Near-grid-scale energy transfer and coherent structures in the convective planetary boundary layer

Details Near-grid-scale energy transfer and coherent structures in the convective planetary boundary layer Near-grid-scale energy transfer and coherent structures in the convective planetary boundary layer

Nov 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phfl...11.3482l&link_type=abstract

Physics of Fluids, Volume 11, Issue 11, pp. 3482-3494 (1999).

Physics

6

Boundary Layer Structure And Processes, Fluctuation Phenomena, Random Processes, Noise, And Brownian Motion

Scientific paper

Coherent structures associated with forward- and backward-scatter energy transfer in the convective planetary boundary layer are studied using large-eddy simulation. A box filter is adopted for calculation of the resolved near-grid-scale stress tensor (Lij), strain-rate tensor (Sij) and dissipation rate (LijSij). Results of conditional sampling at two different heights are presented. The conditional events are resolved dissipation rate and vertical velocity fluctuation. The latter is to distinguish the forward and backward scatter associated with upward and downward motions, respectively. Near the surface, the forward-scatter event with positive vertical velocity fluctuation is physically associated with large-scale elongated updrafts. Therefore, the contribution of spanwise component L22S22 to the resolved dissipation rate is most significant. In the outer layer, the forward-scatter event with positive vertical velocity fluctuation occurs at the top of rising updrafts, consistent with negative S33 and the dominance of L33S33. In like manner, the forward scatter with negative vertical velocity fluctuation is found ahead of downdraft motions where S33 is negative and L33S33 dominates. Near the surface, the off-diagonal component L13S13 becomes dominant as the result of downdraft motions to the surface. For the backward scatter, the vertical diagonal component L33S33 is strongest except near the surface in regions of downward motions where L13S13 tends to be most intense. The strong L33S33 event is embedded within updrafts and is characterized by minimum pressure. Flow visualization suggests that the backward scatter occurs on the upwash side of the vortex where updrafts pass through. It is argued that the diameters of vortices can be effectively increased by nearby updrafts, representing energy transfer from small- to large-scale structures. The backward scatter associated with downward motions near the surface appears to generate small-scale counter-rotating motions, resembling sweeps.

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