Wind shear and thermodynamic characteristics near the stratocumulus cloud top

Aircraft measurements were made from July 16 to August 15, 2008 during the Physics of Stratocumulus Top (POST) experiment off the coast of central California with the objective to study the interfacial properties at the top of marine stratocumulus. The aircraft measurements was were made primarily in field of unbroken stratocumulus with multiple porporsing sounding legs through the cloud top to detect fine-scale thermodynamic and turbulent properties at the interface. Our analyses have focused on the sounding profiles. The interface can be identified in several different approaches. The cloud top interface (Zqc) is determined as the highest level where liquid water content approaches a threshold value (0.04 gkg-1 )used here. The maximum gradient interface (Ztgm) marks the largest vertical gradient of virtual potential temperature and the turbulent mixing interface (Zmix) is defined as the upper boundary where atmospheric boundary layer air can reach via turbulence. The observations indicate that in most cases a sharp interface layer exists at the top of stratocumulus. The mean differences between heights of Zmix and Ztgm are less than 10 m for all flight days and they are statistically higher than height of Zqc with near or more than 20 m for about 30 % of all flight days. The geographical distributions of the interfaces are non-uniform. The strong mean wind shear in vicinity of cloud top is important for formation of separation of above two interfaces (Zmix and Ztgm) from cloud top. The turbulence mixture in vicinity of cloud top induced by strong mean wind shear could induce variation of the vertical variation of liquid water content to deviate from it adiabatic values.