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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.
