Revisiting haboobs in the southwestern United States: An observational case study of the 5 July 2011 Phoenix dust storm

Convectively-driven dust storms (or haboobs) are common phenomena in the southwestern United States. However, studies about haboobs in this region are limited. Here, we investigate the state and fate of a massive haboob that hit Phoenix, Arizona on 5 July 2011 using satellite, radar, and ground-based observations. This haboob was a result of strong outflow boundaries (with peak wind gusts of 29 m s−1) from storms that were initiated in the southeast of Tucson. In particular, we find three major outflow systems (based on radar data) that were generated by forward propagating storms, ultimately merging near Phoenix. This resulted in peak hourly PM10 and PM2.5 concentrations of 1974 μg m−3 and 907 μg m−3 at US EPA stations near Phoenix. The high PM concentration is consistent in space and time with the dust wall movement based on our analysis of radar data on hydrometeor classification. Enhanced aerosol loadings over metropolitan Phoenix were also observed on 6 July from NASA Terra/Aqua MODIS aerosol optical depth (AOD) retrievals (AOD > 0.8). We infer from CALIOP vertical feature masks and HYSPLIT back trajectories that remnants of the haboob were transported to northwest of Phoenix on 6 July at 2–4 km above ground level. Ratios of PM2.5 to PM10 from IMPROVE stations also imply low-level transport to the east of Phoenix on 8 July. Finally, we find that this haboob, which had local and regional impacts, is atypical of other dust events in this region. We note from this analysis that extreme events such as this haboob require an integrated air quality observing system to provide a more comprehensive assessment of these events.