Michael W. Douglas, National Severe Storms Laboratory/NOAA

Malaquias Peña, CIMMS/University of Oklahoma,

Norman, Oklahoma USA


Since April 1997 special upper air observations have been made in Latin America as part of a program to improve the understanding of climate variability over the region. These observations, sponsored by the NOAA Office of Global Programs as part of its Pan American Climate Studies (PACS) research program, originally had specific research objectives related to the explanation of rainfall variations over Central America (Douglas, et al 1999). However, a variety of events have affected the evolution of the sounding network (SONET) activities during the past 2 years; this paper presents a summary of the recent activities and the near-future plans.

Figure 1. The original network of PACS-SONET pilot balloon stations operating from May-October 1997.



The initial focus of the PACS-SONET was the identification of warm season circulation patterns associated with wet and dry spells over Central America, with an attempt to determine the accuracy of the NCEP reanalysis over the region. To do this, a network of 12 pilot balloon stations was established from southern Mexico to northern Peru to help map the windfield on a daily basis (Fig. 1). Observations were made twice-daily so as to identify the amplitude of the diurnal cycle in the windfield, associated mostly with land-sea breeze circulations. This was important because many upper-air climatologies are based on observations made only once-daily. Such observational climatologies could be difficult to compare with Reanalysis-based climatologies if these soundings are strongly affected by local diurnally-forced circulations.


During the initial set-up of the Ecuadorian and Peruvian stations in later April and early May 1997 it became apparent that the early phase of an El Niño event was underway. As the boreal summer progressed it became apparent that the El Niño event was extraordinarily intense, and that the 6-month measurement program planned for PACS-SONET might not be representative of "normal" warm season conditions over Central America. It had been hoped that conditions from 1997 might be sufficiently close to the long-term climatology over the region to make meaningful generalizations; by the end of the original observing program in October it was clear that this would not be possible. This conclusion, together with the heightened interest in the regional effects of the strong El Niño along the coasts of Ecuador and Peru, led the PACS-SONET to evolve and adapt in response to these meteorological conditions.

Additional support for temporary pilot balloon stations in Ecuador and Peru was obtained on short notice from PACS program managers, and two additional sites were established in Ecuador and five in northern Peru (Fig. 2). These sites were established in December and January 1997-8, and generally functioned until May 1998. A number of logistical details, mostly associated with the very heavy rains over the region, prevented a complete data record from being obtained at all stations. March 1998 was the month of most complete observations, with most stations making twice-daily observations.

In addition to the enhanced pilot balloon network, a special raingauge network, employing simple raingauges, was established in December 1997. Approximately 80-100 additional stations were established in far northern Peru, mostly at public health clinics scattered throughout the countryside. Volunteers from the clinics made the rainfall measurements once daily. These observations proved essential for describing the variations in daily rainfall over the region (Douglas et al, 2000).

One small extension of the 1998 observations that has had a major role in the development of the observing network in South America is the observations made at Santa Cruz, Bolivia, as part of the special El Niño observations. Although these observations were only made for 3 months, they showed a strong and variable low-level jet at Santa Cruz, which had not been well described from observations previously. These observations indirectly stimulated further observations from a more extensive network during 1999, and have led to the region being highlighted for continuous monitoring in the future (see below).

Figure 2. The PACS-SONET stations during the intensive El Niño phase January-May 1998. Not all stations were operating simultaneously.



In late 1999 support was obtained for extension of the PACS-SONET for an additional 3-year period. In addition to an extension in time of the program, an expansion of the network is underway. This expansion will take several forms. In terms of geographical extension, new pilot balloon sites are being established in Paraguay (2), Bolivia (6), and possibly in neighboring Chile and Argentina. The primary objective of these additional observations is to describe the variability of the low-level flow east of the Andes and the circulation over the Bolivian altiplano. In North America at least 6 sites will be operated in Mexico, including two in northwestern Mexico to improve the description of the low-level flow along the Gulf of California.

In addition to more observing sites, a major effort is being made to make the network a real-time data collection and distribution activity. This requires efforts to upgrade communications at many sites, and the development of procedures to ensure the flow of data not only to research institutions, but to all interested operational forecasting institutions. Although the PACS-SONET is funded as a research activity, it is widely recognized that long-term sustainability of the network requires access to the data in real-time, as the greatest perceived benefit of the observations at the national level is in their use for daily weather forecasting activities.


The limitations of pilot balloon observations, especially those due to cloudiness, has been recognized for many years. The initial emphasis of the PACS-SONET on the use of pilot balloon observations was based on the recognized importance of wind measurements in the tropics, but even more importantly, on the recognition that funds would never be available for a greatly expanded radiosonde network over the region of interest. Unfortunately, there are many important meteorological topics that require a knowledge of the profiles of humidity or temperature. The PACS-SONET has recognized this need, and is proceeding, with support from the US National Weather Service International Activities Division and the National Severe Storms Laboratory, to develop recoverable radiosonde technology that can be used to enhance the PACS-SONET. This work has taken two forms. The first is the development of radiosondes carried aloft by model airplanes. The second is the lifting of gliders by balloons; when the glider detaches from the balloon it returns gliding to the point of origin for recovery (Howard, et al 1998). Both of these systems have been undergoing field tests over the past several years, and it is hoped that they will become part of the PACS-SONET as soon as their development merits field deployment. This is anticipated during the year 2000.

Acknowledgments: The PACS-SONET project is supported by the Pan American Climate Studies program of the NOAA Office of Global Programs. Support of the PACS program managers is greatly appreciated, as is support from PACS community scientists. Additional specific acknowledgments can be found on the PACS-SONET homepage at: http://www.nssl.noaa.gov/projects/pacs


Douglas, M.W., M. Peña, N. Ordinola, L. Flores, J. Boustead, and J. L. Santos, 2000: Synoptic and spatial variability of the rainfall along the northern Peruvian coast during the 1997-8 El Niño event. Sixth Conference of Southern Hemisphere Meteorology and Oceanography, Santiago, Chile, April 3-7, 2000

Douglas, M.W., W. Fernandez and M. Peña, 1999: Design and evolution of the PACS-SONET observing system in Latin America. Third Symposium on Integrated Observing Systems, Dallas, Texas, January 10-15, 1999, pp 131-134.

Howard, K.W., M.Douglas, S. Fredrickson, I.Winge, D. Egle, D. Smith, and N. Renno, 1998: The development of a recoverable radiosonde: The glidersonde project. Tenth Symposium on Meteorological Observations and Instrumentation. Phoenix, Arizona, 11-16 January, 1998.

Corresponding author: Dr. Michael Douglas, NSSL, 1313 Halley Circle, Norman, OK 73069 USA

email: mdouglas@nssl.noaa.gov