WMOD00277 - Laserfiche WebLink

Home Browse Search

apparent decreases again at the warmest LemperaLure::; . The Climax and Wolf Creek Pass curves are somewhat similar. The major differences appear to be a slight shift of the mean daily snowfall peak toward warmer temperatures and a less pro- nounced peak around -14C to -15C for the Climax data. The reduction of mean daily snowfall at the warmer temperatures is less pronounced for the Wolf Creek Pass sample. This may be due to accretion which is observed with greater frequency in the Wolf Creek Pass area. The shift toward warmer temperatures of the mean daily snowfall peak at Climax is anticipated by the model. Figure 3 shows that for representative Climax conditions the 500 mb temperature mode that provides optimum ice nuclei concentrations is from -2lC through -23C, Assuming the secondary peaks of daily snowfall around -14C to -15C are a result of dendritic growth, the more pronounced effect at Wolf Creek Pass might be explained by a greater frequency of cloud systems having higher supersaturations and somewhat more maritime characteristics. The distribution of natural precipitation with respect to 500 mb temperatures suggests a deficiency of effective ice nuclei may exist in the warmer cloud systems. This deficit may on occasion be alleviated by an ice crystal multiplication process in the dendritic range around -15C. This is much more in evidence at Wolf Creek Pass than at Climax. A ratio of ice crystal concentration to corresponding ice nuclei concentration that increases exponentially with temperature is not reflected in the natural precipita- tion data from Wolf Creek Pass and Climax. There- fore, artificial augmentation of natural snowfall at warmer cloud top temperatures and when high <:loud water supply rates exist appears probable. g. The accretion process Equation 10 can be written as Nc = (k)f (w /r) G(T) (13) where Nc is the optimum ice crystal concentration, k is a constant, w is the vertical motion in pressure coordinates, r is the radius of the ice crystal and G(T) is a function of the cloud system temperature. As pointed out previously, the crystal size is free to adapt to changes in the cloud water supply due to variations in the upward speed. Thus, additional ice crystals may not be needed to utilize increases in cloud water if larger crystal sizes can be grown. A limiting condition comes into play, however, when the crystal size required to maximize the precipitation process becomes too large. Under these conditions the cry stals may be unable to remain in the cloud system for a sufficient growth period or the accretion process may begin to contribute sj~g- nificantly to cloud water removal. The residence time of ice crystals is frequently limited by the relatively small horizontal extent of orographic clouds over the mountains. Therefore, there is frequently an upper limit placed on the crystal size by the g8Uli1etl'Y uI the Ul'ugl'dIJl1ic cloud. Thi::; re::;Lric Lion on crystal size not only tends to limit the amoun t of cloud water that can be removed by diffusional growth of ice, but also affects possible accretional growth. The reason is that the amount of cloud water removed by accretion is also a function of ice crystal size. This is seen in the expression for the rate of mass growth of a falling crystal by accretion usually given by 2 'dm/dt = nr E(V - Vd) Q c c where r is crystal radius, Q is liquid water content, (V - V ) the difference in fan velocities between crjstaldand supercooled droplet, and E the collection efficiency. (14) In utilizing equation (14), it is usually assumed that V ,< < V. The collection efficiency E of ice crystals ?s notCwell known. It is not only a function of crystal size but of crystal habit and there- fore is related to cloud system teniperatures. The crystal radii associated with the onset of riming are generally noted to be in the range of 1 O~ to 400 II but large crystals (1000 II ) without riming are frequently observed. The studies at Climax indicate that accretional growth definitely plays a subordinate role to that of diffusional growth at that location. There is evidence that this may not be true in some of the warmer storms over the San Juan barrier, This is seen in Figure 5 where the mean daily snowfall on Wolf Creek Pass does not show an abrupt decrease at the warmer cloud top temperatures. The fact that nature relies on accretion for the removal of cloud water does not necessarily mean this process should be utilized in an operational precipitation management program. In general we have the relationship I = 0 +0 + L (15) c D Acc where I is the input condensate formed in lifting the , airmasEf over the mountain barrier, 0 is the precipitation output due to diffusional B-owth of ice cryst81s, 0 is the precipitation output due to accretion orS'bud droplets to ice crystals, and Lis the cloud water lost to precipitation through inef- ficient processes involving evaporation to the atmosphere. The core of the problem is to reduce the loss (L) to a minimum. Whether the cloud water is brought to the ground by diffusional or accretional growth is of no importance in itself. However, there will be times when the naturally occurring accretion process has the capability of bringing more cloud water to the ground than if the prec ipitation process was converted entirely to one of diffusional growth. The accretion process might help solve a particular targeting pr oblem. The higher fall velocities of the rimed crystals might be utilized to optimize precipitation as near the upwind portion of the cloud as possible. This need might arise under 11