Shear-induced turbulent mixing in stratocumulus layer

The influence on strong wind shear on turbulent mixing at the top of a stratocumulus layer is investigated with the “Airborne Cloud Turbulence Observation System” - ACTOS. In contrast to airplane measurements the payload is designed for helicopter-borne measurements with a low true air- speed of 15 - 20 m s^-1 resulting in a spatial resolution up to cm scale. Beside meteorological parameter as temperature, pressure and absolute humidity the cloud droplets, interstitial aerosol and high resolution turbulence are measured. The data set presented here was sampled over the Baltic Sea during a campaign at the airport of Kiel/Germany in October 2007. The measurements were performed around the top of a stratocumulus layer consisting of several subsequent ascents and descents in and above the cloud layer (so called “dolphin flights”). Above the cloud top - defined by the absence of cloud droplets - an interfacial layer of 40 - 50 m thickness was found with similar turbulence and thermodynamic properties compared to the cloud region with droplets. The top of this interfacial layer is characterize by the maximum gradient of different parameter such as temperature, absolute humidity, gradient richardson number and wind shear. We conclude that shear-induced mixing is followed by cloud droplet evaporation at cloud top which results in the development of the interfacial layer. Such a process is important for cloud life time and radiative properties of the cloud top.