Measurements of temperature, relative humidity, wind speed and direction, and pressure, belong to the basic input quantities to analyze and describe actual weather patterns, and serve as input for the initialization of short- and medium-range operational numerical weather forecast models. These parameters are measurable simultaneously by radiosondes. The basic concept is the use of sensors designed for in-situ measurements in direct contact with the air, and immediate transmission of the measured values to a ground station by radio. The data are then disseminated through the GTS (Global Telecommunications System). The sonde and sensor technology has developed continuously since the first working radiosondes were developed in the late 20s of the last century. Since many decades, radiosondes are launched worldwide and internationally coordinated in daily routine, primarily by national weather services. Operational soundings reach heights of 20 km to 35 km, depending on balloon size and local atmospheric conditions. Different sonde types from manufacturers of about 10 countries are nowadays in use, and provide data with appropriate quality, primarily for weather applications. The near real-time data availability, high vertical resolution, high potential for automatization, as well as comparatively simple applicability and moderate costs, make radiosondes an essential data source for that purpose.
Beyond the weather-related applications which primarily lean on tropospheric observations, radiosondes supply valuable data from the higher altitude range of the upper troposphere through the tropopause into the lower stratosphere (UTLS). The instruments may therefore significantly contribute to the generation of data appropriate for studies of climate related processes, for example analyses of trends of essential climate variables in connection with climate change. However, the extremely dry and solar radiation influenced conditions and the low pressure at altitudes above the tropopause are still a challenge for the radiosonde sensor technologies. One aim of GRUAN with respect to radiosondes is to assess the data quality which at best is achievable from sonde measurements (reference quality). This is done by means of comprehensive manufacturer independent characterization of operational sondes, including mandatory comprehensive error and uncertainty quantification applying metrological methods, and including all available information.
Recently, eight GPS radiosonde models from six manufacturers of six countries are in use within GRUAN, six of which have a GRUAN Data Product available or under development. Basic technical specifications, actual launch statistics, and links to further information for each of the sondes is given here.