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

Figures & data

Fig. 1 The advanced IAGOS Capacitive Hygrometer bases on a compact capacitive sensor measures relative humidity and differs from the MOZAIC Capacitive Hygrometer by the thin-film polymer composition. The improved hydroactive polymer composition of Vaisala HUMICAP® type H has a larger capacitance to increase the sensitivity. For signal processing, the Vaisala HMT333 transmitter unit is used. Courtesy of Vaisala Oyj, 2015.

Table 1. Field study data overview for cruise altitude

	No. of flights	Period	Temperature range (°C)	H2O VMR (ppmv)	RHliquid (% RH)	In/out of cirrus (min)
CIRRUS-III	4	24–29/11 2006	−63 – −42	16–360	10–78	217/537
AIRTOSS-ICE	7	29/08–05/09 2013	−57 – −40	6–530	1–83	60/193

Table 2. Participating hygrometers during CIRRUS-III^a and AIRTOSS-ICE^b

Instrument	Detection quantity	Measurement technique	Response time	Uncertainty	Reference
MCH^a and ICH^b	RHliquid (% RH)	Capacitive sensor	LT: 1 s MT: 10 s UT/LS: 1 min	±5% RH	Neis et al. (2015)
FISH^a,b	VMR (ppmv)	Lyman-α photo-fragment fluorescence	1 s	7%±0.3 ppmv (precision 1%)	Meyer et al. (2015)
OJSTER^a	VMR (ppmv)	Open-path TDL	1 s	10–15%	May and Webster (1993)
SEALDH-II^b	Molar density	Direct TDLAS	0.5 s	4.3%±3 ppmv 0.083 ppmv at 1 s	Buchholz et al. (2013)

MCH and ICH: MOZAIC and IAGOS Capacitive Hygrometer; FISH: Fast in situ Stratospheric Hygrometer; OJSTER: Open-path Julich Stratospheric TDL ExpeRiment; SEALDH-II: Selective Extractive Airborne Laser Diode Hygrometer; LT: lower troposphere; MT: middle troposphere; UT: upper troposphere; LS: lower stratosphere; for further information see Bange et al. (2013).

Fig. 2 Probability distribution functions (PDF) show the distributions of the measured sensor temperatures during a) 15 yr of MOZAIC measurements, b) CIRRUS-III and c) AIRTOSS-ICE. The red line indicates the lower limit of the sensor calibration. The grey line illustrates the most probable value of MOZAIC distribution.

Fig. 3 RH_liquid and T _sensor (blue dotted line) sequence of an AIRTOSS-ICE flight. Small-scale fluctuations of the measured reference RH_liquid (black line) can be resolved by the ICH (red line) in warmer air masses (white background). In colder air masses with T _sensor<−20°C, the ICH seems to measure the smoothed background.

Fig. 4 Example to demonstrate the way to determine the most probable response time in the temperature bin T=243 K for AIRTOSS-ICE flight 3.

Fig. 5 Determined response times per temperature bin and flight for the CIRRUS-III (blue squares) and AIRTOSS-ICE (red stars) flights. The robust exponential fit (black line) through the data leads to the MCH and ICH characteristic temperature-dependent response time τ [see eq. (2)].

Fig. 6 From top to bottom: RH_liquid measured by the ICH (red) and the measured reference (black), that is, FISH (clear sky) and SEALDH-II [in-cirrus (blue dots) and above 1000 ppmv]; RH_liquid measured by the ICH (red) and the smoothed reference (EMA including the temperature-dependent response time, black); Δ RH_liquid (ICH and reference) as envelope for the measured reference and as green line for the smoothed reference, as a function of flight time during the AIRTOSS-ICE flight on 3 September 2013; and sensor temperature T _sensor (black) as well as the lower temperature limit of the calibration (red).

Fig. 7 (a) presents comparisons between the RH_liquid data from MCH and the measured reference, that is, FISH (clear sky) and OJSTER (in-cirrus), during CIRRUS-III. (b) shows results of comparisons between ICH and the measured reference, that is, FISH (clear sky) and SEALDH-II (in-cirrus and above 1000 ppmv), during AIRTOSS-ICE campaign; the straight line indicates the linear regression line, while the dashed lines illustrate the sensor uncertainty range ±5% RH_liquid. In the transition area measurements of both reference instruments can occur.

Fig. 8 Similar to Fig. 7, but the reference RH_liquid data are smoothed prior to the comparisons of (a) CIRRUS-III and (b) AIRTOSS-ICE.

Fig. 9 Frequency of occurrence of the ratios of MCH to reference RH_liquid (a) and of ICH to reference RH_liquid (b), respectively. Both figures are divided into the PDF of original (left panel) and smoothed (right panel) reference data.

Bange J. , Esposito M. , Lenschow D. H. , Brown P. R. A. , Dreiling V. , co-authors . Wendisch M. , Brenguier J.-L . Measurement of aircraft state and thermodynamic and dynamic variables. Airborne Measurements for Environmental Research: Methods and Instruments. 2013; Wiley-VCH Verlag GmbH & Co KGaA. 7–75. DOI: http://dx.doi.org/10.1002/9783527653218.ch2 .

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