Introduction
The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS;
) measured limb emission spectra between 685 and
2410 cm-1. The instrument was a Fourier transform spectrometer
run by the European Space Agency (ESA) on the Environmental Satellite
(ENVISAT). It covers two measurement periods: from June 2002 to March 2004 it
measured with a theoretical resolution of 0.025 cm-1 (after a
“Norton–Beer Strong” apodisation, , the spectral
resolution is 0.0483 cm-1). This period is called full spectral
resolution (FR). Between January 2005 and April 2012 it measured with
a reduced spectral resolution (RR, theoretical
resolution: 0.0625 cm-1, apodised resolution:
0.121 cm-1) but with a finer tangent altitude spacing. The
tangent altitudes and their relative spacing depend on the measurement mode.
Only spectra from the MIPAS nominal measurement mode have been used
for this work. The limb scans have 17 tangent altitudes between 6 and
68 km for the full-resolution period and 27 spectra with tangent altitudes
between 6 and 73 km in the reduced-resolution period.
Retrievals of temperature and its horizontal gradient, vertical pointing of
the line of sight and many trace gases are conducted with the research data
processor developed at the Institute of Meteorology and Climate Research in
cooperation with Instituto de Astrofísica de Andalucía (CSIC)
. Earlier versions of the retrieval of CH4
and N2O with this processor for the full-resolution period were described
by , and for the reduced-resolution period by
. The retrieval setup of the latest data versions
V5R_CH4_224, V5R_CH4_225, V5R_N2O_224, V5R_N2O_225, V5H_CH4_21 and
V5H_N2O_21 can be found in . Data versions
V5R_CH4_224 and V5R_CH4_225 are practically equivalent, each covering
a different time period. The same holds for V5R_N2O_224 and V5R_N2O_225.
The only technical difference between each two of these versions is that
ECMWF analyses from a different source were used as a priori temperature
profiles for the preceding retrieval of the temperature profiles which in
turn were used for the retrieval of CH4 and N2O. This has no noticeable
effect on the data products of CH4 and N2O. The only purpose of
different version numbers is to guarantee full traceability and repeatability
of the retrieval.
A Tikhonov first-order finite-differences constraint in combination with an
all-zero a priori profile is used. This serves to smooth the retrieved
profile, instead of pulling it towards the a priori profile itself
. For CH4 there are additional diagonal elements
in the regularisation matrix for altitudes at 70 km and above (these are
altitudes above the highest tangent altitude), where the profile is hence
pulled towards zero.
The new retrieval setup versions rely on the usage of an updated
spectroscopic data set, an improved handling of continuum contributions to the
spectra, some smaller changes in the constraint, revised selections of
microwindows and the additional jointly fitting of HNO3 and H2O. The
profiles are retrieved on a fixed-altitude grid between 0 and 120 km. The
grid spacing between 4 and 70 km is 1 km, outside that range it is
coarser. For the comparisons, only profile points have been used, where the
diagonal element of the averaging kernel is above 0.03 and the visibility
flag is 1. The latter is a value which indicates for one profile point,
whether the retrieval actually used measured data which was emitted at the
altitude of this profile point. It is 0 if there are no spectra available
either because the measured spectra in that altitude are influenced by
emissions of clouds, or because there are no measurements available for that
altitude. This is the case for all altitudes outside the MIPAS scan range,
which is smaller than the range of the retrieval altitude grid. The
resolution of the data products in the stratosphere ranges from 2.5 to
7 km, (for details see ). The error
profiles of the retrieved mixing ratios of CH4 and N2O resulting from
instrument noise are reported for each measurement.
A comparison of MIPAS IMK/IAA CH4 to profiles measured by other
instruments can be found in . They discuss data
versions CH4_V5R_222 and CH4_V5R_223 which cover the MIPAS reduced-resolution period only. These versions are the direct predecessors of the
CH4_V5R_224/CH4_V5R_225 versions under discussion in this work. The
retrieval setup of versions CH4_V5R_222 and CH4_V5R_223 can be found in
. found the MIPAS CH4
profiles below 20–25 km to be biased high and give 14 % as the most
likely value.
In this work we compare the new data versions V5R_CH4_224, V5R_CH4_225,
V5R_N2O_224, V5R_N2O_225, V5H_CH4_21 and V5H_N2O_21 to various
satellite instruments and additionally to surface data. These comparison
instruments are described in Sect. . The documentation
of the comparison method is given in Sect. , while
Sect. contains the validation itself. In
Sect. an attempt is made to gain some knowledge about
how well the data products from the two different MIPAS measurement periods
agree by using comparisons to other instruments.
Section gives a summary of our results. Since the
bulk of the MIPAS data is from the reduced-resolution period, we
discuss these profiles before those measured during the full-resolution
period.
Description of the comparison instruments
ACE-FTS
The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS)
on board SCISAT measures spectra between 750 and 4400 cm-1 in
solar occultation mode . A scan consists of several
spectra measured with tangent heights from cloud top to 150 km. From these
spectra a retrieval of atmospheric temperature, pressure and abundances of
various trace gases including CH4 and N2O is performed. The irregular
retrieval grid is defined by the tangent altitudes. The retrieval setup is
reported by for versions 2.1 and 2.2. For version 3.0 the
changes in the retrieval setup are described in and
chiefly address temperature issues. The N2O data used for the comparison
shown here was version 3.5, which is nearly identical to version 3.0, but
solves a problem with the temperature and pressure a priori, affecting
profiles measured after September 2010. For this version (and all latter
versions), an altitude-dependent status flag is available ;
for the comparison, we only use data where this flag is zero. Since the
altitude flag is on an interpolated regular altitude grid, and we use the
data on the retrieval grid, we assumed a data point on the retrieval grid to
be valid, if both the flag at the grid point directly above and below were
valid. The reported random errors are derived from the least-squares fit and
hence represent noise in the measured spectra. Additionally, they include a
CO2 term depending on the relative difference between retrieved and
a priori CO2 profile.
For CH4, a research version of the 3.5 retrieval was used. It is improved
over version 3.5 in the treatment of the different isotopologues. The ACE-FTS
profiles of the 3.5 research product have slightly lower mixing ratios than
those of version 3.5. This difference is largest at 15 km and below,
where it amounts to about 0.03 ppmv. The difference declines between 15
and 25 km. Since MIPAS has higher mixing ratios, the differences
between MIPAS and ACE-FTS as shown in this work are slightly larger
than when MIPAS CH4 is compared to ACE-FTS version 3.5.
ACE-FTS is the only comparison instrument which covers profiles of both
CH4 and N2O for both the MIPAS FR and RR measurement periods.
The vertical resolution of the profiles are of the order of 4 km. For the
comparison to MIPAS profiles, data measured from 21 February 2004 on
were used. The profiles for the comparison with the MIPAS full-resolution period all are measured between 60 and 90∘ N. For the
RR period, the profiles are distributed more evenly over the latitudes.
SCIAMACHY
The Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY
(SCIAMACHY; ) is an UV/VIS/NIR/SWIR spectrometer on
board of ENVISAT. The instrument measures in several different geometries.
This comparison was made with CH4 profiles retrieved from the solar
occultation mode. This mode provides one measurement per orbit, between 50
and 70∘ N. The retrieval is conducted at the University of Bremen;
the data version 4.5.2 has been used. The retrieval setup for these profiles
is described by . The valid data range for this
product is 17–45 km and its vertical resolution is of the order of 4 km.
An error estimate based on the residual of the spectral fit is
provided.
HALOE
From September 1991 to 21 November 2005, the HALogen Occultation Experiment
(HALOE) on board of the Upper Atmosphere Research Satellite (UARS) measured
profiles of CH4 in occultation geometry . We compared
MIPAS profiles to HALOE data version v19. This version has already
been compared to ACE-FTS v2.2 profiles by , where HALOE data
were shown to have mixing ratios about 5–10 % lower than ACE-FTS.
found the CH4 ACE-FTS version 3.0 mixing ratios to be
reduced at some altitudes compared to version 2.2. And since the version 3.5
research CH4 data have even lower mixing ratios than version 3.0, the
difference between HALOE v19 and the latest ACE-FTS version (3.5 research)
should be smaller than those in the earlier versions found by
. The typical vertical resolution of the profiles is given
as 3–5 km . An error estimate covering instrument noise
is provided along with the data. The profiles used for the comparison to
MIPAS data from the reduced-resolution period were measured from
January to August 2005 and are distributed over all latitude bands. For the
FR period the temporal overlap is larger and the collocated profiles are
distributed over all latitudes and seasons.
Aura-MLS
Since July 2004, the Microwave Limb Sounder (MLS) on the Aura satellite
measures N2O in the 640 GHz region . Details
for the retrieval version v2.2 can be found in . The
vertical resolution is between 4 and 5 km. The retrieval algorithm derives
an error estimate based on the instrument noise.
For the comparison data version v3.3 was used. Here, for the temperature and
tangent pressure the values retrieved in previous retrieval steps were used,
while in version v2.2 those were retrieved jointly with
N2O. The following selection criteria were implemented (as suggested by
): valid pressure range, 100–0.46 hPa, estimated
precision is positive, the status flag is an even number, the quality field
is larger than 1.4, the convergence field is below 1.01 and any profiles are
discarded, where at 68 hPa the N2O mixing ratio exceeds 350 ppbv.
Since there is no data overlap with the MIPAS full-resolution period,
comparisons could be made for the reduced-resolution data only.
Odin-SMR
The Sub-millimetre Radiometer (SMR, ) on board of the Odin
satellite measures profiles of N2O in the 502.3 GHz region.
Measurements are available from February 2001 until the time of this writing,
thus covering the complete temporal range of the MIPAS–ENVISAT
data set. We use data from the Chalmers University of Technology product
version v2.1. The retrieval uses an optimal estimation approach. Details can
be found in . The vertical resolution is of the
order of 1.5 km. The profiles cover an altitude of 15–70 km. We only
used data where the measurement response variable exceeded 0.9 and the
quality flag is 0 or 4. There is an error estimate available for the mapping
of the instrument noise on the profile.
GCASN surface data
The Global Cooperative Air Sampling Network (GCASN) is a international
project by the National Oceanic and Atmospheric Administration (NOAA) of the
US Department of Commerce, operated by the Global Monitoring Division (GMD)
at the Earth System Research Laboratory (ESRL). It measures amount of
substance fractions of CO2, CH4 and several other trace gases.
The surface air flask samples are taken at baseline observatories, additional
fixed locations and ships and are analysed at measurement laboratories.
Information on the CH4 product can be found in and
. For our comparisons we used CH4 mean data
derived from 77 stations. These stations are located at latitudes between
89.98∘ S and 82.45∘ N. The data version is 3 August 2015
.
HATS surface data
The Halocarbons and other Atmospheric Trace Species Group (HATS) of NOAA/GMD
provide surface flask measurements of various atmospheric trace gases. We
compared MIPAS N2O to the Combined Nitrous Oxide data product from
the GMD at NOAA/ESRL . The measurements of 13 sites
stationed at latitudes between 89.98∘ S and 82.45∘ N were
used to calculate a global mean.
Description of the comparison method
To compare the various satellite instruments to MIPAS, the mean of
several collocated pairs of profiles were taken. For the selection of the
collocations, criteria of maximum spatial and temporal distance were applied.
We used a maximum radius of rmax=500 km and a maximum temporal
deviation of Δtmax=5 h. For a comparison of MIPAS
data with any of the instruments, the selection of matching pairs was
unambiguous in a sense that only one profile complied with the candidate
MIPAS profile and vice versa. The total number of matched pairs for each
instrument and the temporal coverage of the matches are displayed in
Table .
For the comparisons, the profiles of the instruments were interpolated to the
MIPAS altitude grid. ACE-FTS, SCIAMACHY, HALOE and Odin-SMR provide the
profiles on a geometric grid and hence a linear interpolation was used. For
Aura-MLS the vertical coordinate of the profiles is pressure. These profiles
were interpolated linearly in the log(pressure) domain to the MIPAS grid
using the MIPAS pressures.
Number of collocations n between the instruments and MIPAS
and their temporal ranges.
Instrument
MIPAS comp.
Temporal range
n
product
ACE-FTS
CH4 full res.
Feb 2004–Mar 2004
253
SCIAMACHY
CH4 full res.
Jul 2002–Mar 2004
1232
HALOE
CH4 full res.
Jul 2002–Mar 2004
2306
ACE-FTS
CH4 red. res.
Jan 2005–Apr 2012
8301
SCIAMACHY
CH4 red. res.
Jan 2005–Apr 2012
7440
HALOE
CH4 red. res.
Jan 2005-Aug 2005
157
ACE-FTS
N2O full res.
Feb 2004–Mar 2004
253
Odin-SMR
N2O full res.
Jul 2002–Mar 2004
38 739
ACE-FTS
N2O red. res.
Jan 2005–Apr 2012
8307
Odin-SMR
N2O red. res.
Jan 2005–Mar 2012
174 198
Aura-MLS
N2O red. res.
Jan 2005–Apr 2012
830 575
To avoid sampling problems due to the different vertical extent of the
profiles, only data were used to calculate the mean profiles where both
instruments in the respective pair provide valid values. The number of data
points from which the mean is calculated is hence a function of the altitude.
Typically the lower parts of the mean profiles contain fewer data points than
the means at higher altitudes. This is due to the fact that MIPAS
spectra containing a cloud signal beyond a certain threshold are excluded
from the analysis. We discarded mean profile points which were calculated
from less than 10 individual profile points to get meaningful statistics.
To compare two instruments, the mean over the data was calculated for each
instrument. Additionally, the standard deviation for each instrument was
determined to check whether atmospheric variations are reproduced by both
instruments consistently. To examine the bias between the instruments, the
difference (both absolute and relative) of the mean profiles was calculated.
Also the standard error for the mean (absolute) difference has been derived
to estimate the significance of the bias .
For all the instruments, some kind of estimated error was available,
representing statistical uncertainties (for details on these see
Sect. ). For MIPAS, this error estimate covers
the influence of the instrument noise on the profiles. The combined error
σcombined for two instruments
σcombined=σref2+σmip2
could be derived from the given error estimates (σref
and σmip) with Eq. () and compared
to the standard deviation of the difference. Since for the difference of
collocated measurements, atmospheric variability should largely cancel out,
the standard deviation of the difference describes the statistical
uncertainty of the difference. If the error estimates were perfect, and the
instruments sampled exactly the same air mass, the combined error estimate
should equal the standard deviation of the difference.
For MIPAS, there also is an extended error estimate available for some
selected measurements. It includes propagated errors of the preceding fitted
variables temperature and ozone mixing ratio as well as estimates of the
uncertainties of the line of sight, the spectral shift, the calibration and
the instrument line shape, the zonal temperature gradient, and the mixing
ratios of all other gases where climatological values were used for the
radiative transfer calculations during the retrieval. These estimated errors
have been added quadratically to the MIPAS instrument noise error.
A combination of this extended MIPAS error and the other instrument's
error is shown as well. These extended error estimates are not mean values
over the sample, but just a representative example of a typical error budget
for one sample scan. Hence perfect agreement to the standard deviation of the
difference cannot be expected.
In general, the vertical resolutions of MIPAS and the different
instruments do not differ very much, hence one could assume that the
MIPAS averaging kernels would not be needed to be applied. However,
the MIPAS profiles' resolution is poorest at the boundaries, and the
profiles of CH4 and N2O show large variations in the gradients at the
lower boundaries. This could lead to comparison artefacts at the lower
boundaries of the profiles. Since the lower part of the MIPAS profiles
is of particular interest for this study because previous versions of CH4
and N2O from MIPAS show the largest bias in that altitude region,
we decided to apply the MIPAS averaging kernels to the other
instruments.
In the case of ACE-FTS the original profiles were degraded with the
MIPAS averaging kernels (AKs) to remove artefacts in the differences,
caused by their better altitude resolution. We used profiles
vmrACEnative on the native ACE-FTS
retrieval grid which consists of the tangent altitudes and hence is variable
from profile to profile. This grid is coarser than the MIPAS grid. To
apply the MIPAS AK (AMIP), we interpolated the
ACE profiles using the interpolation matrix W from the native ACE
grid to the MIPAS grid. To obtain the degraded ACE profile on its own
grid and to remove any finer structures which might be introduced by the
using of the finer gridded MIPAS AK, we further applied the matrix
which re-samples from the fine to the coarse grid V=(WTW)-1WT to the
result. A final interpolation to the MIPAS grid (using W)
enables the calculation of the mean and taking the difference to the
MIPAS profile. Since the MIPAS retrieval uses an a priori which
is set equal to zero , the comparison profile of
ACE-FTS degraded with the MIPAS AK is given by
vmrACEAK=WVAMIPWvmrACEnative.
The AK degraded value at one altitude zi is
vmrACEAKi=∑jaijvmrACEnativej,
where aij are the matrix elements of WVAMIPW.
The AK can only be applied to a profile point at an altitude zi if
a valid data point of the reference instrument
vmrACEnativej is available for all the
elements aij which are not zero. Since the AKs in reality are never
exactly zero, we only discarded profile points, if the absolute of at least
one AK element where no valid reference profile point is available is larger
than a threshold of 0.01. That means if for the calculation of the degraded
mixing ratio at zi, profile elements
vmrACEnativej are needed in the sense
that the absolute of aij is above this threshold, and there is at least
at one of those profile elements no valid data point of the reference
instrument, then the calculation of this profile point is not carried out and
the mixing ratio at zi is marked as missing instead. However, if there are
valid data points of the reference instruments, then the elements where the
absolute of aij are below the threshold of course are not excluded from
the calculation.
Due to this approach, the vertical extent of the profile is reduced after the
degradation with the MIPAS AK. However, for ACE-FTS this alters the
number of valid data points in the mean profiles only slightly because the
vertical extent is larger than for MIPAS and only data points where
both instruments show valid data are used to calculate the mean profiles. The
ACE-FTS mixing ratios of CH4 did not change noticeably due to the
degradation with the MIPAS AKs. The same holds for N2O in the full-resolution period. But for N2O in the reduced-resolution period, the
profiles show much lower mixing ratios at the lower end after the
degradation. Those differences amount to about 17 ppbv at 9 km and
decrease with altitude up to 18 km, where both the degraded and the
original ACE-FTS profiles show the same mixing ratios. This makes the
application of the MIPAS AKs for N2O in the reduced-resolution
period essential for the bias estimates. This is due to the MIPAS AKs
being asymmetric at the lower end of the profile. Most of the information for
the profile points at 12 km and below is in fact derived from spectra from
altitudes above.
For Odin-SMR and Aura-MLS we used the same approach, since these are
available on coarser grids than MIPAS as well. For both instruments
the data loss due to the border effects of the application of the
MIPAS AKs is more pronounced than for ACE-FTS, but still quite small:
on average, about 3 km of the lower and upper ends of the profiles are
lost. For Odin-SMR the extent of the mean profile is not altered, but the
profile values below 22 and above 50 km are based on fewer data points.
For Aura-MLS the upper end of the mean profile is reduced from 59 to
54 km. At the lower end, the extent of the mean profile is not altered,
but the mean below 21 km is based on fewer data points. Both instruments
show slightly different mixing ratio differences to MIPAS at the lower
profile ends after the application of the AKs. For Odin-SMR, the largest
differences between original and degraded profiles for the reduced-resolution
period occur at around 18 km and amount to around 8 ppbv, declining
both below and above until 20 km where both profiles show the same mixing
ratios. For the full-resolution period the differences between the two
Odin-SMR profiles are similar, but their maximum is at 16 km. For Aura-MLS
the differences are almost 60 ppbv at 14 km but decline quickly with
altitude up to 16 km where the two Aura-MLS profiles show almost the same
values. Between 17 and 19 km the differences amount to about 5 ppbv,
above the degraded and the original profiles agree.
The HALOE profiles are given on a finer grid than MIPAS.
W′ is the interpolation matrix from coarse grid
(MIPAS) to fine grid (HALOE). Then the HALOE profiles degraded with the MIPAS
averaging kernels are given by
vmrHALOEAK=AMIPV′vmrHALOEnative,
where V′=(W′TW′)-1W′T. The
degraded profiles do not differ much from the original profiles. For the full-resolution period the differences are around 0.03 ppmv between 12 and
17 km and for the reduced-resolution period up to 0.08 ppmv between 14
and 16 km, with no differences outside these altitude ranges. By
application of the averaging kernels, the altitude coverage of HALOE profiles
on average is reduced by 4 km. However, the total extent of the mean
profile does not alter, there just are fewer data points in the lower-most
7 km.
The SCIAMACHY profiles are given on the same grid as MIPAS. Hence no
interpolation is needed for the application of the MIPAS AKs to
SCIAMACHY profiles. However, due to the limited altitude range of SCIAMACHY
profiles, few data points remain after the application of the AKs. The only
difference between the original SCIAMACHY mean profile and the profile where
the MIPAS AKs have been applied is a slight oscillation with an
amplitude of about 0.06 ppmv at the lower end (18–21 km in the
comparison for the full-resolution period. Since the mean profile in that
altitude region relies on very few data points, we think it is not
representative. For the reduced-resolution period, the difference between
profiles where the MIPAS AKs were applied and where not, is very
small (0.02 ppmv at 17 km, declining to zero at 20 km and
0.04 ppmv at 43 km, else zero). Because of the few data points
left after the application of the AKs, we prefer to show the profiles without
the MIPAS AKs.
To enable comparisons with data obtained by a totally independent measurement
principle from the satellite instruments we also compared MIPAS measurements
to data measured at the Earth surface. Their high precision (CH4:
1.5 ppbv precision ; N2O: 0.2 ppbv precision
) compared to the various satellite instruments, makes
them a valuable reference. MIPAS does not measure trace gas volume
mixing ratios at the surface, which makes a direct comparison to surface data
difficult. We assumed that the relatively long lifetime of CH4 and N2O
in the atmosphere in combination with the well-mixed troposphere and the
absence of sources of CH4 and N2O in the free troposphere allow the
comparisons of MIPAS mixing ratios in the (upper) troposphere to
surface values.
The comparison with the surface data networks GCASN (CH4) and HATS
(N2O) was done using global monthly means. For the surface data, monthly
means vmr‾i for each individual station were
used. Then an area-weighted global average
vmr‾global was taken for each month
according to
vmr‾global=∑icosφivmr‾i∑icosφi,
where φi is the
latitude.
For the comparison to surface data, we used the MIPAS mixing ratio at
the altitude grid point 3 km below the tropopause to make sure to have
a tropospheric value. For the determination of the tropopause from the MIPAS
temperature profiles, we used two different approaches. In the latitude band
between 25∘ S and 25∘ N we used the altitude where the
potential temperature Θ equals 380 K. Outside that latitude band,
we used the WMO criterion . In some cases this approach
failed, possibly because the resolution of the MIPAS temperature profiles
(including a priori information) is too coarse, so a manual post-selection
has been applied. A total of 1 % of the profiles have been discarded
from this analysis, most of them measured during Arctic winter conditions.
The remaining MIPAS data were used to calculate monthly means for
latitude bands 10∘ wide. For each station of the comparison surface
data network, the derived MIPAS value in the corresponding latitude
band was taken to calculate a global monthly mean mixing ratio, using the
same latitude-dependent weighting function (Eq. ) as for
the surface data.
Possible errors of our method are that the value measured by MIPAS 3 km
below the tropopause could still be influenced by stratospheric values; and
that the tropospheric profile could not be sufficiently constant. Considering
that there are no atmospheric sources for CH4 or N2O, the latter would
mean that the actual concentrations in the upper troposphere should be lower
than the surface values. Both errors would essentially lead to an
overestimation of the satellite concentration. So even if we cannot expect a
perfect agreement between the tropospheric MIPAS value and the surface
concentration, we would expect the satellite measurement to be roughly lower
than the surface values.
Validation
Validation of reduced-resolution methane
In Fig. mean profiles of CH4 both of
MIPAS for the reduced-resolution period and ACE-FTS are shown. The
profiles of the two instruments agree quite well with respect to their shape
and values. ACE-FTS shows a steady decline of mixing ratio with altitude over
the entire profile. Above 12 km, this is the case for MIPAS as
well. Between 9 and 12 km MIPAS shows a slight increase with
altitude. Above around 13–14 km a strong negative vertical gradient is
observed by both instruments, which becomes less pronounced above
approximately 40 km. Between 15 and 46 km the MIPAS profile
shows slightly higher values than ACE-FTS. Above that altitude the profiles
agree well.
Comparison of CH4 from ACE-FTS and MIPAS reduced resolution
(V5R_CH4_225 and V5R_CH4_224).
Left panel: mean profiles of MIPAS (black) and its standard deviation (horizontal bars) and of
ACE-FTS (red) and its standard deviation (horizontal bars).
Middle panel: mean difference MIPAS minus ACE-FTS (blue solid), standard error of the difference (blue dotted),
mean relative difference MIPAS minus ACE-FTS relative to ACE-FTS (green, upper axis).
Right panel: combined mean estimated statistical error of the difference
(pink dotted, contains MIPAS instrument noise error only),
combined mean estimated statistical error of the difference (pink dashed, contains MIPAS example random
error budget), standard deviation of the difference (pink solid).
The standard deviation of the profiles is mostly similar, but especially in
lower altitudes MIPAS shows a slightly higher variation. The general
agreement of the two standard deviations serves as an additional indicator
that the two instruments describe the same physical distribution.
The mean differences (middle panel) show that the MIPAS profiles have
higher mixing ratios below 45 km, with maximum differences of around
0.15 ppmv at 13–15 km. Between 47 and 56 km MIPAS has slightly
lower mixing ratios than ACE-FTS, further above MIPAS values are
higher again. The relative differences do not exceed 10 %. The
standard error of the mean difference generally is very small, indicating
that the bias between the two data sets is significant. Only the profile
points between 8 and 9 km show areas where the bias is smaller than its
uncertainty and hence is statistically insignificant.
The combined estimated error is smaller than the standard deviation of the
difference almost over the entire profile, so one or both of the instruments
underestimates its errors. This is not surprising, because the available
error estimates do not cover the total random error, but only measurement
noise. Any (random) parameter errors (e.g. from previously fitted profiles)
are not accounted for. Including the MIPAS extended random error in the
estimate (pink dashed curve in third panel of
Fig. ) leads to higher values, but above 20 km,
there remain unexplained discrepancies between errors estimated and standard
deviations observed. Below 20 km the extended random errors seem to be
overestimated.
Comparison of CH4 from HALOE and MIPAS reduced resolution
(V5R_CH4_225 and V5R_CH4_224).
Details as in Fig. .
In Fig. the comparison of CH4 reduced-resolution data to the HALOE profiles is shown. Both mean profiles have small
kinks at 17 and 18 km, and a local maximum at 17 km. Above they show
a steady decrease with height over the entire altitude range. However, below
35 km, the HALOE profile shows a smoother decline, while the MIPAS
profile's vertical gradient has more oscillations, even though there are no
actual local extrema. Above that altitude, the vertical gradients of the two
profiles are almost identical. Over the entire profile, HALOE's mixing ratios
are smaller than those of MIPAS. The bias is statistically significant
everywhere. The maximum differences occur around 17 km and are around
0.2 ppmv. Above that altitude, they look quite similar to the differences
between MIPAS and ACE-FTS. The differences have a minimum at 28 km
and a secondary maximum at around 35 km of about 0.1 ppmv. In ACE-FTS
such a secondary maximum in differences is present as well, however it is
located in slightly lower altitudes at 31 km and the differences are
smaller (0.05 ppmv). The combined error of both instruments is clearly
underestimated above 20 km, even taking the extended MIPAS error
into account. Below that altitude the combined extended random error estimate
is slightly larger than the standard deviation of the difference, while the
error estimate with noise only for MIPAS is still below the standard
deviation.
Comparison of CH4 from SCIAMACHY and MIPAS reduced resolution
(V5R_CH4_225 and V5R_CH4_224).
Details as in Fig. .
Figure shows the mean profiles from SCIAMACHY
compared to MIPAS. The profiles from both instruments show a steady
decrease with altitude. However, while the SCIAMACHY profile declines much
less between 25 and 27 km, the MIPAS profile declines more
smoothly. Between 17 and 25 km, SCIAMACHY mixing ratios are about
0.15 ppmv lower. Then the differences decrease, above 27 km there are
small differences with alternating signs but in general there is a good
agreement between the instruments. Over the entire profile, the bias is
significant. The combined error estimate is lower than the standard deviation
of the difference for the estimate using the noise error only. With the
extended MIPAS error budget, the combined error below 22 km is
larger than the standard deviation, indicating again that the extended
MIPAS error budget is probably overestimated. However, considering
that the extended error budget is for an example measurement only, the
agreement between the standard deviation of the difference and the estimated
error seems to be reasonable.
The comparison of MIPAS with the GCASN surface data is shown in
Fig. .
MIPAS mixing ratios are higher than those of GCASN, the differences
average to 0.05 ppmv. While the GCASN data show a clear positive trend
over the observation period, which is well documented (e.g.
) for MIPAS data this does not seem to be the
case. This could be due to a negative drift overlaid on MIPAS CH4
measurements. This would be in agreement with recent findings by
and who found that MIPAS
measurements are prone to an instrument drift due to changing detector
nonlinearities. The analysis of proves the resulting drift
due to changing detector nonlinearities to be negative in most latitudes for
CH4 in the upper troposphere and lower stratosphere. It is of the order of
0.04 ppmvdec-1.
Comparison between volume mixing ratios of CH4 from GCASN and
MIPAS reduced resolution (V5R_CH4_225 and V5R_CH4_224).
Large black circles/continuous line: monthly mean mixing ratios for MIPAS,
small black circles/dashed line: standard deviation for MIPAS;
red continuous line: monthly mean mixing ratios for GCASN;
red dashed line: standard deviation for GCASN.
The comparisons of MIPAS reduced-resolution CH4 profiles to the
three satellite instruments are not conclusive. Between 30 and 35 km HALOE
and ACE-FTS show lower mixing ratios than MIPAS, while SCIAMACHY is
slightly higher. The latter instrument has comparatively low values at
25 km, where the agreement between ACE-FTS and HALOE to MIPAS
is quite good. Below 25 km the comparisons to ACE-FTS, HALOE and SCIAMACHY
show that MIPAS has the highest mixing ratios. Hence a positive bias
in that altitude region is likely. It is largest below 20 km where it is
between 0.1 and 0.2 ppmv. This bias is qualitatively confirmed by the
comparison with the GCASN surface data, however here the difference is only
0.05 ppmv. The largest difference occurs with HALOE at 17 km, where
MIPAS mixing ratios are around 0.2 ppmv higher. Overall versions
V5R_CH4_224 and V5R_CH4_225 have improved significantly compared to
versions V5R_CH4_222 and V5R_CH4_223 where found
differences to HALOE of up to 0.35, and 0.2 ppmv to ACE-FTS (version 3.5)
and SCIAMACHY in the lower part of the profile. Considering that the ACE-FTS
version 3.5 used in the comparison of provides higher
values than the 3.5 research version used here (about 0.03 ppmv at those
altitudes), the reduction of the values in the newer MIPAS version is
of the order of 0.08 to 0.15 ppmv.
All the comparisons show the combined random error estimate to be larger than
the standard deviation of the difference in the lower altitudes. This could
indicate that the selected MIPAS example measurement, for which the
extended random error estimate was conducted, is less representative for the
entire data set than hoped for. At higher altitudes, however, the combined
error estimate is smaller than the standard deviation of the difference.
Comparison of N2O from ACE-FTS and MIPAS reduced resolution
(V5R_N2O_224 and V5R_N2O_225).
Details as in Fig. .
Validation of reduced-resolution nitrous oxide
The comparison for the MIPAS reduced-resolution period N2O profiles
to ACE-FTS is shown in Fig. . Both profiles show
a steady, smooth decrease with altitude. Below 30 km the profiles from
ACE-FTS have lower mixing ratios than MIPAS. The largest differences
between the two instruments occur at 10 km and are around 30 ppbv. The
differences decline with altitude until around 33 km, where MIPAS
has slightly lower mixing ratios. In general the agreement between the two
instruments above 30 km is good.
Comparison of N2O from Odin-SMR and MIPAS reduced resolution
(V5R_N2O_224 and V5R_N2O_225).
Details as in Fig. .
Comparison of N2O from Aura-MLS and MIPAS reduced resolution
(V5R_N2O_224 and V5R_N2O_225).
Details as in Fig. .
In Fig. the MIPAS N2O profiles are
compared to those measured by Odin-SMR. The agreement between the two
instruments is good. Their shapes are identical. Below 25 km MIPAS
is slightly higher. The differences are largest at 17–18 km and are just
below 10 ppbv. Above 25 km MIPAS has slightly lower mixing
ratios than Odin-SMR. In the upper part of the profile, the relative
differences are quite high, with MIPAS showing lower values. But since
this occurs at altitudes where the absolute volume mixing ratios are very
low, this does not indicate any severe problems with the data sets. The
combined errors of the two instruments are underestimated below 36 km,
even with the extended MIPAS error budget (although it clearly is an
improvement over the simple noise-only variant). Above, the estimated errors
are larger than the standard deviation and hence probably overestimated, but
in general the agreement is good. At the regions below 17 and above 58 km
the combined errors are very large.
The comparison of MIPAS profiles to Aura-MLS
(Fig. ) demonstrates that the two instruments
generally agree with respect of the shape of the profile. The profiles show
a maximum at 17 km and an almost steady decrease above that – only between
18 and 19 km are the mixing ratios of both instruments slightly
increasing. Below 17 km, the profiles show declining values towards lower
altitudes. MIPAS measured higher mixing ratios between 15 and 27 km
and slightly lower values between 27 and 41 km. The differences are
largest at 17–21 km and amount to approximately 17 ppbv.
The maximum in the mean profile is caused by the altitude-dependent sampling.
Those profiles which have values at lower altitudes are typically measured
outside the Tropics and measure lower mixing ratios than those which measure
in tropical regions. A mean based on entire profiles (within a fixed altitude
range) does not show this maximum, but either the vertical extension or the
amount of measurements in the mean is diminished, depending on the chosen
altitude range. The combined error estimate is too low between about 19 km
and too high above 31 km. Outside that altitude range, the error estimates
are larger than what would be expected by the standard deviation. This is
true, as much for the estimate without as for that with the extended error
budget. The latter decreases the difference to the standard deviation of the
difference, especially in the lower part of the profile, hence a reasonably
good agreement can be achieved.
Comparison between volume mixing ratios of N2O from HATS and
MIPAS reduced resolution (V5R_N2O_224 and V5R_N2O_225).
Details as in Fig. .
Figure shows the comparison of global monthly
means of MIPAS and HATS surface data.
On average, MIPAS data prove to be 12 ppbv higher than the surface
measurements. Similarly to the comparison for CH4, the trend over the
period agrees poorly. As for CH4 the MIPAS trend is smaller than
that for the surface data, possibly due to negative instrument drift, which
would be in agreement to the findings of for N2O in the
upper troposphere and lower stratosphere (around 7 ppbvdec-1).
While there are noticeable differences between MIPAS and ACE-FTS and
Aura-MLS, the agreement with Odin-SMR is better. MIPAS seems to have
a tendency to have a high bias in the lower part of the profile and low bias
in the upper part compared to the other instruments. But the latter is true
only for the comparisons to ACE-FTS and Odin-SMR; compared to Aura-MLS the
MIPAS mixing ratios are higher above 40 km.
Validation of full-resolution methane
The comparison of ACE-FTS data to MIPAS CH4 profiles from the full-resolution period (Fig. ) uses only data from the
northern high latitudes, due to the short data overlap and its non-uniform
monthly latitude sampling. Both profiles have a similar shape and show
a steady decrease with altitude. Differences of up to 0.12 ppmv at
17 km occur and in the lower part of the profile MIPAS has slightly
higher mixing ratios than ACE-FTS. Above 19 km the agreement between the
two instruments is excellent.
Comparison of CH4 from ACE-FTS and MIPAS full resolution
(V5H_CH4_21).
Details as in Fig. .
Comparison of CH4 from HALOE and MIPAS full resolution
(V5H_CH4_21).
Details as in Fig. .
As in the reduced-resolution period, the combined error estimate of the
instruments using the MIPAS extended error budget is slightly too low,
except for altitudes below 10 km, where the random errors seem to be
slightly overestimated. Using the MIPAS extended error budget improves
the situation, but the resulting estimate still is lower than the standard
deviation of the difference.
In Fig. the MIPAS full-resolution CH4
product is compared to HALOE. Both the instruments measured a steady decline
with altitude, but the HALOE profile is smoother than MIPAS, for which
the vertical gradient shows more variation. Over almost the entire profile,
MIPAS measures higher mixing ratios than HALOE. The largest
differences occur below 20 km; at its maximum MIPAS is about
0.2 ppmv higher than HALOE.
The standard deviation of the difference indicates that the combined errors
for the instruments are underestimated, even taking the extended MIPAS
error budget into account.
The comparison of MIPAS full-resolution CH4 profiles to SCIAMACHY
(Fig. ) is very similar to that of the reduced-resolution period. Below 25 km the MIPAS mixing ratios are higher;
the differences are below 0.1 ppmv. Between 25 and 35 km the agreement
is very good, MIPAS showing slightly lower mixing ratios. Above
35 km MIPAS has slightly higher mixing ratios than SCIAMACHY. The
combined errors are slightly underestimated.
Comparison of CH4 from SCIAMACHY and MIPAS full resolution
(V5H_CH4_21).
Details as in Fig. .
Comparison volume mixing ratios of CH4 from GCASN and MIPAS full
resolution (V5H_CH4_21).
Details as in Fig. .
From comparisons of MIPAS global mean data to GCASN surface
measurements (Fig. ), we find that MIPAS
measures mixing ratios that are on average 0.07 ppmv higher than the
surface data.
The comparisons with different instruments offer no easy conclusions. While
the MIPAS profiles agree well with ACE-FTS, they have higher mixing
ratios than HALOE. They are higher than SCIAMACHY below 25 km and slightly
lower above that altitude, yet higher again above 35 km. However, at
around 17 km the differences to ACE-FTS, HALOE and SCIAMACHY have their
maximum, so it is likely that MIPAS has a high bias at this altitude.
Between 25 and 35 km MIPAS agrees well with ACE-FTS and SCIAMACHY,
and even while lower mixing ratios were measured by HALOE, the bias between
MIPAS and HALOE is lower than at other altitudes.
Comparison of N2O from ACE-FTS and MIPAS full resolution
(V5H_N2O_21).
Details as in Fig. .
Validation of full-resolution nitrous oxide
In Fig. the MIPAS full-resolution N2O
profiles are compared to those measured by ACE-FTS. While the general
agreement between the instruments with respect to the shape of their profiles
is good, the MIPAS profile below 20 km shows more bumps and kinks.
The differences between the two profiles are of the order of 10 ppbv and
have alternating signs. Their maximum is at the lower end of the profiles and
exceeds 15 ppbv. Below 18 km MIPAS provides mostly higher mixing
ratios than ACE-FTS; above 18 km ACE-FTS is higher, especially between 25
and 32 km. In between 18 and 25 km and above 32 km there is good
agreement between the instruments. The bias is significant over the entire
altitude range. The estimate of the combined error is lower than what would
be expected from the standard deviation of the difference.
The comparison of MIPAS to profiles measured by Odin-SMR generally
looks good (Fig. ). Below 19 km MIPAS is
higher; the largest difference occurs at 16 km and is just below
15 ppbv. Between 19 and 35 km the agreement is almost perfect. Above
that altitude the absolute differences remain very small, but MIPAS is
slightly lower, which leads to notably relative differences. As for the
reduced-resolution period, the combined error estimates are lower than the
standard deviation of the difference in the lower part of the profile, and
higher in the upper part.
Comparison of N2O from Odin-SMR and MIPAS full resolution
(V5H_N2O_21).
Details as in Fig. .
Figure shows the comparison of global monthly
means from MIPAS to those from the HATS surface data for the full-resolution period. The MIPAS mixing ratios are 24 ppbv higher.
Comparison between volume mixing ratios of N2O from HATS and
MIPAS full resolution (V5H_N2O_21).
Details as in Fig. .
Especially around 17 km MIPAS seems to measure N2O volume mixing
ratios which are too high by 10–15 ppbv. The differences to the two
satellite instruments are smaller than the difference to the surface data. At
higher altitudes the comparisons are not conclusive. In general the absolute
differences are small, but there are rather large relative differences where
MIPAS N2O has lower mixing ratios than ACE-FTS and Odin-SMR.
Comparison between full- and reduced-resolution period
The comparisons in Sect. have been done independently for the
two MIPAS measurement periods. However, since some of the comparisons
are available for both the MIPAS full- and reduced-resolution period,
we can draw some conclusions about the consistency of the two data sets in
doing cross-comparisons between the differences to other instruments for the
two MIPAS measurement periods.
Consistency check for CH4
For CH4 the three comparison instruments ACE-FTS, HALOE and SCIAMACHY all
cover both MIPAS measurement periods. However for ACE-FTS, due to the
small overlap between the two instruments, only data between
60–90∘ N in February and March are available. To avoid sampling
artefacts, in Fig. the V5R_CH4_224 and
V5R_CH4_225 products in the same latitudinal range in February and March
only are shown.
Comparison of CH4 from ACE-FTS and MIPAS reduced resolution
(V5R_CH4_224 and V5R_CH4_225) at 60–90∘ N
in February and March.
Details as in Fig. .
This enables a comparison with Fig. without any
effects resulting from seasonal or latitudinal sampling discrepancies.
However, the data for the full-resolution period cover only measurements
from 2004, while the reduced-resolution comparison uses data from 7 years
(2006–2012), which makes this comparison vulnerable to meteorological
anomalies, especially in 2004. The differences between MIPAS and
ACE-FTS for the two MIPAS measurement periods look similar. In general
the differences of the MIPAS reduced-resolution comparison look
smoother than for the full-resolution period. This is probably due to the
different sample size. The MIPAS reduced-resolution data provide
slightly higher differences to ACE-FTS almost over the entire profile. Only
the spike at 17 km in the MIPAS full-resolution profile shows
a similar difference as the reduced-resolution data, where no such spike is
visible.
The comparison between HALOE and MIPAS full-resolution data covers all
the seasons, while for the reduced-resolution data only profiles from January
to August 2005 were measured. Hence we also compared means of full-resolution
data only using profiles in this seasonal range as well. However, the
differences then reproduced the differences shown in
Fig. . For this reason this extra comparison is
not shown here. Comparison of the differences between HALOE and MIPAS
for the two measurement periods (Figs. and
) reveals that over most of the profile, the
MIPAS reduced-resolution data lead to slightly higher differences
than the full-resolution data, while the shape of the differences is similar.
In the full-resolution data however, at 17 km there are particularly high
values in the MIPAS profile. This is similar to the spike in the same
altitude at the comparison of the MIPAS full-resolution data with
ACE-FTS.
The comparison of the differences between SCIAMACHY and the two MIPAS
measurement periods (Figs. and
), shows that at the lower altitudes MIPAS
produces slightly higher mixing ratios for the reduced-resolution period than
for the full-resolution data. At altitudes above 35 km, the full-resolution period seems to lead to higher mixing ratios than the reduced-resolution period.
In the comparisons of MIPAS with the GCASN surface data set
(Figs. and ), the
differences are very similar, the bias for the full resolution is slightly
higher.
In conclusion, all the satellite comparisons suggest that the MIPAS
reduced-resolution period shows slightly higher mixing ratios (about
0.05 ppmv) than the data for the full-resolution period, at least in the
lower part of the profile. An exception seems to be the kink in MIPAS
full-resolution data. Hence there could be some bias between the measurement
periods. The surface data comparison, however, hints at a better agreement
between the two MIPAS data sets in the troposphere than at the
altitudes above.
Consistency check for N2O
N2O profiles from ACE-FTS and Odin-SMR cover both the MIPAS
measurement periods, hence we use those comparison to draw conclusions about
the consistency of the two MIPAS data sets.
For reasons described in Sect. , for the cross-comparison using ACE-FTS, collocated profiles from February and March between
60 and 90∘ N have been used for N2O MIPAS reduced-resolution data (Fig. , for the full-resolution period, see Fig. ). Below 20 km, the
MIPAS reduced-resolution data provides higher mixing ratios than the
full-resolution data; the latter seem to agree better with the ACE-FTS
instrument. Both differences to ACE-FTS have some oscillations, but they are
not correlated.
The comparisons to Odin-SMR both look good (Figs.
and ). For the full-resolution period there are
small differences at 17 km, where MIPAS shows slightly higher
mixing ratios, while for the reduced-resolution period this is less
pronounced. In 19–20 km however, MIPAS mixing ratios in the
reduced-resolution period are slightly higher than Odin-SMR, while for the
full-resolution period the instruments agree well.
As for CH4 the difference between the HATS surface data and MIPAS
for the full-resolution period (Fig. ) is slightly larger
than for the reduced-resolution period (Fig. ).
It is difficult to draw final conclusions, because below 15 km only
comparisons to ACE-FTS are available. They show that, similarly to CH4, it
is likely that MIPAS reduced-resolution spectra lead to higher mixing
ratios than the full-resolution period data. Above 18 km the data from
Odin-SMR suggest that there is hardly any discrepancy between the
MIPAS full- and reduced-resolution periods, while the differences to
ACE-FTS do differ.
Comparison of N2O from ACE-FTS and MIPAS reduced resolution
(V5R_N2O_224 and V5R_N2O_225) in 60–90∘ N
in February and March.
Details as in Fig. .