Morning Transition of Complex-Terrain Flows

A field experimental program, known as the Phoenix Air Flow Experiment (PAFEX II), was conducted in the summer of 1998 to investigate the flow and contaminant dispersion in the complex-terrain airshed of Phoenix, Arizona. A major thrust of the experiment was to study vertical mixing during the morning transition, wherein stably stratified nocturnal boundary layer transforms into daytime convective boundary layer. Typically, this transition is associated with the growth of the convective boundary layer upon sunrise, but detailed measurements during PAFEX II showed the possibility of another major breakdown mechanism involving shear instabilities. At night, the flow is mainly down-slope and down-valley (contributed by both the local and long-range drainage flows) and during the morning transition an up-slope flow develops near the ground while the upper level winds continue to flow down-slope. The ensuing velocity shear between the (lighter) upper and (denser) lower layers causes instabilities to set in, breakdown of which causes strong vertical mixing. This mechanism becomes operational before the development of the convective layer near the ground. The above scenario is supported by tethered-balloon observations, ozone and aerosol sampling as well as by simulations carried out using the HOTMAC meso-scale airflow model. (This work was supported by the NSF EGB Initiative)