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produced by seeding. Seeding could conceivably decrease the radar signal while increasing the precipitation rate. Thus, radar is not suitable for detecting seeding effects except in special cases such as reported by Hobbs et al. (1981). However, radar scanning can be very valuable in monitoring natural variations in cloud structure over the entire region of the target. Such variations can mask, or be mistaken for, real seeding signatures and it is important that they be documented. Doppler radar sets can provide important wind measurements over the experimental area in addition to reflectivity data, The wind data can help interpret the reflectivity patterns. 1.3.5 Precipitation Characteristics. - The most detectable effect of seeding at ground level is usually an increase in the IPC. Changes in ice crystal sizes and habits, and silver content in the snow, also suggest that seeding affected the precipitation process. However, successful seeding usually increases the concentration of ice crystals to levels well above background. Much of the IPC enhancement will likely be at small crystal sizes, less than a millimeter in diameter. Small crystals produced by seeding are often in the form of hexagonal plates. An aspirated particle imaging probe offers the most practical means of continuously monitoring ice crystal characteristics. However, it should be supplemented by photographic documentation which provides more detailed data, though requiring considerable manual reduction. The riming process (also called accretional growth) is the freezing of tiny cloud droplets to the falling ice crystals. It was shown to have been reduced in some of the reviewed experiments. This should be a consequence of seeding if enough ice crystals are created to utilize most of the excess SL W. The degree of riming usually is not discernible from aspirated imaging probe data so manual or photographic observations are required at the surface target. Obtaining snow samples for silver analysis at frequent intervals is useful in evaluation of seeding effectiveness. Enhanced silver levels in the snow do not prove any seeding effect directly since most or all of the silver could result from scavenging by natural snowflakes. However, finding only background silver concentrations very likely means the target was not impacted by seeding. Thus, the silver-in-snow data provide a check against claiming natural variations as seeding effects. Highly sensitive precipitation gauges are needed for physical experiments because the seeding effects may be very brief (e.g., only a fraction of an hour for a single seedline laid down by an airplane), and precipitation rates may be low. For example, the total precipitation amounts from individual seedlines of AgI reported by Super and Boe (1988) ranged from 0.10 to 0.22 mm water equivalent. Conventional weighing gauges have a resolution of 0.25 mm, and are unsuitable for physical experiments unless modified. 1.3.6 General Considerations. - Some of the reviewed studies attempted to "piggy-back" physical experiments on experiments designed for statistical evaluation. This approach was generally unsatisfactory. Most statistical experiments attempt to affect a sizable area for a significant time requiring seeding of a relatively large volume of atmosphere, for example, the release of several seedlines for airborne seeding. Such seeding takes substantial time. Yet, one of the main approaches for analyzing physical experiments is to examine temporal changes in expected characteristics (lPC, precipitation rate, degree of riming, etc.), This is best done by reducing the time required for seeding, say to a single seedline for airborne seeding, thus reducing the effect of natural changes in the constantly varying atmosphere during the course of the experiment. i : 1 'I 8