During June of 1991, data from radar wind profilers, RASSes, sodars, and radiosondes were collected in the Phoenix area as part of an investigation into processes affecting the buildup of ozone precursors in the nocturnal boundary layer and their subsequent processing and mixing in the first few hours after sunrise. On clear mornings not strongly affected by monsoonal flows, anomalous cooling was measured between 0500 and 0700 Local Standard Time (LST) in a layer several hundred meters deep in the vicinity of Sky Harbor Airport near downtown Phoenix even though sunrise occurred at approximately 0525 LST. This cooling was accompanied by a layer of winds, also several hundred meters deep, with an easterly component undercutting the prevailing westerly or northwesterly flows. The easterly flows appear to be late-developing drainage winds from the mountains to the east. At two sites approximately 45 km farther west and northwest, the flows were weaker or absent, and the cooling was also weaker. The resulting convergence and cooling aloft over the Phoenix area may have enhanced vertical mixing in the first few hundred meters above the surface during this period. This hypothesis is consistent with observed time series of CO and ozone made at the surface and at two heights, 50 and 140 m AGL, on a downtown skyscraper, i.e., these time series show evidence for developing vertical transport or mixing even before sunrise.