AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus GmbHGöttingen, Germany10.5194/amt-8-1233-2015Evaluation of the MOZAIC Capacitive Hygrometer during the airborne field study CIRRUS-IIINeisP.p.neis@fz-juelich.deSmitH. G. J.KrämerM.https://orcid.org/0000-0002-2888-1722SpeltenN.PetzoldA.https://orcid.org/0000-0002-2504-1680Forschungszentrum Jülich GmbH, Institut für Energie and Klimaforschung, IEK-8 Troposphäre, 52425 Jülich, GermanyForschungszentrum Jülich GmbH, Institut für Energie and Klimaforschung, IEK-7 Stratosphäre, 52425 Jülich, GermanyJohannes Gutenberg Universität Mainz, Institut für Physik der Atmosphäre, 55099 Mainz, GermanyP. Neis (p.neis@fz-juelich.de)13March2015831233124315July201422September201418December20149February2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://www.atmos-meas-tech.net/8/1233/2015/amt-8-1233-2015.htmlThe full text article is available as a PDF file from https://www.atmos-meas-tech.net/8/1233/2015/amt-8-1233-2015.pdf
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 R2= 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
Tsensor=-40 ∘C (corresponds to ambient temperature
of Tambient=-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.
Introduction
Water vapour is one of the most important variables for weather prediction
and climate research. Particularly, the interaction between the water vapour
in the UT–LS (upper troposphere and lowermost stratosphere) and tropopause
dynamics is not well understood. Thus, in the latest IPCC report
, it is stated that the knowledge about potential trends and
climate feedback mechanisms of upper-tropospheric water vapour is poor because
of the lack of long data records of high quality in this specific region of
the global atmosphere. Neither the global radiosonde network nor satellites
can provide measurements of the required spatial and temporal resolution,
while the regular in situ measurement of upper-tropospheric humidity (UTH) is
still difficult.
Since 1994, the European research programme MOZAIC Measurement of
Ozone by Airbus In-Service Aircraft; and its successor IAGOS
In-service Aircraft for a Global Observing System;
have been providing regular data for relative humidity (RH) and other
meteorological quantities like temperature and pressure as well as data on
atmospheric composition (e.g. ozone and CO) with high spatial and temporal
resolution on a global scale. The long-term observations are obtained by in
situ measurements aboard civil passenger aircraft using the existing
infrastructure of the international air transport system. However, the
continuous high-quality in situ measurements are restricted to the major
global flight routes and to the cruising altitude band of 9–13 km; i.e. the
observations refer to a large extent to the UT–LS region. Relative humidity
data from the MOZAIC programme have been used for various climatological
studies, including the distribution of UTH ,
the distribution of RH with respect to ice
RHice; e.g. and
ice-supersaturation regions e.g. in
the upper troposphere.
Atmospheric RH is measured in the MOZAIC/IAGOS programme through a compact
airborne humidity sensing device using capacitive sensors (MOZAIC Capacitive
Hygrometer: MCH). The sensor itself and the applied calibration techniques
are described in detail by . The first validation studies from
formation flights of a MOZAIC aircraft and a research aircraft are reported
by , while present an approach for a
potential in-flight calibration of the MCH. A reanalysis of the global MOZAIC RH
data set for the period 2000–2009 was performed recently .
In order to assess the validity of the long-term water vapour data and their
limitations, provided an in-flight comparison of MOZAIC
and POLINAT water vapour measurements.
However, this formation flight intercomparison was difficult to analyse
because the twin-engine research aircraft Falcon 20 had to follow the MOZAIC
Airbus A340-300 with changing time lags and distances whereby sampling of
identical air masses can not always be ensured. In 2006, there was the
opportunity to participate in the aircraft campaign CIRRUS-III along with
high-precision research-grade instruments for measuring the water vapour
volume mixing ratio (VMR). The in-flight single-platform measurements
permitted a blind intercomparison of the MCH with high-performance water
vapour instruments by measuring the same air masses and under different
atmospheric conditions. A similar analysis of the improved IAGOS Capacitive
Hygrometer is in preparation and will be published elsewhere.
MOZAIC Capacitive Hygrometer
A detailed description of the MOZAIC Capacitive Hygrometer is given by
and . In the following, we
summarise the original descriptions.
The compact airborne MCH consists of a capacitive sensor (Humicap-H, Vaisala,
Finland) whose capacitance depends on the relative humidity 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
basic measurement process is based on the diffusion-limited adsorption of the
H2O molecules by the dielectric membrane of the sensor. Since diffusion is
strongly temperature-dependent, the sensor response slows down with
decreasing temperatures. Figure shows how both sensors are
mounted in the used air sampling housing Model 102 BX, Rosemount Inc.;
see. The relative humidity and temperature signals are fed
into a microprocessor-controlled transmitter unit (HMP230, Vaisala) which
passes the signals to the data acquisition system. The data conversion from
capacitance signals to relative humidity values is performed offline in a
separate data quality assurance and analysis step.
Cross section of the airborne capacitive sensing element. Right
angle protects against particles, and control holes in the side wall minimise
internal boundary layer effects .
In its original MOZAIC mounting position aboard an Airbus A340-400 the sensor
housing is placed ca. 7 m downstream of the aircraft nose on the left
side with a 7 cm distance from the aircraft skin to avoid possible
contaminating interferences of the aircraft skin. Inside the Rosemount
housing the air flow is separated into the main flow, which traverses
straight through the housing, and the minor flow, which follows a sharp right
angle into a smaller channel where the sensors are placed. The housing is
equipped with small holes in the side wall to neglect internal boundary layer
effects by sucking the internal boundary layer air through the holes. The
right angle of the minor flow protects the RH and T sensors against dust,
water and particles.
Due to the strong speed reduction in the inlet part of the housing, the
sampled air flow is significantly heated through adiabatic heating. Assuming
100 % conversion of kinetic energy into heat during flow deceleration, the
ambient temperature Tambient (static air temperature, SAT) increases
to the temperature at the sensor inside the housing, i.e. the sensor
temperature Tsensor (total air temperature, TAT).
Given the fact that the
adiabatic conversion of energy is not exactly 100 %, the latter's quantity
(Tsensor) is calculated from the actually measured sensor
temperature, i.e. the typically 0.1–1.0 K colder recovery temperature
(total recovery temperature, TRT), and the so-called recovery factor. This
aircraft-speed-dependent and empirically determined factor is provided by the
housing manufacturer.
Sampled air flow is heated through adiabatic heating effects when
entering the inlet. ΔTemperature describes the increase relative to
the ambient temperature Tambientstatic air temperature, SAT;
see for several aircraft speeds, i.e. the Mach number M, by
assuming 100 % conversion of kinetic energy to heat during flow
deceleration.
The relationship between Tambient and
Tsensor is a function of the aircraft speed, i.e. its
Mach number M:
Tsensor=Tambient⋅1+cp-cv2cv⋅M2,
where cp (= 1005 J kg-1 K-1) and cv
(= 717 J kg-1 K-1) are the specific heat of dry air at
constant pressure and volume, respectively. The resulting difference between
Tsensor and Tambient at 10–12 km cruising altitude for
different Mach numbers is displayed in Fig. : for the
MOZAIC-typical aircraft speed of M= 0.81 the adiabatic
heating effect is ca. 30 K. Tambient is derived from
Eq. () with an uncertainty of less than ±0.5 K resulting
from uncertainties in Tsensor (±0.25 K) and M. Because of
the strong temperature increase, the detected dynamic relative humidity
RHdynamic is significantly lower than the static relative humidity
RHstatic of the ambient air at Tambient:
RHstatic=RHdynamic⋅TambientTsensorcpcp-cv⋅es,liquid(Tsensor)es,liquid(Tambient),
where es,liquid is the water vapour saturation pressure over
liquid water at Tsensor and Tambient, respectively. The
water vapour saturation pressure over liquid water es,liquid
follows the formulation of saturation water vapour pressure
over a plane surface of pure water or ice, which was recommended by the World
Meteorological Organization and adapted to the international
temperature scale 1990 (ITS-90) by . For fast, high-flying
aircraft the relation RHstatic/ RHdynamic reaches a
factor of ca. 13, which leads to the fact that the RH sensor operates in
the lowest 10 % of its full dynamic range. Since the sensor is operating in
the lower part of its full dynamic range, an individual calibration of each
sensor is necessary, which is accomplished in the atmospheric simulation
chamber at Jülich before installation on the aircraft and
after detachment past 500 h of flight. This corresponds to about 4–6 weeks
between installation and deinstallation. These calibrations are made over a
sensor temperature range between -40 and +20 ∘C against
(i) a Lyman-α fluorescence hygrometer at water vapour
mixing ratios below 1000 ppmv relative accuracy
±4 %, and (ii) a dew/frost point hygrometer (General
Eastern, Type D1311R) at water vapour mixing ratios above 1000 ppmv with an
accuracy of ±0.5 K. The relative humidity of a calibrated sensor
(RHC) at constant temperature T is found to be linearly related
to the uncorrected output value (RHUC) provided by the HMP230
transmitter unit.
RHC(T)=a(T)+b(T)⋅RHUC(T)
In Sect. the calibration procedure of the MCH is
described which was used during the CIRRUS-III field study. It combines the
standard procedure based on and the in-flight calibration
described by .
Evaluation of 9 years of pre- and post-flight calibrations in MOZAIC has
shown that the offset a(T) is the most critical parameter in determining
the uncertainty of the measurements with a shift of about -5 % RH, while
the sensitivity (slope) b(T) is less critical and only changes by about
-2 %.
CIRRUS-III flight overview at cruise altitude. Air masses are
divided into “troposphere” and “stratosphere” with the ozone VMR
threshold of 125 ppmv.
FlightDateTake-off/TemperatureH2O VMRIn/out ofStratosphere/no.landing (UTC)rangerangecirrustroposphere124 Nov10:47/14:53-62.6 to -52.8 ∘C24–107 ppmv95/96 min2/190 min228 Nov08:22/12:07-62.4 to -44.1 ∘C17–138 ppmv4/160 min11/153 min328 Nov13:31/17:25-60.0 to -42.4 ∘C27–360 ppmv56/124 min5/175 min429 Nov09:16/13:51-61.2 to -45.8 ∘C16–216 ppmv62/158 min2/218 minSum-62.6 to -42.4∘ C16–360 ppmv217/537 min19/735 min
To extend the performance assessment of the MCH from the formation flight
intercomparison , the sensor was operated aboard a Learjet 35A
twin-engine business-jet aircraft as part of the
CIRRUS-III field study, which was coordinated by Forschungszentrum Jülich.
The overarching goals of CIRRUS-III were to understand the formation
mechanism of cirrus clouds in different background conditions, their
radiative effects and the microphysical properties of the cirrus cloud
particles. In total six flights were conducted in the period between 23 and
29 November 2006 at mid-latitudes (45–70∘ N; see Fig. )
and at flight altitudes between 7 and 12 km. These flights in the UT–LS were
launched from the Hohn Air Base in northern Germany with the Learjet 35A
operated by enviscope GmbH.
For the sensor intercomparison studies CIRRUS-III provided 4 flights (see
Table ). The data set consists of ca. 13 flight hours in air
masses colder than -40 ∘C at cruise altitude, ca. 4 flight
hours in cirrus clouds and 9 flight hours out of clouds. Furthermore,
stratospherically influenced air masses were sampled for 19 min with
ozone VMR above 125 ppmv, measured by the dual-beam UV-absorption ozone
photometer JOE (Jülich Ozone Experiment) .
Two flights had to be discarded due to inlet heating problems at the
reference instrument FISH (Fast in situ Stratospheric Hygrometer). An overview of the individual flights is provided
in Table .
Instruments and parameters used during the CIRRUS-III field campaign.
FISH: Fast in situ Stratospheric Hygrometer; OJSTER: Open-path
Jülich Stratospheric TDL ExpeRiment; MCH: MOZAIC Capacitive Hygrometer; LT:
lower troposphere; UT: upper troposphere; LS: lower stratosphere; for further
information see .
Time series of water vapour volume mixing ratio (VMR) from MCH
(red), FISH (blue) and OJSTER (grey) during the CIRRUS-III flight on
28 November 2006. Ice saturation is shown in cyan, while pressure (black) and
ambient air temperature (green) are plotted with dashed lines.
Instrumentation
During the CIRRUS-III field campaign, high-precision research-grade
instruments were operated aboard the aircraft to characterise the air masses
probed during flight patterns in frontal cirrus clouds. An important part of
the instrumentation was dedicated to the measurement of gas-phase and total
water. The instrumentation included a MCH and an open-path tunable diode
laser (TDL) system Open-path Jülich Stratospheric TDL ExpeRiment,
OJSTER; MayComm Instruments, to measure gas-phase
water vapour VMR. Simultaneously, total
water VMR (i.e. gas-phase plus ice water) was measured by the reference
measurement instrument FISH . The closed-cell
Lyman-α fluorescence hygrometer was equipped with a forward-facing
inlet to sample gas-phase water in clear sky and total water inside cirrus
clouds. To determine whether a data point is in a cirrus cloud or not, the
ratio of RHice from FISH (total water) and OJSTER (water vapour)
was used see. FISH was calibrated using a laboratory
calibration facility with the capability to simulate realistic atmospheric
conditions, i.e. water vapour VMR from several hundred to a few ppmv and
pressure from 1000 to 10 hPa. During the calibration, the water vapour
mixing ratio was determined using a commercial dew point hygrometer (MBW
DP30). The instruments and the parameters derived from their measurements are
listed in Table .
Prior to the CIRRUS-III campaign the MCH were (pre-flight) calibrated in
the simulation chamber at Forschungszentrum Jülich following the procedures
briefly described in Sect. 2. Unfortunately a post-flight calibration was not
possible due to sensor failure after deinstallation of the MCH from the
Learjet aircraft at the end of the campaign. From long-term experiences of
MOZAIC pre- and post-flight calibrations it is well known that over the
3-month period between the pre-flight calibration and the end of the campaign
the offset a(T) can change significantly by about 5 % RH while the slope
b(T) changes by less than 2 % on the relative scale see
Eq. and. In order to determine the potential change
of the offset a(T) between pre-flight calibration and the end of the
campaign, the so-called in-flight calibration method was
applied.
From top to bottom: VMR measured by the MCH (red) and the reference (blue),
i.e. FISH (clear sky) and OJSTER (in-cirrus); RHliquid and Δ
RHliquid (MCH and reference), as a function of flight time during
flight 2 on 28 November 2006; and sensor temperature Tsensor (black) as
well as ambient temperature Tambient (green). The blue-shaded area represents air masses with
high humidity and possible cirrus cloud. Air masses with sensor temperatures
at and below the calibration limit are shaded in red. The grey-shaded
sequence illustrates the effect of increasing response time as
sensor temperatures decrease.
Thus, the sensor offset a(T) at lowest relative humidity was
determined from the measurements themselves as obtained during periods of
the aircraft flying in the lower stratosphere, where the water vapour
mixing ratio reached well-defined minimum values. In our case, the minimum
value in stratospherically influenced air masses was about 20 ± 1 ppmv
as measured by the FISH instrument. Its resulting contribution to the
RHliquid signal of the MCH is minimal. Compared to the pre-flight
calibration an offset change of (4.5 ± 1) % RHliquid was
found. The RHliquid flight data of the MCH obtained during the
CIRRUS-III campaign were corrected for this offset drift. The resulting
overall uncertainty of the RH measurements by the MCH, including
contributions from temperature uncertainties, is about ±5 %
RHliquid, which is in good agreement with the mean uncertainty range
obtained from long-term MOZAIC measurements .
Frequency of occurrence for observations of Tsensor during
ca. 15 years of MOZAIC (top panel) and CIRRUS-III (bottom panel).
Results – assessment of MCH performance
The instrumentation deployed in CIRRUS-III allows an in-flight
intercomparison of all water vapour instruments. Figure
illustrates an example of the kind of data collected from one research flight
on 28 November 2006 (Flight 2). Data from the water vapour sensing
instruments used for the intercomparison are shown as VMR. The
Tambient encountered during the flight ranged from
-44.1 to -62.4 ∘C for relevant measurement altitudes. The respective
water vapour VMR covered the range from 17 ppmv at the tropopause to ca.
150 ppmv in the free troposphere and even higher values during ascent from
and descent into the airport.
For the instrument intercomparison we analysed the sensors with respect to
RHliquid since this is the parameter the MCH is calibrated against
in the sensor temperature range (see Sect. ). Further, data for
water vapour VMR > 1000 ppmv were excluded from this study because the
FISH instrument becomes optically opaque and thus insensitive to changes in
VMR .
In Fig. , we compare VMR data and RHliquid data from the MCH
and gas-phase reference, i.e. OJSTER data in cloud or otherwise FISH data, for a
complete validation of the MCH for flight 2. Largest deviations of the MCH to
the reference are found in clear-sky air masses for cold conditions with
Tsensor<-40 ∘C (this corresponds to
ambient temperature below ca. -60 ∘C at M= 0.70) and at
transition sequences around the cirrus cloud. Except for these extreme
conditions, the difference between the MCH and the reference is of the order
of 10 % RHliquid or less. Given the fact that during CIRRUS-III
the MCH was operated at its lower limit of performance, the agreement with
the research-grade reference instruments is remarkably good.
Differences in relative humidity RHliquid of MCH and both
reference instruments, i.e. FISH (top panel, clear sky) and OJSTER (bottom
panel, in-cirrus), are scattered against the sensor temperature
Tsensor. A drift towards too-dry MCH measurements below the
calibration limit of -40 ∘C is clearly seen. The median values (red
lines in the box) of the 1 ∘C binned data as well as the 25th and
75th percentiles are within the calibration limits.
Comparison cross plot between reference, i.e. FISH (blue dots, clear
sky) and OJSTER (red dots, in-cirrus), and MCH RHliquid (left
panel) and RHice (right panel) each displayed as a scatter plot with
robust fitting curve (dashed line).
An analysis of MCH performance at the limit of its operation range is
provided in the example of flight 2 in Sect. .
MCH performance against reference instruments
In order to prepare a data set for evaluation of the MCH performance, we
introduced three filter operations to reduce the CIRRUS-III data set to MOZAIC
typically operational conditions. First, it has to be noted that regular
operation conditions of the MCH aboard long-haul passenger aircraft with a
cruising speed of ca. M= 0.81 are characterised by Tsensor≥-35 ∘C (see
Fig. a), which is within the lower MCH calibration limit of
-40 ∘C (see Sect. ). However, during the operation
aboard the slower-flying Learjet 35A (cruising speed <M= 0.70),
Tsensor values significantly lower than
-40 ∘C were reached (see Fig. b).
Consequently, data with Tsensor<-40 ∘C were excluded from the analysis.
This fact is illustrated in Fig. , which shows the difference in
RHliquid between MCH and FISH data for clear-sky conditions and
OJSTER data for in-cirrus conditions, according to
Tsensor. Furthermore, the maximum
Tambient was set to the level of instantaneous freezing of
-40 ∘C in order to minimise the perturbation of measurements by
erroneously sampled liquid water droplets in warm clouds.
Finally, flight sequences of the Learjet 35A with steep ascents and descents
were excluded, since these flight conditions are not comparable to conditions
aboard long-haul passenger aircraft. To obtain information about the MCH
performance relevant for the MOZAIC data set, i.e. for nearly constant flight
levels with moderately slow changes in temperature and humidity, the flight
altitude for CIRRUS-III was smoothed over 90 s time intervals, and when
altitude changes exceeded Δz> 6 m in 5 s the respective data
points were excluded from the intercomparison. These filtering operations
lead to a data set with MOZAIC typically operational conditions with a
remaining fraction of about 36 % of campaign data (see Table
for more details).
The correlation between MCH and reference RHliquid data and
RHice data from FISH (clear sky) and OJSTER (in-cirrus) is shown in
Fig. . The scatter plots for the 1 Hz data sets reduced to
MOZAIC-relevant conditions (hereafter referred to as “reduced data set”)
show similar scattering around the line of unity. Linear regression analysis
confirms this with similar results for both cases: a correlation coefficient
of R2= 0.92 with a slope of virtually unity. The offset for the
RHliquid regression is 0.18 ± 0.09 % RHliquid,
and for the RHice regression it is 0.36 ± 0.15 %
RHice.
Correlation of RHliquid data from MCH and the reference,
i.e. FISH (clear sky) and OJSTER (in-cirrus), during CIRRUS-III; the straight
line indicates the linear regression line, while the dashed lines illustrate
the sensor uncertainty range ±5 % RHliquid. In the transition
area both reference instruments can occur (see Fig. ). The top
panel shows the number of data points per 5 % RHliquid bin.
A more statistically based view on the data set is shown in Fig. ,
where the correlation between the sensors averaged for 5 %
RHliquid bins is shown. The MCH agrees very well with the reference
instruments over the entire range of values measured in the cloud-free
atmosphere. Inside cirrus clouds, i.e. RHliquid> ca.
70 %, the sensors deviate as expected as a result of the increased response
time of the MCH. Small-scale supersaturations are smoothed out, while OJSTER
can detect these with response time of ca. 1 s. Linear regression
analysis weighted with the number of occurrences provides a correlation
coefficient of R2= 0.99 with an offset of -0.15 ± 1.29 %
RHliquid and a slope of 1.02 ± 0.03. Median values and almost
all of the 25th and 75th percentiles fall within the ±5 %
RHliquid range around the linear regression line, which confirms
the previously determined MCH uncertainty of 5 % RHliquid
(see also Table ).
Fraction of remaining data after filtering the data set of MOZAIC
atypically operational conditions.
For a better understanding of an uncertainty of 5 % RHliquid,
Fig. shows water vapour VMR as a function of temperature for
5 % and 10 % RHliquid for pressure levels at typical passenger
aircraft flight altitudes. As an example, at Tambient= 215 K
and pressure = 220 hPa, a measured RHliquid= 5 % with an
uncertainty of 5 % RHliquid corresponds to a VMR of ca.
5 ± 5 ppmv.
Median, 25th/75th percentile values and counts of
ΔRHliquid (MCH and reference). Data were classified into
5 % RHliquid bins relating to the reference, i.e. OJSTER data in
cloud or otherwise FISH data.
The consistency of the MCH RHliquid data is shown in
Fig. . The probability distribution functions (PDFs) for
RHliquid derived from MCH data agree very well with those derived
from the reference for the entire data set. Larger deviations at higher
values of RHliquid, e.g. at possible cirrus cloud edges, reflect the
fact of the longer response time of the MCH. The sensor behaviour for those
conditions at the limit of the sensor operation specifications is analysed in
detail in the following section.
Limits of MCH operation
The comparison between the MCH RHliquid data and the reference
RHliquid data, i.e. OJSTER data in cloud or otherwise FISH data,
during the CIRRUS-III field study shows a remarkably good agreement for the
reduced data set. However, the performance of the MCH sensor in conditions at
its limits of operation, e.g. close to the lower calibration limit of
Tsensor=-40 ∘C or during strong humidity changes,
has to be analysed in detail in order to assess the sensor's operation range.
For this purpose, the time series of flight 2 is revisited in
Fig. , where the individual RHliquid time series, the
difference of both RHliquid time series and the
Tambient as well as the Tsensor time series are applied.
Water vapour volume mixing ratio (VMR) as a function of ambient
temperature for 5 % (solid lines) and 10 % RHliquid (dashed
lines), respectively. The different pressure levels represent typical
passenger aircraft flight altitudes. The inner box shows a zoom of the lower
temperature and VMR values.
Number of data points (top panel) and frequency of occurrence
(bottom panel) for observations of RHliquid during CIRRUS-III; blue
and red lines refer to data from reference, i.e. FISH (clear sky) and OJSTER
(in-cirrus), and MCH, respectively. The number of counts of both data sets
agrees in almost all 5 % RHliquid bins. The exponential decline at
higher values is in accordance with the result of . A
bimodal distribution can be seen clearly in the probability density function
(PDF) view of the data sets, where there is a clear-sky section at lower
values and a cirrus section at higher values. The differences
in the PDF can be mainly explained by the longer response time
of the MCH into and out of the clouds.
Probability density functions (PDFs) of the complete
(a–c) and reduced (d–f) MCH
(a, d) and reference (b, e), i.e. FISH
(clear sky) and OJSTER (in-cirrus), water vapour volume mixing ratio (VMR)
data related to the ambient temperature Tambient.
Water vapour volume mixing ratio is binned in the logarithmic space between 0
and 8.8, with a bin size of 0.8 and temperature in 1 ∘C bins.
(c) and (f) show the difference of the MCH and reference
PDFs for the complete and reduced data set, respectively.
The following three sequences of interest have to be analysed:
Sequence 1 lasts from 08:40 to 09:10 UTC, when the MCH still shows a good
response at higher sensor temperatures of about -20 ∘C and agrees
within 5–10 % RHliquid with the reference. However, at
decreasing sensor temperature, the response time of the MCH increases
significantly. This results in a delay causing higher humidity values and
greater differences in the comparison with the reference. Because of the
dominating van der Waals forces the adsorption of new water molecules by the
dielectric membrane of the sensor occurs faster than desorption. For that,
the response to positive humidity gradient is faster than to negative
gradient, which can be seen in the behaviour of the RHliquid
differences in time.
Sequence 2 illustrates a strong humidity change between 09:40 and
10:00 UTC, while flying through a cirrus cloud. Because of slower MCH sensor response at
colder sensor temperatures, the MCH RHliquid values can not follow
the rapid changes in RHliquid as observed by the reference.
Sequence 3 refers to a section of the flight between 11:00 and
11:40 UTC, when Tsensor reaches values below the sensor calibration limit of
Tsensor=-40 ∘C, i.e. ambient temperatures below
-70 ∘C at commercial aircraft speed of M= 0.81.
The MCH shows an increased response time and a loss of signal fine structure, and
increasing deviations between the MCH and the reference instruments occur.
Despite delayed sensor response for conditions at the limit of its operation
range, the MCH shows a very good overall performance during the CIRRUS-III
field study. Figure illustrates the PDFs of water vapour VMR data
as a function of Tambient (panels a to c for the complete data set
and panels d to f for the reduced data set) according to
. The frequencies of occurrence are calculated in
1 ∘C bins for the MCH data set (panels a and d), the reference data
set (panels b and e) and the deviation of the MCH and reference PDFs for the
complete and reduced data set (panel c and f). The water vapour
VMR is binned in the logarithmic space between 0.0 and 8.0 with a bin size of
0.8. The colour bars are binned in 5 % spaces for a better interpretation
of the contour plots.
The MCH seems to remain at dryer values for the coldest temperatures of
Tambient≅-60∘C, which is again a result of the
delayed sensor response at sensor temperatures below the calibration limit.
Further, small deviations at lower temperatures are also observed. In
summary, data sets for both cases show a similar behaviour in the water vapour VMR
distribution with only small deviations; these deviations have no
statistically significant relevance.
Conclusions and recommendations
The CIRRUS-III (2006) aircraft campaign provided a data set for the
evaluation of the MOZAIC Capacitive Hygrometer in a blind
intercomparison with high-performance water vapour instruments based on
tunable diode laser absorption spectrometry (OJSTER, in-cloud reference) and
Lyman-α fluorescence detection (FISH, clear-sky reference).
Except for conditions at its operation limit (e.g.
Tsensor<-40 ∘C and during rapid changes in
RHliquid), the MCH performs with a difference of 10 %
RHliquid or less to the references.
In order to obtain a representative result for the MCH's uncertainty for its
regular deployment aboard passenger aircraft, the data set was restricted to
conditions corresponding to regular sensor operation aboard MOZAIC aircraft:
data with sensor temperatures below -40 ∘C were excluded due to the
calibration limit. In MOZAIC less than 1 % of RH observations are made at
sensor temperatures colder than -40 ∘C. Strong ascent and descent
sequences of the aircraft were removed, and the maximum
Tambient was set to -40 ∘C to exclude effects of warm
clouds.
The 1 Hz correlation yielded a robust linear fit with a slope of unity, with
no statistically significant offset and a correlation coefficient of
R2= 0.92, which was confirmed by the correlation of the binned
RHliquid data. The RHliquid data grouped in 5 %
RHliquid bins agree very well for the MCH and reference instruments
over the entire cloud-free range and for most of the cirrus cloud
sequences, and they yield a MCH uncertainty of 5 % RHliquid.
Comparing the MCH's and references' PDFs
for RHliquid shows no statistically significant effect of delayed
sensor response at conditions beyond the operation range. Neither strong
humidity changes nor operation at the lower calibration limits causes
considerable sensor failures. The main limitation for the use of MCH
RHliquid data is related to sensor temperatures below the
calibration limit of Tsensor=-40 ∘C. However, these
temperatures are encountered only rarely in the MOZAIC programme as long as
the flight routes do not reach polar air masses with ambient temperatures
below -70 ∘C. In summary, the MCH is highly suitable for
climatology analyses in the MOZAIC programme even if the sensor is not
applicable to high-time-resolution measurements.
A value for the limit of detection is not appropriate for the MCH, but the
variable to describe its performance is the here determined uncertainty of
the RHliquid measurements. RHliquid measurements below
5 %, which are common in the lowermost stratosphere, have to be used
carefully because these data are close to the sensor uncertainty range,
which, as shown before in Sect. , results in a relative
deviation of 100 %.
Acknowledgements
The authors gratefully acknowledge Peter Spichtinger (Mainz Univ.) for
fruitful discussions. The support by enviscope GmbH to the technical
organisation of the field study is also appreciated. Part of this work was
funded by the German Federal Ministry for Research and Education (BMBF) in
the framework of the joint programme IAGOS-D under grant no.
01LK1223A. The service charges for this
open-access publication have been covered by a Research Centre
of the Helmholtz Association.
Edited by: S. Malinowski
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