Evening transition observations in Phoenix, Arizona

Past research has suggested that the evening transition in complex topography typically has several main features, such as (a) continued weak upslope flows persisting 3-5 h after sunset (if the sidewalls of the valley prevent Coriolis-induced turning of winds), thus signifying delayed transition; (b) unsteady local stagnation and vertical mixing within tens of meters above the surface; and (c) transition of stagnation fronts to downslope/downvalley gravity currents during the evening hours, especially at higher-elevation (steeper) slopes, and their arrival at adjoining low-elevation gentle slopes as "slope breezes" This transition process typically occurs in locales such as Phoenix, Arizona, which has expansive exposure to plains in one direction (to the west and south) and is adjacent to abrupt change in the terrain in other directions (primarily to the north and east). An analysis of wind records from several automated weather stations and a radar wind profiler for selected characteristic periods representing all four seasons and data from a previous major field campaign in the greater Phoenix valley illustrate (i) the shallow nature of transition flows that develop on a year-round basis during frequent clear, calm nights in the desert Southwest and their seasonal sensitivity; (ii) a spatial variation of transition times relative to the only first-order National Weather Service station in the region (Sky Harbor International Airport); (iii) the dependence of transition time (and hence the delay of transition) on the exposure, the elevation, and the magnitude of slope; and (iv) a possible heat-island influence. These observations are quantified using theoretical estimates, and the results are placed in the context of multiscale flows in urban basins.