A214-0001
Using Cloud Radar to Retrieve Entrainment Rates in Stratocumulus Clouds

Wednesday, 16 December 2020
Poster
Emina Maric, University of Utah, Salt Lake City, UT, United States, Steven K Krueger, University of Utah, Atmospheric Sciences, Salt Lake City, UT, United States, Pavlos Kollias, Stony Brook University, Stony Brook, NY, United States and Zeen Zhu, Stony Brook University, School of Marine and Atmospheric Sciences, Stony Brook, NY, United States
Abstract:
Entrainment is the process by which a turbulent flow incorporates adjacent fluid that is nonturbulent. Entrainment in stratocumulus-topped boundary layers (STBLs) occurs at cloud top. In global climate models(GCMs), 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 velocity, we , is small and difficult to measure. Differences between currently used parameterizations of we in GCMs 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.

We have developed a new method to retrieve entrainment rates in STBLs which uses vertical velocity retrievals from surface-based cloud radar, in conjunction with other surface-based measurements. These new estimates will allow a more comprehensive evaluation of parameterizations of entrainment rate for STBLs. The new method will also provide ground-based estimates of entrainment in STBLs over long time periods.

We use retrievals of vertical velocity and liquid water content profiles to estimate the turbulent liquid water flux, which is then used in combination with observed cloud-top jumps of temperature, water vapor, and calculated radiative flux profiles to estimate the entrainment rate. We have already made extensive use of 25 large-eddy simulations, each 10 days in duration, performed by van der Dussen et al. (2015), to evaluate the retrieval method. We found that the liquid-water flux jump method does indeed produce excellent estimates of we when applied at levels 80 to 100 m above the lowest cloud base. The figure shows that these estimates are as good as those obtained for levels from 20 m to 100 m below cloud top, the levels for which aircraft flux measurements are typically made.

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