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Radiosondes have for several decades been the primary means of obtaining atmospheric vertical profile data from the surface to the lower stratosphere. They are routinely used as input to the operational meteorological analyses that are used in numerical weather prediction and meteorological diagnostics. In the absence of other in situ measurements, radiosonde observations have recently been used to assess trends of atmospheric conditions above the surface, even though they were not designed for this purpose. The instrument packages carried aloft by balloons are generally equipped with temperature, humidity, and pressure sensors, whose measurements are radio-transmitted to a ground receiving station. Wind data are also obtained by tracking the position of the instrument during ascent. Temperature sensors vary according to the manufacturer and model of the radiosonde; most contemporary instruments carry a thermocapacitor, wire resistor, thermocouple, or bimetallic sensor. An excellent overview of radiosonde instruments, including discussion of measurement error characteristics, is provided by the World Meteorological Organization (WMO, 1996).

Currently, the global radiosonde network nominally includes about 900 upper-air stations, of which about two-thirds make observations twice daily (at 0000 and 1200 Coordinated Universal Time (UTC)). The network is predominantly land-based and favors the Northern Hemisphere (Figure 2.6). Radiosondes can achieve heights of about 35 km, although many soundings terminate below 20 km because less expensive balloons burst at a lower altitude. There has been some deterioration in the radiosonde network in recent years. The loss of navigational systems used to track the sondes has led to at least temporary closing of some stations, particularly in Africa. Efforts to reduce operating costs have led to station closures and reduced observing schedules in some parts of the former Soviet Union and elsewhere.

Measurements are made and transmitted with approximately 10–50 m resolution during ascent, but archived sounding data may contain only about 20 data levels per sounding. Radiosonde-based data sets for climate monitoring come from two basic data products: individual soundings containing all reported data (Angell, 1988), or monthly mean data (known as CLIMAT TEMP reports) at mandatory pressure levels only (Parker et al., 1997). Only about 45% of stations provide CLIMAT TEMP reports in addition to the daily sounding data. Missing daily datacontinue

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