Regional Simulation of Summertime Precipitation over the Southwestern United States

Using results taken from a finescale (25 km), regional modeling simulation for the summer of 1999, along with contemporaneous daily surface observations, synoptic variations in summertime precipitation over the southwestern United States are described and analyzed. Two separate techniques for characterizing and evaluating large-scale summertime precipitation patterns within the observed and simulated systems are presented; in addition, these evaluation/characterization techniques are used to analyze the hydrologic forcings associated with observed and simulated modes of rainfall variability. Overall, two robust spatiotemporal precipitation patterns are identified involving 1) precipitation over the western portion of the Rocky Mountain plateau centered on eastern Arizona and southern Utah, and 2) precipitation located over the eastern portion of the plateau and the elevated orography of eastern New Mexico and southern Colorado. Time series associated with these two precipitation regimes are correlated with low-level and midlevel circulation patterns in order to investigate the related large-scale environmental conditions. It is found that for both regimes intraseasonal precipitation is related to the intrusion of midtroposphere, midlatitude low-pressure anomalies over the southwestern United States, resulting in synoptic-scale shifts in the position of the climatological midtroposphere monsoon ridge. The interaction between the resultant midtroposphere pressure fields and the quasi-stationary monsoon surface pressures found over the Rocky Mountain plateau during the summertime produce large-scale vertical velocities consistent with the observed and simulated rainfall patterns associated with each regime.