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These observations of cloud-droplet spectrum, liquid water contents and hydrometeor sizes and types, as well as <br />inferences about cloud updrafts, are to be used to determine the impact of the CCN particulates on in-cloud <br />processes, most notably coalescence. Since measurements were also made at higher elevations in the region of <br />ice processes, it will be possible to infer the effect of the aerosols on these processes as well. <br /> A high-performance twin-engine aircraft (Cheyenne II), equipped with state-of-the-art cloud <br />microphysics instrumentation, was deployed within and in proximity to operational rain-enhancement “target” <br />areas in semi-arid West and South Texas during the 2-month time frame beginning on August 4, 2004. The <br />capabilities of this aircraft are addressed in Appendix A.Measurements of CCN were also made in western and <br />southern Oklahoma, as well as in other sectors of Texas where cloud seeding has not been performed in recent <br />years. In all, the research aircraft conducted 34 missions during the 9-week field program, logging a total of 75 <br />hours. The results of SPECTRA I are presented in a separate document. <br />2.2 SPECTRA II <br /> SPECTRA II to document the effect of hygroscopic salt seeding on internal cloud properties was <br />conducted during May and early June of 2005 as documented herein. It is important first, however, to establish <br />the scientific basis for seeding with hygroscopic particles. <br />3.0 SCIENTIFIC BASIS FOR HYGROSCOPIC SALT SEEDING <br /> Recent experiments have sought to determine if hygroscopic seeding of individual convective clouds <br />can improve their precipitation efficiency by enhancing the coalescence process within them. Randomized <br />. <br />experiments in South Africa using hygroscopic flares (Mather et al, 1997; Bigg, 1997) and in Thailand using <br />hygroscopic salts introduced in bulk into the clouds (Silverman and Sukarnjanaset, 2000) have produced <br />statistically significant increases in radar-estimated rainfall from the seeded clouds, ranging from 30 to 60 <br />percent. Numerical simulations of the growth of the salt particles to precipitation-size particles support the field <br />results (Cooper et al., 1997). Most impressive has been the replication of the South African results in Mexico <br />(Bruintjes et al., 1999, 2001; Bruintjes, 1999). The method, involving the production of hygroscopic salts from <br />burning flares affixed to the seeder aircraft circling in updrafts at cloud base, has not yet been tested over a large <br />area, nor has it been tested in a meaningful way in the U. S. <br /> Seeding at cloud base with hygroscopic material to produce precipitation increases is predicated on the <br />assumption that the rain-producing process evolves in the following manner: (1) the introduction at cloud base <br />of large and giant cloud condensation nuclei (CCN) produced by burning hygroscopic flares in racks mounted <br />to the wings of the seeder aircraft; (2) preferential activation of the larger CCN from the flares, leading to a <br />broadening of the cloud droplet distribution; (3) growth of the large cloud droplets into raindrops via natural <br />coalescence processes, in clouds which could not otherwise have grown raindrops through warm-rain processes; <br />(4) the transport of the raindrops into the supercooled portion of the cloud where the raindrops freeze due to <br />their larger size; (5) invigoration of the cloud due to released latent heat and growth of the frozen drops to large <br />graupel by accretion of the cloud water; and (6) increased radar-estimated rainfall at cloud base and <br />presumably more rainfall at the ground, when the enhanced water mass moves downward through the cloud <br />(Mather et al., 1997). Most of these links in the conceptual model guiding the hygroscopic seeding <br />experimentation have not yet been documented satisfactorily. Further, the conceptual model remains in a state <br />of flux. <br />13 <br />