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

Coherent structures associated with forward- and backward-scatter energy tr ansfer in the convective planetary boundary layer are studied using large-e ddy simulation. A box filter is adopted for calculation of the resolved nea r-grid-scale stress tensor (L-ij), strain-rate tensor (S-ij) and dissipatio n rate (LijSij). Results of conditional sampling at two different heights a re presented. The conditional events are resolved dissipation rate and vert ical velocity fluctuation. The latter is to distinguish the forward and bac kward scatter associated with upward and downward motions, respectively. Ne ar the surface, the forward-scatter event with positive vertical velocity f luctuation is physically associated with large-scale elongated updrafts. Th erefore, the contribution of spanwise component L22S22 to the resolved diss ipation rate is most significant. In the outer layer, the forward-scatter e vent with positive vertical velocity fluctuation occurs at the top of risin g updrafts, consistent with negative S-33 and the dominance of L33S33. In l ike manner, the forward scatter with negative vertical velocity fluctuation is found ahead of downdraft motions where S-33 is negative and L33S33 domi nates. Near the surface, the off-diagonal component L13S13 becomes dominant as the result of downdraft motions to the surface. For the backward scatte r, the vertical diagonal component L33S33 is strongest except near the surf ace in regions of downward motions where L13S13 tends to be most intense. T he strong L33S33 event is embedded within updrafts and is characterized by minimum pressure. Flow visualization suggests that the backward scatter occ urs on the upwash side of the vortex where updrafts pass through. It is arg ued that the diameters of vortices can be effectively increased by nearby u pdrafts, representing energy transfer from small- to large-scale structures . The backward scatter associated with downward motions near the surface ap pears to generate small-scale counter-rotating motions, resembling sweeps. (C) 1999 American Institute of Physics. [S1070-6631(99)01811-5].