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2. Radiosonde data

Radiosonde data are available from several sources, but the most complete archive is probably the Comprehensive Aerological Research Data Set [Eskridge et al., 1995], the source of observations for this study. We employed data from 83 radiosonde locations in the tropical belt (30N-30S), shown in Fig. 1. The focus of this abstract is climatological fields for the most recent climatic normal period 1961-90. The complete data record at some stations spans 1948-97, and we have employed data through 1997 in examining interannual variability and trends, which are not discussed here.

At each of the tropopause levels defined above, we retain or compute pressure (p), geopotential height (Z), temperature (T), potential temperature (theta), and qs. Monthly means were computed from individual soundings at each radiosonde site. Separate statistics for the 0000 and 1200 UTC observations were obtained whenever possible, and climatological monthly values were obtained by averaging values for each calendar month over the 30-yr period of record 1961-90. On average, 18 years of data were available for each station, and more than 80% of the stations contributed data for at least ten years. Although this network provides vastly better spatial coverage of the tropics than in earlier studies, serious data voids remain: 54% percent of the grid is unsampled, with few stations in the eastern Pacific, the Indian Ocean, and Africa.

Climatological values have been gridded at 10 degree latitude by 15 degree longitude resolution, with generally one or two stations per sampled gridbox (Fig. 1). For presentation purposes, gridded climatological means for 0000 and 1200 UTC were averaged, and data void regions were assigned the zonal-mean value based on the sampled boxes within the relevant latitude band.

Fig. 1. Locations of radiosonde stations used in this study. Red circles indicate only 0000 UTC data were used, blue squares indicate only 1200 UTC data were used, and filled diamonds indicate data at both observation times were used.