Measurements of the spectrum of the atmospheric emission in the far-infrared
(FIR) range, between 100 and 667 cm-1 (100–15 µm) are
scarce because of the detection complexity and of the strong absorption of air
at ground level preventing the sounding of the FIR from low
altitude. Consequently, FIR measurements need to be made from high-altitude
sites or on board airborne platforms or satellites. This paper describes the
dataset of FIR spectral radiances of the atmosphere and snow surface emission measured in the 100–1000 cm-1 range by the Far-Infrared Radiation
Mobile Observation System (FIRMOS) instrument during a 2-month campaign
carried out from the ground at about 3000 m of altitude on the top of
Mt. Zugspitze in the German Alps in 2018–2019. This campaign is part of
the preparatory activity of a new space FIR mission, named
Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM), which is
under development by the European Space Agency (ESA). The dataset acquired
during the campaign also includes all the additional measurements needed to
provide a full characterisation of the observed atmospheric state and the
local atmospheric and surface conditions. It includes co-located spectral
measurements in the infrared range from 400 to 1800 cm-1; lidar
backscatter profiles; radio soundings of temperature, humidity and aerosol
backscatter profiles; local weather parameters; and snow/ice microphysical
properties. These measurements provide a unique dataset that can be used to
perform radiative closure experiments to improve modelling parameters in the
FIR that are not well-characterised, such as water vapour spectroscopy,
scattering properties of cirrus clouds, and the FIR emissivity of the surface
covered by snow. The consolidated dataset is freely available via the ESA
campaign dataset website at 10.5270/ESA-38034ee.
Introduction
The far-infrared (FIR), defined here as the longest wavelength region of the
infrared spectrum covering the wavenumber range from 667 cm-1 (or
equivalently 15 µm wavelength) down to 100 cm-1
(100 µm), contains more than 50 % of the energy emitted by
the Earth toward space. This spectral region is modulated by the properties of
the most relevant components of the climate system, such as water vapour,
carbon dioxide, clouds, and snow surface emissivity . The
characterisation of the FIR radiative properties of these components is
therefore essential to improve gaseous modelling of spectroscopy
, cirrus cloud radiative properties , and
surface emissivity , which in turn will allow for a better
understanding of the Earth radiation budget and reduce uncertainties in
climate models . Nevertheless, due to
technical challenges, systematic and global measurements of the FIR from space
are still missing and are scarce from ground and airborne platforms.
To fill this observational gap, a space mission, named
Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM)
, is under development by the European Space Agency (ESA)
as the ninth Earth Explorer mission to be launched in 2026
(https://www.forum-ee9.eu/, last access: 6 May 2021). This mission will measure with high accuracy the spectrum of the
outgoing infrared radiation from 100 to 1600 cm-1
(100–6.25 µm) covering, for the first time with high spectral
resolution, the FIR portion of the spectrum. In preparation for this mission,
an instrument demonstrator, named Far-Infrared Radiation Mobile Observation
System (FIRMOS), was developed for field applications from ground-based (and
in perspective airborne) platforms to verify with real measurements the
sounding capability provided by FIR observations.
At ground level the atmosphere in the FIR spectral region is very opaque
because of the high concentration of water vapour. For this reason, sounding
of the FIR with ground-based observations is possible only from high altitude
sites where the water vapour content is sufficiently low to make the
atmosphere transparent at frequencies below 667 cm-1. This requires
the deployment of FIR instrumentation on high mountain sites, around
3000 ma.m.s.l. or at higher altitude
. Observations during the
wintertime are preferred in order to have the water vapour content at its
seasonal minimum, thus improving the capability to sound the lowest
wavenumbers, with potential semi-transparency down to 250 cm-1. The
sounding of the remainder of the FIR, below 250 cm-1, is only
possible from airborne platforms or high-altitude aircraft and
balloons flying in the upper troposphere
or the lower stratosphere.
In this paper, we describe the measurements acquired during the first field
deployment of FIRMOS between the end of 2018 and the beginning of 2019 at the
Alpine observatory of Mt. Zugspitze in the south of Germany at about 3000 m
altitude, together with all the ancillary measurements needed to provide a
complete characterisation of the observed atmospheric and surface states.
Zugspitze Observatory
The Zugspitze site was chosen because of the very dry atmospheric conditions
frequently encountered at the Summit Observatory, with values of the
integrated water vapour (IWV) as low as 0.1 mm and median of
2.3 mmsee more details in. The minimum IWV is
approximately a factor of 40 lower than at typical lowland mid-latitude
sites. In this respect the Zugspitze is comparable to the driest sites in the
world, i.e. remote locations like the Atacama Desert or polar stations. In
spite of that, the Zugspitze Observatory is reachable via cable car within
10 min from a car park and offered a suitable infrastructure for the
deployment of FIRMOS due to the presence of a complementary and comprehensive
set of instruments. This provided independent information of the observed
atmospheric and cloud states to be used as reference for the comparison with
FIRMOS. The Zugspitze Observatory comprises two stations: the Summit station
(47.421∘ N, 10.986∘ E; 2962 ma.m.s.l.) and the
Environmental Research Station Schneefernerhaus (47.417∘ N,
10.980∘ E; 2675 ma.m.s.l.), which is located on the south
slope of the mountain, 300 m below and 680 m southwest of the
Summit (see Fig. ).
Summit and Schneefernerhaus stations at Mt. Zugspitze, Germany.
Instrument description and performed measurements
During the campaign, measurements were carried out using instruments deployed
at the Zugspitze Summit, at the Schneefernerhaus station, and at the Karlsruhe
Institute of Technology (KIT) in Garmisch-Partenkirchen.
The emission spectrum was measured in the FIR with FIRMOS, which was developed
at CNR-INO with the support of ESA and the Italian Space Agency (ASI) and
installed in November 2018 on the terrace of the Summit station. The
instrument is a Fourier transform spectrometer (FTS) designed on the base of
a similar instrument, named Radiation Explorer in the Far
InfraRed – Prototype for Applications and Development (REFIR-PAD)
, permanently installed in Antarctica in 2012
. The FTS consists of an interferometric layout with
double-input and double-output ports in a Mach–Zehnder configuration, which
increases the measurement reliability and the calibration precision
. The instrument is able to cover the FIR radiation using
wideband germanium-coated biaxially oriented polyethylene terephthalate
beam splitters and room-temperature pyroelectric detectors. It was mounted
within a plastic box to protect the optics and electronics from environmental
conditions, such as wind and snowfall, when installed on the site. The full
field of view (FOV) of FIRMOS is 22 mrad.
At the Summit station, KIT operates an Extended-range Atmospheric Emitted
Radiance Interferometer (E-AERI) system , manufactured by
ABB Bomem Inc. (Quebec, Canada), which covers part of the far, mid- and near-infrared spectral regions. Like FIRMOS, E-AERI is zenith looking, its FOV is
46 mrad.
The spectrum of the downwelling longwave radiation (DLR) was therefore
measured in the FIR and mid-infrared spectral ranges from 100 to
1800 cm-1 (100–5.56 µm) using both FIRMOS, covering the
100–1000 cm-1 spectral range, and the first channel of E-AERI,
covering the 400–1800 cm-1 range. Spectra are provided without
apodisation, with a sampling resolution corresponding to the sinc response
function of Δσ=0.3cm-1 for FIRMOS (maximum optical
path difference (MOPD) = 1.66667 cm) and Δσ=0.48215cm-1 for E-AERI (MOPD = 1.03703 cm). The
instrument line shape (ILS) of FIRMOS is a linear combination of sinc and
sinc2 (as in the case shown in ) with a full width
at half maximum (FWHM) of about 0.36 cm-1, whereas for E-AERI the
ILS is an “ideal” sinc with FWHM = 0.58 cm-1. More information about ILSs is provided with the
dataset.
Figure shows FIRMOS' installation close to the E-AERI
instrument, which is permanently installed on the top of the shelter,
4 m above FIRMOS, and routinely performs measurements of spectral DLR
when the weather conditions allow operation. A comparison between the average
of 310 measurements of the spectral DLR in clear sky conditions performed
simultaneously with FIRMOS and E-AERI is also shown in the figure.
FIRMOS and E-AERI installed at the Summit station (a). The comparison
between the average spectra of 310 simultaneous observations acquired by the
two instruments in clear sky during 2019 campaign is shown in (b).
KIT also operates two lidar systems for aerosols and clouds ,
as well as water vapour and temperature profiling, installed at
the Schneefernerhaus station. The advantage of mounting the lidar systems at
Schneefernerhaus and the spectrometer at the Summit is that the onset of the
measured lidar profiles is typically about 300 m above the laser,
which coincides with the location of the spectrometers at the Summit. Cirrus
cloud properties are detected with the stratospheric aerosol lidar, which is a
pure backscatter lidar operating at the wavelength of 532 nm. This
system was operated in a semi-automatic mode, with a sequence of one profile
every 4 or 10 min and an integration time of 1 min. Water
vapour and temperature profiles were measured alternately with a Raman lidar
system using a XeCl laser at 308 nm for water vapour and with a Nd:YAG
laser at 355 nm for temperature. The latter measurements were carried
out manually in order to switch between one laser and the other. Water vapour
profiles were recorded with 1 million laser shots, an integration time of
1 h, and a vertical resolution of 30 m near ground
(4 kma.m.s.l.) and 250 m at
15 kma.m.s.l. Temperature profiles were derived from density
profiles measured only in absolute clear sky conditions with an integration
time of typically 1 h and a vertical resolution of 30 m near
ground (4 kma.m.s.l.) and 2 km at
80 kma.m.s.l. The lidar in operation at night and an example of
measurements are shown in Fig. .
Lidar measurements at the Schneefernerhaus station. The plot shows an
example of the range-corrected signal (RCS) of the backscatter lidar, detecting
a cloud layer, and the water vapour profile of the Raman lidar measured on 6 February 2019, 19:21 UTC.
Radio soundings from Garmisch-Partenkirchen of temperature, relative
humidity (RH), and colour index profiles for the launch on 6 February 2019,
17:33 UTC. A single cloud layer during the ascending leg (blue curve) and a
double layer during descending (red curve) are shown in the colour index plot.
Specific balloon radio soundings for in situ water vapour
profiles and cirrus cloud microphysics were performed in 2 d (5–6 February 2019) under clear sky and thin cirrus cloud conditions during
nighttime (Fig. ). The antenna for communication with the balloon
was built up on the Summit station, and the preparation of the sonde payload
and launch took place at KIT Campus Alpin, in Garmisch-Partenkirchen
(47.476∘ N, 11.062∘ E; 730 ma.m.s.l.) about
8 km northeast of Mt. Zugspitze. The balloon sondes are equipped with
a standard radiosonde (RS; Vaisala RS41-SGP), a Cryogenic Frostpoint Hygrometer (CFH) for measuring water vapour profiles with high accuracy, an ozone sonde,
and the Compact Optical Backscatter Aerosol Detector (COBALD)
, developed at the Swiss Federal Institute of Technology in
Zurich, that provides backscatter profiles at two wavelengths (940 and
455 nm). The colour index (CI) within the cloud layers, defined as the
aerosol backscatter ratio (ABSR=backscatter ratio-1) in
the red channel (940 nm) divided by ABSR in the blue channel
(455 nm), is also provided. The combined balloon payload is well
tested and also regularly used by the Global Climate Observing System (GCOS)
Reference Upper-Air Network (GRUAN) see, for example,. The CFH
has an uncertainty of about 2 %–3 % in the troposphere and
less than 10 % in the lower stratosphere. The CFH is especially
suitable for measuring water vapour under the dry conditions at the tropopause
and in the stratosphere up to altitudes of 28 km. The accuracy of the
backscatter ratio after post-processing the COBALD raw data is better than
5 %, while the precision is smaller than 1 % in the upper
troposphere/lower stratosphere .
The Summit station is served with standard pressure, temperature, relative
humidity (RH), and wind (PTHW) meteorological observations, provided by Deutscher
Wetterdienst (DWD) at the website
https://opendata.dwd.de/climate_environment/CDC/observations_germany/climate/, last access: 28 February 2019, which for completeness were also included in the
available dataset for the period covered by the field campaign. Useful
standard atmospheric soundings with operational radiosondes available near
Zugspitze can be freely downloaded via the University of Wyoming website
(http://weather.uwyo.edu/upperair/sounding.html, last access: 26 October 2020) for the station of Muenchen-Oberschleissheim (Station
No. 10868, daily soundings at 00:00 and 12:00 UTC), 100 km
north of the Summit, and the Innsbruck-Flughafen station (Station No. 11120,
daily soundings at 03:00 UTC), 33 km southeast.
Finally, the properties of snow samples were characterised in terms of snow
grain type, density (kgm-3), and specific surface area (SSA,
m2kg-1), using the DUal Frequency Integrating Sphere for Snow SSA
measurement (DUFISSS) sensor, which retrieves the SSA by measuring the
reflectance at 1310 nm.
Field campaign
The aim of the Zugspitze field campaign was to collect FIR measurements of the
DLR to be used to provide evidence of the FIR capability to retrieve vertical
profiles of water vapour, temperature, and cirrus cloud properties, as well as
to perform a side-by-side validation of the FIRMOS measurements in the
spectral range in common with the E-AERI instrument permanently installed on
the site.
The campaign took place from 29 November to 18 December 2018 and from
21 January to 20 February 2019. Measurements were performed when the weather
conditions allowed operations; this occurred for a total of 33 d. The
main specifications of the measurements, which are provided in the dataset,
are summarised in Table .
Instrumentation deployed at Zugspitze and measurements performed
during the FIRMOS campaign and provided in the available dataset.
InstrumentLocationType of measurementIntegration/Date/no. of measurementsrepetition timeFIRMOSZugspitze SummitstationDLR spectrum FOV = 22 mrad100–1000 cm-1128/256 s29 November 2018 to 18 December 2018 1197 spectraΔσ=0.4cm-1Δσ=0.3cm-1210/420 s21 January 2019 to 15 February 2019 838 spectraFIRMOSZugspitze SummitstationIce/snow spectrumFOV = 22 mrad100–1000 cm-1Δσ=0.3cm-1210/420 s16 February 2019 to 20 February 2019 152 snow + 283 sky spectraE-AERIZugspitze SummitstationDLR spectrum FOV = 46 mrad400–1800 cm-1Δσ=0.48215cm-1214/440 s28 November 2018 to 20 December 20181479 spectra11 January to 20 February 2019 3509 spectraMeteorological station from DWDZugspitze SummitstationLocal pressure, temperature, relative humidity, and windEvery 10 minContinuousBackscatter lidarSchneefernerhaus stationExtinction profile1/4–10 min16 November 2018 to 19 February 2019Raman lidarSchneefernerhaus stationH2O and T profiles1 h16 November 2018 to 19 February 2019Dedicated RSIMK-IFU, Garmisch-PartenkirchenPressure, temperature, relative humidity, O3, cloud extinction, and colour index profiles–5 February 2019 to 6 February 2019 Four launches under clear sky and thin cirrus cloud conditions during nighttimeDUFISSSZugspitze SummitstationSnow specific surface area–18 February 2019 to 19 February 2019 Two measurements every FIRMOS acquisitions
During the first part of the campaign in November–December 2018, FIRMOS was
installed and made operational. The weather conditions during that period were
not very good, and only a few days of clear sky to test acquisitions occurred
(only 43 useful sky observations). This first part is considered an
engineering campaign for the deployment and preparation of the full suite of
instruments. For completeness, these measurements were included in the
dataset, but their usefulness is quite limited.
The second part of the campaign in January–February 2019 was dedicated to the
acquisition of the DLR spectra, both in clear sky and in cirrus cloud
conditions, and of all the ancillary information provided by lidars,
radio soundings, and meteorological stations. Lidar measurements can be used to retrieve
cirrus properties such as cloud geometry (cloud bottom and top heights),
extinction profile, and optical depths. These properties, together with
vertical soundings of humidity and temperature, provide a full
characterisation of the atmospheric state that can be used to check and to
refine radiative models of water vapour spectroscopy and cirrus ice-particle
properties in the FIR and to explore the effect on the retrieval performance
of cirrus microphysics, as shown in . An example of
spectral measurements in the presence of cirrus cloud and the associated lidar
profile is shown in Fig. .
FIRMOS and E-AERI spectra simultaneously acquired in presence of a
cirrus cloud on 6 February 2019 and the corresponding lidar RCS profile.
Snow measurements at Zugspitze Summit. FIRMOS in a slant angle during
snow emissivity measurement (top left) and the average spectrum with
uncertainties of 11 spectra acquired measuring the 18-2136 sample (top right). DUFISSS
snow specific surface area (SSA) measurement (bottom left) and density and SSA
properties of the snow samples (bottom right, red labels indicate day and time of FIRMOS acquisition). The upper-left grey box corresponds to an ice sample whose
parameters were only roughly estimated.
Four days at the end of the campaign were finally devoted to measure snow and
ice samples with the objective to characterise the FIR spectral emissivity,
which is not well-characterised in the FIR range . We
performed systematic snow measurements on 18 and 19 February 2019; FIRMOS was
adapted to observe in a slant direction close to nadir (Fig. ,
top left). In this way the main contributions to the measured radiance were by
the surface emission and by the sky reflection. Each sample was measured by
FIRMOS for roughly 90 min and was successively characterised in terms of
its surface properties. An example of FIRMOS measurement with error estimates
is shown in Fig. (top right). To attain diversity in the snow
properties, eight samples of snow and one sample of ice were collected in the
vicinity of the Zugspitze Summit and close to Zugspitzplatt stations and were
characterised with DUFISSS (Fig. , bottom left). One measurement
with artificial patterns of surface roughness was also
conducted. Figure (bottom right) shows the density and SSA
values retrieved on the measured samples. For the ice sample, the figure shows
only a rough estimate of the parameters (grey box) since a precise
measurement could not be performed with the available equipment.
FIRMOS validation
The FIRMOS dataset includes calibrated spectra and noise estimates, calculated
starting from the FTS-acquired interferograms and following the procedure
described in . Each spectrum is the average of four sky
observations, and it is calibrated with four calibration measurements (two looking
at a hot back body reference source and two looking at a cold one). The noise
is characterised by a spectrally uncorrelated component (independent from one
spectral channel to the other), called noise-equivalent spectral radiance
(NESR) due to the detector error, and a spectrally correlated component,
called calibration error (CalErr) mainly due to the temperature uncertainty
of the calibration sources. The NESR is obtained from the error propagation
through the calibration procedure of the 1σ detector random error,
whereas the CalErr is calculated through the error propagation of the
temperature accuracy error measured on each reference blackbody
. These noise estimates show highly resolved structures
which are due to the absorption lines of gases inside the interferometric
path. They depend on the actual working conditions of the instrument that can
vary from measurement to measurement. An example of the FIRMOS products is
shown in Fig. . In the figure, the NESR (red line) is also
compared with the standard deviation (STD) of four sky measurements (brown line)
measured with constant sky conditions. Compared to the NESR, the STD estimate
does not contain the noise coming from the calibration function measurements, but it
contains the effect of possible radiance variations coming from the observed
scene. Therefore the STD can be smaller than the NESR estimate when the
observed scene is constant.
Example of FIRMOS DLR spectrum and the associated noise estimates
acquired on 25 January 2019, 16:03 UTC.
FIRMOS spectral measurements were validated against E-AERI in the common
spectral region in order to qualify FIRMOS with a standard commercial
spectrometer. The noise and the calibration accuracy for E-AERI were calculated,
using the standard procedure provided by the manufacturer and outlined in
, from the variance of measurements observing constant
sources and taking the average over 25 cm-1 wavenumber bins across
the spectrum. To compare these features with the analogous features of
FIRMOS, the same 25 cm-1 average was also operated. The comparison
is shown in Fig. in brightness temperature error at
280 K. The FIRMOS noise equivalent delta temperature (NEDT) was
calculated from the STD using a similar approach as for the E-AERI noise
estimate. More information about the spectral and radiance calibration of the
two instruments is provided with the dataset.
FIRMOS (red lines) and E-AERI (blue lines) brightness temperature
error estimates at 280 K, averaged over 25 cm-1 wavenumber bins across the
spectrum.
A measurable error of the spectral frequency scale due to the uncertainty of
the metrology laser was found for both FIRMOS and E-AERI by fitting the
measurements with synthetic spectra. FIRMOS shows a frequency scale error of
about +50ppm between simulations and observations
(σreal=(1+5×10-5)×σfirmos),
whereas E-AERI has the same error but with different sign
(i.e. σreal=(1-5×10-5)×σe-aeri). The frequency scale of
spectra was therefore recalibrated with this correction before the comparison.
Furthermore, since the two instruments have different instrument line shapes
and measurements are provided on different sampling grids, for this comparison
spectra are equalised by applying the same apodisation function (Norton–Beer
strong function) with 0.968 cm-1 of FWHM
resolution and resampled to a common spectral grid of
0.5 cm-1. Figure shows the comparison between the
mean spectra of 420 simultaneous measurements selected with the temporal
distance between FIRMOS and E-AERI measurements being less than 10 min. A
good agreement between the two instruments is found in the common spectral
range from 400 to 1000 cm-1 with the difference (shown in the
bottom panel) that is within the total uncertainty calculated using both the
noise and the calibration accuracy (summed in quadrature) of both
instruments. The difference is larger in the strong water vapour and
CO2 bands because the slightly different location of the two
instruments. However, only in a few lines in the range of
400–450 cm-1 does the difference between spectra exceed the
uncertainty estimate; this is probably due to the different FOV of the two
instruments, which on average might observe a slightly different scene.
Comparison between FIRMOS- and E-AERI-averaged spectra of 420
simultaneous acquisitions for the 2019 campaign. Spectra were apodised with the
Norton–Beer strong function with FWHM = 0.968 cm-1 and resampled to a common spectral grid
of 0.5 cm-1. The bottom graph shows the difference between the average spectra
(in green) compared with the total measurement uncertainty (in black).
To make another assessment of FIRMOS measurements, an indirect approach was
also used by comparing values of IWV retrieved from FIRMOS spectral
measurements with the same parameter retrieved from E-AERI measurements, using
in both cases only the common 400–600 cm-1 spectral region. The
IWV was calculated using the KIT algorithm that minimises the FIRMOS (or
E-AERI) vs. LBLRTM (Line-By-Line Radiative Transfer Model) spectral residuals.
While details will be the subject of an upcoming publication, we briefly state
here that we found only a small overall bias
(FIRMOS-E-AERI) = 0.0002 mm which is negligible compared to the
measured atmospheric IWV states, ranging from 0.2 to 2 mm. This proved
that there are no indications of significant calibration errors between the
two instruments in the FIR spectral domain of the water vapour rotational
band.
Data availability
The full dataset of the 2-month campaign, including infrared
spectra (FIRMOS and E-AERI) and all the additional information (lidars, local
PTHW, dedicated RS, snow SSA), is available via the ESA campaign dataset
website
https://earth.esa.int/eogateway/campaigns/firmos (last access: 20 May 2021)
10.5270/ESA-38034ee. ESA requires a free
registration to inform users about issues concerning data quality and news on
reprocessing. Information about the data formats are reported in README files
within each data subdirectory.
Conclusions
In summary, the unique spectral measurements provided by the combination of
FIRMOS and E-AERI covering the relevant spectral region of the thermal emission
of the atmosphere from 100 to 1800 cm-1, together with the other supporting
measurements (lidars, radio soundings, atmospheric state, and surface properties),
provide a complete dataset that can be used to constrain radiative properties of
water vapour, cirrus ice particles, and snow/ice emissivity over almost all the
infrared emission, including the under-explored FIR spectral range.
Author contributions
LP designed the experiment and was chief scientist for the
field campaign. RS was responsible for the local deployment. LP, MB, GB,
FDA, AM, SV, RS, MR, HV, CR, DK, and BC ran the instruments during the campaign. LP,
CB, GDN, RS, MR, HV, CR, BC, SDB, MG, and FGW carried out data analysis and
measurement validation. DS and HO were responsible for the campaign
organisation. LP prepared the manuscript with contributions from all
co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
The authors acknowledge the reviews from Xianglei Huang and the anonymous referees who provided valuable comments to improve the paper.
Financial support
The study has been supported by the European Space Agency (ESA) and the FIRMOS project
(ESA–ESTEC contract no. 4000123691/18/NL/LF) for the development
and the deployment of FIRMOS, as well as the Italian Space Agency (ASI) and the
research projects SCIEF (Italian acronym of Development of the National
Competencies for the FORUM experiment – ASI contract no. 2016-010-U.0) for the
preliminary development of a few subsystems. The dedicated balloon activities
were also partly supported by funding from the Helmholtz Association in the
framework of MOSES (Modular Observation Solutions for Earth Systems).
Review statement
This paper was edited by Qingxiang Li and reviewed by Xianglei Huang and two anonymous referees.
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