Surface layer counter-gradient motion in and around an urban area

Quadrant analysis of turbulent momentum flux, u'w', typically focuses on updrafts of slow fluid (quadrant 2, ejections) and downdrafts of fast fluid (quadrant 4, sweeps). The counter-gradient contributions of quadrant 1 (upward motion of fast fluid) and quadrant 3 (downward motion of slow fluid) to u'w' are typically small. Analysis of 3D sonic anemometer data from JU2003 found persistent quadrant 1 and quadrant 3 events corresponding to eddy sizes on the order of 100m. Although these events had a significant effect on less than 1% of the momentum flux calculations for the entire month of the field campaign, they may be relevant to transport and dispersion. Also discussed are the conditions prevalent during these events and some possible mechanisms which give rise to significant quadrant 1 and quadrant 3 motions.