Pseudovertical Temperature Profiles and the Urban Heat Island Measured by a Temperature Datalogger Network in Phoenix, Arizona

As part of an air-quality field campaign conducted in Phoenix, Arizona, during the summer of 2001, a network of temperature dataloggers and surface meteorological stations was deployed across the metropolitan area for a 61-day period. The majority of the dataloggers were deployed along two intersecting lines across the city to quantify characteristics of the urban heat island (UHI). To obtain pseudovertical temperature profiles, some of the instrumentation was also deployed along a mountain slope that rose to 480 m above the valley floor. The instrumentation along the mountain slope provided a reasonable approximation of the vertical temperature profile of the free atmosphere over the valley center during the night and a few hours after sunrise. Mean differences of 0.63 and 0.92 K and standard deviations of 1.33 and 1.45 K were obtained when compared with the in situ radiosonde and remote radio acoustic sounding system measurements, respectively. The vertical temperature gradients associated with temperature inversions within 200 m of the surface during the morning were also close to those obtained from the radiosondes. The average UHI during the measurement period was between 2.5° and 3.5°C; however, there was significant day-to-day variability, and it was as large as 10°C during one evening. The peak UHI usually occurred around midnight; however, a strong UHI was frequently observed 2–3 h after sunrise that coincided with the persistence of strong temperature inversions obtained from the radiosonde and the pseudovertical temperature profiles. The nocturnal horizontal temperature gradient was somewhat different than that reported for other large cities, and the UHI did not decrease with increasing wind speeds until the wind speeds exceeded 7 m s−1.