The Pan American Climate Studies (PACS) Program of NOAA funded the establishment of a regional atmospheric sounding network (SONET) in Latin America to investigate the rainfall variability over Central America during the summer of 1997. The planned sounding network, PACS-SONET, consisting initially of 12 pilot balloon sites in Mexico, Nicaragua, Costa Rica, Panama, Colombia, Ecuador and Peru, together with one radiosonde station on Cocos Island in the eastern Pacific Ocean, was established during April and May 1997 and was to operated for six months. Nearly all sites used relatively inexpensive optical theodolites and pilot balloons to obtain wind soundings; the suitability of this relatively low-technology alternative for the region was based on previous field program experience in Mexico. The observing program planned for twice-daily observations, so that the amplitude of diurnal wind variations could be estimated. This would permit an assessment of the uncertainties inherent in climatological upper-air wind analyses that aregenerally based upon only once-daily soundings.
Shortly after the first stations were established in April 1997, it became apparent that a significant warm event in the eastern and central Pacific was developing. By mid-summer, the extreme magnitude of the event had become apparent. This led to two conclusions: 1) the observations being made during the summer of 1997 would be very anomalous over Central America, thus putting into question the extrapolation of potential scientific findings to other years, and 2) there existed the possibility of obtaining an unprecedented data set to describe the evolution of the windfield associated with the developing El Niņo. Therefore, the observational plan was modified to: 1) establish additional pilot balloon stations in Ecuador and Peru to better sample conditions in the region of anticipated heavy rainfall, 2) extend the observational period at many stations through the end of October 1998, and 3) include the installation of several hundred inexpensive raingauges in coastal Ecuador and Peru to assure reasonable depiction of daily rainfall, for comparison with the wind soundings and with satellite observations.
The financial, logistical, and technical difficulties involved in establishing and expanding the network (ultimately to 17 pilot balloon sites) will be described. In addition, limitations of the network observations for helping with real-time forecasting activities will be highlighted. The prospects for maintaining or even expanding the network in the future will be briefly discussed.