Upper atmosphere humidity measurement with the APS sensor—1st progress report on the Vaisala Reference Radiosonde Program

While the current radiosonde networks support normal weather observation needs rather well, they do not provide accurate enough information for climate change observations. In January 2009 Vaisala launched a program to develop an operational reference-grade radiosonde for climate studies and other applications where enhanced radiosonde sensor performance is required. The program is targeting especially the needs of the planned GCOS Reference Upper Air Network.

Vaisala is now collaborating with the international scientific community, concentrating first on upper-air measurements of humidity, the most abundant and most important greenhouse gas in the earth's atmosphere. However, humidity is also one of the most difficult parameters to measure with high precision and accuracy, especially in the upper troposphere and stratosphere where conditions are extremely cold and dry.

The first version of the operational reference radiosonde is based on the Vaisala RS92 radiosonde sensors and Vaisala's Advanced Polymer Sensor (APS), a new capacitive sensor measuring extremely low dewpoints. The APS is designed to observe humidity in the upper troposphere and lower stratosphere up to 30 km. It has dewpoint range from -30 °C to -90 °C, thus supplementing well the RS92 Humicap sensor by providing an independent measurement in the upper atmosphere. In this presentation, the sonde design is described and the results of the first field test campaigns are analyzed.

After piloting, the instrument will be further developed according to the feedback gained from the international scientific community. In parallel, Vaisala intends to continue development of the reference radiosonde to also provide even more precise measurements of lower troposphere humidity, temperature and pressure until climate science needs are met.