1. Air Parcel Trajectory Analysis
Abstract
This study investigates the role of increased atmospheric humidity in occurrences of wet days in the southwestern United States during the monsoon seasons of , using spatiotemporal analyses of ground-measured precipitation, spatial analyses of surface and 700-hPa humidity, and air-parcel trajectory analyses. A precipitation regionalization indicates that the Gila River basin in southern Arizona/southwest New Mexico should be divided into a western region and an eastern region. A rainfall peak occurred in late August/early September for the western region; however, similar to that of the core monsoon zone in northwestern Mexico, the eastern region of the basin had a rainfall peak in late July/early August. Wet days in the western (eastern) region were associated with a large (moderate) peak in dew point temperature in the southwestern (south central) portion of the basin. The middle troposphere was more humid than normal on both sets of days, with the anomalies for western region wet days being larger and located more over the Gila River basin than anomalies for the eastern region wet days. The Sierra Madre Occidental was the most likely source of middle troposphere moisture for both regions; however, the Gulf of Mexico may have been a significant contributor to rainfall in parts of the eastern region. The Gulf of California probably was the dominant source of low-level moisture for western region wet days, with gulf surges likely causing the late August/early September peak.
Precipitation Regionalization
Dispersed point pattern of 35 precipitation stations used for regionalization of Gila Basin (middle right, top)
Analysis of scree plot showed that first two components explained 63% of variance (middle right, bottom)
Result of regionalization was division of Gila Basin into western and eastern precipitation regions (above)
Regions have differing seasonal rainfall peaks (far right)
Rainfall peak variability also reflected at two high-elevation locations (Bradshaw and White Mountains) and within two “intense monsoon zone” (IMZ) regions
Air Parcel Trajectory Analysis
Western region wet days linked to southward back trajectories
Eastern region wet days showed few back trajectories intersecting the Sierra Madre Occidental
Likely that western region wet days are driven by gulf surge events
Bradshaw Mountains
White Mountains
IMZ
IMZ
Two-day back trajectories ending at 3180 m (above) and 5890 m (below) above sea level. Trajectories are shown for Bradshaw (a), White (b), western IMZ (c), and eastern IMZ (d).
Smoothed time series of typical percentage of seasonal precipitation for (a) western region (black) and eastern region (grey); (b) Bradshaw Mountains (black) and White Mountains (grey); and (c) western IMZ (black) and eastern IMZ (grey)
Study Area
Monsoonal precipitation is key component of Gila Basin hydrologic cycle (up to 50% of annual total in some areas)
Heavy rainfall events linked to local hazards (e.g., flash flooding, severe erosion, transportation and safety concerns)
Gila Basin has robust spatial and temperature coverage of precipitation measurements and large array of stakeholder groups, thus making it ideal for analyzing monsoon variability
Surface Humidity Variability
Western region wet days had dew-point temperature peak in southwestern portion of Gila Basin (above, top and bottom left)
Eastern region wet days had dew-point temperature peak in south-central portion of Gila Basin (above, top and bottom right)
Entire Gila Basin more humid on western region wet days
Results and Conclusions
Precipitation regionalization of Gila River basin indicates that study domain can be divided into western and eastern regions
Eastern region has rainfall peak in late July/early August, with western region peak occurring in late August/early September
Positive dew point temperature anomalies are greatest in south-central portion of basin on eastern region wet days, and in southwestern portion of basin on western region wet days
Back trajectories suggest stronger linkage of Gulf of California moisture to western region wet days than eastern region
Gulf surges likely a strong control on western region wet days
Study demonstrates spatio-temporal complexity of monsoon activity at northern periphery of North American monsoon
Mean monsoon-season values of dew point temperature (a) and 700-hPa specific humidity (b). The shaded region is the Gila River basin.
Specific-humidity anomalies at 700-hPa for wet days in the Bradshaw Mountains (a), White Mountains (b), western IMZ (c), and eastern IMZ (d).