding marine boundary-layer decoupling

Science

Most prior studies on the 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.

Impact

We have found:

(1) The coupling state of a stratocumulus-topped boundary layer (STBL) is sensitive to the strength and direction of temperature advection.

(2) A STBL is more coupled in cold advection than in warm advection.

(3) When cold advection becomes extremely strong, a STBL becomes more decoupled again, but still more coupled than that in warm advection.

These results offer us a more general paradigm for understanding STBL decoupling: the dynamics of decoupling across the whole spectrum of horizontal temperature advection (not just cold advection). 

Summary

Using ground observations from three DOE/ASR marine campaigns, we found that a STBL undergoing warm advection is 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 the 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.