Abstract
Motivated by air quality and numerical modelling applications as well as recent theoretical advancements in the topic, a field experiment, dubbed transition flow experiment, was conducted in Phoenix, Arizona to study the evening transition in complex terrain (shift of winds from upslope to downslope). Two scenarios were considered: (i) the flow reversal due to a change of buoyancy of a cooled slab of air near the ground, and (ii) the formation of a transition front. A suite of in-situ flow, turbulence and particulate matter (PM) concentration sensors, vertically profiling tethered balloons and remote sensors were deployed, and a mesoscale numerical model provided guidance for interpreting observations. The results were consistent with the front formation mechanism, where it was also found that enhanced turbulence associated with the front increases the local PM concentration. During the transition period the flow adjustment was complex, involving the arrival of multiple fronts from different slopes, directional shear between fronts and episodic turbulent mixing events. The upward momentum diffusion from the incipient downslope flow was small because of stable stratification near the ground, and full establishment of downslope flow occurred over several hours following sunset. Episodic frontal events pose challenges to the modelling of the evening transition in complex terrain, requiring conditional parametrizations for subgrid scales. The observed increase of PM concentration during the evening transition has significant implications for the regulatory enforcement of PM standards for the area.
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Acknowledgments
The authors wish to thank Randy Redman and Peter Hyde (Arizona Department of Environmental Quality), Jennifer McCulley, Adam Christman, Dragan Zajic, Charles Retallack, Martin Weeden, Richard Montenegro and Anna Haywood (Arizona State University), Steve Shackleford (Federal Aviation Administration), Mark Sandomir (Mountain View High School), and the Maricopa County Air Quality Department team without whom TRANFLEX would not have been possible. In addition, the authors thank Niyati Parikh Yagnik for her work on the pattern recognition algorithm. The authors also wish to acknowledge the three anonymous reviewers of this manuscript for making very valuable comments. The financial support for the experiments was provided by the National Science Foundation (NSF-ATM) and Arizona Department of Environmental Quality. The data analysis was performed with the support of NSF (CMG) and the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program, which is supported by the Office of Naval Research.
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Fernando, H.J.S., Verhoef, B., Di Sabatino, S. et al. The Phoenix Evening Transition Flow Experiment (TRANSFLEX). Boundary-Layer Meteorol 147, 443–468 (2013). https://doi.org/10.1007/s10546-012-9795-5
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DOI: https://doi.org/10.1007/s10546-012-9795-5