Laserfiche WebLink
<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />A scanning Doppler radar will be located on top the barrier. During precipitating periods the <br />radar will monitor the wind component toward/away from the radar. Estimates of horizontal <br />wind velocity will be made using the VAD (Velocity-Azimuth Display) approach for a single <br />Doppler radar (Browning and Wexler, 1968). These wind estimates will be useful in targeting <br />assessment. The radar also will document natural precipitation variations in time and space, <br />which need to be known to analyze possible seeding effects. A natural pulse of increased <br />precipitation can coincide with a seeding experiment, either masking a real seeding effect or <br />leading to a false claim of a seeding effect. Radar scanning of the entire experimental area will <br />help guard against such incorrect evaluations. <br /> <br />A radar profiler will monitor the vertical distribution of horizontal winds above the mountain <br />barrier. The profiler's RASS technique will provide continuous information on atmospheric <br />stability over the barrier by monitoring the vertical profile of virtual temperature (Schroeder, <br />1990). <br /> <br />Additional atmospheric information will be collected during direct detection seeding <br />experiments by aircraft sampling. For example, cloud droplet size distributions will be <br />measured by a FSSP (forward scattering spectrometer probe), which, along with other sensors, <br />will monitor liquid water amounts and distributions. Periodic rawinsonde releases from the <br />upwind valley will provide the vertical structure of temperature, moisture, and wind in the <br />atmosphere. These observations, in combination with other noted measurements, should yield a <br />reasonable portrayal of the environment that ice crystals are exposed to along their <br />growth/fallout trajectories. <br /> <br />4.6.7 Snowflake fallout trajectories <br /> <br />The time available for ice crystal growth and fallout to the target will be estimated with the <br />assistance of the numerical model discussed in section 4.4, and the wind/temperature/moisture <br />observations noted above. Model predictions will be compared with aircraft observations within <br />the seeded zone over the target and surface observations on the mountain barrier. <br /> <br />A number of uncertainties exist in model calculations, partially because of incomplete <br />knowledge of physical processes and partially because of observational inadequacies. <br />Uncertainties also exist regarding the modes and rates of ice nucleation, especially with limited <br />observations of the spatial distributions of SL W and transient supersaturations. Calculated ice <br />particle and snowflake growth rates have uncertainties, particularly when accretion and <br />aggregation are involved. Questions about snowflake trajectories exist, caused not only by <br />growth rate uncertainties, but also by limited knowledge of how the vertical wind component <br />varies between the seeding sites and the target. In spite of these uncertainties, models can <br />provide reasonable first approximations of reality. Comparison with the field observations will <br />improve the approximations. Although numerical models have limitations, they provide the <br />best means of comprehending the complex interactions involved in winter storms. <br /> <br />4.6.8 Seeding-induced snowfall on the target <br /> <br />The final link in the chain of physical events is precipitation accumulation on the ground. This <br />observation will be monitored by high resolution gauges spaced at about 1.5-km intervals, along <br />lines perpendicular to the prevailing wind at two downwind distances. One line will be located <br />on top the barrier near the downwind edge. The other will be located on the lee slope. The <br /> <br />29 <br />