68 Anisotropy of Observed and Simulated Turbulence at Marine Stratocumulus top

Monday, 9 July 2018
Regency A/B/C (Hyatt Regency Vancouver)
Szymon P. Malinowski, University of Warsaw, Warsaw, Poland

Anisotropy of turbulence near the top of the marine stratocumulus-topped boundary layer (STBL) is
studied using airborne measurements from the POST and DYCOMS-II field campaigns and largeeddy
simulation (LES). Focusing on turbulence ~100 m below the cloud top, we see remarkable
similarity between daytime and nocturnal flight data covering different inversion strengths and freetropospheric
conditions. With λ denoting wavelength and zt cloud-top height, we find that
turbulence at λ/zt ≈ 0.01 is weakly dominated by horizontal fluctuations, while turbulence at λ/zt > 1
becomes strongly dominated by horizontal fluctuations. Between are scales at which vertical
fluctuations dominate.
Typical-resolution LES based on POST flight 13 and DYCOMS-II flight 1 capture observed
characteristics of below-cloud-top turbulence reasonably well. However, using a fixed vertical grid
spacing of 5 m, decreasing the horizontal grid spacing and increasing the subgrid-scale mixing
length leads to increased dominance of vertical fluctuations, increased entrainment velocity, and
decreased liquid water path.
Our results support the notion that entrainment parameterizations into STBL (e.g., in weather and
climate models) could be improved by accounting for anisotropy of turbulent eddies in the cloudtop
region. While LES has the potential to facilitate improved understanding of anisotropic cloudtop
turbulence, sensitivity to grid spacing, grid-box aspect ratio, and subgrid-scale model needs to
be addressed.
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