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

Abstract

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-α 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 = 0.92 and slope m = 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.−70 to −40 • C. The use of MCH data is limited to sensor temperatures above the calibration limit of T sensor = −40 • C (corresponds to ambient temperature of T ambient = −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.

Quality assessment of MOZAIC and IAGOS capacitive hygrometers: insights from airborne field studies

“…As a main conclusion, Neis et al (2015) reported a good agreement of the MCH with the reference time series (see Section 3.2), with an uncertainty of 5% RH with respect to liquid water (RH liquid ). During sequences with decreasing sensor temperature, the MCH showed a decreasing sensitivity as a consequence of the increasing response time (see Section 4.1), that is, small-scale fluctuations across strong RH gradients could not be resolved.…”

“…Another reason for classifying the field study data by the sensor temperature is the fact that Neis et al (2015) have observed an improvement in the agreement with the reference humidity data with increasing sensor temperature, that is, with decreasing response time of the hygrometer.…”

“…A detailed description and evaluation of the MOZAIC Capacitive Hygrometer is given elsewhere (Helten et al, , 1999Smit et al, 2014;Neis et al, 2015). The ICH uses a capacitive sensor (Vaisala HUMICAP † of type H, Finland), the capacitance of which depends on the RH of the dielectric layer of the condenser, and a platinum resistance sensor (Pt100) for the direct measurement of the temperature at the humidity sensing surface.…”

“…The MCH itself, the applied calibration techniques and evaluations are described in detail by Helten et al (1998) and Neis et al (2015), while Smit et al (2008) presents an approach for a potential in-flight calibration.…”

A tandem approach for collocated measurements of microphysical and radiative cirrus properties

“…The accuracy is ±0.5 • C for the temperature measurement and ±5 % for the detection of the relative humidity. Evaluation and measurement methods of the MCH are described in detail in Neis et al (2015).…”