Evaluation of the MOZAIC Capacitive Hygrometer during the airborne field study CIRRUS-III

Abstract. 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 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 MOZAIC-relevant conditions, the 1Hz relative humidity (RH) cross correlation between MCH and reference instruments FISH (clear sky) and OJSTER (in-cirrus) yielded a remarkably good agreement of R2 = 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 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.