ENTRAINMENT PROCESSES IN THE CONVECTIVE BOUNDARY LAYER WITH VARYINGWIND SHEAR

Large-eddy simulations (LES) are performed to investigate the entrainment and the structure of the inversion layer of the convective boundary layer (CBL) with varying wind shears. Three CBLs are generated with the constant surface kinematic heat flux of 0.05 K m s?¹ and varying geostrophic wind speeds from 5 to 15 m s?¹. Heat flux profiles show that the maximum entrainment heat flux as a fraction of the surface heat flux increases from 0.13 to 0.30 in magnitude with increasing wind shear. The thickness of the entrainment layer, relative to the depth of the well-mixed layer, increases substantially from 0.36 to 0.73 with increasing wind shear. The identification of vortices and extensive flow visualizations near the entrainment layer show that concentrated vortices perpendicular to the mean boundary-layer wind direction are identified in the capping inversion layer for the case of strong wind shear. These vortices are found to develop along the mean wind directions over strong updrafts, which are generated by convective rolls and to appear as large-scale wavy motions similar to billows generated by the Kelvin?Helmholtz instability. Quadrant analysis of the heat flux shows that in the case of strong wind shear, large fluctuations of temperature and vertical velocity generated by large amplitude wavy motions result in greater heat flux at each quadrant than that in the weak wind shear case.