Pan American Climate Studies Sounding Network

The Pan American Climate Studies Program - Sounding Network (PACS-SONET) is a research project funded by the NOAAís Office of Global Programs. This research activity (Principal Investigator Mike Douglas) was approved in December 1996, with a primary objective of determining the atmospheric circulations associated with wet and dry spells in Central America during the rainy season (Peña & Douglas 2002). Another important objective was to help determine whether the current National Centers for Environmental Prediction (NCEP) operational analyses over Central America and the eastern tropical Pacific are accurate. The project has been extended a number of times to adapt to changing circumstances. It has been expanded to a broader geographical region and new objectives have been established. This document presents a brief history of the project and provides links to pages describing some of its aspects.

Why pilot balloon observations?

Inexpensive pilot balloon stations, rather than radiosonde stations, have been the cornerstone of the PACS-SONET. The main advantage of pilot balloon observations is that they are inexpensive. Typically, a 30 g pilot balloon observation is only ~10% the cost of a radiosonde observation. In addition, pilot balloon observations require a relatively simple instrument (a meteorological theodolite), require no electric power, stations can be established easily, and training takes little time. The theodolites for PACS-SONET were obtained without charge from a US National Weather Service storage and reconditioning facility; this greatly reduced the cost of the project, since new theodolites cost ~$8,000 each.

The suitability of this relatively low-technology approach for the region was based on field experiment experience of NSSL personnel in Mexico during the early 1990's. The 1990 SWAMP (Southwest Area Monsoon Project) and 1993 SWAMP-93 (EMVER-93 in Mexico) were experiments of 1-2 month duration that each employed a network of about 10 pilot balloon stations, among other observations. For a short description of these experiments, data, figures and links to other pages related to SWAMP and EMVER, see

History of the Project

The original PACS-SONET configuration consisted of 12 pilot balloon sites (Fig. 1). The stations were established during April and May of 1997 in Mexico, Nicaragua, Costa Rica, Panama, Colombia, Ecuador and Peru, together with a combined pilot balloon and radiosonde station on Cocos Island in the eastern Pacific Ocean. These observations, made twice-daily, were to describe the conditions associated with wet and dry spells over Central America and to help evaluate the quality of the NCEP reanalyses windfields over the region - analyses that were (and continue to be) a mainstay of tropical climate research activities. The intended duration of the observations was 6 months, ideally extending through October, and close to the end of the rainy season in much of Central America.

Fig. 1. PACS-SONET initial configuration.

Due to the strong El Niño of 1997-98, the observational plan was modified to extend the period of observations at many stations through the mid-year 1998 to establish additional pilot balloon stations in Ecuador and Peru to better sample conditions in the region of anticipated heavy rainfall. This allowed an unprecedented data set to be obtained to describe the wind field along the Ecuadorean and northern Peruvian coasts, associated with the warm event in the eastern Pacific, El Niño. See for a discussion of El Niño 1997-98 as seen from PACS-SONET observations.

Pilot balloon observations were also made at Santa Cruz, Bolivia, as part of the special El Niño campaign. 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 previous observations. These observations indirectly stimulated further observations from a more extensive network in Bolivia during 1999. See for a description of the 1999 atmospheric mesoscale campaign.

Fig. 2. PACS-SONET extensions due to El Niño 1997-98 (white dots),SALLJEX pilot balloon stations (gray dots) and current PACS-SONET (red circles). Station coordinates and network updates can be found at

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 took place.. This expansion has taken several forms. In terms of geographical extension, new pilot balloon sites were established in Paraguay (2) and Bolivia (6). The primary objective of these additional observations was to describe the variability of the low-level flow east of the Andes and the circulation over the Bolivian altiplano. Three additional sites were established in Mexico in March 2000 to bring to 6 the number operating (one more was added in April 2002). The stations in northwestern Mexico, were designed to improve the description of the low-level flow along the Gulf of California. . In March 2001 one station was established in Venezuela at San Fernando de Apure, a location in the very flat llanos north of the Orinoco river. Figure 2 shows the current PACS-SONET configuration and other stations that have been operational with financial or logistic support from the project.

In addition to more observing sites, a major effort has been made to make the network a real-time data collection and distribution activity. This required 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. The latter is accomplished mainly through the development of the projectís web page, in which the observations are made available in near real time, in the form of both raw data and plotted maps at select levels (See

More recently, the PACS-SONET stations in Bolivia and Paraguay served as the ramp-up network for the upper-air component of the South American Low Level Jet Experiment (SALLJEX) that started on November 15, 2002. The NSSLís SALLJEX page at can be visited for more information, data, procedures and additional links to pages related to the experiment.

Fig. 3. Pictures from two special observing campaigns conducted in Bolivia during SALLJEX around Lake Titicaca (left) and in the Salar de Uyuni (right).

In 2003 a second extension of the PACS-SONET was approved for an additional 3 years. Additional changes in the network configuration and mode of operation are expected.

Educational component

An essential aspect of the PACS-SONET continues to be that of educational activities. The project has explored a variety of measures to increase awareness of the data set to the countries and institutions involved. Short courses (1-3 week) have described PACS-SONET in Peru in 1999, in Bolivia in December 2000, and July-August 2001 in Panama. In February 2002, a workshop in San Ignacio de Velasco, Bolivia was intended to introduce the participants to many aspects of the observational campaign planned for SALLJEX. The following are links to pages describing some of the activities related to PACS-SONETís educational component:

New types of observations

The limitations of pilot balloon observations, especially those due to cloudiness, have 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. There are many important meteorological topic, however,s that require knowledge of the profiles of humidity or temperature.

To address the problem of obtaining more complete, yet affordable radiosonde-type soundings over the global tropics, work was started more than 6 years ago on the development of "recoverable radiosonde" technology. This involves at two related systems. The first system is known as the "glidersonde", which is basically a glider that carries a suite of radiosonde (or similar) sensors and is lifted to high altitude by a meteorological balloon. After the glider detaches from the balloon at a predetermined altitude, it returns to the launch point (or another location) by gliding and navigating with the help of an onboard GPS system and microcomputer. There are various technical and operational considerations involved with the deployment of such a system, but the concept has already been demonstrated (Howard et al 1998).

The second version of recoverable radiosonde involves use of a small autonomously piloted model aircraft ("Powersonde"). This can ascend and descend autonomously, while making an atmospheric sounding. Either radiosonde sensors or other meteorological sensors can be used in principle.

Field testing of the powersonde and glidersonde developed at NSSL was carried out just south of Pretoria, Republic of South Africa (RSA), between th whom NSSL is collaborating. These experiments were carried out during January 2002. The field tests were sponsored in large part by the International Activities Office of the US National Weather Service.Results and pictures from the test flights in South Africa are available at

Fig. 4. Prototype of powersonde being developed at NSSL and tested in Irene, Republic of South Africa in January 2002.