A small number of sites within the GRUAN network have been providing these ground checks in order to characterize the instruments and their uncertainty prior to launch. This dataset is essential to improve confidence in the long-term trends and estimates of climate change.
The ground check data provide important pieces of information, which has previously not been available. First, sondes, which do not meet manufacturer specifications can be identified and rejected prior to use. Second, small changes in the manufacturing process, which may not have been communicated by the manufacturer, but which may impact the measurement accuracy, can be identified. Third, for large changes in the observing system, such as changes in radiosonde suppliers, this information can be used to reduce the discontinuity between the observations of these different instruments.
A couple of sites within GRUAN currently use a standard humidity chamber (SHC), which test the humidity and temperature sensors at saturation and thus provides SI tracabilty references for temperature and humidity. Proper ventilation within the SHC ensures that the ground check measurements are comparable to typical sounding conditions. In contrast to a 0 %RH check, the humidity check at saturation is very sensitive to batch dependent variations.