Evaluating Measurement Uncertainty and Performance of Vaisala Reference Radiosonde Prototype

In 2009, Vaisala launched a program to develop a reference grade radiosonde for climate studies, especially targeting the needs of the Global Climate Observing System (GCOS) Reference Upper Air Network (GRUAN). The program's initial focus is to improve upper-air measurements of water vapor.

To date, the project has produced a prototype of an operational reference radiosonde RR01, which is built around the DRYCAP® advanced capacitive humidity sensor, capable of measuring extremely low humidites in the upper troposphere and lower stratosphere (UTLS). Conventional Vaisala RS92 radiosonde technology is used for telemetry and measuring pressure, temperature and tropospheric humidity along with GPS wind finding. In 2012, the program entered the beta testing phase. The final target is a reference radiosonde that is considerably easier to operate and more economical than the current reference grade instruments, thus enabling more frequent climatological soundings.

In line with the GRUAN recommendations, we have defined uncertainty estimates for RR01 humidity measurement. Each frostpoint measurement by the RR01 is accompanied by an individual uncertainty estimate, which consists of components of uncertainty from all stages of the measurement process: instrument calibration, storage, corrections applied during humidity calculation, and the actual sounding event. According to preliminary results, typical uncertainty estimates of the DRYCAP® measurements, expressed in frostpoint temperature, display stable values around 1.0°C below 20 km in altitude and increase gradually to 2.0°C at 30 km (with k=2, or 95.5% confidence level).

During 2012, a group of beta testing partners involved with the GRUAN program has been conducting independent field testing of the Vaisala RR01 using chilled mirror instruments as a reference. Test sounding sites include Boulder (Colorado, USA), Payerne (Switzerland), Sodankylä (Finland), as well as Vaisala's own test sites in Helsinki (Finland) and Penang (Malaysia). A summary of the results and an analysis of the current performance of RR01 will be presented.