WMOD00293 - Laserfiche WebLink

Home Browse Search

- . . . available. Aggregation of high concentrations of ice particles appeared to be the mechanism for greater precipitation rates in the plume. Little snow was left to fall over the eastern ridge of the Plateau. Winds were atypically light during this experiment so that seeded snowfall for windier cases might be expected to settle further eastward. . Comparisons of precipitation totals between seeded and nonseeded gauges were suggestive that seeding may have produced a limited snowfall increase over the period of the experiment of perhaps 0.5 mm at some gauges. However, one eastern seeded gauge received less snowfall than two nonseeded gauges also on top of the Plateau. Analyses of observations and physical reasoning suggest that most seeded snowfall settled to the Plateau top before crossing to its east edge, and perhaps before reaching the middle of the Plateau top at gaugeTARSW. In retrospect, one or more gauges would have been useful along the upwind highway a few km north ofRRS. Updrafts predicted by the Clark model were associated with the windward edge ofthe Plateau and were generally limited to 0.5 m S.I near the surface. There were areas of stronger down drafts over the Plateau, reaching speeds of -1.5 m S.l. The complex gravity wave pattern forced a secondary maximum of tracer material several km south of TAR and some additional condensation over the northern portions of the eastern flight track. The model suggests the case had weaker transport and diffusion of tracer material over the Plateau than indicated by field measurements. The plume transport time over the Plateau was slower than reality; however, the model ultimately simulated transport of AgI into liquid water cloud over the Plateau. The heights to which the simulated plumes were transported were in reasonable agreement with aircraft observations, but the simulated lateral spread was excessive at the surface. The model's smoothed terrain likely contributed to the less organized transport because the simulated canyons were shallow enough to limit the funneling of the poorly organized surface winds. The actual terrain consisted of a pronounced canyon downwind of AHS which likely limited crosswind horizontal dispersion within the canyon. The simulated clouds and ice were weak and shallow because buoyancy was not part of their formation. It is believed that buoyancy from solar heating did contribute to the observed clouds, and for that reason also . - contributed toa more vigorous vertical plume transport than was simulated. -Overall, the model simulation produced a reasonable first-approximation of reality. In summary, AgI seeding during this experiment produced strong microphysical evidence of markedly enhanced ice particle concentrations on the Plateau top and at aircraft levels. Plumes released on the windward slope were readily transported over the Plateau in spite of limited horizontal winds. Plume widths were limited, suggesting seeding generators at similar high altitude sites should be spaced no more than 5 km crosswind. Horizontal dispersion from other high altitude sites could be more or less than from AHS depending upon local topography. Seeding apparently increased the snowfall, especially on the western portion of the Plateau top, but accumulations were limited. Seeding appeared to significantly increase the aggregation of snowflakes along the upwind highway. Best estimates of snowfall rates caused by seeding are 0.4 rom h-l based on 2D-C estimates along the upwind highway (Table 2) and less than half that rate from gauges on the central and eastern portions of the Plateau top (Sec. 6) Of course, SL W amounts were quite limited during this week and diminishing final portion of the storm, so seeding potential would be expected to be quite limited as well. Overall, the results of this experimentation are in general agreement with the current physical understanding of how winter orographic seeding should work. 59 L