Introduction
Changes in the atmospheric ozone distribution originating from natural and
anthropogenic sources have an effect on the Earth's climate. These changes
are altitude dependent, thus requiring continuous monitoring of the
vertical distribution of ozone in the atmosphere. Ozone depletion events in
the Antarctic and occasionally also in the Arctic region, as well as
long-term changes in the midlatitude ozone, can be monitored by satellites
that provide vertical ozone profiles. These satellite-based vertical ozone
profiles have almost global coverage and can be applied for various
applications and research areas from local phenomena to global evolution.
Ozone profiles have been retrieved from various spaceborne instruments with
different sensor types and measurement techniques since the 1970s
.
This paper discusses the quality of the operational ozone profiles derived
from the nadir-viewing Global Ozone Monitoring Experiment-2 (GOME-2) aboard
the first of a series of polar orbiting Meteorological operational (Metop)
satellites Metop-A. The GOME-2 instrument is
a successor to Global Ozone Monitoring Experiment (GOME) (launched 1995). At
present, there are two GOME-2 instruments flying: one on Metop-A (launched
2006) and one on Metop-B (launched 2012). The third GOME-2 instrument will
be launched on Metop-C (planned 2017).
The global performance of the optimal estimation-based Ozone ProfilE
Retrieval Algorithm (OPERA) using measurements from GOME on board the
European Space Agency's (ESA) second Earth Remote Sensing satellite (ERS-2)
was studied by , focusing on the convergence behaviour of
the algorithm. The impact of the first guess on the retrieved profiles as
well as the choice of the ozone climatologies and ozone cross-sections were
also analyzed. After the further development of OPERA,
have shown that the algorithm can also be applied to other nadir instruments
measuring in UV–VIS range such as OMI and SCIAMACHY.
A comprehensive validation of GOME-2/Metop-A ozone profiles has been done
within the framework of the EUMETSAT's Ozone Monitoring and Atmospheric
Composition Satellite Application Facility (O3M SAF) project using balloon
ozone sondes, lidars and microwave radiometers. The validation with the
balloon and ground-based instruments covers the altitude range from the
Earth's surface up to about 60 km. The overall outcome is that, in
general, the relative difference meets the operationally required target
value of 30 % in the troposphere at most of the reference stations. In
the stratosphere the 15 % target value is met below 37 km, above
which the differences increase and the GOME-2 ozone values are
systematically lower compared to the ground-based instruments. The increasing
negative bias in the GOME-2 ozone profiles above 37 km at all the
reference stations was noticed to start after November 2008. In general, the
validation with ozonesondes shows better agreement at midlatitude stations
than at higher latitudes and the tropical region. The detailed validation
report can be found at http://o3msaf.fmi.fi.
In this paper, the quality of the ozone profiles retrieved by OPERA using
GOME-2/Metop-A measurements is assessed by comparison with profiles
retrieved from limb-viewing satellite instruments: Global Ozone Monitoring by
Occultation of Stars (GOMOS), Optical Spectrograph and Infrared Imager System
(OSIRIS) and Microwave Limb Sounder (MLS). The limb-viewing instruments have
a significantly better vertical resolution than nadir-looking instruments,
which allows probing the uncertainties related to the coarse vertical
resolution of GOME-2. The selected limb-viewing instruments practically do
not use a priori information in the inversion. They provide ozone profiles
with very high accuracy and have a small bias with respect to the
ground-based measurements (e.g. ). The altitude region
for the comparison is between 15 and 60 km and the comparison period
covers almost 4 full years of collocated data from March 2008 to
December 2011.
The paper is organized as follows. The characteristics of the ozone profile
data involved in the comparison are presented in Sects. and
. The methodology for the comparison of ozone
profiles is presented in Sect. . Finally, the comparison
results are presented and discussed in Sect. , and
conclusions are provided in Sect. 6.
GOME-2 ozone profiles
Metop-A was launched on 19 October 2006 into a Sun-synchronous polar orbit at
an altitude of about 840 km and is the first satellite in EUMETSAT's
Metop series. GOME-2 aboard Metop-A is a nadir-viewing scanning
spectrometer using four channels in the ultraviolet (UV) and visible (VIS)
range between 240 and 790 nm with a spectral resolution of
0.2–0.4 nm. The standard footprint size of the ground pixel is
usually 640km×40km for the UV part and
80km×40km for the VIS part of the spectrum. The
equator crossing local time is 09:30 a.m. for the descending node. The
GOME-2 measurements are used to retrieve total ozone column and ozone
profiles, surface UV radiation, aerosols and total columns of NO2,
SO2, BrO, HCHO and H2O as well as tropospheric
subcolumns of NO2 and ozone .
The GOME-2 ozone profiles are retrieved using the UV–VIS spectral range
between 265 and 330 nm. The retrieved profiles are available from
January 2007 onwards. The ozone profiles are generated by the OPERA algorithm
developed at the Royal Netherlands Meteorological Institute (KNMI). The
algorithm uses an iterative approach when fitting state vector (ozone
profile) to the measured radiances using an optimal estimation
method and the LIDORTA radiative transfer model
. The ozone climatology by is used as
a priori data. The error analysis shows that the dominant errors exceeding
the 5 % level originate from uncertainties in the spectral calibration,
ozone a priori profile, temperature profile, cloud top pressure and forward
model errors see ATBD for the details.
For users there are two types of products available: a Near-real-time Ozone
Profile product consisting of 3 min data blocks within 3 h of sensing and
an Offline Ozone Profile product consisting of data blocks as whole orbits
within 2 weeks. Both of these products are retrieved in a coarse resolution
using 640km×40km ground pixels and also in
a high resolution using 80km×40km ground
pixels see the ozone profile product user manual. The
ozone profile products are produced at KNMI and available to users in NRT via
the EUMETCAST system and offline via the O3M SAF archive
(http://o3msaf.fmi.fi). One orbit file contains the observations from
the sun-lit side of the Earth. Global coverage can be achieved in 1.5 days
with about 14 orbits daily.
In this paper, we use the operational ozone profile data from the coarse
resolution Level 2 Offline Ozone Profile product. Our data set covers years
2008–2011 retrieved with nine OPERA software versions 1.14–1.24. The
Level 2 ozone profile product contains a priori profile, averaging kernels,
full error covariance matrix, retrieval noise covariance matrix and other
relevant information .
We have used the GOME-2 ozone profiles with a quality processing flag
indicating successful retrieval . The ozone profile is
given as partial columns in Dobson units (DU) at 40 layers between
logarithmically spaced pressure levels between the surface and
0.001 hPa . The cloud-top pressure
replaces the surface pressure level in cloudy and partially cloudy scenes.
The vertical resolution of the ozone profile retrieved using OPERA is between
7 and 15 km .
Figure shows averaging kernels for a one
example retrieval at pixel (42.7∘ N, 44.5∘ W) on
10 February 2010. The nominal altitude of each averaging kernel is marked by
a circle (left panel). Typically, the averaging kernels are not peaked at
their nominal altitudes; this suggests that other altitudes contribute
information to ozone value at individual retrieval altitude. The row sums of
averaging kernel matrix (in the middle panel) show the altitude ranges where
the observations are sensitive to the profile. The degrees of freedom for
signal (DFS) in this example pixel is 3.5 that reveals total number of
independent pieces of information. The cumulative DFS (right hand panel)
indicates that the retrieved ozone profile has collected information from the
measurements between 8 and 50 km.
Reference spaceborne ozone profiles used in the comparison
In this comparison study we have used reference measurements from three
satellite instruments: GOMOS/Envisat, OSIRIS/Odin and MLS/Aura. These
instruments have a long-term data record with ozone profiles available for
a time period between 2002 and 2012 for GOMOS, from 2001 to present for OSIRIS
and from 2004 to present for MLS. Table gives a
short summary of the data used in the comparison; a more detailed description
is presented below.
GOMOS ozone profiles
The GOMOS instrument on board the ESA's Envisat satellite was launched in
2002 and it monitored ozone, other trace gases (such as NO2,
NO3, H2O and O2) and aerosols until 2012 using stars
as light sources . The spectral range of the spectrometers
is in the ultraviolet-visible wavelengths (248–690 nm) and in the
near-infrared (755–774 and 926–954 nm).
GOMOS utilized a stellar occultation technique to measure the vertical
distribution of ozone at the altitudes between 10 and 100 km
. The retrieval is based on the maximum likelihood method
and it uses only minimally a priori information of the vertical distribution
of ozone, namely smoothness that results in vertical resolution of 2–3 km
.
Averaging kernels (AK) of GOME-2 retrieval for one pixel at
(42.7∘ N, 44.5∘ W) on 10 February 2010. Rows of AK matrix
(left panel), row sums of AK matrix (middle) and cumulative DFS (right panel)
for the first 38 retrieval levels. The circles (left panel) represent the
nominal altitudes of the averaging kernels.
The accuracy of the ozone profile derived from the GOMOS retrieval depends on
the star magnitude and temperature of the star
. The most accurate ozone profiles are
retrieved using the hot and bright stars. The GOMOS nighttime measurements
have high precision with a retrieval error in the stratosphere around
0.5–4 % and in the mesosphere around 2–10 % .
The stratospheric ozone precision estimates from the GOMOS nighttime
measurements have been validated by using the differential method presented
by . The summary of the geophysical validation of GOMOS
measurements is presented in .
Description of ozone profile data used in this paper.
Instrument
GOME-2
GOMOS
OSIRIS
MLS
Principle
nadir scatter
star occultation
limb scatter
limb thermal emission
Local time
∼ 09:30 a.m.
∼ 10:00 p.m. outside polar area
∼ 06:00 a.m. and 06:00 p.m.
∼ 01:45 p.m.
Algorithm version
OPERA v1.14–1.24 and v1.32 a
IPF 6.0 b
FMI v3.2 c
MLS v3.3 d
Ozone unit
layer partial column in DU
number density
number density
volume mixing ratio
Altitude range
0–70 km
10–100 km
10–70 km
261-0.02 hPa
Vertical levels
41
100–150
25–60
37–47
Vertical resolution
7–15 km, depending on altitude
2–3 km, depending on altitude
2–3 km, depending on altitude
2.5–3 km
Native vertical grid
logarithmically spaced between surface
tangent altitudes
∼ 1–3 km
12 per decade between
and 0.001 hPa
(vertical sampling < 1.7 km)
1000 and 1 hPa
Random retrieval
∼ 6 % in lower troposphere
0.5–4 %e
0.5–3 %
2–100 % (261–0.1 hPa)d
uncertainty
< 2 % in stratosphere
> 3 % in lower stratosphere
a ,
b , c ,
d ,e .
In this comparison study, we have used nighttime (with solar zenith
angle > 107∘) ozone profiles from the GOMOS processor version 6
data (Table ). We have selected only stars that
are medium to bright (magnitude Mv≤2) and stars that are hot
(temperature ≥7000 K). Additionally, GOMOS data were filtered for
outliers and unreliable data using the recommendations of the readme document
(https://earth.esa.int/web/sppa/mission-performance/esa-missions/envisat/gomos/products-and-algorithms/products-information).
OSIRIS ozone profiles
The OSIRIS instrument on board the Swedish Odin satellite, launched in 2001,
measures limb scattered solar light at the 280–810 nm wavelength
region with around 1 nm spectral resolution .
OSIRIS also includes an infrared imager with three channels but in this study
we only use the spectrograph data. The OSIRIS spectrograph data can be used
to retrieve various trace gases such as ozone, NO2, OClO, BrO, and
aerosols. OSIRIS measures the atmosphere near 06:00 p.m. local solar time on
the ascending node and near 06:00 a.m. local solar time on the descending
node.
The OSIRIS ozone profile data used in this paper are retrieved using an onion
peeling type inversion method developed at the Finnish
Meteorological Institute (FMI). The FMI-OSIRIS Level 2 version is 3.2 and it
uses the Level 1 version 55092 data. In the FMI-OSIRIS ozone retrieval,
around 300 wavelengths between 280 and 680 nm are used to produce ozone
profiles between 10 and 70 km. Generally, the FMI-OSIRIS ozone
profiles agree within ∼5 % with the GOMOS night time and MLS data
. The difference between the FMI-OSIRIS
ozone and the other OSIRIS ozone product retrieved using the SaskMART
algorithm is typically a few percentage points in the
stratosphere .
In this comparison we only use OSIRIS morning measurements, which is
convenient because GOME-2 measures in the morning too. There were some
OSIRIS ozone profiles containing unrealistic ozone concentration values which
were omitted from the analysis.
MLS ozone profiles
The MLS instrument on board the Earth Observing System (EOS) Aura satellite,
launched in 2004, measures microwave thermal emission from the limb
. The Aura satellite's equator crossing time is near
01:45 p.m. The MLS instrument observes emissions in spectral regions
centred near 118, 190, 240, 640 GHz and 2.5 THz. The
retrieved vertical profiles of trace gases include ozone, BrO,
ClO, CO, H2O, HCl, HCN, HNO3,
N2O and SO2. The MLS limb-viewing measurements are processed
using an optimal-estimation-based retrieval method .
Ozone is retrieved using the 240 GHz retrieval. The vertical ozone
profiles are reported as mixing ratios at pressure levels. The comparison in
this paper includes MLS version 3.3 Level 2 ozone profiles
. The useful pressure range for the MLS version 3.3 ozone
profiles is between 261 and 0.02 hPa (∼ 10–80 km).
The MLS ozone comparison in the stratosphere with other profiles from
satellite, balloon, aircraft and ground-based data resulted in an overall
agreement of 5–10 %
.
We used MLS ozone profile data filtered using data quality metrics provided
along with the profile data and as recommended in the document by
. In addition, we only used MLS data with the SZA<80∘. High SZA observations often have additional uncertainties,
but with MLS we can avoid these measurements because the spatiotemporal
coverage is very good, anyway.
Collocation criteria
We selected collocated profiles with the spatiotemporal separation presented
in Table . They are the following: ±12 h, ≤400km
for GOMOS, ±6 h, ≤200km for OSIRIS and ±6 h, ≤100km for MLS. The geographic distance between GOMOS, as well as
OSIRIS, and the GOME-2 ground pixel centre is calculated approximately at
30 km tangent point location. For MLS, the distance is calculated
between the GOME-2 pixel centre and MLS geolocation. We would like to note
that the geophysical distance in the atmosphere can differ from the ground
distance by 20–30 km .
For each reference instrument, the collocation criterion is a compromise of
having the smallest spatiotemporal separation whilst having sufficient
amount of collocations. The separation in time is dictated by the local time
of the measurements. For the spatial separation with GOMOS (separation less
than 400 km), we used the effective horizontal resolution of the considered
limb/occultation measurements (e.g. ). Decreasing of the
allowed geographic distance or time difference reduces the number of
collocated ozone profile pairs with GOMOS significantly, and also influences
the spatial coverage by the collocated profiles and statistical significance
of the results. More dense measurements by OSIRIS and MLS allow tighten the
collocation criterion. Formally, the same collocation criterion (as for
GOMOS) can be applied for all reference instruments, but this will lead to
multiple collocations. In order to get statistically independent pairs, only
the closest one in time or in space should be used in the analysis. Thus,
even if the collocation criteria would be formally the same, the real
spatiotemporal difference will be smaller in collocations with denser
samplers. Figure shows latitude-time distribution
of the collocated measurements. Similarity of biases (see
Sect. ) with respect to all reference instruments
indicates that the selected collocation criteria are adequate for the
evaluation of the GOME-2 profiles.
Used coincidence criteria to define profile data pairs matched in time and space.
Instrument
Time
Geographic
Number of
difference
distance
collocations
(±h)
(km)
GOMOS
12
400
514 470
OSIRIS
6
200
267 101
MLS
6
100
1 435 449
Comparison methodology
The retrieved GOMOS and OSIRIS vertical ozone data are ozone number density
values, whereas the retrieved MLS vertical ozone data are volume mixing ratio
values (Table ). The reference ozone profiles are
transformed to GOME-2 representation in DU on 40 pressure layers. For GOMOS
and OSIRIS, we used pressure profiles from ECMWF (European Centre for
Medium-Range Weather Forecasts), while MLS data have necessary information
for such conversion.
We have smoothed the high-resolution limb profiles to the GOME-2 vertical
resolution using the GOME-2 averaging kernels as :
xsref=xa+A(xref-xa),
where xsref is the resulting smoothed reference profile,
xref is the actual reference profile interpolated to the
GOME-2 pressure layers, xa is a priori profile and
A is the averaging kernel matrix in the GOME-2 retrieval. The
reference ozone profiles have significantly higher vertical resolution;
therefore smoothing of GOME-2 data using averaging kernels of high-vertical
resolution instruments is not effective and can be omitted
.
Latitude-time distribution of collocated data between GOME-2 and
GOMOS (red), between GOME-2 and OSIRIS (green) and between GOME-2 and MLS
(blue).
We have performed the comparison between temporally and spatially coincident
profiles only. The collocation criteria for each reference instrument are
described in Sect. (see also Table ).
While the calculation is done at the GOME-2 pressure layers, the results
are shown in the altitude range of 15–60 km. As the altitude layers
in km vary slightly among the retrieved GOME-2 profiles, we have presented
in figures the ozone layer amounts at the midpoints of the averaged altitude
layers.
The relative difference (%) used here is defined as
RD=GOME-2-REFREF×100%,
where the ozone profiles GOME-2 and REF are in the unit of
partial ozone columns in DU at 40 pressure layers.
Comparison results and discussion
Latitude dependent difference
The agreement between GOME-2 and the reference instruments for the whole
time period is shown in Fig. . The ozone profiles were
retrieved from GOME-2 measurements, which are not corrected for instrument
degradation (see Sect. about degradation correction).
Top: the mean relative differences (%) at 10∘ latitude
bins of GOME-2 and the smoothed GOMOS (left), OSIRIS (middle) and MLS
(right) profiles, for the full comparison period from March 2008 until
December 2011. The mean profile is missing if there are less than 10
collocated profile pairs. The black line shows the mean tropopause altitude.
Bottom: the number of collocated profiles with GOMOS, OSIRIS and MLS, at each
altitude layer.
The vertical pattern of the mean relative differences across the tropical and
midlatitude regions is similar with all three instruments. The comparison
shows an agreement of GOME-2 ozone profiles with those of GOMOS, OSIRIS and
MLS within ±15 % in the altitude range from 15 km up to ∼ 35–40 km depending on latitude. On average, the GOME-2 ozone
profiles have a positive bias from the surface up to ∼25 km and
a negative bias above ∼30 km apart from the higher latitudes.
The agreement between the GOME-2 and reference ozone profiles is the best
at midlatitudes. Around the equator, between 20∘ S and
20∘ N, there is a higher positive bias reaching 20 % at
altitudes below ∼20 km. The black line (upper panels) shows the
mean climatological tropopause altitude. We can notice that the high positive
relative bias occurs ∼3 km above the tropopause and below.
At southern high latitudes the averaged relative differences between GOME-2
and the reference profiles are significantly different from the midlatitudes
and tropics, and they are also different from each other. In comparisons with
both GOMOS and MLS, a positive bias of ∼ 10 % is observed at
altitudes 30–40 km, and a negative bias ∼ 20 % is observed
at altitudes below 30 km (Fig. ). These features
at high southern latitudes are not seen in comparison with OSIRIS
(Fig. , center). In addition, a positive bias in the
zone ∼ 55–75∘ S at altitudes 18–28 km is observed
solely in the comparisons with MLS. These differences can be explained by
sampling issues related to the seasonal dependence at high latitudes as
verified and discussed below in Sect. . Note that
southern high latitudes have non-uniform seasonal coverage by collocated
data, especially for GOMOS and OSIRIS.
Seasonal dependence and temporal evolution of the difference
The seasonal variation of the mean relative differences between GOME-2
ozone profiles and the averaging kernel smoothed GOMOS, OSIRIS and MLS
profiles are shown in Fig. . As in
Sect. , the GOME-2 ozone profiles are based on
measurements that are not corrected for instrument degradation. In these
plots, the collocated data are divided into five latitude zones. The seasons
are defined as DJF (December, January, February), MAM (March, April, May),
JJA (June, July, August) and SON (September, October, November). Here we
consider only 2 years of data: 2010 and 2011. A period of 2 years enables
a good representation of the latitudinal and seasonal dependent relative
differences without excessive averaging. Note that due to large samples the
detected biases are statistically significant, except a few cases of small
number of collocated profiles (e.g. Fig. upper
left corner, N=10 for OSIRIS).
The mean relative differences (%) between GOME-2 and the
reference ozone profiles classified according to the latitude zones (rows)
and the seasons (columns) covering years 2010–2011 of collocated data. The
comparison results are shown against the smoothed GOMOS profiles (blue),
OSIRIS profiles (green) and MLS profiles (red). The dashed lines show the 1σ standard deviation around the mean. The number of collocated profile
pairs is given in the upper right corner of each panel. The number of
collocations is not always valid for the lowest altitude layers.
The seasonal variation of the bias with respect to all reference profiles is
very similar, especially at the tropical and midlatitudes. At the high
latitudes the divergence in comparison results may occur due to the different
temporal sampling by the collocated profiles. The different air masses, being
inside or outside polar vortex, can also contribute to the varying mean
relative differences in the polar spring season.
In general, at tropical latitudes (30∘ S–30∘ N) the
seasonal variation of the mean relative difference is low. However, the
negative peak in the bias just above 40 km is on average about
10 % deeper in DJF than in the other seasons. The reason for this
seasonal increase in the relative difference is not yet understood. At
midlatitudes, there is a moderate seasonal variation of the GOME-2 biases
in the stratosphere, yet more evident at southern midlatitudes.
The seasonal dependence of the mean relative differences can be clearly seen
at the high latitudes. There is a negative bias with respect to all reference
data just above 20 km in JJA (i.e. local winter) at southern high
latitudes. The same vertical structure in the relative difference is seen at
the northern high latitudes in the local winter (DJF).
In SON (30–90∘ S) and in MAM (60–90∘ S), the shape of the
bias profile is a bit different between the comparisons with the separate
reference instruments. In SON (30–90∘ S) the collocated data with
GOMOS includes only September, whereas the collocated data with both OSIRIS
and MLS include all three “SON” months, and the comparison results from
November are very different (not shown here). In MAM (60–90∘ S),
collocated data for MLS includes March and April, whereas for GOMOS and
OSIRIS only March is included (note that biases with respect to OSIRIS and
GOMOS are very close to each other).
Monthly median relative differences between GOME-2 and smoothed
GOMOS ozone profiles, in different latitude zones in the time period of
2008–2011. The monthly median is missing if there are less than 10
collocated profiles. The vertical lines indicate moments of the product
software version (sv) updates 1.20 and 1.23.
At the northern high latitudes (60–90∘ N) the mean relative
differences vs. MLS data at the upper altitude levels deviate from others in
MAM and JJA. In addition, in SON (60–90∘ N) the bias above
40 km is higher against OSIRIS and MLS than against GOMOS. The reason
for this might be that collocations for GOMOS includes only September.
At lower altitudes, the accuracy of the ozone profiles from limb-viewing
instruments worsens due to lower signal-to-noise ratio thus causing a wide
standard deviation around the mean relative differences
(Fig. ). When the comparison is done with the
actual reference profiles, i.e. without averaging kernel smoothing, the bias
is very similar than can be seen in Fig. except
the highest altitude layers and the tropical lowest altitude layers, and this
can be expected.
Figures –
show a temporal evolution of the monthly median of relative differences
(%) starting from March 2008 for each latitude zone with respect to the
smoothed GOMOS, OSIRIS and MLS profiles, respectively. The GOME-2 ozone
profiles in these figures are based on measurements that are not corrected
for instrument degradation.
These figures confirm that the relative difference has a definite seasonal
cycle at polar and midlatitudes. This can be seen especially in
Figs. and
since both OSIRIS and MLS have temporally
denser data set. In the tropics, the seasonal variations in the GOME-2
biases are milder, as expected.
We can notice a change to the better agreement in the upper stratosphere and
the lower mesosphere after October 2009, and even more clearly in the whole
stratosphere after the end of April 2010. These changes correspond to OPERA
algorithm versions 1.01 and 1.10 updates, and software version 1.20 and 1.23
updates .
As Fig. , but for the monthly
median relative differences between collocated GOME-2 and smoothed OSIRIS
ozone profiles.
There can be several possible reasons for the mainly systematic differences
encountered with respect to the reference data. The GOME-2 ozone profiles
and the reference profiles used from the GOMOS, OSIRIS and MLS instruments
are collocated at different local solar times (see
Table ). It has been shown that the natural
variability of ozone is affected by the photo-chemical reactions using solar
light above 40 km e.g.. The large
relative differences in the upper stratosphere with respect to GOMOS data,
and to a lesser extent against OSIRIS and MLS data, could be partly caused by
this diurnal effect (see Fig. ).
The comparisons done here with occultation and limb measurements show
a turning point of bias with the maximum relative differences at around
40–50 km. In the paper by it is discussed that
above 2 hPa (∼45 km) the implemented additive offset
(to partially correct the influence of instrument degradation) has the
largest influence on the number of pixels passing the quality control. In the
validation report it has been noticed that in the course of
time the GOME-2/MetopA data become noisier due to the instrument
degradation, mainly at higher altitudes above 40 km. The effect of
degradation correction and its influence on observed biases is discussed
below in Sect. .
As Figs.
and , but for the monthly median
relative differences between collocated GOME-2 and smoothed MLS ozone
profiles.
The diurnal variation of ozone between the latitudes 20 and
30∘ N illustrated by the Whole Atmosphere Community Climate Model
(WACCM). The local solar times of OSIRIS, GOME-2, MLS and GOMOS
measurements are marked with dashed vertical lines.
Solar zenith angle dependent difference
We have studied the dependence of GOME-2 bias on the solar zenith angle (SZA).
The SZA is provided with GOME-2 ozone profile retrieval. Typically the bias
increases with increasing SZA. Figures
and show the SZA dependence of the GOME-2 bias
compared to smoothed GOMOS and OSIRIS data for the altitude layers located
around 23, 30, 37 and 45 km at the southern high latitudes
(60–90∘ S). We show here bias only for one latitude zone,
60–90∘ S, since only there do exceptionally high bias values
that depend on SZA occur. The results were similar for all years; thus we only show
data from 2010.
The relative differences (%) with respect to smoothed GOMOS
profiles as a function of GOME-2 SZA, at four altitude layers around
23 km (upper left), 30 km (upper right), 37 km
(bottom left) and 45 km (bottom right). The profiles are from the
southern high latitudes (60–90∘ S) covering 1 year (2010) of
comparison data. The grey dots represent the same GOME-2 pixels at all the
altitude layers. They indicate high disagreement values (except at altitude
layer around 30 km).
At the southern high latitudes the negative bias increases with increasing
SZA around 23 km altitude. This is also observed at northern high
latitudes, 60–90∘ N (not shown here). A similar dependence on SZA
at tropical and midlatitude regions was registered around 45 km (not
shown here). On the contrary, at the southern midlatitudes around
37 km there is a tendency for the (negative) bias to get smaller; i.e.
there is better agreement when the SZA increases (not shown here).
In Figs. and exceptionally
large positive relative differences with respect to both GOMOS and OSIRIS
data appear above ∼30 km when SZA>85∘ and
at the same time large negative differences occur around 23 km. We
have marked these pixels by grey and they represent the same pixels at all
the altitude layers. We can notice that at ∼30 km (top right
panel in Fig. ) these pixels do not have divergent
relative difference values. In addition, the very high positive relative
differences with respect to OSIRIS profiles are encountered when SZA is
between 68 and 80∘ (marked with grey dots in
Fig. ). This happens only in the southernmost
latitude levels around 87∘ S during the southern polar spring and
summer months. The reason for these exceptionally high bias values is not
known at the moment; this feature will be investigated in the future studies.
The collocated pixels with GOMOS data have the southernmost latitudes around
69∘ S thus these high difference values are not seen against GOMOS
data (Fig. ).
As Fig. , but against smoothed OSIRIS
profiles.
Arctic ozone depletion 2011
In order to demonstrate the usefulness of the GOME-2 data we considered the
ozone profiles retrieved from the GOME-2 measurements during the Arctic
ozone depletion in spring 2011. In the Antarctic region the ozone hole event
recurs annually, but in the Arctic region severe ozone loss happens less
frequently. The exceptionally reduced transport of ozone from midlatitudes
into the Arctic region together with enhanced chemical ozone loss inside
polar vortex caused the anomalously low ozone concentration over the North
Pole in March 2011 . The Arctic ozone
depletion is not represented by the typical a priori profile used for this
season and latitude and hereby offers a chance to verify the retrieved
profiles under the challenging circumstances.
As the air masses inside the polar vortex can be very different compared to
outside polar vortex atmosphere, the comparisons have been done separately
for the pixels being inside and outside the polar vortex. We have done the
comparison over Arctic region (60–90∘ N) with collocated OSIRIS
data and used 475 K potential vorticity associated with OSIRIS data
to determine the collocated GOME-2 pixel to be outside or inside polar
vortex area. In this special case, we require pixels to a SZA≤65∘ because the disagreement with reference data increases as SZA
increases at high latitudes when considering the altitude range around
20 km (see previous Sect. ). The number of
collocated profiles inside vortex is 17 and outside vortex 96, respectively.
In reality, as the division is not the same at all altitudes, some part of
the profile can be inside, and the rest of the profile can be outside of the
polar vortex.
Monthly averages of the collocated GOME-2 (blue), OSIRIS (black),
smoothed OSIRIS (red) and a priori (green) ozone profiles inside polar vortex
(left panel) and outside polar vortex (middle panel) in the high northern
latitudes (60–90∘ N) in March 2011. The individual GOME-2 ozone
amounts at each altitude layer are denoted by blue dots. The relative
differences (%) between outside and inside polar vortex monthly means for
GOME-2, OSIRIS and smoothed OSIRIS are shown in the right panel. In this
same figure are plotted corresponding mean GOME-2 relative retrieval errors
outside and inside polar vortex (dashed lines).
Figure shows the monthly averages for March 2011 of
collocated ozone profiles from GOME-2 and OSIRIS measurements separately
for profiles inside (left hand panel) and outside polar vortex (middle
panel). The retrieved GOME-2 mean profile inside polar vortex is far away
from a priori profile and is rather close to the OSIRIS profile. Outside
polar vortex, all profiles are close to each other. Interestingly, in this
case the OSIRIS mean profiles agree better with a priori values, especially
in outside polar vortex situation. Probably, this happened by chance.
The difference between inside and outside polar vortex monthly mean values
scaled by outside polar vortex monthly mean is evident as seen for GOME-2
(blue) and OSIRIS (red and black) (Fig. right
panel). In this figure are shown also the mean GOME-2 relative retrieval
errors outside and inside polar vortex separately.
This case study shows that the GOME-2 can observe the ozone depletion well
and thus provides very useful information. We would like to note that the
horizontal coverage by GOME-2 is much more dense than by limb-viewing
instruments.
Monthly averages of the relative differences (%) with respect to
the collocated smoothed GOMOS (blue), OSIRIS (green) and MLS (red) ozone
profiles in March 2008. The results are shown for three latitude zones
separately when using the operational GOME-2 data (dashed curves) and data
with the new experimental correction for instrumental degradation (solid
curves).
Correction for instrumental degradation
The GOME-2 vertical ozone profile algorithm depends strongly on good
absolute-calibrated solar and earthshine spectra. Unfortunately, GOME-2 is
affected by instrument degradation that is strongest in UV
. This instrumental degradation increases over time, thus
setting additional challenges to the algorithm to adapt to these changes. An
experimental correction has been developed, which aims to rectify an initial
bias and the time dependent degradation for a number of relevant wavelengths
and scan angles. This degradation correction is applied to the measurement,
leaving all other Opera ozone profile algorithmic features intact. The data
produced with the degradation correction is only used for comparisons in this
section, not in the operational data shown earlier in this paper. This
limited reprocessed GOME-2 data set was generated with OPERA version 1.32.
To demonstrate its potential we have done some comparisons of GOME-2
vertical ozone profiles using a newly developed experimental degradation
correction with the GOMOS, OSIRIS and MLS data. The preliminary results for
March 2008, shown in Fig. , indicate good
functionality of the implemented correction. The large underestimation of
ozone above 40 km has been turned into a smaller overestimation,
rectifying the deviation considerably. The operational ozone profiles (dashed
curves) used here are from the offline ozone product (OOP) with software
version 1.14 in coarse resolution, whereas the degradation corrected profiles
(solid curves) have been retrieved in higher resolution. Thus the number of
collocated profiles in comparison is higher when using higher resolution
GOME-2 data. Since the beginning of the O3M SAF, the operationally
available vertical ozone profile product has been under continuous
development and improvement . This can be seen also in
timeline comparison results of the upper stratosphere, e.g. after 2010
(Figs. –).
Conclusions
We have evaluated the accuracy of the GOME-2 ozone profiles based on
measurements that are not corrected for instrument degradation. This has been
done on a global scale by comparing GOME-2 ozone profiles with
high-vertical resolution spaceborne ozone profiles by GOMOS, OSIRIS and MLS.
In this comparison study, we have used spatially and temporally coincident
profiles for the time period from March 2008 to December 2011.
The bias with respect to all reference instruments is very similar; it
depends on altitude, latitude and season. The overall agreement of ozone
profiles from GOME-2 and other instruments is within 15 % below
35–40 km. The comparison shows that, in general, there is a negative
bias in the GOME-2 ozone profiles above around 30 km. On the other
hand, GOME-2 systematically overestimates ozone in the lower stratosphere
at tropical and midlatitudes. At high latitudes, the bias has more clear
seasonal-dependent variations. Typically the bias increases with increasing
SZA.
The methodology for the GOME-2 instrumental degradation correction is in
the experimental phase. The preliminary evaluation done with the limited
degradation corrected data set has shown dramatic reduction of biases at
upper altitudes, thus indicating good functionality of the implemented
correction.
The detailed investigation considering the exceptional ozone depletion at
Arctic during the spring 2011 showed that the GOME-2 ozone profiles
captured the unusual ozone vertical structure in spite of an a priori ozone
profile that was not representative of that situation. This indicates that
GOME-2 data can provide valuable geophysical information.