Abstract
To assess the realism of large-eddy simulation (LES) of the stratus-topped boundary layer and its predicted turbulent structure, we performed detailed data analyses on a LES (which has a 12.5 m grid size in all three directions), in a manner similar to those used by Nicholls (1989) on aircraft measurements. The first analysis retrieves the primary convective elements, i.e., the negatively buoyant downdrafts, which are driven mainly by cloud-top radiative cooling, through a conditional sampling technique. Comparison shows that the LES of this resolution reflects most of the observed downdraft features; most of the discrepancies that exist between the obervations and the LES can be explained by decoupling of the cloud layer from the underlying flow that exists in the former but not in the latter. The second analysis shows the vertical velocity spectrum and its agreement with the measurements. In the third analysis, showing the turbulent kinetic energy budgets, the discrepancy in the turbulent transport term (i.e., the divergence of the third-moment quantity\(\overline {wE} \), the turbulent-kinetic-energy flux) between the LES and measurements exists even with such a fine resolution LES. This discrepancy is related mainly to the different behavior in\(\overline {w^3 } \) between the LES and observations, which may again be associated with decoupling.
An advantage of LES over aircraft observations is that the former can provide three-dimensional flow structure at any instant. In this paper, we examined the instantaneous flow structure and observed closed cellular patterns near the cloud top in which updrafts occupy the broad centers and relatively strong downdrafts occur in the narrow edges. In the intersections of these cell boundaries, there exist weak downdrafts, consisting of relatively cold and dry air, that are the most likely origins of the strong downdrafts extending throughout the mixed layer.
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The National Center for Atmospheric Research is sponsored by the National Science Foundation.
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Shen, S., Moeng, CH. Comparison of a computer-simulated stratus-topped boundary layer with aircraft observations. Boundary-Layer Meteorol 65, 29–53 (1993). https://doi.org/10.1007/BF00708817
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DOI: https://doi.org/10.1007/BF00708817