Upper Atmosphere Humidity Measurement with Drycap Sensor

Atmospheric water vapor is a significant greenhouse gas. To gain proper understanding of the impact of water vapor on the climate, atmospheric water vapor profiles should be measured with high precision and accuracy. Due to challenging environmental conditions including extremely low temperatures and humidity concentrations, few research grade instruments are capable of measuring humidity accurately in upper troposphere and stratosphere. Laborious utilization or inadequate resolution of the existing methods necessitates a more feasible instrument for frequent use. To provide a user-friendly precision instrument for atmospheric measurements, Vaisala has launched a reference radiosonde program. In the first phase of the program, the standard Vaisala RS92 radiosonde was equipped with a Drycap174 humidity sensor module. Drycap174 is a highly sensitive capacitive polymer sensor originally developed for measuring ultra-dry gases in challenging industrial conditions. In the reference radiosonde prototype RR01, the specified measurement range of the Drycap174 sensor is -30 ... -90 °C in frost point temperature. This thesis describes the main branches of reference radiosonde development during 2009-2013. Various mechanical structures to protect the sensor from turbulence were studied by modelling and field testing. Compensation algorithms were developed to eliminate systematic measurement error. Measurements were conducted at various sites around the globe to examine the performance in different environmental conditions. The results show that Drycap174 is a promising technology for measuring small concentrations of water vapor in upper troposphere and stratosphere. The project succeeded in improving the performance of the prototype. The residual measurement bias relative to the chilled mirror instrument could not be completely eliminated and should be addressed in future work.