Transition from VIZ/Sippican to ROTRONIC - A new humidity sensor for the SWISS SRS 400 Radiosonde

This report describes the implementation of a new humidity sensor for the Swiss radiossonde SRS 400 to improve humidity profile measurements. Since the introduction of the SRS 400 radiosonde in 1990, MeteoSwiss uses a fast response carbon resistive hygristor of the US company VIZ/Sippican. With rising demands on the quality of atmospheric water vapour measurements for numerical weather prediction models and climate change observations, the VIZ/Sippican hygristor does not respond to the increasing requirements any more. MeteoSwiss and Meteolabor, the manufacturer of the SRS 400, therefore decided to investigate solutions for a possible substitution of the VIZ/Sippican carbon hygristor. The goals set forth for the new sensor were to improve humidity profile measurements in the lower and mid troposphere and to have a cost effective solution. Another constraint was to find a sensor that fits in the electronic design of the SRS 400 radiosonde.