The MOZAIC Capacitive Hygrometer (MCH) is usually operated onboard of passenger
aircraft in the framework of MOZAIC (Measurement of Ozone by AIRBUS In-Service
Aircraft). In order to evaluate the performance of the MCH, it was operated aboard
5 a Learjet 35A aircraft as part of the CIRRUS-III field study together with a closed-cell
Lyman-α fluorescence hygrometer (FISH) and an open path tunable diode laser system
(OJSTER) for water vapour measurement. After reducing the data set to MOZAICrelevant
conditions, the 1 Hz relative humidity (RH) cross correlation between MCH and
reference instruments FISH (clear sky) and OJSTER (in-cirrus) yielded a remarkably
good agreement of R
2
10 = 0.97 and slope m = 0.96 and provided the MCH uncertainty
of 5 % RH. Probability distribution functions of RH deduced from MCH and reference
instruments agreed well over the entire range of observations. The main limitation for
the use of MCH data is related to sensor temperatures below the calibration limit of
Tsensor = −40 ◦C (corresponds to ambient temperature of Tambient = −70 ◦C at typical
15 cruising speed of long-haul passenger aircraft), which causes a delay in the sensor’s
time response. Good performance of MCH for clear sky as well as for in-cirrus conditions
demonstrated the sensor robustness also for operation inside ice clouds.