| OCR Text |
Show 35 minutes and the coherence value was very nearly significant ai: the 5% level. Taken together with the physical evidence of high nuclei count and presence of silver in the snow, these facts are indicative of a seeding effect in the southern portion of the network. These are typical of the results obtained in many storm periods; an effect is indicated but not conclusively proven by the analysis of data from a single precipitation period. The analysis results are often indicative of an effect, but not conclusive. There are three major factors which weaken the effectiveness of the technique. The first is that, often, even fairly high generator release sites have an effective lid put on vertical transport by the presence of inversions. These typically occur to at least half the relief height of major barriers for a significant portion of the seasonal snowfall periods. Obviously, if the nucleating material does not reach a level where the major portion of the target precipitation originates, it cannot be effective. This condition is not peculiar to this scheme of analysis, of cours~, as no scheme can be effective if the nucleating agent does not reach the de s ired nucleating levels in the desired concentrations. The second factor which hampers this type of analysis is that the transport characteristics, namely wind speed and direction, do not remain constant long enough to obtain a time series of sufficient length for analysis. As an extreme case hypothesize a periodic linear effect transported past the m easuring sensor at a constantly changing velocity. The periodic component of the measured output would be largely destroyed, and therefore undetectable by the power spectrum analysis technique. The third and final factor limiting the sensitivity of this technique, is that the periodicity of the input forcing function is in some measure destroyed by the dispersion characteristics of the atmospheric transfer function. At the rather high frequency dictated by the analysis technique, the periodicity of the seeding square wave tends to be smeared by filling in of the off periods by nucleating agent. This is not as serious as the second problem, since most of this smearing or filling in of pulses occurs at the lower levels where the treatment is not as important to the total effect. Taken in concert, these three factors severely limit the effectiveness of the power spectrum analysis technique. All three of the limitations could be partially overcome by the proper use of aircraft seeding. 5. 3 Case Study Results With the change in operational routine during the 1968-69 season, there was also a change in the method of evaluation. One of the new evaluation methods that was utilized consisted of making case studies (in various degrees of detail, depending on data quality) for each of forty-four six-hour operational periods. For each of the six-hour blocks, computations of timing and location of seeding effect were made as outlined previously. This allowed collected data on precipitation rates and totals, silver-in-precipitation concentration values, snow crystal habit, riming, and Agl nucleation to be divided into seeded and unseeded 291 |
