Efficiency of Precipitation Enhancement from Convective Clouds at Salt Powder Modification

Hygroscopic particles from pyrotechnic flares are widely used in a number of projects aimed at enhancement of precipitation from convective clouds (Cooper et al., 1997). At modification with this method hygroscopic particles are introduced into an updraft in the cloud base. The concept of modification is in the transformation of cloud drop spectrum at the initial stage of condensation in such a way that at future evolution of the cloud coagulation processes will be intensified and precipitation formation processes will be enhanced. Hygroscopic particles generated by pyrotechnic compounds have the sizes from 0.3 to 1 μm. The use of larger particles (I – 3 μm), as the investigation results have shown, makes it possible to obtain a more significant modification effect, but the consumption of the agent is by more than the order of the value higher than that of pyrotechnic compounds. Rosenfeld D. (2005, 2010) proposed to use a polydisperse salt powder NaCl with rather broad particle size distribution functions for cloud modification. Such a powder was tested in the Big Cloud Chamber of SI RPA ‘Typhoon’ (Russia) (Drofa et al., 2010). The results of the tests supported by the results of numerical simulations demonstrate significant advantages of the powder for obtaining additional precipitation amounts. The essential effect of modification by the salt powder manifesting itself at mass concentrations of the powder introduced is by an order of the value less than at the use of pyrotechnic flares. When the salt powder is used, precipitation can be obtained from small usually non-precipitating warm convective clouds. The present paper gives (on the basis of numerical simulations) the estimates of modification effect on convective clouds caused by polydisperse salt powders and describes the conditions necessary for maximum positive effect attained with this method. The analysis of a scenario of cloud drop formation in a convective cloud shows that at the initial stage of condensation the main cloud droplet size distribution spectrum is formed on hygroscopic nuclei with the radii from 0.015 to 0.65 μm. Cloud spectrum maximum is formed on the nuclei of the radii of about 0.02 μm. Just such hygroscopic particles are active condensation nuclei for which unlimited growth at the initial condensation stage is realized. For artificial stimulation of coagulation processes in clouds it is necessary at the initial stage of condensation to transform the droplet spectrum in such a way that the number of large drops will be increased without changes in the conditions of cloud drop formation on atmospheric condensation nuclei. For this hygroscopic particles with the radii over 0.5-0.6 μm should be introduced. The number of additional particles needed for intensification of coagulation events, and the subsequent precipitation formation is determined by the results of numerical simulation. The size distribution of hygroscopic particles at numerical simulations is described by the function of the form: f(r)=a r -μ exp(-(r/ro)). (1) The salt powder particles spectra are of such a form. For the powder presented in (Drofa et al., 2010) the distribution parameters (1) are equal to: μ = 1.5, r0 = 5 μm; for the powder (Drofa et al., 2011) μ = 0.5, r0 = 5.3 μm. An example of calculations of the initial stage of cloud medium microstructure formation at the introduction of the salt powder into the subcloud layer is given in Fig. 1. Here shown are the calculation results of