Precipitation Regionalization

1

• 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

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 1996-2002, 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.

• Air Parcel Trajectory Analysis

Bradshaw Mountains
White Mountains
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).
IMZ
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)

• 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).