Well-developed low speed and high temperature streaks in association with the alignment of convection cells are observed in a large-eddy-simulation (LES) generated strongly sheared convective boundary-layer flow, which is driven by a geostrophic wind speed of 15 m s?¹ and a surface kinematic heat flux of 0.05 K m s?¹. Vortices that drive streaky structures are identified through an eigenvalue method (λ₂ method) near the surface. These vortices are highly elongated along the quasi-streamwise direction alternating sign of the x-component of vorticity (ω<SUB>x</SUB>). By conditional sampling of fully developed vortices, a statistically significant coherent structure is educed. The educed vortex is elongated to the streamwise direction with the elevation angle of about 17˚ above the horizontal surface. However, the horizontal tilting is not clearly demonstrated in the present simulation. Fluctuation fields in the domain of the educed vortex show the existence of a low speed and high temperature streak as a direct consequence of momentum and heat transport by vortical motions. The strong ejection (upward transport of low momentum or high temperature) occurring at the higher level than that of the strong sweep (downward transport of high momentum and low temperature) can be explained from the spatial distribution of the fluctuation fields of velocity and temperature. The contribution of ejection to the Reynolds stress at z/h₁ = 0.18 is about 75%, which is slightly greater than that (70% at z/h₁ = 0.173) for the neutrally stratified atmospheric boundary layer. Ejection is also found to be dominant for the turbulent heat flux.
Abstract<BR>1. Introduction<BR>2. The LES Experiment<BR>3. Flux Statistics<BR>4. Identification of Vortices<BR>5. Results and Discussion<BR>6. Concluding Remarks<BR>Acknowledgements<BR>References<BR>
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