Physics of Stratocumulus Top (POST): turbulent mixing across capping inversion

摘要

High spatial resolution measurements of temperature and liquid watercontent, accompanied by moderate-resolution measurements of humidityand turbulence, collected during the Physics of Stratocumulus Topexperiment are analyzed. Two thermodynamically, meteorologically andeven optically different cases are investigated. An algorithmicdivision of the cloud-top region into layers is proposed. Analysisof dynamic stability across these layers leads to the conclusionthat the inversion capping the cloud and the cloud-top region isturbulent due to the wind shear, which is strong enough to overcome the high static stability of the inversion. The thicknessof this mixing layer adapts to wind and temperature jumps such thatthe gradient Richardson number stays close to its criticalvalue. Turbulent mixing governs transport across the inversion, butthe consequences of this mixing depend on the thermodynamicproperties of cloud top and free troposphere. The effects ofbuoyancy sorting of the mixed parcels in the cloud-top region aredifferent in conditions that permit or prevent cloud-top entrainmentinstability. Removal of negatively buoyant air from the cloud top isobserved in the first case, while buildup of the diluted cloud-toplayer is observed in the second one.