A case study of the development of nocturnal slope flows in a wide open valley and associated air quality implications

This paper documents the development of nocturnal flows in the wide open Phoenix, Arizona (U.S.A) valley (30 km x 100 km) that is bordered by a large nearly flat plain to the west and high mountains to the north and east. Local thermally driven winds concomitant with the absence of significant synoptic pressure gradients dominate typical winter conditions in the Phoenix valley. The purpose of the Phoenix Air Flow Experiment (PAFEX-1) was to study the development of thermally driven flows during the evening transition in a sloping valley and describe the general pattern of transport and dispersion of contaminants during transition periods and at night. Measurements were made using a tethered balloon, sonic anemometer, balloon-based aerosol sampler. radiation sensors, cup anemometers, thermistors and humidity sensors in conjunction with data collected from 44, standard meteorological stations located throughout the valley. Over the period of 15 days in January and February 1998 the general diurnal flow patterns were repeatable, but varied substantially around the valley. This paper focuses on a case study of the evening transition, nocturnal circulation and morning break-down of the nocturnal circulation on the night of 31 January and morning of 1 February. Central valley measurements were consistent with the notion that the evening transition is associated with a moving front, followed by intense mixing and the movement of the front to establish down-valley winds. Flows originating from different slopes led to the arrival of fronts at the various measurement locations at different times. These flows intrude into the valley and interact with each other, often causing multi-layered vertical structure. The intrusions respond to the evolving stratification and cause striking variability of these layers, for example, periodic wind and temperature disturbances corresponding to the arrival of new intrusive fronts. The evolution of the boundary layer was found to have a direct beating on the pollution concentrations in the Phoenix air shed.