Using Vertical Velocity Retrievals to Estimate Entrainment Rates in Stratocumulus Cloud Systems

Advances in aircraft measurements, ground-based remote sensing, and numerical simulations since the 1970s, when stratocumulus-topped boundary layers (STBLs) were extensively studied by Wayne Schubert and colleagues, have led to new insights, especially into the entrainment process. Entrainment is the small-scale process by which a turbulent flow incorporates adjacent fluid that is nonturbulent. Entrainment in STBLs occurs at cloud top. In most atmospheric models, this entrainment must be parameterized.

A STBL is a shallow convecting layer in which both the updraft and the downdraft branches of the convective circulation are saturated (cloudy) in the upper part of the boundary layer. The convective circulation is vigorous, yet the cloud-top entrainment rate is small, and therefore difficult to measure. Even though the entrainment rate is small, differences between currently used parameterizations of it in global climate models can lead to STBLs that differ by as much as a factor of two in climatologically important properties such as liquid water path and boundary layer depth. Moreover, differences among proposed entrainment parameterizations yield STBLs with different equilibrium sensitivities; some parameterizations are more sensitive to divergence while others are more sensitive to variations in the sea-surface temperature.

We have developed and tested a new method to estimate entrainment rates based on the cloud-topped mixed layer concepts introduced by Schubert and colleagues. The method uses in-cloud vertical velocity near cloud base or cloud top and a measure of horizontal liquid water content variability (such as cloud base height variations, liquid water path variations, or liquid water content profile variations) to estimate the turbulent liquid water flux, which is then used in combination with cloud-top jumps of temperature, water vapor, and radiative flux to estimate the entrainment rate. The in-cloud vertical velocity and cloud base height, liquid water path, and liquid water content profiles are all obtained using ground-based remote sensing. Such retrievals are available from the Eastern North Atlantic ARM (Atmospheric Radiation Measurement program). We are also using LES (large-eddy simulation) to further test and evaluate this method to estimate cloud-top entrainment rates in STBLs.

In my presentation, I will link current research on cloud-topped mixed layers with the concepts developed by Schubert and colleagues.