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Tropospheric Moisture and Monsoonal Rainfall over the Southwest United States
David P. Brown
Department of Geography and Anthropology, Louisiana State University
Jeremy E. Diem
Department of Geosciences, Georgia State University
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.
Study Area
Precipitation Regionalization
Bradshaw
Mountains
Air Parcel Trajectory Analysis
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).
• 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
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)
Surface Humidity Variability
• 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
Results and Conclusions
• 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
• 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
Specific-humidity anomalies at 700-hPa for wet days in the
Bradshaw Mountains (a), White Mountains (b), western IMZ
(c), and eastern IMZ (d).
Mean monsoon-season values of dew
point temperature (a) and 700-hPa
specific humidity (b). The shaded
region is the Gila River basin.
• 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 southcentral 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