WMOD00489

• Title: Modeling Regional Controls of Watershed Precipitation for Climate Change Studies
• Date: 1/13/1991
• Weather Modification - Doc Type: Report

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Fig. 3. Regional airflow pattern on January 17, 1979, at
1800 UTC from the MM4 for the 700-mb level (3) and at
500 m agl (b). Locations of the standard synopti.c
rawinsonde sites at Desen Rock, NV (DRA), Wmslow, AZ
(INW), and Tucson, AZ (TUS) ~"abel~. .M~4
sounding points selected for an uutial preclplt.BtJon
modeling study are shown by ( J..). and the am ~f the.
MogoUon Rim is shown by (A-). Two CCM gnd pomts
(.) lie within this limited region of the MM4. UOIts are
the same as in Figure 2.

Large variations in the horizontal gra~ients of
moisture and wind flow evolved throughout this storm.
These gradients controlled the moisture Dux .~d .
resulting distribution of condensate and preClpltaUon.
Figure 3 focuses on the region affe~ing th~ Ver.de and
Salt River Basins. It shows the detaIled gnd pawt
structure of airflow at sigma = 0.95 (boundary layer
air at 500 magI) and at 700 mb over Arizona at 1800
UTe. Note that the surface pressure in this region
varies from 900 to 850 mb. Our analysis concentrates
on the low-level airflow which is strongly influenced by
the terrain and which is largely responsible for the
precipitation. A weU-defin~d cyclonic flow is shown by
the wind vectors (see Fig. 3). This region had a
100m S-I change in wind speed over a north-south
distance of 150 km at 700 mb (see Fig. 3a). This
mesoscale pattern of winds resulted in a wen-
organized zone of convergence from the bound:uY
layer to 700 mb with divergence at 300 mb, w~lch
produced strong lifting over. the region of m~lmum
precipitation. The divergence patterns (see FIg. 4)

show the spatial continuity and mesoscale structure
over north-central Arizona at the time of maximum
precipitation on January 17 at 1800 llJ'C. This
pattern moved from southern Arizona across the
region and into Colorado on the 18th.

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Fig. 4. Regional divergence pattern on Janua.ry 17, 1979, at
1800 UTC at 300-mb (3) and 700-mb levels (b) illustrating
the mesoscale continuity of the storm dynamics. Units are
10-6 S.I. Dashed lines indicate convergence. The grid
points are as in Figure 2.

Significant spatia] variations in moisture
occurred in the mixing ratio fields from 500 m to
700 mb at 1800 ure. The boundary layer maximum
over southern Arizona shown in Figure S resulted in a
decrease of 3 g kg-I from south to north aver a
distance of 250 km across the watersheds of interest.
Aloft at 700 mb, the tongue of maximum moisture
passed over southeast Arizona resulting in an east.-to-
west decrease of 1.5 g kg-I along the Mogollon Rim.
The 40-percent decrease in mixing ratio over ~e .
domain combined with a 50-percent decrease 10 w10d
speed would result in large errors in computations of
precipitation from unrepresen~tive ~undiJllgs at .
standard synoptic stations shown in FIgure 3. Dunng
this period, the v-component of the wind all 700 mb
increased nearly S to 8 m S-I over the rim :in 6 hours.
Similarly significant changes in mixing ratio of about
1-2 g kg-I occurred, suggesting the importance of high
temporal resolution to describe the duration and
intensity of precipitation. These variations in moisture
supply for a specific watershed clearly show the
importance of defining the regional scale structure of
storms. Similar complex. nonlinear, mesoscale
patterns evolved over other watersheds in t.he Western
United States during January 1979.