IGRA consists of quality-assured soundings at over 1500 globally distributed stations with varying periods of record. Although the overall period of record is 1938 to present, the length and completeness of record vary widely among stations, and the vertical resolution, vertical extent, and completeness of soundings improve considerably over time. Mandatory levels generally include geopotential height, temperature, wind direction, and wind speed. Beginning in 1969, dewpoint depression is usually also available in the lower and middle troposphere but becomes scarcer in the upper troposphere because of the general practice to discontinue humidity measurements at temperatures less than 40°C (Elliott and Gaffen 1991; Garand et al. 1992). Temperature and dewpoint depression are also available at significant thermodynamic levels (which usually do not include geopotential height). Wind observations are reported at significant thermodynamic levels or at separate levels whose elevation is defined by pressure and/or height.
The dataset contains slightly more than 28 million soundings with a total of 800 million levels. Approximately 20 million of the soundings from roughly 1250 stations contain temperature measurements, with the remainder consisting of only pibal observations. As shown in Table 1, 82% of the soundings originate from the GTS-based core data sources, while the other large-scale sources and country-specific datasets contribute 6% and 12%, respectively. The most frequent observation times available are 00 and 12 UTC beginning in 1958 and 03 and 15 UTC before that year. The majority of stations take observations twice daily at or near those observation times, and some provide observations even more frequently; however, a number of stations only have one sounding per day for extended periods due to a lack of equipment or observers.
As indicated by Figure 1a, IGRA contains stations in most areas of the globe. The spatial coverage is most complete in Europe and sparsest in northern Canada, interior Antarctica, and equatorial Africa. However, the total number, spatial distribution, and temporal completeness of stations vary considerably over time (Figure 2). For each year between 1938 and 2003, Figure 2 displays the total number of stations (dashed line) and the number of stations reporting one or more soundings on at least 80% of possible days (solid line). During the early part of the record, the number of stations increases from one station (in Tasmania) in 1938 to several hundred in the early 1960s, when most of the stations report data on more than 80% of the days. By the time the number of stations peaks in 1991, approximately 840 of the available 1180 stations report at least one sounding on at least 80% of the days. Station closings are responsible for the decline in the number of stations in recent years. Relative to the map of all stations (Figure 1a), the distribution of the 937 stations active in 2003 (Figure 1b) exhibits the most pronounced deficit in western equatorial Africa.
The jumps in the number of stations in 1946, 1963, and 1973 (Figure 2) are related to changes in the number or type of data sources contributing to IGRA (Table 1). Before the beginning of the first GTS data source in September 1963, the U.S. Air Force and country-specific U.S. sources each account for nearly half of the soundings (approximately 48% and 44%, respectively), with the remainder provided by the country-specific sources for Australia (nearly 8%) and Argentina (<1%). Consequently, during this early period, IGRA stations are concentrated in the contiguous United States, Alaska, and the former Soviet Union, with additional stations in parts of the North Atlantic, southeast Asia, Argentina, and coastal Australia. With the jump in 1963, coverage of western Europe, China, and Japan begins, while many stations in Africa, Brazil, Central Asia, and India do not become available until the late 1960s or early 1970s.
The change in vertical resolution and extent over time is illustrated by time series of the average number of mandatory and total levels per sounding (Figure 3) as well as time series of the percentage of soundings reaching up to at least 100 or 10 hPa (Figure 4). Before the 1960s, soundings consist primarily of mandatory levels below 100 hPa. By the early 1960s, most of the soundings contain observations up to the 100-hPa level and include some significant levels. The addition of large numbers of stations with varying degrees of data completeness accounts for the overall drop in the percentage of soundings reaching into the stratosphere during the late 1960s and 1970s. Overall, however, the vertical resolution of soundings continues to improve, as indicated by the rather monotonic rise in the total number of levels per sounding. By 2003, the average sounding consists of 11 mandatory and 35 additional levels, and 74% (35%) of all soundings reach at least a 100-hPa (10-hPa) level.
Elliott, W.P., and D.J. Gaffen, 1991: On the utility of radiosonde humidity archives for climate studies. Bull. Amer. Meteor. Soc., 72, 1507-1520.
Garand, L., C. Grassotti, J. Hall, and G.L. Klein, 1992: On differences in radiosonde humidity-reporting practices and their implications for numerical weather prediction and remote sensing. Bull. Amer. Meteor. Soc., 73, 1417-1423.