Atmospheric Rivers and Cool Season Extreme Precipitation Events in the Verde River Basin of Arizona

Inland-penetrating atmospheric rivers (ARs) can affect the southwestern United States and significantly contribute to cool season (November-March) precipitation. In this work, a climatological characterization of AR events that have led to cool season extreme precipitation in the Verde River basin (VRB) in Arizona for the period 1979/80-2010/11 is presented. A bottom up approach is used by first evaluating extreme daily precipitation in the basin associated with AR occurrence, then identifying the two dominant AR patterns (referred to as Type 1 and Type 2, respectively) using a combined EOF statistical analysis. The results suggest that AR events in the Southwest do not form and develop in the same regions. Water vapor content in Type 1 ARs is obtained from the tropics near Hawaii (central Pacific) and enhanced in the midlatitudes, with maximum moisture transport over the ocean at low levels of the troposphere. On the other hand, moisture in Type 2 ARs has a more direct tropical origin and meridional orientation with maximum moisture transfer at midlevels. Nonetheless, both types of ARs cross the Baja Peninsula before affecting the VRB. In addition to Type 1 and Type 2 ARs, observations reveal AR events that are a mixture of both patterns. These cases can have water vapor transport patterns with both zonal and meridional signatures, and they can also present double peaks in moisture transport at low- and midlevels. This seems to indicate that the two types can be interpreted as end points of a range of possible directions.