As part of an ozone study, a boundary-layer measurement campaign was conducted in the Phoenix area during four weeks in May and June of 1998. Because of the lack of routine upper-air measurements, the characteristics of the boundary layer in the Phoenix region have not been examined in detail until now. An array of three 915 Mhz radar profilers was deployed to obtain continuous profiles of wind velocities to heights of 3000 m AGL or more. Each profiler site was equipped with a Radio Acoustic Sounding System to obtain virtual temperature profiles to heights on the order of 1000 m AGL. Deeper vertical profiles of temperature and humidity were obtained at two of the wind profiler sites with radiosondes launched five times per day (0800, 1000, 1200, 1400, and 1700 LST) during 13 intensive measurement days. Supplementary soundings were obtained in the mountains southeast of Phoenix at 1700 LST. DOE’s G-1 aircraft measured wind speed, direction, temperature, and humidity in addition to various chemical species over the metropolitan area. The RAMS mesoscale model was used prior to the experiment to simulate expected wind patterns and help determine locations for the placement of the boundary-layer measurement instruments.
The mesoscale model was also used to predict the growth of the boundary layer and the thermally-driven circulations. The model results indicated that in the presence of light ambient winds a deep boundary layer would grow and a mountain-plain circulation could develop that would transport pollutants from Phoenix into the mountains to the northeast. Subsequent nocturnal drainage flow patterns would then bring pollutants released on one day back over the city on the following day. Preliminary findings show that boundary layers grew as deep as 2500-3500 m, with no appreciable differences in mixed layer heights between the city and nearby mountains. Moderate to strong westerly to southwesterly winds were observed by all of the profilers above the convective boundary layer throughout the 4-week period, but winds over Phoenix were generally light in the convective boundary layer. Evidence of shallow upslope winds was found at the two profiler sites located close to the foothills. Along-valley southerly winds developed around noon at one site; these winds gradually turned to the west during the afternoon as the flow became coupled to the larger-scale mountain-plain circulation. Northwesterly upslope winds were usually observed at the other foothills profiler site the entire day. In contrast with the preliminary modeling results, no strong near-surface drainage winds were detected by the profilers. Weak drainage flows that developed after midnight within 300 m of the ground were measured only on a few nights. The data also suggest that the basin atmosphere was well ventilated during this period and that little recirculation occurred. This result contradicts predictions made by the model but is consistent with airborne and ground-based ozone measurements obtained during the experiment. The results of the boundary-layer measurements and modeling will be used to help interpret the ozone data.