Sedimentation effects on stratocumulus lifetime more important than previously thought

In a new study published in the “Journal of Advances in Modeling the Earth System” Dr Bernhard Schulz and Prof Juan Pedro Mellado, scientists in the Max Planck Research Group “Turbulent Mixing Processes in the Earth System” at the Max Planck Institute for Meteorology (MPI-M), investigated two of the key processes that render the mixing of cloudy and free-tropospheric air uncertain. The first process is the gravitational settling of cloud droplets suspended in the cloud – referred to as droplet sedimentation – which weakens cloud-top mixing. The second process is wind shear located in the cloud-top region which amplifies cloud-top mixing. By employing direct numerical simulations with a grid spacing of around 20 cm the authors showed that for subtropical conditions both processes can be equally important for determining cloud-top mixing. While droplet sedimentation diminishes the rate of cloud-top mixing by up to 40%, wind shear does the opposite and enhances the rate of cloud-top mixing by up to 40%. This result challenges previous large eddy simulations, which suggest that wind-shear effects are more important than droplet-sedimentation effects.

As the droplet size distribution determines the strength of droplet sedimentation, changing the shape of the droplet size distribution can substantially alter the rate of cloud-top mixing and thus cloud lifetimes. This shows that droplet sedimentation can alter cloud lifetimes not only because of its effects on rain formation but also because of its effects on cloud-top mixing. A second implication is that parameterizations of the rate of cloud-top mixing should pay equal attention to droplet-sedimentation and wind-shear effects.dscherungseffekte gleich wichtig sind.