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

Ching-Long Lin

doi:10.1063/1.870206

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 (L ij ), strain-rate tensor (S ij ) and dissipation rate (L ij S ij ). 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 L 22 S 22 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 S 33 and the dominance of L 33 S 33 . In like manner, the forward scatter with negative vertical velocity fluctuation is found ahead of downdraft motions where S 33 is negative and L 33 S 33 dominates. Near the surface, the off-diagonal component L 13 S 13 becomes dominant as the result of downdraft motions to the surface. For the backward scatter, the vertical diagonal component L 33 S 33 is strongest except near the surface in regions of downward motions where L 13 S 13 tends to be most intense. The strong L 33 S 33 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.

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

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