Boundary-Layer Meteorol (2013) 147:443–468 DOI 10.1007/s10546-012-9795-5 ARTICLE The Phoenix Evening Transition Flow Experiment (TRANSFLEX) Harindra J. S. Fernando · Brett Verhoef · Silvana Di Sabatino · Laura S. Leo · Seoyeon Park Received: 20 March 2011 / Accepted: 12 December 2012 / Published online: 6 January 2013 © Springer Science+Business Media Dordrecht 2013 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) concen- tration 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 tran- sition 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 sta- ble 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 H. J. S. Fernando · S. Di Sabatino · L. S. Leo Environmental Fluid Dynamics Laboratories, Department of Civil & Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, South Bend, IN 46556, USA e-mail: Harindra.J.Fernando.10@nd.edu L. S. Leo e-mail: LauraSandra.Leo.13@nd.edu S. Di Sabatino (B ) Laboratorio di Micrometeorologia, Dipartimento DiSTeBA, Universita’ del Salento, 73100 Lecce, Italy e-mail: silvana.disabatino@unisalento.it B. Verhoef · S. Park Center for Environmental Fluid Dynamics, Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ 85287-9809, USA e-mail: brettverhoef@hotmail.com S. Park e-mail: ksunp@kepco-enc.com 123