WMOD00309

• Title: Applications of the Clark Model to Winter Storms Over the Wasatch Plateau
• Prepared For: Utah Department of Natural Resources Division of Water Resources
• Prepared By: James A. Heimbach, Jr.
• Date: 7/1/1993
• State: UT
• Weather Modification - Doc Type: Report

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A 30 gm hr-l release from the Birch Creek Canyon mouth was modeled. Fig. 11 shows the surface contours
at the 3 hr (1 hr initial integration + 2 hr) point The directional shear at the surface has spread the plume
substantially. There is transport over the Plateau, but the rigid funneling up the canyon observed during the field
experiment is not modeled. This is likely due to the smoothed 1 km resolution not faithfully reproducing the canyon.
Figure 12 is the modeled plume at 2.9 km. This level is below the terrain over the southern portion of the Plateau
and below the minimum IFR flight level of approx. 3.7 km. There were no contours in the next highest level
analyzed, 4.2 km. At the 2.9 km level the model gave about an order of magnitude lower concentrations of AgI than
those inferred by SF6 sampling 0.8 km above this level. The use of average SF6 concentrations is somewhat
ambiguous because the plume edges are held hostage by the minimum detectable signal. If one assumes good
discrimination of the true SF6 edges, then an average value could bring the inferred AgI concentration down by a
half order of magnitude (see, for example, Griffith et al., 1992, p 56). Another issue is the initial spreading of the
modeled AgI material through a grid bin, which for the innermost domain is 1 km by 1 km by 0.1 kIn. The 1 km
corresponds to 10 or more seconds of flight time and represents a substantial initial dilution. This can be countered
by decreasing the grid size which impacts required computer resources.
Mean SF6 concentrations measured on the surface by the NA WC van averaged 20 to 100 ppt in the
afternoon (none detected in the morning). The mean afternoon temperature at the DOT site was approximately -4.5
oC and assuming a pressure of 770 mb gives an inferred co-released AgI concentration on the surface ranging from
8.9X1o-s to 4.4XIO-4 pgm m-3 which is within an order of magnitude of what the model predicted for the surface.
The horizontal extent of the modeled plume is south of the average position on the west flight track, but
agrees well with the sampling on the east flight track. The model correctly predicted veering or clockwise turning
of the transport.
d. Valley AgI Releases for 2 March 1991
On the 2nd, AgI had been continuously released at all the valley sites at 8 gm hr-1 since 28 February. There
were no other AgI releases in the experimental area. The aircraft sampling found IN during both flights. The edges
and mean positions are depicted in Figs. 13 and 14. On these figures, an edge is indicated by a "+" and the direction
of flight for the edge encounter is labeled just to the right; "N" for north, etc. If there were two few IN detected
to meet the edge criteria, than a mean lagged position of IN for the pass is labeled with a box and associated flight
direction.
Though these are admittedly coarse positions, the IN on the west flight track are clearly being transported
over the northern part of the experimental area. In the afternoon on the east flight track, there are two plume
penetrations south of the main body of IN. This is probably a curving transport pattern rather than movement of
IN from the southern release points, since nothing of these was detected on the west track.

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