The PARC project is a four-year scientific program to evaluate the viability of increasing rainfall through the introduction of hygroscopic seeding material at cloud base. This is a cooperative program between the Secretaria de Fomento Agropecuario and NCAR, with significant participation by Altos Hornos de Mexico. The objectives of PARC follow on work done in South Africa and trial studies done in Arizona. During the field project of the summer of 1996, the emphasis was on characterizing the development of convection and precipitation in central Coahuila, with preliminary studies on establishing techniques for detecting potential effects from hygroscopic seeding. A wide variety of cloud conditions were encountered, but a significant fraction of the storms that develop in Coahuila appear to be susceptible to positive changes from hygroscopic seeding; that is, they have an inefficient coalescence process. During the summer of 1997 (1 July through 30 September), the field project focussed on: 1) beginning the randomized seeding experiment; 2) continuing the collection of meteorological data for further evaluation of the randomized experiment and other physical studies; and 3) fostering long-term collaborations with educational and research organizations in Mexico, such as the Universidad Autonoma de Coahuila, the Universidad Nacional Autonoma de Mexico, the Instituto Mexicano de Tecnologia del Agua (IMTA), and others.
The primary components for data collection during the field project included a 5-cm Enterprise radar, a Piper Cheyenne II aircraft, sounding and surface observations at the operations site, two additional mesonet sites, and an IMTA raingage network of 10 stations. Communications, data ingest, forecasting, data analysis, and logistical support were coordinated through the operations center located at the international airport near Monclova, Coahuila.
The main objectives of each operation were to select suitable candidate storms, randomly seed or not seed them, and make the measurements that are required to detect and analyze any seeding effects. A secondary objective was to conduct additional cloud microphysical studies that might further characterize precipitation development processes and their variability in Coahuila storms. Forty-six cases from 26 flights were selected during the 3-month field project. Twenty-four were seeded (seed cases) and twenty-two were not seeded (control cases). An additional 20 flights were made either searching for suitable cases, making cloud penetrations for other physical studies, or performing instrument calibration work. The data are currently being analyzed by D. Breed, R. Bruintjes, M. Dixon and B. Brown, but it is clear that 46 cases are not enough to detect a potential seeding effect. Particularly unique to this project is the additional information collected from an aerosol probe and cloud condensation nucleus counter that were made available to PARC by colleagues at Atmospheric Environment Service of Canada. Plans for the next two years will emphasize: 1) data collection from cases, randomly seeded or not seeded, following the same selection criteria as this year; 2) defining the number of cases needed for statistical significance; 3) utilizing results from the physical studies in an attempt to stratify the cases; and 4) beginning modeling studies of convective development in central Coahuila and of precipitation development in the variety of clouds sampled thus far.