AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus GmbHGöttingen, Germany10.5194/amt-8-4657-2015 Methane and nitrous oxide retrievals from MIPAS-ENVISATPlieningerJ.johannes.plieninger@kit.eduhttps://orcid.org/0000-0001-9096-5962von ClarmannT.StillerG. P.https://orcid.org/0000-0003-2883-6873GrabowskiU.GlatthorN.KellmannS.LindenA.HaenelF.KieferM.HöpfnerM.https://orcid.org/0000-0002-4174-9531LaengA.LossowS.https://orcid.org/0000-0003-2833-0522Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research, Karlsruhe, GermanyJ. Plieninger (johannes.plieninger@kit.edu)5November20158114657467023April201528July201521October201530October2015This 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://amt.copernicus.org/articles/8/4657/2015/amt-8-4657-2015.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/8/4657/2015/amt-8-4657-2015.pdf
We present the strongly revised IMK/IAA MIPAS-ENVISAT
CH4 and N2O data products for the MIPAS full-resolution
(versions V5H_CH4_21 and V5H_N2O_21) and for the reduced-resolution
period (versions V5R_CH4_224, V5R_CH4_225, V5R_N2O_224 and
V5R_N2O_225). These data sets cover both MIPAS measurement periods
from June 2002 until March 2004 and from January 2005 to April 2012.
Differences with older retrieval versions which are known to have a high bias
are discussed. The usage of the HITRAN 2008 spectroscopic data set leads to
lower values for both gases in the lower part of the profile. The improved
correction of additive radiance offsets and handling of background radiance
continua allows for aerosol contributions at altitudes in the upper
stratosphere and above. These changes lead to more plausible values, both in
the radiance offset and in the profiles of the continuum absorption
coefficients. They also increase the fraction of converged retrievals. Some
minor changes were applied to the constraint of the inverse problem, causing
small differences in the retrieved profiles, mostly due to the relaxation of
off-diagonal regularisation matrix elements for the calculation of jointly
retrieved absorption coefficient profiles. Spectral microwindows have been
adjusted to avoid areas with saturated spectral signatures. Jointly
retrieving profiles of water vapour and nitric acid serves to compensate
spectroscopic inconsistencies. We discuss the averaging kernels of the
profiles and their vertical resolution. The latter ranges from 2.5 to
7 km for CH4, and from 2.5 to 6 km for N2O in
the reduced-resolution period. For the full-resolution period, the vertical
resolution is in the order of 3 to 6 km for both gases. We find the
retrieval errors in the lower part of the profiles mostly to be around
15 % for CH4 and below 10 % for N2O. The errors above
25 or 30 km increase to values between 10 and 20 %, except for
CH4 from the reduced-resolution period, where the estimated errors
stay below 15 %.
Data versions with temporal coverage and information for which the changes
discussed in Sect. were applied.
The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS,
) is a Fourier transform spectrometer on board the
Environmental Satellite (ENVISAT). This satellite was launched in March 2002
and operated by the European Space Agency (ESA) until April 2012, when it
ceased all communication to ground. During that time, MIPAS measured
atmospheric emission spectra in the infrared between 685 and
2410 cm-1 in limb geometry. There are two MIPAS
measurement periods: during the full-resolution period from June 2002 until
March 2004, the instrument measured with a theoretical spectral resolution of
0.025 cm-1 (0.0483 cm-1 after a “Norton–Beer
strong” apodisation; ). Due to a malfunction in the
interferometer slide system, there is a data gap until January 2005 when the
instrument recommenced measuring with a so-called reduced
resolution
ESA uses the term “optimised resolution” for this
period in their product names.
of 0.0625 cm-1
(0.121 cm-1 after the apodisation). The instrument has been
operated in several different measurement modes with different tangent
altitude patterns. For this study, only spectra from the MIPAS nominal
measurement mode have been used; this covers about 80 % of the total
measurement time. For the full-resolution period, one limb scan contains
17 different spectra with tangent altitudes from 6 to 68 km. A limb scan
in the reduced-resolution period consists of 27 different spectra and their
tangent altitudes are latitude-dependent. The lowest tangent altitude ranges
from 6 km at the polar regions to 9 km at the equator and the
highest tangent altitude increases from 70 km in polar regions to
73 km at the equator.
The processor developed at the Institute of Meteorology and Climate Research
in cooperation with the Instituto de Astrofísica de
Andalucía (CSIC) retrieves profiles of temperature and various trace
gases. The retrieval of CH4 and N2O with this processor was
first described in for the full-resolution period and
in for the reduced-resolution period.
Various studies showed CH4 and N2O profiles retrieved from
MIPAS-ENVISAT to have a positive bias in the lower part of the
profiles. In the largest CH4 and N2O
values are compared to tropospheric climatological data and found to be
0.7 ppmv (39 %) higher for CH4 and 80 ppbv
(25 %) higher for N2O. compared
MIPAS full-resolution CH4 and N2O profiles, versions
V3O_CH4_9 and V3O_N2O_9, to those measured by ACE-FTS in version 2.2. This
study uses profiles measured during northern polar winter. Below 15 to
18 km, MIPAS CH4 profiles showed a high bias of around
0.2 ppmv (15 %), while for N2O a MIPAS bias of
around 25 ppbv (10 %) was found below 15 to 18 km.
Von Clarmann et al. (2009) state that the bias in the reduced-resolution
period is smaller than in the full-resolution period, but still present.
did a detailed validation of MIPAS reduced-resolution V5R_CH4_222, comparing it to profiles from ACE-FTS, SCIAMACHY,
HALOE, SOFIE, Mark IV balloon interferometer and cryosampler in situ
measurements. They found MIPAS profiles to be biased high below
25 km by about 0.2 ppmv (14 %).
The aim of this study has been the improvement of the MIPAS IMK/IAA
CH4 and N2O products.
General retrieval descriptions
Here we discuss the latest retrieval setup of CH4 and N2O.
For the full-spectral-resolution period, this refers to data versions
V5H_CH4_21 and V5H_N2O_21 and for the reduced-resolution period to data
versions V5R_CH4_224, V5R_CH4_225, V5R_N2O_224 and V5R_N2O_225. For
simplicity, in the following only the last part of the version code will be
used. This simplification, however, is not applicable beyond this work
because version identifiers are gas-specific. Versions 224 and 225 are
almost identical. Version numbers are different only to guarantee complete
traceability. The only technical difference is the source of the ECMWF
analysis used to constrain the preceding temperature retrievals. This has no
discernible effect on the CH4 and N2O profiles. Thus, we do
not discuss these versions separately. The data sets of these versions are
disjoint in a sense that one observation is either 224 or 225. Thus the data
sets are complementary. For one level-1 product, either a 224 or
a 225 product exists, but not both. All this applies also to older pairs of versions
i.e. 220 and 221, as well as to 222 and 223. Table
offers an overview of the data versions discussed.
Volume mixing ratios (vmr) of CH4 and N2O are retrieved in
MIPAS channel B in several microwindows between 1220 and
1320 cm-1 in the P-branch of the ν4 band of CH4
with the MIPAS-ENVISAT data processor developed at IMK and IAA
. The level 1b data used for the retrievals
were from version MIPAS/5.02–5.06.
For each MIPAS limb scan, profiles of CH4 and N2O are
retrieved. Since there are strong cross-interferences between the CH4
and the N2O lines, both species are retrieved simultaneously in one
retrieval step to minimise mutual error propagation. In addition, mixing
ratios of the interfering species HNO3 and H2O are jointly
fitted to improve the spectral residual. The profiles of temperature,
pressure and ozone as well as the spectral shift correction and the tangent
altitudes are known from previous retrieval steps and are not treated as
variables in the CH4 and N2O retrieval, but their retrieved
values are used within the radiative transfer calculations. An additive
radiance offset correction is retrieved for each microwindow. It is constant
for all tangent heights. Additionally, for each microwindow, continuum
absorption coefficient profiles are retrieved that account for continuum
spectral contributions of atmospheric aerosol, uncertainties in the modelling
of the continua of O3, H2O and N2 and contributions
of distant lines which sum up to a quasi-continuum .
A first-order Tikhonov finite differences constraint
e.g. is used to fight ill-posedness and to
reduce vertical oscillations. The a priori profiles for both gases are zero.
Therefore, not the mixing ratios themselves, but differences between adjacent
profile values with respect to the a priori profile are constrained. Since
the a priori profile is chosen all zero, this type of regularisation acts as
a smoothing constraint. For methane, above 70 km a diagonal element
in the regularisation matrix is used additionally. It pulls the profile in
that height region towards zero. The regularisation matrix entries related to
the continuum absorption coefficient contain some off-diagonal elements to
prevent neighbouring microwindows from differing too much.
The retrieval is done on a fine vertical grid with a spacing of 1 km
between 4 and 70 km. Above that range, the grid gets coarser and
consists of the following grid points: 75, 80, 85, 90, 100 and
120 km. Since not the entire altitude range is covered by
measurements, some data points do not represent the real atmosphere at the
respective altitude. Hence we recommend the applications of two altitude-dependent filter criteria: first, to neglect profile points where the diagonal
element of the averaging kernel is less than 0.03, and second, to discard
points where the visibility flag is “false”. The latter indicates that
MIPAS has not seen the atmosphere at respective altitudes.
The retrieval setup version 2 for the full-resolution spectra is an
adaption of the reduced-resolution spectra setup 224 and 225. All
changes described in Sect. are applied to these data
versions as well. The only difference is a slightly different selection of
the spectral microwindows.
Changes in the retrieval setup
As mentioned in the introduction, the main drawback of previous data versions
was the high bias of CH4 and N2O below 25 km. In
order to improve the data, a number of major modifications in the retrieval
setup were adopted. To make our data traceable, all modifications since
versions 220 and 221 are reported and discussed in the following.
The first part (Sects. and )
deals with the changes which led to the data product versions 222 and 223.
The second part (Sects. to )
explains the further changes which were included in the current product
versions 224 and 225. In Table the included
changes are listed for each data version.
To illustrate the influence of the individual changes a test data set of
110 orbits from the reduced-resolution measurement period with 10 439 limb scans
is used. These spectra were measured between 5 June and 18 August 2010.
Figure shows the zonal mean vmr distributions for
this data set.
Almost all the changes implemented in the 224 and 225 setup for the reduced-resolution
spectra have been included in the new 21 setup for the full-resolution spectra as well. Only the selection of the spectral microwindows
is slightly different. A data set of 1054 measurements from 16 orbits between
10 January and 20 February 2004 was calculated to investigate this new
retrieval setup. In Fig. the latitude-dependent
mean vmr profiles for those calculations are shown.
Zonal mean distributions for reduced-resolution spectra, measured
between 5 June and 18 August 2010. The black lines are the interpolated
isopleths. White areas show regions with no valid data at all. The upper
panel shows results for CH4 (version V5R_CH4_224); the lower panel shows results for N2O
(version V5R_N2O_224).
As Fig. , but for full-resolution spectra,
measured between 10 January and 20 February. The upper panel shows results for
CH4 (version V5H_CH4_21); the lower panel shows results for N2O (version
V5H_N2O_21).
Zonal mean difference distributions. Difference calculation with
HITRAN 2008 spectroscopy of CH4 and N2O minus calculation
with older spectroscopic data sets (for details see text). The upper panel
shows results for CH4; the lower panel shows results for N2O.
Usage of HITRAN 2008
Previous retrievals of CH4 and N2O (up to versions 220
and 221) have relied on the HITRAN 2000 spectroscopic data set with updates from
2001 for N2O and HITRAN 2004 for CH4. The spectroscopic
data sets of CH4 and N2O received an update in the HITRAN 2008
release . Hence in the new CH4 and N2O
retrieval setup, these new line data sets have been used. For CH4 all
the differences in the spectroscopic data set are described in and
references therein. For N2O the updates in HITRAN 2008
have not affected the spectral region of the MIPAS retrieval. In the HITRAN 2004
version, many updates were introduced of the N2O spectroscopy
over the HITRAN 2000/2001 data set which had previously been used by the
MIPAS retrieval. Almost the entire data set has been revised; details can be
found in and references therein.
In Fig. the influence of the usage of
the updated spectroscopic data set is presented. Areas where the new
spectroscopy leads to higher mixing ratios are red; those where it produces
lower mixing ratios are blue. Both in CH4 and N2O, the mixing
ratios are smaller in the lower part of the measured profiles. For
CH4 the main changes are below 20 km in the tropics and below
15 km in the polar regions. The differences are most pronounced in
the troposphere at the lower boundary of the data product, where new
spectroscopic data lead to values of up to 0.18 ppmv lower than the
old ones. These findings agree with who examined the
influence of the HITRAN 2008 spectroscopy on the CH4 profiles
retrieved from the NASA AURA Tropospheric Emission Spectrometer (TES,
) and found the values to be lower with the new data set. The
N2O profiles with the HITRAN 2008 spectroscopy show smaller values up
to almost 35 km. Only the lowermost part of the valid data has
a difference of less than -10 ppbv, the minimum in differences is
-19 ppbv. The main differences in the retrieved CH4 and
N2O result from updates of spectroscopic parameters for lines already
existing in the database. Only very small differences between the old and new
spectroscopy can possibly be attributed to new lines added within the used microwindows.
To determine, whether the changes of the profiles are due to either the
spectroscopic changes of CH4 or N2O, retrievals where for
only one of the gases an updated spectroscopy was used, were carried out. It
turned out, that the changes in CH4 are mostly due to the changes in
the CH4 spectroscopy, and to a smaller extent to changes in the
N2O spectroscopic data set. The changes in N2O profiles are
mostly due to the changes in the N2O spectroscopy, but the larger
changes at the very lower boundary in southern latitudes are due to the
updates of the CH4 spectroscopy.
Radiometric offset and continuum contributions
For the data versions 220 and 221 the continuum absorption coefficient
was fitted up to an altitude of 32 km, which was the original
suggestion of for the retrieval of
temperature. However, in some publications (e.g. )
relevant aerosol abundances above that altitude are
discussed. To account for contributions from particles in this altitude
range, we extended the upper boundary for the fit of the continuum absorption
coefficient to 60 km.
As Fig. , but difference
retrieval with new offset and continuum setup minus reference with old
setup.
The residual radiance offset in versions 220 and 221 has been fitted
independently for each individual tangent height. In combination with the
fitting of the continuum absorption coefficient, this leads to an ill-posed
inverse problem, where the contributions of both these variables can hardly
be distinguished: continuum absorption coefficients that are too high can nearly
compensate radiance corrections that are too low and vice versa. This leads to
linearly dependent rows in the Jacobians of the retrieval. Thus, increasing
the altitude range for the fit of the continuum absorption coefficient alone
leads to even bigger oscillations. To avoid these instabilities, we now use
a constant additive radiative offset over all tangent heights, a hard
constraint of the radiance offset which IMK/IAA retrievals of most other
species had already applied. This approach avoids any oscillating
compensations between radiometric offset and the continuum absorption
coefficient and leads to smoother profiles of the latter. Convergence does
slightly improve. Instead of 10 378, now 10 403 of a total of 10 439 cases
do converge. The scientific analysis of the origin of the retrieved
background radiation, probably aerosols, remains to be done.
The degrees of freedom both in the profiles of methane and
nitrous oxide prove to increase slightly (by 3.9 % for CH4, by
5.9 % for N2O). The difference between the retrieval with
constant radiometric offset and continuum absorption coefficient fitted up to
60 km and the setup versions 220 and 221 is shown in
Fig. . Both the volume mixing ratios of
CH4 and N2O are decreased in the altitude range below
20 km. Hence the continuum handling in the old setup led to an effect
of downward error propagation and to higher values in these altitude regions.
Both gases show an increase at roughly 30 km (in the southern polar
regions this feature is at slightly lower altitudes).
Constraint
The IMK/IAA CH4 and N2O retrievals use an altitude-dependent
Tikhonov-like smoothing constraint based on squared first-order finite
difference matrices L1. Up to data versions 222 and 223
the altitude dependence was implemented as suggested by . This
constraint has been replaced by a constraint using a regularisation matrix
R=L1TAL1+D, where
A is a diagonal matrix controlling the altitude dependence of the
constraint and D is zero except for some diagonal entries above
70 km (increasing with height) affecting methane only. At those
altitudes, they pull the profile of methane towards the a priori which is
zero. It is introduced because otherwise the retrieved methane profiles tend
to show negative values above 70 km, even in the zonal averages,
which clearly is an artefact. Adding the diagonal element in the constraint
leads to values close to zero. However,some slightly
negative retrieved volume mixing ratios still remain in the zonal mean at 75 km.
Our analysis shows that the introduction of the diagonal element to the
regularisation matrix R does not alter the volume mixing ratio
profiles below that altitude. Hence these physically still erroneous values
above do not affect the quality of the data product below.
A further change was applied with respect to the off-diagonal matrix elements
which pull the continuum absorption coefficient profiles of neighbouring
microwindows towards each other. These entries have been considerably
reduced, which allows a more pronounced spectral structure of the background
emission. This leads to slightly decreased values of CH4 at
20 km altitude and N2O below 15 km. This is shown in
Fig. , where the difference between
the calculations with the new constraint setup minus those with the old setup
is plotted. The degrees of freedom of CH4 were reduced by 0.25 and of
N2O were increased by 0.25, while the continuum absorption
coefficient profiles gained about 14 degrees of freedom.
Spectral microwindows
Usually, high-resolution spectroscopy retrievals do not invert the entire
measured spectra, but only narrow spectral windows containing lines of the
target species, so-called microwindows e.g.. The
selection of these spectral microwindows has been changed compared to
previous data versions. The microwindows from 1270 cm-1 towards
higher wavenumbers have been restricted to higher altitudes because the
spectra in that region were saturated around the position of the line centres
below. Additional microwindows at 1225, 1239 and 1245 cm-1 and
in the range of 1257 to 1270 cm-1 have been introduced to
compensate the related loss of information at the higher wavenumbers and to
stabilise the joint fit of H2O (see Sect. ).
As Fig. , but difference
retrieval with Tikhonov constraint and relaxed constraint between
neighbouring microwindows minus reference with old setup.
Selected microwindows of the setups V5R_CH4_220, V5R_CH4_221,
V5R_N2O_220 and V5R_N2O_221 as well as V5R_CH4_222, V5R_CH4_223,
V5R_N2O_222 and V5R_N2O_223 (upper panel) and V5R_CH4_224,
V5R_CH4_225, V5R_N2O_224 and V5R_N2O_225 (lower panel) as a function of
wavenumbers and tangent altitudes. The microwindows are marked as black
patches.
As Fig. , but difference
retrieval with new selection of microwindows minus reference with old
setup.
The selection of the microwindows used in versions 220, 221, 222 and 223
are compared to those of versions 224 and 225 in Fig. .
Figure shows the impact of
the new microwindow selection on the mean profiles of CH4 and
N2O. For both species the volume mixing ratios below 20 km
did decrease at all latitudes. In the subtropics and tropics between 20 and
30 km, an increase in volume mixing ratio can be observed. Convergence
of the iterative retrieval was achieved in a larger number of cases: instead of
10 399 in setup versions 222 and 223, now 10 421 measurements (of
10 439 total) converged. The changes in the microwindows led to a slight
decrease (by 6 %) in the root-mean-square difference between the measured
and the best-fit spectra (RMS) and to an increase in degrees of freedom for
methane (by 2.3 %). For N2O the degrees of freedom dropped
slightly (by 2.7 %).
Joint fit of water vapour and nitric acid
To improve the fit and to reduce the systematic residuals in the best fit
spectra, the mixing ratios of HNO3 and H2O are retrieved
additionally to CH4 and N2O, continuum absorption coefficient
and radiometric offset (joint fit approach) in the latest versions.
Versions 222 and 223 and earlier versions used the volume mixing ratio profiles
which were retrieved in previous retrieval steps as constant parameters. The
new approach can compensate for any spectroscopic inconsistency between the
spectral microwindows of the specific gas retrievals (in this case
HNO3 and H2O) and those used in the setup for the retrieval
of CH4 and N2O.
The influence of the joint fit of HNO3 and H2O on the results
for CH4 and N2O is shown in
Fig. . Both the
profiles of CH4 and N2O are moderately reduced below
20 km altitude. The RMS does hardly change and the degrees of freedom
for CH4 and N2O decrease slightly.
Retrieval characterisation
The CH4 and N2O profiles from the reduced-resolution period
derived with the new setup versions 224 and 225 show significantly
reduced oscillations in polar regions compared to those retrieved with
versions 222 and 223. The fraction of converged retrievals of the
entire data set has significantly increased compared to the older versions.
With the new setup, 0.37 % of the retrievals did not converge, compared to
1.02 % of the profiles with setup versions 220 and 221, and 1.27 % with
versions 222 and 223.
As Fig. , but difference
retrieval with joint fit of HNO3 and H2O minus reference with
old setup.
Mean vertical resolution in km for profiles from reduced-resolution
spectra (versions V5R_CH4_224 and V5R_N2O_224). The upper panel shows results for
CH4; the lower panel shows results for N2O.
Selected rows of the averaging kernel matrix for the measurement in
orbit 43202 at 39.4∘ N, 78.9∘ E on 5 June 2010,
05:02:29 UTC (derived from reduced-resolution spectra, version
V5R_CH4_224). The black crosses highlight the diagonal terms of the
averaging kernel matrix. The blue dashed-dotted line gives the integral value
of the rows (upper axis). The upper panel shows results for CH4 and the lower panel shows results for
N2O.
Estimated error contributions for the measurement in orbit 43202 at
39.4∘ N, 78.9∘ E on 5 June 2010, 05:02:29 UTC
(derived from reduced-resolution spectra, versions V5R_CH4_224 and
V5R_N2O_224). Left panels: gas profiles, right panels: contributions of
different errors. All contributions are absolute values, except for the total
relative error, which is given as a percentage (%; upper axis). The upper panels show results for
CH4; the lower panels show results for N2O.
The mean vertical resolution of the test data set (see
Sect. ) is shown in
Fig. both for CH4 and N2O.
The values are full widths at half maximum of the rows of the averaging
kernels. For CH4 a resolution of about 2.5 to 7 km can be
obtained below 60 km. Above this level, it becomes rather coarse, due to the
larger tangent height spacing. At altitudes below 20 km there is
a slightly degraded resolution. The best resolved part of the profile is the
altitude range between 25 and 35 km, where the resolution reaches up
to 2.2 km. N2O is resolved well between 20 km
(15 km in mid-latitudes) and 50 km, where the resolution is
about 2.5 to 6 km. Above this range the volume mixing ratios are too
small to allow for a good signal in the spectra. For both gases the mid-latitude regions have a better resolution than the tropical or polar areas.
This is due to the colder tropopause in the latter regions, which produces
lower emissions and hence a weaker signal.
In Fig. a subset of the
averaging kernel for a sample profile is shown. This reduced-resolution scan
was measured in orbit 43202 at 39.4∘ N, 78.9∘ E on 5 June 2010,
05:02:29 UTC. The plot shows the rows of the averaging kernel:
each black cross on one curve denotes the nominal altitude of the related
averaging kernel. The rows of the averaging kernels represent the weights of
the true atmospheric states at various altitudes in the retrieval at the
nominal altitude. In the case of CH4, for most of the curves the
black cross lies at the maximum position, i.e. the atmospheric state at
a certain altitude has the largest impact on a retrieved value in the same
grid height. The curves are roughly symmetric to this point in shape. This
allows a straightforward interpretation of the retrieved profiles. Above
55 km and below 15 km, the curves do not always have their maximum on the
nominal grid point and/or are asymmetric in shape. Without considering the
averaging kernels, this can cause biases, and thus, interpretation of the
retrieved profile should be conducted more carefully. The dashed-dotted blue
line gives the integral of the averaging kernel rows. Due to the
Tikhonov-type regularisation, the integral values are around 1 below
60 km. Above 60 km they decline due to the diagonal term in the
regularisation matrix, indicating that some of the information for these
altitudes is not based on the measurement.
The lower panel of Fig. shows
the averaging kernels for the retrieval of N2O for the same
measurement. Roughly symmetric curves can be found between 20 and
50 km. All the information for the retrieved values above
50 km are almost entirely dependent on the values below. Outside this
range the data without explicit consideration of the averaging kernels are
prone to misinterpretation. The integrals of the averaging kernel rows are
1 at all altitudes, since there are no diagonal elements in the
regularisation matrix.
Along with the retrieval, for each profile the impact of the instrument noise
on the retrieved profile is calculated as a routine data product. However
there are various additional error sources which have to be considered. These
errors have been estimated for certain example profiles. The following errors
have been assumed: for the uncertainty of the line of sight (los)
0.15 km vertical pointing ; for the
spectral shift 0.005 cm-1; for the instrumental calibration
error (gain) 1 % ; for the instrumental line shape
error (ils) 3 % (F. Hase, personal communication, 2015); and for the temperature gradient in
latitudinal direction 0.01 K km-1 (constant with respect to
altitude). The spectroscopic errors were extracted from the HITRAN database
, with a correction of the actual line intensity in
dependence of the rotational quantum number as suggested by
. Resulting line intensity uncertainties are between
2 and 5 % for CH4 and between 4 and 7 % for
NO2 for low J values. The representative uncertainties in air
broadening coefficients have been estimated at 15 % for CH4 and
3.5 % for N2O. Since no information on the error correlations
between the individual transitions was available, these errors were assumed
to be fully correlated, which implies that the error estimation is on the
conservative side. The profiles of temperature and O3 are known from
previous retrieval steps; related retrieval errors were propagated onto the
CH4 and N2O results. The contribution of CO2,
SO2, NO2, HOCl, HCN, H2O2, C2H2,
COF2, CFC-14, N2O5 and ClONO2
to the spectra was calculated based on climatological abundances.
For these gases, estimated profiles of 1σ were used to estimate corresponding
CH4 and N2O retrieval errors.
As Fig. , but for profiles from
full-resolution spectra (versions V5H_CH4_21 and V5H_N2O_21).
As Fig. , but for the
measurement in orbit 10324 at 46.0∘ N, 144.5∘ W on
20 February 2004, 07:48:31 UTC (derived from full-resolution spectra,
version V5H_CH4_21).
As Fig. , but for the
measurement in orbit 10324 at 46.0∘ N, 144.5∘ W on
20 February 2004, 07:48:31 UTC (version V5H_N2O_21).
Profiles and total errors calculated from error budgets for
different atmospheric conditions measured during the reduced-resolution
period. Left panels: profiles, middle panels: absolute total errors, right
panels: relative total errors. The colours indicate the atmospheric conditions
annotated in the legend. The black curves titled “example” are for the
error budget shown in Fig. . The upper
panels show results for CH4; the lower panels show results for N2O.
The estimated error contributions of all these sources are shown in
Fig. for the measurement at 5 June 2010,
05:02:29 UTC along with the derived profiles. Below
15 km the instrument noise is the most contributing source to the
error for both gases. Above, the other errors have larger contributions.
Especially the estimated spectroscopic error is very large and dominates the
error budget between about 17 and 42 km for CH4 and between
20 and 38 km for N2O. It has, however, to be mentioned that
these spectroscopic error estimates are speculative because the
inter-transition correlations of the errors are not known. The assumption of
full correlations may be over-conservative. Inter-transition correlations of
less than unity would lead to partial randomisation of this kind of error and
the resulting uncertainty would be largely reduced. The second largest error
contribution is the uncertainty of the vertical pointing of the line of
sight. For CH4, below 60 km the relative total error is
between 5 and 17 %. The relative total error of N2O below
30 km is around 5 to 10 %; above 30 km it increases with height to
values in the order of 10 to 15 %, until it further increases above
40 km. Tables and
give numeric values for the more important error contributions for a few
selected altitude grid points.
For the full-resolution spectra, oscillations in the CH4 profiles
were considerably reduced in version 21. However, a larger
fraction of the retrievals did not converge (8.50 % instead of 2.78 %).
Figure shows the mean vertical resolution
of a test data set for the full-resolution spectra. Between 15 and
40 km the resolution is in the order of 3 to 6 km for both
gases. The resolution is not as good as in the period of reduced spectral
resolution, because each limb scan consists of fewer tangent altitudes.
The averaging kernels of the test measurement between 15 and 50 km
generally look well-behaved (Fig. ), but in some cases,
particularly at very high or low altitudes, are slightly off-centre. This
should be kept in mind for further interpretation of the data. In most cases
the retrieved data points are most sensitive to atmospheric volume mixing
ratios slightly below the altitudes they are calculated for (between 1 and
2 km). The integral values of the rows show a similar behaviour as
for the measurements in the low-resolution period.
The error budget shown in Fig. makes
clear that the instrument noise is larger than the other errors at the
altitudes below 15 km. Again the largest contributor to the error
above that altitude is the spectroscopic error, followed by the uncertainty
of the tangent altitude pointing (line of sight). The relative error of
CH4 is between 5 and 20 %. For N2O it is between 5 and
10 % below 25 km and between 5 and 20 % between 25 and
45 km. As said before, the spectroscopic error may be overestimated,
because the assumption of inter-transition correlations of these errors may
be too pessimistic. Numeric values for certain altitude grid points and the
more important error contributions are reported in
Tables and .
Error budget for CH4 for the measurement in orbit 43202 at
39.4∘ N, 78.9∘ E on 5 June 2010, 05:02:29 UTC (derived from
reduced-resolution spectra, version V5R_CH4_224). Relative values are given as percentages.
As Fig. , but for profiles measured during
the full-resolution period.
To estimate how representative the error analysis is, we calculated the
extended error budget for additional profiles. To cover different states of
the atmosphere, we used profiles in the northern and southern mid-latitudes
and in the tropics, each for night and day. We also used a polar winter and a
polar summer profile. For each case, we selected the profile as
representative which shows the least sum of the quadratic deviations from a
mean profile for that case. The total errors for these cases are shown in
Fig. for the reduced-resolution period and in
Fig. for the full-resolution period. Mid-latitude
errors in the lower part of the profiles are usually larger than the
examples. However, most of the differences occur in areas where the profiles
have no valid information. Around 20 km the example profiles show
similar values. Between 23 and 34 km the example profiles for the
reduced resolution have smaller relative errors than the additional error
budgets; above that, the profiles show similar errors. For the low-resolution
period, the example errors are, in general, similar to the errors of other
mid-latitude profiles, but there are areas where either one or the other
error is larger. For most cases polar winter profiles show lower absolute
errors, but higher relative errors than the examples, especially for the
profiles in the reduced-resolution period, where those profiles are measured
in the stronger southern polar vortex. Polar summer errors are similar to
those in the mid-latitudes. The tropical profiles have larger errors in most
cases. Exceptionally large errors due to the spectroscopic uncertainties can
be found for the tropical night profile measured during the full-resolution
period (Fig. ).
The largest component of the error budget us usually the spectroscopic error
(which might be overestimated as stated above). The biggest differences of
the error budgets for individual profiles are usually due to the error of the
pointing of the line of sight. It shows a bigger variation than the other
errors. Especially sensitive is the tropopause region, where the profiles
show strong changes of the gradient. Quite often, the error budget shows a
spike in that region as well, which is caused by the error due to the
uncertainty of the line of sight. This can be seen for example in the total
error of the southern mid-latitude daytime profile for N2O for the
full-resolution period.
Error budget for CH4 for the measurement of in orbit 10324 at
46.0∘ N, 144.5∘ W on 20 February 2004, 07:48:31 UTC (derived
from full-resolution spectra, version V5H_CH4_21). Relative values are given as percentages.
The new MIPAS-ENVISAT CH4 and N2O profiles
versions 21, 224 and 225 are now available for the complete
MIPAS measurement period. The usage of the HITRAN 2008 spectroscopic
data set improved continuum and offset handling, minor changes in the
constraint, inclusion of H2O and HNO3 to the retrieval vector
and different selection of spectral microwindows overall lead to improved
data products where the known high bias has been reduced. Averaging kernels
are found to be symmetric in the stratosphere. The vertical resolutions there
are in the order of 2.5 to 7 km for CH4, 2.5 to
6 km for N2O during the reduced-resolution period and in the
order of 3 to 6 km for both gases during the full-resolution period.
The relative errors in the lower part of the profiles are mostly around
15 % for CH4 and below 10 % for N2O. They
increase above 25 or 30 km to values between 10 and 20 %, except
for CH4 from the reduced-resolution period, where the error remains
below 15 % over almost the entire profile below 60 km. It turned
out that knowledge of the air broadening coefficients and line intensities of
the individual lines is insufficient to reliably estimate the propagation of
spectroscopic errors on the retrieved vmr profiles. In addition, information
on the inter-transition correlations of these errors is needed. The extended
error budget itself depends on the atmospheric state, hence the absolute
given numbers can not be simply attributed to any other profile; but
nevertheless, they give a good estimate about the general qualitative nature
of the errors discussed. Comparisons to other instruments will be the subject
of an upcoming paper.
Acknowledgements
This work is a contribution to the “Helmholtz Climate Initiative REKLIM”
(Regional Climate Change), a joint research project of the Helmholtz
Association of German research centres (HGF). We thank ESA for providing the
MIPAS level-1b data.
We acknowledge support by Deutsche Forschungsgemeinschaft and Open
Access Publishing Fund of Karlsruhe Institute of Technology.
The article processing charges for this open-access publication
were covered by a Research Centre of the Helmholtz Association.
Edited by: M. Riese
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