ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-6701-2016Measurements of global distributions of polar mesospheric clouds during 2005–2012 by MIPAS/EnvisatGarcía-ComasMayahttps://orcid.org/0000-0003-2323-4486López-PuertasManuelhttps://orcid.org/0000-0003-2941-7734FunkeBerndhttps://orcid.org/0000-0003-0462-4702Jurado-NavarroÁ. AythamiGardiniAngelaStillerGabriele P.https://orcid.org/0000-0003-2883-6873von ClarmannThomasHöpfnerMichaelhttps://orcid.org/0000-0002-4174-9531Instituto de Astrofísica de Andalucía, CSIC, Granada, SpainKarlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Karlsruhe, GermanyM. López-Puertas (puertas@iaa.es)3June20161611670167194February201626February201629April201613May2016This 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://acp.copernicus.org/articles/16/6701/2016/acp-16-6701-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/6701/2016/acp-16-6701-2016.pdf
We have analysed MIPAS (Michelson Interferometer for Passive Atmopheric Sounding) infrared measurements of PMCs for the summer seasons in the
Northern (NH) and Southern (SH) hemispheres from 2005 to 2012. Measurements
of PMCs using this technique are very useful because they are sensitive to
the total ice volume and independent of particle size. For the first time,
MIPAS has provided coverage of the PMC total ice volume from midlatitudes to
the poles. MIPAS measurements indicate the existence of a continuous layer of
mesospheric ice, extending from about ∼81 km up to about 88–89 km on
average and from the poles to about 50–60∘ in each hemisphere,
increasing in concentration with proximity to the poles. We have found that
the ice concentration is larger in the Northern Hemisphere than in the
Southern Hemisphere. The ratio between the ice water content (IWC) in both
hemispheres is also latitude-dependent, varying from a NH / SH ratio of
1.4 close to the poles to a factor of 2.1 around 60∘. This also
implies that PMCs extend to lower latitudes in the NH. A very clear feature
of the MIPAS observations is that PMCs tend to be at higher altitudes with
increasing distance from the polar region (in both hemispheres), particularly
equatorwards of 70∘, and that they are about 1 km higher in the SH
than in the NH. The difference between the mean altitude of the PMC layer and
the mesopause altitude increases towards the poles and is larger in the NH
than in the SH. The PMC layers are denser and wider when the frost-point
temperature occurs at lower altitudes. The layered water vapour structure
caused by sequestration and sublimation of PMCs is present at latitudes
northwards of 70∘ N and more pronounced towards the pole. Finally,
MIPAS observations have also shown a clear impact of the migrating diurnal
tide on the diurnal variation of the PMC volume ice density.
Introduction
Polar mesospheric clouds (PMCs), also called noctilucent clouds (NLCs), occur
in the coldest regions of the atmosphere near the summer polar mesopause.
PMCs normally form a layer extending vertically for several kilometres,
peaking near 83 km, located at latitudes polewards of 50∘. In this
region the temperature frequently drops below the frost point which, for
mesospheric pressures and humidities, is about 150 K. They mainly consist of
water ice particles with radii ranging from a few nm to about 100 nm
.
PMCs modify the ambient plasma of the D-region and gives rise to intense
radar echoes, the so-called PMSE (polar mesospheric summer echoes)
. It is now generally accepted that larger ice particles are
located near the bottom of the layer, while smaller ones are more likely to
be near the top of the layer .
PMCs have been intensively studied using ground, rocket, and space
observations using SNOE/UVS (Student Nitric Oxide Explorer/Ultraviolet
Spectrometer, SBUV (Solar Backscatter Ultraviolet), Odin/OSIRIS (Optical
Spectrograph and InfraRed Imager System), Envisat/SCIAMACHY (SCanning Imaging
Absorption SpectroMeter for Atmospheric CHartographY), GOMOS (Global Ozone
Monitoring by Occultation of Stars), AIM/SOFIE (Aeronomy of Ice in the
Mesosphere/Solar Occultation For Ice Experiment), and AIM/CIPS (Cloud Imaging
and Particle Size).
e.g.;
as well as sophisticated models e.g.. A
good review of our knowledge on PMCs up until 2006 was compiled by
. A more recent review, including a comparison with
mesospheric clouds on Mars, was conducted by .
PMCs are being discussed as potential early indicators of global climate
change in the middle atmosphere because they are
very sensitive to temperature and water vapour concentration. Since enhanced
CO2 amounts see, e.g. are expected to lead to an
eventual cooler upper mesosphere/lower thermosphere, and higher CH4
amounts to enhanced H2O near the mesopause
, they may both lead to an increase
of PMC occurrence, which might be interpreted as an effect of climate change
in the middle atmosphere. There is not, however, a consensus in the
scientific community about this aspect . The
recent study of SBUV data from 1979 to
2013 by has shown, in addition to the clear solar cycle
signal, a good correlation with stratospheric ozone variations. Also, they
have found that PMC ice water content in bright clouds increased rapidly from
1979 to the late 1990s and has been approximately constant from the late
1990s to 2013. Similar results were found by using
SBUV data and a different method for calculating the ice water content (IWC).
These authors also compared SBUV and SOFIE data and found good agreement in
average IWC if an appropriate threshold was applied to the SOFIE data set and
consistent day-to-day and year-to-year variations between both data sets were
used.
looked at trends in the northern midlatitude noctilucent
cloud occurrences using satellite data and model simulations and found a
significant increase in the PMC occurrences at midlatitudes from 2002 to
2011. This result differs somewhat from the insignificant trend found by
for a similar period but at higher latitudes.
analysed trends in mesospheric ice layers in the high-latitude Northern Hemisphere for the 1961–2013 period with model
simulations. They reported a generally good agreement between long-term PMC
variations from the MIMAS model and the SBUV satellite observations. They
found that the modelled trends in ice water content are latitudinally
dependent with no clear trend at midlatitudes (50–60∘ N) but with
a clear positive trend at high latitudes (74–82∘ N) and also in
extreme PMC events.
studied the solar-induced 27-day variations in polar
mesospheric clouds using 15 seasons of SOFIE data and suggested that the
27-day variations in the PMCs are due to 27-day variations of vertical winds.
As described above, a large fraction of the observations taken so far were
performed by measuring the scattered light, in the visible or UV, of the
solar radiation (in the case of instruments from space) or lidar light
(in case of ground instruments). This technique usually observes the ice
particles with radii larger than about 20 nm but lacks sensitivity for
smaller particles see, e.g.. A different technique,
however, has been developed recently by the AIM/SOFIE instrument. These measurements have provided key
characteristics of PMCs such as their frequency, mass density, particle
shape, and size distribution, as well as their seasonal evolution and
altitude dependence see,
e.g.. Furthermore these
satellite data have supplied critical information about the relationship of
the ice density distribution with mesopause temperature and water vapour
concentration see, e.g..
While PMCs emit thermal radiation, their infrared (IR) emissions are very
difficult to observe due to the low ice particle volume density and the very
cold polar summer mesopause temperatures. In fact, only three IR emission
observations have been reported to date: those taken by CRISTA
, by SPIRIT and those taken by the
Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)
. This technique has the advantages of being able to
measure PMCs in dark conditions, thus providing a better spatial and temporal
coverage, and of being sensitive to the total ice volume density, regardless
of particle size, and hence include the very small particles.
In a previous paper reported the detection of
infrared emissions from PMCs taken by the MIPAS instrument on board Envisat
(Environmental Satellite), and provided further evidence of the water ice
nature of the PMC particles. We also described the retrieval of the ice
particle volume density and reported the analysis of the retrieved densities
for 19–21 July 2005. In this paper we present the global distribution
(by altitude, latitude, and longitude) of ice volume density measured by MIPAS
for several days in each of the Northern (NH) and Southern Hemisphere (SH)
seasons from 2005 to early 2012. We also analyse several aspects of the
PMCs such as (i) the mean altitude of the layer, the ice water content and
its hemispheric dependence; (ii) the correlation of ice volume density with
the frost-point temperature and the water vapour concentration; and (iii) the
diurnal variation of the ice volume density. MIPAS, as well as SOFIE, has the
advantage of measuring the whole content of ice particles (all sizes) in the
mesosphere. Hence, a comparison with SOFIE observations, version 1.3, is also
shown.
Days of MIPAS observations of PMCs in the different
modes.
MIPAS is a high-resolution limb sounder on board the Envisat satellite,
launched on 1 March 2002. It took measurements until 8 April 2012, when the
Envisat satellite failed. MIPAS measurements covered a wide spectral range
with a high spectral resolution, operating at 0.025 cm-1 from
2002 to 2004 and 0.0625 cm-1 from 2005 to the end of the mission. It
also operated with a high sensitivity, allowing measurements of most of the
atmospheric emissions in the mid-infrared over a large altitude range
. MIPAS operated with a global latitudinal coverage
(pole-to-pole) and performed measurements irrespective of day- or night-time.
The instrument carried out most of its observations in the 6–68 km altitude
range (the nominal mode), but it also regularly made observations at higher altitudes
in its middle-atmosphere (MA), noctilucent cloud (NLC), and upper-atmosphere (UA)
modes .
In the MA mode, the spectra are available at limb tangent heights from about
20 up to 102 km with a vertical sampling of 3 km. The UA mode ranges from
about 42 to 172 km, and has a vertical sampling of 3 up to 102 km, and
5 km above this altitude. The NLC mode is a variant of the MA
mode specifically tailored for measuring the PMCs during the summer seasons
. In this mode the spectra cover
tangent heights from 39 up to 78 km at 3 km steps; then from 78 up to
87 km at 1.5 km steps, and from 87 up to 102 km again in 3 km steps. The
horizontal field of view (FOV) of MIPAS is approximately 30 km. The days of
PMC measurements in the different observation modes are listed in
Table , and a summary of the distribution of these days within
the different seasons is shown in Table .
Distribution of MIPAS days of observation of PMCs
per season∗ .
NLC MA UA Total YearNHSHNHSHNHSHNHSH20053––412462006––22113320074–33219420083–564612122009336466151320103364641511201133516714112012–3–2–4–9Total1912272626317269
* For the NH season the days correspond to June–August of
the listed year. For SH season the days correspond to December of the
preceding year and January–February of the listed year.
The method used for the retrieval of PMC ice volume density from the MIPAS
spectra has been described by . A brief excerpt is
included here. The spectra analysed in this work were all taken with the
optimized spectral resolution of 0.0625 cm-1. The ice volume density
was retrieved from the radiance profiles obtained by integrating the spectra
from 770 to 920 cm-1. The profiles were corrected for an offset
variable in altitude, latitude, and time. The noise equivalent spectral
radiance in this spectral region is about 20 nW/(cm2 sr cm-1), and
the corresponding noise in the integrated radiances of a single scan is
∼60 nW/(cm2 sr).
The ice volume density was retrieved from the spectrally integrated radiance
profiles using a linearly constrained least squares fitting, where the
Jacobians were calculated using the Karlsruhe Optimized and Precise Radiative transfer Algorithm (KOPRA) radiative transfer algorithm
. The inversion was constrained by a Tikhonov-type scheme
using a squared first-order differences matrix to obtain
a reasonably smoothed vertical profile of volume densities. The ice
refractive indices were taken from .
In this analysis we have included the following improvements and updates with
respect to : (i) The more recent version 5
(5.02/5.06) of MIPAS L1b spectra has been used
; (ii) an updated version of the
temperature is used for the retrieval of ice density (see below); (iii) the
altitude registration of the L1b spectra has been improved using the
information from the retrieved temperature and LOS (line of sight) instead of
the engineering information included in the L1b files
; (iv) the offset correction of the
integrated radiance profiles was improved by taking into account its altitude
and latitudinal variations; (v) the ice density profiles were retrieved only
for scans with converged pressure-temperature profiles (no
interpolation over latitude/longitude was done); and (vi) due to a mistake in the calculation of the
volume of the particles distribution, the volume densities presented here are
nearly double those previously reported by .
The temperature and LOS required to retrieve the ice density have been
retrieved from the CO2 emission near 15 µm, recorded in the same
MIPAS band A as the PMC emission. Non-local thermodynamic equilibrium
(non-LTE) emissions were taken into account. The detailed description of the
method and the characterization of the inverted temperature profiles are
described by . The upgrades in the retrieval of the
temperature used here (version vM21) and a validation of the results are
reported by . Briefly, these authors include an
updated version of the calibrated L1b spectra in the 15 µm region
(versions 5.02/5.06); the HITRAN (HIgh-resolution TRANsmission molecular absorption) 2008 database for CO2 spectroscopic data;
the use of a different climatology of atomic oxygen and carbon dioxide
concentrations; the improvement of several aspects of the retrieval set-up
(temperature gradient along the line of sight, offset regularization, and the
spectral apodization); and some minor corrections to the CO2 non-LTE
modelling as detailed by . This version of MIPAS
temperatures corrects the main systematic errors of the previous version and
show, in general, a remarkable agreement with the measurements taken by
ACE-FTS Fourier transform spectrometer,
MLS (Microwave Limb Sounder), Odin-OSIRIS, TIMED-SABER (Thermosphere
Ionosphere Mesosphere Energetics Dynamics - Sounding of the Atmosphere using Broadband Emission Radiometry), AIM-SOFIE, and the
Rayleigh lidars at Mauna Loa and Table Mountain . In
the region of interest here, however, there are still significant
differences, with MIPAS polar summer mesopause temperatures differing by
5–10 K from the other instruments, being warmer than SABER, MLS, and OSIRIS
and colder than ACE-FTS and SOFIE.
Since MIPAS measures PMCs in IR emission, knowledge of the temperature of the
ice particles is crucial. There is still disagreement on the temperature
of the particles, particularly on whether they are warmer or colder than
the ambient atmosphere. Using SOFIE measurements, have
found that the ice particles are about 5–20 K cooler than the ambient
atmosphere. They suggested, however, that the V1.022 SOFIE CO2
temperatures they used might have a warm bias of 5–10 K near the polar
summer mesopause. , using infrared solar occultation
measurements from the ACE instrument, also found that the ice particles are
cooler than the ambient temperature. They argue that this might be caused by
inhomogeneities in the temperature along the instrument field of view, with
the ice particles sensing only the cold(er) parcels where they are present,
while the gas temperature is representative of the whole (warmer) air mass
along the FOV. Physical considerations, however, would suggest that the
particles are warmer than the surrounding gas because they are heated by
absorption of radiation . For example, for a
particle distribution with a mean radius between 30 and 50 nm and an
accommodation coefficient of 0.5, found that the ice
particles are warmer than the ambient gas by about 1 K at 80 km and 2 K at
90 km. Analogously, the model calculation of , when applied
to a normal distribution of ice particle size with a mean radius varying from
40 nm at 80 km to 15 nm at 90 km, gives a temperature increase of 0.7 K
at 80 km and 2.7 K at 90 km. As suggested by these models, we applied a
temperature correction of the emitting particles that varies linearly from
1 K at 80 km to 2 K at 90 km. In principle, MIPAS measurements should
also be affected by the problem pointed out by . However,
our observations do not support that finding. If we assume that ice particles
are cooler than the retrieved gas temperature we obtain very high (and
unreasonable) concentrations of ice particles (see
Sect. ).
Zonal mean ice volume density during four days, two in
the SH and two in the NH, as measured by MIPAS in different observation modes
(MA, UA, and NLC, see labels). The solid red lines indicate the frost-point
temperature. The red dashed line is the mesopause as derived from MIPAS. The
black solid line is an estimated mean altitude (weighted with the ice density
to power of 4) of the PMC layer. The number of measured profiles, “#sc”,
is also shown. The noise error of the mean volume density plotted here,
estimated by the standard error of the mean, is about
0.3 × 10-14 cm3 cm-3.
Polar maps of ice volume density at 84 km for the
same days as in Fig. . The solid red lines indicate the frost-point temperature. The diamonds represent the geolocations of the MIPAS
measurements.
The vertical resolution of the ice density profiles, in terms of the
half-width of the columns of the averaging kernel matrix, depends on the
observational mode. For the over-sampled NLC mode, it varies from
∼2.5 km at 81–82 km to ∼3 km at 86 km, and to 3.5–4 km at
90 km. For the middle- and upper-atmosphere modes (MA and UA, or together
MUA), it is coarser, with values ranging from 3.5 to 4 km. The error in the
absolute pointing is about 200 m. The averaging kernels shown in
are for the NLC mode measurements that have a
sampling step (i.e. tangent altitude increment) of 1.5 km. For the MA and UA
modes the averaging kernels are wider because of the coarser sampling of
3 km.
The random single-profile error of the retrieved ice volume density is about
60 %, including both the instrumental noise and the temperature noise
error. The systematic error is about 25–30 % and is mainly due to the
temperature error in the summer mesopause region .
More details of the retrieval of the ice volume density can be found in
.
Ice volume density distributions
Figure shows typical daily zonal means of ice volume density
retrieved from MIPAS for four days in SH and NH summer seasons in different
observation modes. The thick solid red line is the frost-point temperature
contour, and the red dashed line is the mesopause altitude. The solid black
line is an estimated altitude of the PMC layer (i.e. the altitude weighted
with the 4th power of the density). Note that MIPAS is sensitive to all ice
particles, including those with a small radius. Noise errors in these plots,
estimated by the standard error of the mean, are about
0.3 × 10-14 cm3 cm-3. The PMCs are generally
located at regions colder than the frost-point temperature for almost all
conditions. Note also the large variability in latitude and altitude of the
ice density, particularly on 6 July 2009 (bottom right panel), where the PMCs
reach latitudes as low as 60∘ N. Weak PMCs located at latitudes
equatorwards of about 60∘ and outside of the frost-point temperature
contour are likely false detections caused by instrumental (most likely
offset) errors.
Anomalous low-altitude detection of weak PMCs (i.e. below ∼80 km and
outside of the Tfrost region) could be due to the limb nature of
the measurements. Emissions from isolated clouds located in the LOS far away
from the tangent point are reported at abnormally low tangent altitudes (see,
e.g. , their Fig. 11). Also, the FOV can affect the height
of the lower and upper boundaries of the layer. have shown
that the bottom and top altitudes as measured by SOFIE, which has a FOV of
1.5 km, can be smeared out in about 1–1.5 km. These two effects, along
with the temperature error, can explain why MIPAS observes occasional ice volume concentration at the bottom of the layer at temperatures warmer than
the frost-point temperature.
The latitudinal and longitudinal distributions of ice volume density at 84 km for
corresponding days are shown in Fig. . As shown before for the
zonal means, the PMC layer is almost always confined to regions with
temperature below the frost-point temperature. The variability of the
latitudinal and longitudinal spread is also large. Although the PMCs are generally
around the pole, they are sometimes far away (see top right panel in
Fig. ) and their distribution could be controlled by 2-day
and/or 5-day planetary waves . In
particular the distributions of the days 12 January 2005 (top/left), 16 January
2009 (top/right), and 6 July 2009 (bottom/right) seem to be affected by
wavenumber-1 planetary waves.
Zonal mean ice volume density for all measured days in the Southern
(left panels) and Northern (right panels) hemispheres for the NLC (top
panels) and the MUA (lower panels) MIPAS modes (see Table ).
The solid black line is an estimated mean altitude (weighted with the ice volume density to power of 4) of the PMC layer. The noise error of the volume
density plotted here, estimated by the standard error of the mean, is about
0.08 and 0.04 × 10-14 cm3 cm-3 for the NLC and MUA
measurements, respectively.
Zonal mean ice mass density for the NLC (left
panel) and for the MA and UA (MUA) (right panel) MIPAS modes (see
Table ) for the Northern Hemisphere. The solid black line is an
estimated mean altitude (weighted with the ice volume density to the 4th
power) of the PMC layer. The noise error of the mass density plotted here,
estimated by the standard error of the mean, is about 0.8 and
0.4 ng m-3 for the NLC and MUA measurements, respectively.
Figure shows the zonal mean ice volume density averaged for
all measured days in the Southern (left) and Northern (right) hemispheres for
the NLC (top panels) and for the MA+UA (MUA) (lower panels) MIPAS modes (see
Table ). These distributions are analysed in detail later, but
we describe the main features briefly here: (1) PMCs are confined to
altitudes between around 81 km and 89 km with maximum concentrations around
84 km; (2) PMCs are confined to latitudes polewards of about 60∘,
with increasing concentration towards the poles; and (3) from these figures it is
evident that the ice particles occur in higher concentrations in the NH and
that the ice layer is located at slightly lower altitudes in the NH. These
figures also show an apparent higher concentration for the measurements taken
in the NLC mode than in the MUA mode. The NLC mode has a better vertical
resolution, which leads to sharper temperature profiles
see and hence to sharper ice particle profiles
and larger ice particle densities. However, not all the differences between
the NLC and the MUA modes can be attributed to the better vertical resolution
of the former because observations in different modes occurred on different
days, with observations in the NLC mode generally occurring closer in time to
the peak of the PMC season than observations in the other modes.
Top altitude
Figure shows that MIPAS observes significant abundances of
ice up to about 88–89 km. A similar behaviour has been found in the SOFIE
measurements . This altitude is about 3–4 km higher than
the average maximum altitude of 84.4 km measured by the lidars
. These authors have shown that, for SOFIE measurements,
the vertical smoothing due to limb-viewing geometry can cause an extension of
the uppermost altitude of about 2/3 of the vertical resolution, i.e.
1.5–2 km for the MIPAS NLC observation mode. This, however, cannot fully
explain that difference. The detection of PMCs by SOFIE and MIPAS at
altitudes higher than the lidars is most likely due to the different
sensitivities of the two techniques. While the lidar signal varies with
r6, the MIPAS (in IR emission) and SOFIE (in IR extinction) signals change
with the total ice volume density. As the ice particle size decreases towards
higher altitudes , MIPAS and
SOFIE are then more sensitive than lidars to clouds at higher altitudes. The
highest altitude of PMCs derived from MIPAS NLC mode measurements is largely
variable, as can be seen in the typical examples shown in Fig. .
At 70∘ N, it is about 88.5 km (Fig. b). Its
variability depends on latitude and takes 1-σ values from 2.7 km near
70∘ N to 1.6 km near the pole. The uppermost altitude derived here
is slightly higher than that obtained by SOFIE of 86.8 ± 2.1 km but
agrees very well with the Community Aerosol
and Radiation Model for Atmospheres (CARMA) model prediction of 88.5 ± 0.5 km
. Thus, as pointed out by
and , MIPAS and SOFIE results
are consistent with the current understanding of temperatures and water
vapour distributions at these altitudes , and the
associated ice particles at high altitudes are likely to be related to polar
mesosphere summer echoes e.g.. This has also been
evidenced more recently by the concurrent observations from the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) wind
(ALWIN) radar and SOFIE measurements .
Bottom altitude
The bottom altitude of the PMC layers measured by the lidar measurements at
69∘ N was found at 82.2 km. SOFIE obtained a slightly lower
altitude of 81.6 ± 1.6 km, which is within the lidar and SOFIE
combined standard deviations . For the NH and similar
latitudes MIPAS in its NLC mode (see Fig. b) measured an
altitude of 80.9 ± 1.2 km, slightly lower than SOFIE. In SOFIE
measurements the PMCs with a peak extinction altitude below 79 km were
excluded . Applying a similar threshold to MIPAS data,
however, does not change significantly the bottom altitude.
The bottom altitude also changes rapidly with latitude from 65 to 75∘
(Fig. b); hence a difference of a few degrees in latitude
might induce a significant change in bottom altitude. Thus, in summary, we
can conclude that they are in good agreement. It is also worth noting that
the bottom altitude derived from the MUA modes, which have a coarser vertical
sampling (3 km), is lower by about 1 km (80.0 ± 1.8 km) than that
derived from the NLC mode (Fig. d). This is very likely due
to the limb-sounding geometry, as discussed above. The bottom altitude in the
Southern Hemisphere is found to be located at about 1 km higher than in the
NH (see Figs. a and c).
Ice mass density
As discussed above, MIPAS and SOFIE are the only two instruments with comparable
ice concentration data because they both measure the total ice volume density, irrespective of the ice crystal size. Although it is not the
aim of this paper to carry out a detailed comparison or validation, we
include some comparisons here. First, we compare the maximum (peak) values of
the PMC layer, then we compare mean profiles for several seasons.
SOFIE measured ice mass densities at the altitude of maximum concentration,
zmax, for the 2007 NH season that ranges from 0.1 to 80 ng m-3
with a mean value of 14.2 ng m-3Fig. 14a and Table 5
in. These SOFIE measurements occurred at latitudes between
∼66∘ N in the early season and ∼68∘ N at
mid-summer, and at ∼74∘ N towards the end of the season. MIPAS
measurements for the 2005–2012 period at latitudes of ± 2 degrees of
SOFIE latitudes have mean values of just above 20 ng m-3 for the NLC
mode and of ∼12 ng m-3 (with a broader peak) for the MUA modes
(see Figs. a and b), which agree well with
SOFIE data for the 2007 NH season. As a result, the conclusion drawn by
from SOFIE applies to the comparison of MIPAS with other
measurements and models. That is, MIPAS ice mass densities are also
significantly smaller than the lidar measurements taken at ALOMAR
(69∘ N), which show an average value of 47.4 ng m-3, and the
lidar results reported by , which show ice mass densities
ranging from 36 to 102 ng m-3. These differences can be explained, at
least partially, by the larger sensitivity of MIPAS (and SOFIE) to the
smaller particles (i.e. being sensitive to smaller amounts leads to lower
mean concentrations). Another reason causing the differences could be, at
least for the lidar observations, the averaging over the relatively larger
atmospheric volumes sampled by MIPAS (and SOFIE). Furthermore, MIPAS, as well
as SOFIE, is also able to detect thinner ice clouds than other IR instruments
measuring the PMCs from space, e.g. HALOE .
Although a detailed comparison between MIPAS data and CARMA has not been
performed, the results reported by suggest that MIPAS and
CARMA are in agreement, at least for the 65–75∘ latitude range. A
thorough comparison with the CARMA model and MIPAS data, including higher
latitude regions, would be very useful but is beyond the scope of this paper.
Comparison of the ice mass density of MIPAS MUA
modes of measurements (see Table ) with SOFIE v1.3 L2 data for
the 2008 to 2011 period and the yearly mean (lower panel) in the NH. The
solid lines show the mean profiles, SOFIE in black and MIPAS in red. The
shaded areas are the standard deviations divided by the square root of the
number of profiles. The means of the IWC are also shown.
Figure shows a more detailed comparison between MIPAS and
SOFIE ice mass densities, Mice, for the coincident days and
latitudes (within ± 2 degrees of SOFIE mean latitude) in the NH season
for the years with more coincident data: 2008–2011. The variation of ice mass density with local time is important see, e.g.and
Sect. below. Since most of SOFIE measurements
were taken at local times between 23 and 24 h in the NH, we have taken only
the evening MIPAS measurements (made at 22:00). The comparison is based on the mean
profiles for all days of measurements for each season/year for each
instrument because of the large variability of MIPAS ice mass density (see,
e.g. Fig. ). The solid black lines represent the mean of SOFIE
measurements and the solid red line the mean MIPAS ice mass density. These
figures show quite a good agreement between the two instruments for 2008 and
2010. For 2009 and 2011, the peak values are also in good agreement but the
vertical distributions are rather different. The average over the four years
(bottom panel in Fig. ) reflects that above about 85 km, MIPAS
values are generally larger than those measured by SOFIE and smaller below
that altitude. Similar behaviour is also seen in the SH (not shown). This
seems to be a clear characteristic of MIPAS measurements but absent in SOFIE.
We do not have a plausible explanation for this difference. A possible reason
could be a negative bias of MIPAS temperature at those altitudes/latitudes
which would result in a higher ice mass density, but such a bias present only
in these localized regions seems unlikely. Another reason could be that the
averaging kernels are wider in the PMC upper region see Fig. 5
in. Note also that this vertical zonal distribution of
the ice density in MIPAS is consistent with the water vapour (gas phase)
latitudinal distribution measured by MIPAS (see Fig. ), since
the depletion of water vapour near 60–70∘ N occurs at higher
altitudes than near the North Pole.
The IWC of both instruments, which are reported in Fig. ,
are in very good agreement. In the case of MIPAS the values are only slightly
larger. The mean IWC of the coincident days for the 2008–2011 period in the
NH is 50 µg m-2 for SOFIE and 51 µg m-2 for
MIPAS evening measurements (see bottom panel in Fig. ). It is
noteworthy that the NH MIPAS observations are in slightly better agreement
than SOFIE with model calculations carried out by (see
their Fig. 5d). The mean IWC values for the 2008–2011 period for the SH are
24 µg m-2 for SOFIE and 27 µg m-2 for MIPAS
measurements including both morning and evening data taken at 10:00 and 22:00 (SOFIE measures
between 01:00 and 03:00 in the SH).
Same as Fig. but in units of ppmv. The noise error
of the H2O ice concentration plotted here, estimated by the standard error
of the mean, is about 0.08 and 0.04 ppmv for the NLC and MUA measurements,
respectively.
Qice
We also show the zonal mean of ice volume
density in Fig. (similar to Fig. ) but in units of ppmv,
Qice; i.e. the partial concentration of water vapour if all the
ice were to sublimate. For that conversion we used the pressure and
temperature measured by MIPAS . As expected
Fig. shows the same general behaviour as discussed
above for the volume density (Fig. ). In NLC mode, which
contains observations during the mid-season period, we note that the amount
of water vapour in the form of ice ranges from 1 to 3 ppmv at latitudes
equatorwards of 70–75∘, and reaches values up to 5–6 ppmv close to
the poles. Again these values are in good agreement with SOFIE measurements.
have shown time series of SOFIE Qice at the
altitude of peak extinction for the 2007–2013 period for the Northern and
Southern hemispheres (their Fig. 2). The NH mid-summer values range from 2 to
3.3 ppmv, which compare well with those shown in the right panels of
Fig. at the latitudes of SOFIE measurements,
∼66–74∘ N. Similarly, for the SH they show values spanning
from 1.5 to 2.5 ppmv, also in good agreement with those of MIPAS shown in
the left panels of Fig. . This point is discussed
further in Sect. .
Mean altitudes of the mesopause
(zmeso), of the PMC layer (zPMC), and the
difference zmeso-zPMC (right y axis) for the SH
(left) and the NH (right) seasons for all measurements. The different colours
indicate the results for the NLC (black) and MUA (red) MIPAS observation
modes (see Table ).
Latitudinal distribution of IWC of the PMC layers
for the SH (left) and the NH (right) seasons for all measurements. The colours
indicate the data for different years and the number of days measured per
year (see Table ).
Correlation between IWC and the altitude of
the lower branch of the frost-point temperature contour (see
Fig. ) for all data including the NLC + MUA observation modes
for the NH (black pluses) and SH (red diamonds) PMCs seasons (see
Table ). The black line is a linear fit to the data and r the
correlation coefficient.
Zonal mean ice volume density (left) and of H2o
concentration anomaly (the mean profile has been subtracted) (right) for
21 July 2005. The solid red lines indicate the frost-point temperature. The
red dashed line is the mesopause as measured from MIPAS. The solid black line
is an estimated mean altitude of the PMC layer (see Sect. ).
Altitude and column density of the PMCs
Figure shows the mean altitude of the PMC layer for the SH
(left) and the NH (right) seasons for all measurements. The altitude of the
PMC layer has been calculated as the altitude weighted with the 4th power of
the volume ice density. We observe that the mean altitude in the NH for the
NLC mode is located around 83.5–84 km, while in the SH it is about 1 km
higher (84.5–85 km). The fact that the mean altitude is higher (in
∼1 km) for the MA + UA modes is attributed to the coarser sampling
and to the broader vertical resolution in the retrieved temperature from
these modes. The different temporal sampling of the NLC and MUA modes might
also have an effect. have shown that PMCs are
located higher at the beginning and end of the season, and lower in the
middle of the season. This coincides with our results since the NLC-mode
measurements are usually taken in the middle of the PMC season while MUA are
taken earlier and later in the season. We should also note from
Fig. that PMCs tend to be located at lower altitudes near
the poles, and at higher altitudes towards midlatitudes (both in NH and SH
but more clearly in the latter).
reported an average value for the mean
altitude of the PMC layer of 83.5 km for NH and 84.7 km for SH in SOFIE
measurements, and of 83.3 km for the NH from concurrent lidar ALOMAR
measurements in northern Norway (69∘ N). The MIPAS mean values for
the mean altitude obtained here for the NH is very close to both
measurements. Also, it is very much in line with SOFIE, locating the maximum
of the layer about 1 km higher in the SH than in the NH.
carried out a multi-year analysis of the Odin/OSIRIS,
SNOE/UVS, AIM/SOFIE, and TIMED/SABER data sets in the polar regions
northwards and southwards of 65∘ N (∘ S) and found that the mean PMC height
is located 3.5 km ± 0.5 km below the mean mesopause height. In the
case of SOFIE measurements, however, this difference is significantly
smaller, by ∼1 km, for most of the season, except around the middle of
the season . We also looked at the difference between the
mean PMC height and the mean mesopause height in the MIPAS PMC measurements
(see Fig. ). In general MIPAS observations are more in line
with SOFIE observations than with the other instruments. For the case of NLC
and MUA MIPAS observation modes in the NH near 70∘ N, the difference
is about 2.5 km, smaller than the mean value of 3.5 km obtained for all
instruments and closer to the SOFIE value obtained by . It
is worth noting that this altitude difference increases towards the North Pole, more clearly in the case of the NLC mode (taken around the middle of
the season) and reaching about 4 km. In the Southern Hemisphere the
difference between the mesopause and mean ice layer altitudes is even smaller
than for NH, with values ranging between 2 and 2.8 km; again in better
agreement with SOFIE than with the other instruments.
Figure shows the latitudinal variation of the ice water
content of the PMC layer for the SH (left) and the NH (right) seasons for all
measurements. The figure shows clearly that PMCs are more abundant in the NH
than in the SH, extending to lower latitudes in the NH. The main reason for
this is the warmer polar upper mesosphere in the SH than in the NH, about a
10 K difference as measured by MIPAS .
Figure is consistent with the zonal mean ice volume density
shown in Fig. , which shows that ice mass density increases
towards the poles. Large variability from season to season is also clearly
visible, which in the case of MIPAS is attributable not only to the yearly
changes but also to the daily variation because of the infrequent temporal
sampling of MIPAS. The ice column is large for the NLC mode (not shown) in
correspondence with the zonal mean fields shown in Fig. . As
mentioned before, this is probably due to the fact that the NLC measurements
are taken around the middle of the season (see Table ). The
NH / SH ratio of the ice water content varies with latitude (not shown),
ranging from about a factor of 2 near 60∘ to 1.4 near the poles, with
a value of 1.7 near 70∘, which is in very good agreement with the
factor of 65 % reported by from SOFIE measurements.
Polar maps of H2O vmr for altitudes of 90 km
(top) and 80 km (bottom) (note the different scales) and of ice volume
density at 83 km (middle panel) for 21 July 2005 (see
Fig. ). The white diamonds represent the geolocations of
MIPAS measurements.
Top and middle panels: zonal mean morning–evening ice volume density differences (in absolute and % of pm, respectively) for the
SH (left) and NH (right). Bottom panels: zonal mean morning–evening temperature
differences as measured by MIPAS for the SH (left) and NH (right).
Figure shows the correlation between the ice water
content and the altitude of the lower branch of the frost-point temperature
contour (see Fig. ) for the data taken in the NLC and MUA
observation modes in the SH and NH PMCs seasons. The correlation is
significant and shows that the PMC layers contain more ice when the frost-point temperature occurs at lower altitudes. We have done the analysis for
each hemisphere and mode separately (not shown) and found a very similar
correlation for all cases except for the NLC mode in the NH. The reason for
this exception could be the smaller sample size of this case or that the
altitude range of the frost temperature in NH for this mode is very small and
hardly reaches altitudes higher than 82 km.
We have also found that the ice volume density is also anti-correlated with
the mean altitude of the PMC layer (not shown), that is, that the denser PMC
layers are located at lower altitudes and the thinner ones at higher
altitudes. This is consistent with the behaviour shown in
Figs. and where the denser layers are
usually found near the poles and at lower mean altitudes.
Correlation of ice volume density with H2O concentration
suggest that, as opposed to HALOE and MLS water vapour
measurements, the SOFIE vertical resolution is well suited for the study of
correlations between water ice and water vapour. This is also the case for
MIPAS. Given the good latitudinal coverage of MIPAS (covering the whole polar
region) and the fact that the instrument is able to measure the ice water
content and the water vapour concentration simultaneously, we have looked at
the zonal mean as well as latitudinal and longitudinal distribution of both quantities
in the polar summer region.
The water vapour concentrations used here have been derived from MIPAS high-resolution spectra in the region around 6.3 µm. We used version
v5r_h2o_M22 retrievals. The retrieval baseline is an extension to the
lower mesosphere of the set-up described by with the updates
described by . The main difference of this extension is
the inclusion of non-LTE emissions from the H2O vibrational levels, which
are important above around 50 km . Additional
microwindows, covering stronger H2O v2 spectral lines, are also
included in order to increase the sensitivity in the upper mesosphere
.
Figure shows a typical case (21 July 2005) of the zonal mean
cross sections of the ice volume density (left) and the H2O concentration
anomaly (right). We can clearly distinguish three distinct altitude zones
near the polar region: a region centred near the peak of the PMC layer
(∼83 km), where the ice volume density is largest; a hydrated region a few kilometres
below, where H2O presents a relative maximum at
latitudes northwards of 70∘ N, more markedly seen in the bottom panel
of Fig. ; and a dehydrated region above the ice layer
around ∼ 90 km, where H2O exhibits a clear relative minimum (see
top panel of Fig. ). This global behaviour fits very well
with the current picture we have of the PMCs, where sequestration of
H2O in the gas phase to form ice leads to a drier atmosphere just above
the ice layer, and where the sedimentation of ice and its subsequent
sublimation enhances the H2O gas-phase abundance at ∼80 km. The
MIPAS water vapour layered structure gets sharper towards the pole. This is in
contrast to findings from , who located the maximum at about
70∘ N.
These features are more clearly observed in the latitude/longitude maps
(Fig. ), where the dry region at 90 km (top), the water ice
layer at 83 km (middle), and the wetter H2O region at 80 km (bottom) are
all well confined to the polar region. This topic has been recently studied
quantitatively by using SOFIE observation of ice content,
water vapour, and temperature at latitudes near 70∘. They found that,
in both hemispheres, the altitude of the peak of the dehydration regions is
∼ 1.8 km above the height of peak ice mass density, and the altitude
of the peak of the hydration region is ∼ 0.3 km above the observed
bottom of the ice layer. Although no general conclusion can be drawn from the
single day of MIPAS data shown here, we have found different results. In
MIPAS the peak altitude of the hydration region is about 1 km below the
bottom altitude of the PMC layer, and the dehydration region is found to be
significantly (about 2–3 km) higher than in SOFIE (see right panel in
Fig. ).
also found that the column abundance of H2O in the gas
phase is roughly equal in the dehydration and hydration regions, but less
than that contained in the ice layer. From the day of MIPAS data analysed
here we find that the excess of H2O gas-phase column in the
hydration region ranges from 5.5 to 9 ppmv × km for
70–90∘ N, while the column of the upper drier region is
significantly smaller, ranging from -1 to -4.5 ppmv × km. We
should note, however, that we use the mean H2O gas profile averaged over all
latitudes as the “background”, which could probably partially
explain the differences with SOFIE. MIPAS and SOFIE, however, agree very well
in that the excess and deficit H2O gas-phase concentrations are
significantly much smaller than those contained in the ice cloud.
Figure shows MIPAS-enhanced values of about 1.5 ppmv in the
hydration layer and a decrease of 0.5 ppmv in the dehydration region, while
the Qice peak is about 6 ppmv. A more comprehensive study using
all MIPAS data should, however, be carried out to confirm these findings.
Diurnal variation of ice volume density
The diurnal variation of PMCs is an important factor to be taken into account
when comparing data sets with different temporal sampling. Several studies
have shown that the IWC may have a significant diurnal variation at latitudes
close to and equatorwards of 70∘, mainly driven by tidal effects in
temperature and in meridional advection at subpolar latitudes
. MIPAS measures PMCs at two local times,
10:00 and 22:00, and hence easily allows for the inspection of
variations due to the diurnal migrating tide see.
Figure shows the absolute (upper panels) and relative
(middle panels) zonal mean differences (morning–evening) of MIPAS ice volume density
averaged over all measurements in the SH (left panels) and NH (right panels).
The morning–evening absolute differences are larger in the NH, partially due to
the larger concentrations in this hemisphere. The relative differences in the
NH are larger at 60–80∘ N, and reach a maximum value of
0.75 × 10-14 cm3 cm-3. The morning enhancement is
in line with the predictions of but not as large as their
calculated factor of 4–5 in the IWC at 69∘ N. At this latitude, we
find a maximum enhancement of about 80 % in the volume density and
40–50 % in the IWC (not shown). Note however that simulations by
correspond only to June 2007. The changes in the ice volume density at 65–75∘ N shown in Fig. result
in the MIPAS NH evening clouds being on average at slightly lower altitudes,
also in agreement with .
The morning–evening difference of ice volume density at 50–60∘ N is
0.25–0.5 × 10-14 cm3 cm-3 at 81–87 km, i.e.
morning / evening ratios lying between 1.5 at 86 km and 7 at 82 km
(Fig. ). These changes result in narrower and thinner
evening clouds, on average, that mainly disappear below 84 km, in agreement with
findings at subpolar latitudes from and
. The IWC morning / evening ratio increases rapidly towards
these lower latitudes and varies in the range of 1.5 to 2.8 at
50–60∘ N (not shown).
The bottom panels of Fig. correspond to 10:00–22:00
differences in the kinetic temperature measured by MIPAS simultaneously with
the ice volume densities. These are a good measure of the temperature
perturbations due to the diurnal migrating tide . The
morning–evening ice volume density differences in the NH (right panels in
Fig. ) are generally anti-correlated with the corresponding
morning–evening kinetic temperature differences. For example, the positive
ice differences at 80–85 km equatorward of 80∘ N
correspond to negative temperature differences. The temperature
differences tend to be positive above 87 km northward of 65∘ N,
which is reflected in the ice volume density differences. Nevertheless, it is
not possible to infer from these anti-correlations alone the extent to which
diurnal temperature perturbations affect the ice volume density. Direct
influence from other factors, like tidal variation of meridional advection
see, e.g. or non-linear behaviour of phase transitions,
cannot be ruled out.
Indeed, the anti-correlation between diurnal variation of the ice density
(upper and middle left panels in Fig. ) and that of
temperature (lower left panel) in the SH is not so clear. In this hemisphere,
the negative morning–evening temperature difference at 50–60∘ S and
80–84 km is weaker and located at lower altitudes than in the NH. The
corresponding absolute diurnal ice change is small but the ice volume
density at these latitudes is also very small (less than
5 × 10-15 cm3 cm-3). The negative ice concentration
difference at 84–88 km corresponds to a positive temperature difference but
only at 60–80∘ S. And most strikingly, the morning–evening temperature
perturbation around 80–84 km at 65–80∘ S is positive but so is
the ice variation. This indicates that a diurnally varying driver other than
temperature more significantly affects the diurnal ice variation at those
latitudes, at least below 84 km. The overall effect on ice density results
in vertically alternating positive and negative changes that lead to lower SH
morning mean cloud altitude. The impact of that driver most likely depends on
altitude and latitude. A deeper analysis involving wind fields is beyond
the scope of this paper and will be the focus of a future study.
Conclusions
We have analysed MIPAS IR measurements of PMCs for the NH and SH summer
seasons from 2005 to 2012. PMCs were measured in the middle IR in emissions
where, due to the small particle size, the signal is only affected by
absorption and not by scattering. MIPAS is therefore sensitive to the total
ice volume, including the very small ice particles that UV–VIS scattering
observations are generally not sensitive to. The measurements cover only a
few days of the PMC season (varying from 3 to 15) but have global
pole-to-pole coverage. In this way, MIPAS measurements show, for the first
time, global latitudinal coverage (from 50∘ to the pole) of the total
ice volume density.
MIPAS measurements indicate mesospheric ice existing as a continuous layer
extending from about ∼81 km up to about 88–89 km on average and from
the poles to about 55–60∘ in each hemisphere. These altitudes are in
very good agreement with SOFIE measurements, with the lowest altitude being
slightly lower (∼ 0.7 km) in MIPAS, and the uppermost altitude
slightly higher (1.7 km), probably caused by the wider MIPAS field of view.
This bottom altitude is also slightly lower than that derived from lidar
measurements but the uppermost altitude is significantly higher (4–5 km on
average). This indicates that both
MIPAS and SOFIE instruments are sensing the small ice particles in the upper
part of the PMC layer. This has also been proved recently by the concurrent
observations from the ALOMAR wind (ALWIN) radar and measurements from SOFIE
.
PMCs are very variable, both in space and time. On average, MIPAS
measurements show that ice mass density increases towards the poles. The IWC
measured by MIPAS at latitudes where SOFIE measurements are available show
a good overall agreement, being in general slightly larger
(∼ 10 %), and also exhibiting a larger variability, probably caused
by the smaller sensitivity of MIPAS. A distinctive feature, however, is that in
general MIPAS shows larger ice volume densities than SOFIE in the region
above ∼ 85 km and smaller below.
The ice concentration is larger in the Northern Hemisphere than in the
Southern Hemisphere. The ratio between the IWC in both hemispheres is also
latitude-dependent, varying from a NH / SH ratio of 1.4 close to the
poles to a factor of 2.1 around 60∘. This also implies that PMCs
extend to lower latitudes in the NH.
We have found that the mean altitude of the PMC layer in the NH for the NLC
mode of MIPAS observations is located around 83.5–84 km, while in the SH it
is about 1 km higher (84.5–85 km). This hemispheric asymmetry is in very
good agreement with SOFIE observations . For those MIPAS
observations taken in the middle- and upper- atmosphere modes, the mean
altitude is higher (by ∼ 1 km). This difference is attributed to the
coarser sampling and to the broader vertical resolution (particularly in the
retrieved temperature) and also to the different temporal sampling of the
modes since the NLC-mode measurements are usually taken in the middle of the
PMC season while MUA-mode observations are taken earlier and later in the
season. A very clear feature in MIPAS observations is that PMCs tend to be at
higher mean altitudes towards lower latitudes (in both hemispheres),
particularly equatorwards of 70∘.
MIPAS observations show that the difference between the mean PMC height and
the mean mesopause height is about 2.5 km in the NH near 70∘ N.
This is smaller than the mean value of 3.5 km obtained from several
satellite instruments by and closer to the SOFIE value
. MIPAS also shows that this altitude difference increases
towards the North Pole, reaching a value close to 4 km. In the Southern
Hemisphere this difference is smaller than for the NH, with values ranging
between 2 and 2.8 km; again the agreement with SOFIE is better than that
with other instruments.
The anti-correlation between the ice water content and the altitude of the
lower branch of the frost-point temperature contour is significant in MIPAS
observations and shows that the PMC layers have larger ice mass densities
when the frost-point temperature occurs at lower altitudes. The simultaneous
observations of MIPAS PMCs and water vapour have confirmed that PMC layers
are surrounded by a hydrated region below and a dehydrated region above.
These regions are more pronounced towards the poles, particularly at
latitudes poleward of 70∘ N. This global behaviour fits very well
with the current picture we have of the PMCs, where sequestration of H2O
in the gas phase to form ice leads to a drier atmosphere just above the ice
layer and where the sedimentation of ice and its subsequent sublimation
enhances the H2O gas-phase abundance at ∼80 km. The analysis of a
single day of water vapour and PMCs measurements of MIPAS has shown different
results than for SOFIE. The peak altitude of the hydration region is about
1 km below the bottom altitude of the PMC layer in MIPAS, while in SOFIE it
is ∼0.3 km above , and the dehydration region is
found to be at ∼2–3 km above the height of peak ice mass density in
MIPAS but ∼1.8 km in SOFIE. Further, MIPAS shows that the column
abundance of water vapour excess in the hydration layer is about twice than
the deficit in the dehydration layer near 70∘ N, while they are very
similar in SOFIE. However, they both agree that both quantities are much
smaller than the water content in the form of ice.
Finally, MIPAS observations, which are taken at 10:00 and 22:00, also
show a diurnal variation in the ice volume density. The IWC is larger at
in the morning than in the evening in the NH, in line with the model predictions of
. This diurnal variation is anti-correlated with
corresponding differences in temperature in the NH, suggesting that it is
driven by the temperature-migrating diurnal tide, but effects from other
factors cannot be ruled out. In the SH, the lack of a clear anti-correlation
with temperature points to a significant impact of an additional driver.
Acknowledgements
We thank Patrick Espy for useful discussions about the temperature of the ice
particles, and Stefan Lossow and two anonymous reviewers for very useful
comments and suggestions. We thank the SOFIE team for providing data, taken
from http://sofie.gats-inc.com/sofie/index.php. The IAA team was
supported by the Spanish MICINN under the project ESP2014-54362-P and EC FEDER
funds. MGC was financially supported by the MINECO under its “Ramón y
Cajal” sub-programme.Edited by: F.-J. Lübken
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