Anisotropy of observed and simulated turbulence in marine stratocumulus

Focusing on turbulence similar to 100 m below the cloud top, we see remarkable similarity between daytime and nocturnal flight data covering different inversion strengths and free-tropospheric conditions. With lambda denoting wavelength and z(t) cloud-top height, we find that turbulence at lambda/z(t) similar or equal to 0.01 is weakly dominated by horizontal fluctuations, while turbulence at lambda/z(t) > 1 becomes strongly dominated by horizontal fluctuations.

Between are scales at which vertical fluctuations dominate. Typical-resolution LES of the STBL (based on POST flight 13 and DYCOMS-II flight 1) captures 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.