Evaluation of Large-Eddy Simulations via Observations of Nocturnal Marine Stratocumulus

Abstract

Data from the first research flight (RF01) of the second Dynamics and Chemistry of Marine Stratocumulus (DYCOMS-II) field study are used to evaluate the fidelity with which large-eddy simulations (LESs) can represent the turbulent structure of stratocumulus-topped boundary layers. The initial data and forcings for this case placed it in an interesting part of parameter space, near the boundary where cloud-top mixing is thought to render the cloud layer unstable on the one hand, or tending toward a decoupled structure on the other hand. The basis of this evaluation consists of sixteen 4-h simulations from 10 modeling centers over grids whose vertical spacing was 5 m at the cloud-top interface and whose horizontal spacing was 35 m. Extensive sensitivity studies of both the configuration of the case and the numerical setup also enhanced the analysis. Overall it was found that (i) if efforts are made to reduce spurious mixing at cloud top, either by refining the vertical grid or limiting the effects of the subgrid model in this region, then the observed turbulent and thermodynamic structure of the layer can be reproduced with some fidelity; (ii) the base, or native configuration of most simulations greatly overestimated mixing at cloud top, tending toward a decoupled layer in which cloud liquid water path and turbulent intensities were grossly underestimated; (iii) the sensitivity of the simulations to the representation of mixing at cloud top is, to a certain extent, amplified by particulars of this case. Overall the results suggest that the use of LESs to map out the behavior of the stratocumulus-topped boundary layer in this interesting region of parameter space requires a more compelling representation of processes at cloud top. In the absence of significant leaps in the understanding of subgrid-scale (SGS) physics, such a representation can only be achieved by a significant refinement in resolution-a refinement that, while conceivable given existing resources, is probably still beyond the reach of most centers.

Comparisons of bispectral and polarimetric retrievals of marine boundary layer cloud microphysics: case studies using a LES–satellite retrieval simulator

“…The ATEX cases are representative of a trade wind cumulus regime in which scattered cumuli rise into a thin, broken stratocumulus layer. The third case (referred to as "DYCOMS-II" hereafter), originally presented in Stevens et al (2005), is an idealized setup based on clouds observed during the second research flight (RF02) of the Second Dynamics and Chemistry of Marine Stratocumulus project (DYCOMS-II) (Stevens et al, 2003). This case is representative of nocturnal marine stratocumulus under conditions of a dry inversion.…”

Droplet growth in warm turbulent clouds

“…A survey of LES of stratocumulus by Stevens (2002) showed that entrainment rates diagnosed from LES results can vary by up to a factor of two. This has led Stevens et al (2005) to suggest that the entrainment rate may not only be sensitive to the energy-containing eddies but also to the small-scale (subgrid) eddies which are not explicitly resolved in LES, either directly or indirectly through the impact of the small-scale (subgrid) eddies on the large-scale eddies. Such a situation would be in agreement with the concept of an entrainment interfacial layer: since this layer is thin, its dynamics are likely to be dominated by the smaller eddies.…”

Simulation of low clouds in the Southeast Pacific by the NCEP GFS: sensitivity to vertical mixing

“…According to Randall (1980) and Deardorff (1980) an appropriate value to be used in the right hand side of (1) is κ 0 = 0.23. As pointed out by many authors (e.g., Kuo and Schubert, 1988;Moeng, 2000;Stevens et al, 2005;Siems et al, 1990), Eq.…”