Laserfiche WebLink
before sufficient supersaturation is reached in the updraft for nucleating the much higher <br />concentrations of natural CCN active at higher supersaturations. If done correctly, this <br />process could render a cloud that would naturally be continental into a more maritime <br />microphysical state. Maritime clouds are more efficient precipitators due to their broader <br />droplet size distribution with larger mean size, accelerating precipitation growth by <br />droplet coalescence. Thus Mather and colleagues hypothesized that this hygroscopic <br />seeding would increase precipitation on the ground from treated convective clouds. <br />Silverman (2003) summarizes some of the initial work, both theoretical and <br />experimental, that followed to investigate this possibility. He concludes that experimental <br />results are suggestive, but not conclusive. <br />More recently, numerical modelling studies by Seifert and Beheng (2005) and van den <br />Heever et al (2005) in which environmental conditions vary, suggest that in some <br />convective environments more vigorous storms and larger storm precipitation totals may <br />occur with continental CCN populations, and in other environments greater precipitation <br />occurs with maritime CCN populations. <br />It often is taken for granted that near-surface air in interior continental regions is <br />characterized by high cloud CCN concentrations, i.e. many hundreds to thousands per <br />cubic centimeter active near 1% supersaturation. Compilations of CCN observations <br />from many locations (e.g. Fletcher, 1969, p. 101; Gotz et al, 1991, pp. 98-99; Pruppacher <br />and Klett, 1997, pp. 288-290) often show these characteristics for continental CCN <br />populations. Such tables should be taken to represent typical conditions, much as a table <br />of climatological average temperatures and rainfall might be used to characterize average <br />conditions in various locations. When a series of CCN observations is made at one site, <br />or in one region, for extended periods of time, a common result is that CCN <br />concentrations vary between maritime (low) and continental (high) levels from day-to- <br />day with changes in the air masses overlying the site or region. Hobbs et al (1985) <br />conducted an extensive series of airborne observations in the summertime boundary layer <br />over the U.S. High Plains in the summers of 1975 and 1976. He observed that even in <br />this continental interior region roughly 25% of the time the CCN population had <br />distinctly maritime characteristics. Vali et al (1982) presented a two-month sequence of <br />CCN observations made at the surface at two sites, one in northeast Colorado and the <br />other in southwest Nebraska, separated by 60 km, in the summer of 1976. In this case, <br />CCN concentrations were more maritime than continental on ~10% of all days. Detwiler <br />(2004) examined cloud droplet spectra in clouds sampled during three later convective <br />storm field seasons on the High Plains, and found more maritime microphysical <br />characteristics on ~25% of those days. <br />It is clear that practitioners of hygroscopic seeding, seeking to render a CCN population <br />more maritime, need to be aware of the background CCN population, and other <br />environmental characteristics, in order to target their efforts for situations where the <br />seeding will have its intended effect. In pursuit of this goal, sampling of CCN <br />characteristics was conducted in western North Dakota as part of this project in <br />2 <br />