OCR Text |
Show pared to ground measurements under non-precipita~ion conditions, and any gross differences in the dispersion characteristics during storm and nonstorm conditions can be detected. The problem of disparity between measured ice nuclei counts and observed ice crystal counts is a nagging problem in weather modification. Although some studies show near one-to-one relationships (Reinking and Grant, · 1968), ther e is enough evidence for higher ratios of crystals to nuclei (Hobbs, 1969) that this remains ·an unresolved question. Two factors have a bearing on these measurements. The first is that most measurements of ice crystal concentrations are made at the surface by sampling a volume of air at the surface, whether directly or by estimating the volume from which a number of crystals have fallen. From numerical modeling studies of the flow and precipitation over an orographic barrier, it is found that the snow crystals in a given volume on a barrier actually originated from a much larger volume, i. e. , the trajectoriei tend to focus on the barrier. Hence, the ice crystalp in a given unit volume on the surface originated in a region containing fewer crystals per unit volumi The second fact is that most, if not all, nuclei counters do not correctly measure the effect of contact nucleation. In the rapid expansions or relativeti short residence times of cold chambers, there is simply not time for a realistic number of collisions to occur. Hence, all that is measured is the nucleation effect of the sublimation nuclei, and the effect of contact nucleation is not realistically assessed. Also, nuclei captured in a u:1embrane filter, and later developed, cannot realistically measure the effect of contact nucleation, This fact may explain the very nearly nuclei-free condition of unseeded air typically measured at Rabbit Ears Pass site with the NCAR nuclei counter. In spite of this measured lack of nuclei, ice crystal concentrations under natural (unseeded) conditions of 10-100 liter- 1 are observed on an orographic barrier in the Rockies. Contact nucleation, missed by most presently used nuclei counters, is a very plausible explanation for the disparity observed be· tween measured nuclei counts and observed ice crystal concentrations. Several case studies of ice nucleus measurements are discussed below. 4. 1. 1 Discussion of Case Studies Case 1, Figure 73. The pulsed characteristic of the nuclei count can be seen in this figure. It is apparent that the Emerald wind (plotted as hourly means) was in the direction of Rabbit Ears (R. E. to Em. azimuth = 294 degrees) an! was aligned with the upper winds. Seeding was started on the half hour and terminated on the hour as indicated by the solid, straight lines. If these po· tential ice nuclei moved with the speed of the wind indicated by the Emerald 122 |