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

Abstract

The MOZAIC Capacitive Hygrometer (MCH) is usually operated aboard passenger aircraft in the framework of MOZAIC (Measurement of Ozone by Airbus In-Service Aircraft) for measuring atmospheric relative humidity (RH). In order to evaluate the performance of the MCH, the instrument was operated aboard a Learjet 35A research aircraft as part of the CIRRUS-III field study together with a closed-cell Lyman-± fluorescence hygrometer (Fast in situ Stratospheric Hygrometer, or FISH) and an open-path tunable diode laser system (Open-path Jülich Stratospheric TDL ExpeRiment, or OJSTER) for water vapour measurement. After reducing the CIRRUS-III data set to data corresponding to MOZAIC aircraft operation conditions, the 1 Hz RH data cross correlation between the MCH and reference instruments FISH (clear sky) and OJSTER (in-cirrus) yielded a remarkably good agreement of R 2 Combining double low line 0.92 and slope m Combining double low line 1.02 and provided a MCH uncertainty of 5% RH. Probability distribution functions of RH deduced from the MCH and reference instruments agreed well between 10 and 70% RH with respect to liquid water in the ambient temperature range of ca. g'70 to g'40 °C. The use of MCH data is limited to sensor temperatures above the calibration limit of T sensor Combining double low line g'40 °C (corresponds to ambient temperature of T ambient Combining double low line g'70 °C at typical cruising speed of long-haul passenger aircraft). Good performance of the MCH for clear sky as well as for in-cirrus conditions demonstrated the sensor robustness also for operation inside ice clouds