Lineal Eddy Features under Strong Shear Conditions

Abstract

Large eddy simulation of a marine boundary layer (BL) with strong wind shear and weak buoyancy flux, over a relatively large horizontal domain (25?18 km) and long time period (16 h), produces lineal eddy structures with along-axis variations. These ?rolls? are transient, their lifetime being roughly 8 h and their horizontal-to-vertical aspect ratio increases with time. At all heights above the surface layer, even above the BL, temporally coherent features propogate with a speed slightly slower than the vertically averaged BL velocity. The length scale of along-roll variation is about four and one-half times the corresponding cross-roll length scale. To examine the importance of the roll circulations, fluxes of momentum, heat, and water vapor are partitioned into roll and non-roll contributions, and additionally separated into contributions from the ascending and descending roll regions, to reveal differences in advective roll transport and in roll modulation of smaller-scale transfer. The roll influence is largest at mid-BL for momentum and moisture fluxes but largest near the BL top for heat flux. The implied K-theory mixing coefficients are quite different for each variable, even if the influence of the roll circulations is removed. Horizontal variations of surface flux are opposite in sign to flux variations within the BL bulk.