ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-5001-2018Middle atmospheric ozone, nitrogen dioxide and nitrogen trioxide in 2002–2011: SD-WACCM simulations compared to GOMOS observationsWACCM–GOMOSKyröläErkkierkki.kyrola@fmi.fihttps://orcid.org/0000-0001-9197-9549AnderssonMonika E.https://orcid.org/0000-0002-8501-3366VerronenPekka T.https://orcid.org/0000-0002-3479-9071LaineMarkohttps://orcid.org/0000-0002-5914-6747TukiainenSimohttps://orcid.org/0000-0002-0651-4622MarshDaniel R.Earth Observation Research, Space and Earth Observation Centre, Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, FinlandMeteorological Research, Meteorological and Marine Research Programme, Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, FinlandAtmospheric Composition Research, Climate Research Programme, Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, FinlandNational Center for Atmospheric Research, Boulder, Colorado, USAErkki Kyrölä (erkki.kyrola@fmi.fi)12April20181875001501911December201720December201721March201821March2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/5001/2018/acp-18-5001-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/5001/2018/acp-18-5001-2018.pdf
Most of our understanding of the atmosphere is based on observations and
their comparison with model simulations. In middle atmosphere studies it
is common practice to use an approach, where the model dynamics are at least
partly based on temperature and wind fields from an external meteorological
model. In this work we test how closely satellite measurements of a few
central trace gases agree with this kind of model simulation. We use
collocated vertical profiles where each satellite measurement is compared to
the closest model data.
We compare profiles and distributions of O3, NO2 and NO3 from
the Global Ozone Monitoring by Occultation of Stars instrument (GOMOS) on the
Envisat satellite with simulations by the Whole Atmosphere Community Climate
Model (WACCM). GOMOS measurements are from nighttime. Our comparisons show
that in the stratosphere outside the polar regions differences in ozone
between WACCM and GOMOS are small, between 0 and 6%. The correlation of 5-day
time series show a very high 0.9–0.95. In the tropical region
10∘ S–10∘ N below 10 hPa WACCM values are up to 20 %
larger than GOMOS. In the Arctic below 6 hPa WACCM ozone values are up to
20 % larger than GOMOS.
In the mesosphere between 0.04 and 1 hPa the WACCM is
at most 20 % smaller than GOMOS. Above the ozone minimum at 0.01 hPa (or
80 km) large differences are found between WACCM and GOMOS. The correlation
can still be high, but at the second ozone peak the correlation falls
strongly and the ozone abundance from WACCM is about 60 % smaller than that from
GOMOS. The total ozone columns (above 50 hPa) of GOMOS and WACCM agree
within ±2 % except in the Arctic where WACCM is 10 % larger than
GOMOS.
Outside the polar areas and in the validity region of GOMOS NO2
measurements (0.3–37 hPa) WACCM and GOMOS NO2 agree within
-5 to +25 % and the correlation is high (0.7–0.95) except in the upper
stratosphere at the southern latitudes. In the polar areas, where solar
particle precipitation and downward transport from the thermosphere enhance
NO2 abundance, large differences up to -90 % are found between WACCM
and GOMOS NO2 and the correlation varies between 0.3 and 0.9. For NO3,
we find that the WACCM and GOMOS difference is between -20 and 5 % with a very high
correlation of 0.7–0.95. We show that NO3 values strongly depend on temperature and the dependency can be fitted by the exponential function of
temperature.
The ratio of NO3 to O3 from WACCM and GOMOS closely follow
the prediction from the equilibrium chemical theory. Abrupt
temperature increases from sudden stratospheric warmings (SSWs) are reflected as
sudden enhancements of WACCM and GOMOS NO3 values.
Introduction
The quality of atmospheric modelling is crucial for making reliable
predictions regarding future climate. The minimum quality requirement for any model
is that previously measured central atmospheric variables can be simulated
with reasonable accuracy. The increasing number of global satellite
missions since the discovery of the ozone hole offers a good opportunity to
compare models with observed data. Various satellite measurements of trace
gases are traditionally compared with validating ground-based instruments
see e.g., but they are now also increasingly
compared with each other see e.g..
This activity has led to an improved understanding of the accuracy of
satellite measurements which is an essential ingredient for a
model–measurement comparison.
In this work, we make use of the Whole Atmosphere Community Climate Model (WACCM)
from the National Center for Atmospheric Research and compare its
results to satellite observations from the Global Ozone Monitoring by
Occultation of Stars instrument (GOMOS). We concentrate on an atmospheric
region ranging from the stratosphere to lower thermosphere (20–100 km) and
on three important minor constituents O3, NO2 and NO3
measured by GOMOS.
Ozone is a central chemical element in the middle atmosphere and essential
for stopping short wave UV light from entering the biosphere. Ozone has
diurnal variability, which in the stratosphere is weak, but at 90–95 km
nighttime ozone can be an order of magnitude more abundant than during
daytime (see e.g. ). Measured satellite
ozone profiles are validated using ozone sondes and ozone lidars see
e.g.. Comparisons to other satellite measurements also
help to establish the data quality. Nitrogen dioxide, as a member of the odd
nitrogen family, participates in the catalytic destruction of ozone especially in
the upper stratosphere . In polar areas precipitation of
charged particles creates a vast amount of NOx which has a long
chemical lifetime in the polar darkness. When isolated by a stable vortex,
enhanced NOx can descend into the upper stratosphere, which then
leads to natural ozone loss when NOx becomes illuminated by
increasing solar light after the winter season e.g.. Polar NOx is also enhanced by polar descent from
the thermosphere and exceptionally large increases have been measured after
so-called sudden stratospheric warming events (SSWs) where the vortex
structure is disturbed see for example,. Nitrogen
trioxide is a part of the O3–NO2–NO chemistry, has a very strong
diurnal variation at all altitudes and is almost absent during daytime
(see e.g. ).
WACCM is the atmospheric component of the Community Earth System Model (CESM)
. WACCM is a chemistry–climate model spanning the range
of altitude from Earth's surface to the lower thermosphere (approximately
140 km) with 88 vertical levels of variable vertical resolution of 1.1 km in the
troposphere to 3.5 km above 65 km . The model's horizontal resolution
is 1.9∘ latitude by 2.5∘ longitude and the model time step is
30 min. In the present analysis version 4 of WACCM was run in specified
dynamics mode by constraining dynamical fields to Modern-Era Retrospective
Analysis for Research and Applications (MERRA) meteorological reanalyses
below 1 hPa. Above the stratopause WACCM dynamics are solved in a free
running mode, i.e. temperature and dynamic fields are self-determined
(although in practice they are still strongly modulated by MERRA). The
version of WACCM used in this work includes chemistry of the lower, D-region
ionosphere with 307 reactions of 20 positive ions and 21 negative ions
see.
WACCM has been evaluated in many model–measurement intercomparison studies.
In and , WACCM's total ozone values and trends
were shown to be in reasonable agreement with satellite observations. Total
ozone biases from different latitude ranges were between -5.5 and 2.3 %.
Comparisons at specific atmospheric conditions have provided more information
on the agreement between WACCM trace gas profiles and observations. In
, the simulated behaviour of the secondary ozone maximum
is compared against SABER measurements during a major sudden warming. The
behaviour during SSWs was found to be similar while the nighttime ozone
amount is generally underestimated by about a factor of 2 in WACCM.
Comparisons of NOx during polar winter, when NOx is
influenced by energetic particle precipitation, have been made in many
studies .
From these studies it seems that WACCM tends to underestimate
mesospheric NOx by a factor of approximately 4.
GOMOS was an instrument on the European Space Agency's
Envisat satellite which was in operation for just over 10 years between 2002
and 2012. The measurement method of GOMOS, stellar occultation, uses light
from the180 brightest stars allowing global coverage of measurements with good
vertical resolution (2–3 km for ozone, 4 km for NO2 and NO3). The
occultation method is self-calibrating because the occulted star's spectrum
is also measured without atmospheric intervention and therefore the
primary source data for retrievals (i.e. transmissions) are, in principle,
stable. GOMOS measured 880 000 stellar occultations during the lifetime of
Envisat. Ozone's relatively large abundance makes it quite an easily
observable constituent from satellite instruments using optical measurements.
GOMOS measurements can be used to retrieve ozone at altitudes ranging from
the troposphere to the lower thermosphere. NO2 and NO3 can be
retrieved in the stratosphere.
Our comparisons of GOMOS measurements with WACCM simulations will be based on
of individual, co-located profile measurements, whereas in many other
model–data studies climatological or other average quantities are used. Our
method avoids the problem of uneven (in geolocation and time) sampling that
accompanies limb and especially limb occultation measurements and distorts
climatologies. In the Coupled Model Intercomparison Project (CMIP) and in the
more specialised Chemistry–Climate Model Initiative (CCMI) several
atmospheric (or more generally earth system) models including CESM/WACCM have
been compared with each other and also with observations
see.
Most of these studies were interested in targeting on future climate
projections especially in the troposphere. In this work we are interested in
seeing how well a model simulates the whole middle atmosphere from the upper
troposphere to the lower thermosphere in a limited time range 2002–2011.
Our study is structured as follows. In Sect. 2 we introduce the GOMOS
instrument and the measurements we are using in this work. In Sect. 3 the main
properties of the WACCM model are introduced. The comparison method is
introduced in Sect. 4 and individual comparisons of O3, NO2 and
NO3 are presented in Sects. 5–7.
GOMOS measurements
GOMOS was a stellar occultation instrument on board Envisat that was
operational from 2002 to 2012 (for GOMOS overviews, see , and https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/envisat/instruments/gomos). GOMOS
measured occultations during both day and night. However, here we use only
GOMOS nighttime occultations. Measurements made during daytime suffer from
scattered solar light, which leads to a low signal-to-noise
ratio of the stellar
signal. Daytime data have problems especially below 55 km, and the quality
depends very strongly on the properties of the GOMOS target star
. An alternative approach to retrieve ozone during
daytime is to use the scattered solar light observed by GOMOS, this method
works well in the stratosphere and lower mesosphere
. But as mentioned above, we restrict
our analysis to nighttime occultation data, partly because they
provide an altitude coverage from stratosphere to lower thermosphere for ozone.
GOMOS nighttime profiles of O3, NO2 and NO3 are retrieved from
the spectral range 248 to 690 nm. The integration time of the measurements is
0.5 s, which provides an altitude sampling resolution of 0.2–1.6 km
depending on the tangent altitude and the azimuth angle of the measurement.
The retrieved ozone profiles have a 2 km vertical resolution below 30 km
and a 3 km resolution above 40 km, whereas NO2 and NO3 have a
4 km vertical resolution at all altitudes. Details of the GOMOS retrieval
algorithms and data quality are discussed in and
. In this work we use GOMOS data from the ESA
processing version 6 in a vertically gridded form (for data access, see
data availability section). We remove data points that have been measured
when Envisat was located in the region of the South Atlantic Anomaly. The
illumination conditions for the GOMOS measurements are determined by two
solar zenith angles controlling solar light at the tangent point and at the
satellite location. At the tangent point we require that the zenith angle is
greater than 104∘. It has been shown that for zenith angles smaller
than 118∘ at the satellite position some stray light can be present,
but we have not found any discernible change in our results ignoring this
restriction altogether. In the GOMOS gridded ozone data there is an
ozone-specific flag that screens stars that do not provide sufficient
signal-to-noise ratio for reliable ozone retrieval in the mesosphere–lower
thermosphere (faint and cool stars). Profiles considered as outliers either
in the stratosphere or in the mesosphere are also flagged. We only use those
profiles where all three flags are equal to zero. The total number of GOMOS
nighttime measurements is then 238 664. For NO2 and NO3 the ozone
flags can be ignored and we get 377 881 measurements. The number of
measurements peaked in 2004 and declined thereafter due to the problems
connected to the steering mechanism of the instrument. During 2005 no
measurements were collected from the period between February and May due to
this steering problem. Note that the polar regions are not covered by
nighttime measurements during summer months. For other latitudes measurements
cover all seasons.
The first comprehensive validation of GOMOS nighttime stratospheric ozone
(ESA data version 4) against ground-based and balloon-borne instruments was
presented in . The results showed that GOMOS nighttime ozone
agrees within a few percent with the correlative data (sondes and lidars) in
the stratosphere outside polar areas. An update of this work was issued by
using the ESA software version 5 and results were similar
to . In this work we are using the ESA software version 6. All
three versions (4–6) provide very similar results. Version 6 has been
under validation in the ESA projects Valid-2 and Multi-TASTE and the
validation reports are available from https://earth.esa.int/web/sppa/mission-performance/esa-missions/envisat/gomos/cal-val/validation-activities.
Recent similar validation results can be found from and
. Results show differences to be within ±3 % between
20 and 45 km. Below 20 km GOMOS ozone data
show increasing positive bias in the tropics,
but in this work we restrict analysis to higher altitudes where such bias is
not observed. GOMOS and SAGE II, the so-called gold standard of satellite ozone profiles,
were compared in and differences within ±4 %
in 23–55 km were observed when the SAGE II sunrise and sunset occultations
were treated separately. The diurnal variation of ozone in the stratosphere
and some sunset–sunrise instrumental factors contribute to these
numbers (see also ). Climatological comparisons of
several limb viewing satellite instruments including GOMOS are presented in .
GOMOS is able to measure ozone up to 100 km when stars with sufficiently high
effective temperature are used. For mesospheric heights there are no real
validation results, but we can get some insight from comparisons to other
satellite measurements. In GOMOS and MIPAS ozone were
found to agree within ±10 % in 25–70 km; similar results were obtained
in . SABER and GOMOS were compared in
, which showed that GOMOS nighttime mesospheric
ozone values are about 20 % lower
than SABER.
GOMOS measurements can nominally be used to retrieve NO2 at altitudes
between 25 and 50 km, while in the polar regions altitudes up to about 70 km
can be reached during winter months due to higher NO2 concentrations.
There is only one publication where GOMOS NO2 measurements have been
compared with in situ measurements. This was carried out via a comparison with balloon-borne
instruments , which indicated an agreement within
±25 %. In addition, several comparisons against satellite-based
observations have been made. found that GOMOS NO2
values are 10–25 % higher than MIPAS. Comparison with ACE-FTS in
showed better than 10 % agreement between 23 and 30 km and
∼ 25 % between 30 and 45 km. At higher altitudes larger differences were found,
but the necessary correction for diurnal variation made results very
uncertain. Nitrogen dioxide has a strong diurnal variation with maximum and
minimum amounts seen during early night and early morning, respectively (for
diurnal cycle from model simulations, see e.g.
and . Climatological comparison with HALOE can be found in .
GOMOS retrieval of NO3 covers the altitude range 25–50 km. During
daytime NO3 almost vanishes via photolysis but rises quickly after
sunset from the reactions between O3 and NO2 (for diurnal cycle
from model simulations, see e.g. ).
There are only few NO3 measurements to which to compare
GOMOS measurements. GOMOS NO3 have been compared with two balloon
measurements in , but with inconclusive results. In
GOMOS measurements were compared with SAGE III lunar
measurements and the agreement was found to be within ±25 %.
SD-WACCM-D simulations
WACCM includes the Ox, NOx, Clx and BrO
families and CH4 with its reaction products. The number of reactions
is 217 with 59 species. Heterogeneous reactions with three types of aerosols
are also included. The model includes orographic and non-orographic gravity
waves see. The upper boundary temperature condition
is given by the MSIS-model by . The same model is used to
specify O, O2, H and N upper boundary
conditions. At the lower boundary observations are used to specify the
surface mixing ratios of CFC gases, CH3, N2O and other
important gases for stratospheric processes. Historical surface
concentrations of greenhouse gases were taken from .
The solar irradiance is provided by the model of which
takes the spectral and flux variations during the solar cycle into account.
WACCM includes ionisation rates from solar proton events (SPEs) and auroral
electrons. More details of the WACCM model can be found from ,
and .
In this work we use SD-WACCM-D version 4, meaning that the model (a) includes
chemistry of the lower, D-region ionosphere required for detailed
energetic particle precipitation (EPP)
simulations see and (b) is run in specified
dynamics (SD) mode by constraining dynamical fields below 1 hPa to Modern-Era
Retrospective Analysis for Research and Applications (MERRA) meteorological
reanalyses see. SD mode allows for realistic
representation of atmospheric dynamics making the simulations directly
comparable to satellite observations, while the D-region ion chemistry has
been shown to improve the polar mesospheric comparisons for many species,
including NOx. In order to provide an ion
source for the low-latitude D-region chemistry, ionisation due to galactic
cosmic radiation is included in our simulations using the Nowcast of
Atmospheric Ionising Radiation for Aviation Safety (NAIRAS) model for
details, see. For this study, we also include the
ionisation due to 30–1000 keV radiation belt electron precipitation in the
energetic particle forcing. For details on the precipitation model and
ionisation rate calculation, see . In this energy
range, electrons add to HOx and NOx production in situ at
60–90 km altitude, directly affecting mesospheric ozone chemistry at
geomagnetic latitudes between 55 and 72∘. The ionisation rates are applied in
WACCM as daily, zonal mean values which depend on the geomagnetic Ap index
and latitude.
Comparison method
In order to compare GOMOS vertical profiles with WACCM simulations each
satellite measurement is paired with the closest WACCM
latitude–longitude–time profile (i.e. no interpolation between different
WACCM grid cells is done). The geolocation of the satellite measurement is
defined by the average value when the line-of-sight of the instrument is
between 20 and 50 km in altitude. In this study, we compare all GOMOS
nighttime measurements from 2002 to 2011 to a WACCM simulation run for the
same period. For the satellite measurements the comparison is complete in the
sense that every measurement finds its model partner with very good
co-location limits: latitude difference smaller than 0.95∘, longitude
difference smaller than 1.25∘ and time difference shorter than 15 min.
This method avoids the problem of uneven (in geolocation and time) sampling
that accompanies limb and especially limb occultation measurements and which
may distort trace gas climatologies and their comparisons.
A retrieved GOMOS constituent profile is given at the measurement's refracted
line-of-sight altitudes that vary from one measurement to another. In this
work we interpolate (linearly) the profiles to a regular geometric altitude
grid with 1 km step. GOMOS constituent abundances are given in number
densities. WACCM runs on a pressure grid and abundances are mixing ratios. In
order to compare satellite measurements with WACCM we need to either change
satellite measurements to the pressure grid of WACCM or to change WACCM
results to the altitude grid used by satellite data. We have selected to work
using the WACCM's pressure grid; therefore, every GOMOS measurement is
interpolated to the altitudes obtained from the geopotential heights of the
WACCM's latitude–longitude cell nearest to the satellite measurement at the
time of the measurement. This brings the number densities of satellites to
the pressure grid of the model. In this work we show results in mixing ratios
as they more suitable for illustrating results. The transformation to mixing
ratios is accomplished by the neutral density distribution of WACCM
(originating in the SD-version from MERRA and internal dynamics).
The method we use for comparing collocated satellite and WACCM profiles and
their differences at each altitude z is to calculate the bias over a
suitable number of profiles in a selected region (time and geolocation) as
B(z)=〈fkW(z)-fkG(z)〉,
where fkW denotes WACCM and fkG GOMOS
collocated vertical profiles. Satellite gridded profiles have some
missing data from flagged data points or from restrictions of the altitude
coverage of measurements. The corresponding WACCM data points are ignored in
the average in order to preserve the complete correspondence of the data
sets. For practical reasons we will also use the bias in a relative sense as
Δ(z)=100%B(z)〈fkG(z)〉.
The scaling factor (denominator) is calculated from GOMOS in the same region
as the bias.
Calculation of the average estimates is based on dividing spatial and
temporal extensions to suitable scales. We average data within 10∘ in
latitude and use zonal averaging. For the polar regions we also show results
from a larger latitudinal range (from 60 to 90∘ south and north). In
the time domain the analysis is based on 5-day time averaging in order to
capture fast polar processes while keeping reasonable statistical accuracy.
The average from the averaging region and period of time is done by first
making averages for each available star (we require at least 10 measurements
from each star) and then averaging over the stars involved. This provides a
more equal contribution from different latitudes covered and no star can
dominate the average by its high number of measurements. We apply a median
filter (|x- median(x)|> 3 × 1.4826 × median(|x- median(x))|)
for the distribution of GOMOS values
from any given star at each altitude. Any GOMOS outlier means that it and its
paired WACCM data are removed. For ozone the number of outliers is less than
1 % except at 0.01 hPa (ozone minimum) and at the polar latitudes where the
number of outliers can reach 5 %. For NO2 and NO3 the number of
outliers is about 1 % and up to 5 % in the polar areas. All averages are
calculated using the median estimator. After eliminating flagged data and
applying minimum number limits we have 231 923 ozone, 358 738 NO2 and
317 653 NO3 WACCM–GOMOS pairs in our comparisons (note that near the
upper and lower altitude limits of the GOMOS retrievals the actual number of
pairs is usually smaller). From the WACCM and GOMOS 5-day time series we
calculate the WACCM–GOMOS mission average biases and the (Pearson)
correlation coefficients C(z). In this step we require that at least five time
steps are included. This eliminates the latitude belt
80–90∘ S altogether. Notice that the time coverage of the
polar latitudes is strongly restricted by the solar zenith condition
(nighttime) applied on the GOMOS data. In the Antarctic
60–90∘ S the coverage is from mid-February to September and
in the Arctic 60–90∘ N from mid-September to mid-April.
In addition to the general data collection rules already explained we
paid special attention to the validity limits in altitude for GOMOS data.
This work includes nighttime measurements from 138 stars and each of them
have their own valid, constituent specific retrieval range. The GOMOS data we
are using already include star specific valid altitude limits for all three
gases focused on in this work. These limits are based on yearly averages. In order to
handle rapidly changing events we need more dynamic determination of the
validity ranges. Therefore, in this study we have calculated for each star, gas,
latitudinal zone and time window (5-days) the average t-value profile (the
median value divided by its uncertainty; see e.g. Eq. 1 in ).
We reject those portions from the average profile that
have t< 2 (this also eliminates negative density averages even if negative
individual values are accepted).
An average profile that passes the t-value criterion usually forms a
continuous chain of density values (with t> 2) in altitude and the rejected
values (with t< 2) are located at the low and high altitude parts of the
profile. Sometimes two or more disconnected t> 2 regions are also present.
These regions may represent the real atmospheric state or they can be
generated by noise. In the ozone minimum region around 0.01 hPa (80 km)
density values are so small that the t> 2 condition is not usually achieved but
t values recover again at higher altitudes. This minimum structure seems to
be omnipresent and we will always include the minimum region in our ozone
comparisons. In the polar regions large NO2 values above the normal
validity range of NO2 are observed after a solar storm hit the
Earth. This extension of the profile is short-lived and we apply a t test to
monitor its upper limit.
Disconnected noise generated t> 2 regions are typically found at altitudes
where the density of a retrieved gas approaches zero. When the density
decreases the WACCM's distribution of density values (from an averaging
domain) changes from an approximate normal distribution (natural variation)
to a nearly lognormal type distribution because of the physical lower
limit zero in the model. The GOMOS retrieval approach does not limit the
retrieved gas values by a positivity condition as this could lead to bias. As
the density approaches zero the GOMOS' distribution of density values
remains nearly normal also covering negative values. Ideally this
distribution would settle down around zero with t∼ 0 and with the width
given by the noise in data. Unfortunately, sometimes this does not happen and
we see the distribution average become
positive with t> 2. These “ghost”
detections may, for example, be generated by the interference of the other
gases retrieved at the same time. As a precautionary measure against these
ghosts we reject altitudes where the GOMOS distribution (from a given
star, region, time, altitude) includes more than 20 % negative values. For
polar latitudes we apply a more relaxed limit of 33 %, which allows our
analysis to capture fast developing processes.
The procedures explained prevent GOMOS average densities from obtaining values too
close to zero, whereas the corresponding WACCM averages are not constrained. For
ozone the lowest values are obtained from the ozone minimum and they are
about 0.05 ppm for both WACCM and GOMOS. NO2 is removed from the lower
Antarctic stratosphere during July–August before the Antarctic ozone hole.
The lowest WACCM values (in the present work) are about 0.000015 ppb, whereas
at the same altitudes the lowest GOMOS values are about 0.04 ppb. For
NO3 at low altitudes WACCM shows 0.4 ppt, and GOMOS 1.7 ppt.
Ozone yearly median mixing ratio profiles and median relative
differences from GOMOS Sirius occultations and from paired WACCM profiles
from 2002 to 2011 in the 40–60∘ S latitude band. Occultations
take place during late August to mid-September. The vertical axis is
pressure. (a, b) GOMOS profiles (solid lines) and WACCM profiles
(dashed lines). The colour coding in the legend shows the
measurement year and the number of measurements is in parentheses.
(c) Relative median difference WACCM–GOMOS/median(GOMOS). Above
0.04 hPa differences are divided by 10. The colour coding for (c)
follows (a, b), but 2007–2011 lines are
dotted.
Ozone
As an example of retrieved satellite ozone profiles and paired WACCM
profiles, we show observations from the brightest
star in the sky, Sirius in Fig. . It provides the best signal-to-noise ratio at all
wavelengths of GOMOS stellar occultations. These measurements were taking
place every year from late August to mid-September. In Fig.
we show the yearly median profiles from both the GOMOS
observations and the WACCM simulation. It is evident that the observations
and the model simulations generally agree well at all altitudes except in the
neighbourhood of the second ozone peak (around 0.001 hPa, 91 km), where large
differences and yearly variations are evident. The mission average 2002–2011
relative uncertainty of the GOMOS and WACCM Sirius profiles is better than
2 % in the altitude range 0.05–50 hPa. The relative uncertainty grows to
10 % at and around the ozone minimum at 0.01 hPa, but it again reaches 2 % at
the second peak and diverges at higher altitudes. The WACCM–GOMOS relative
difference stays inside ±10 % between 0.05 and 50 hPa, but increases up to
60 % at the second peak and grows still at higher altitudes. Differences are
statistically sound in the mesosphere, whereas in the lower atmosphere the
differences fluctuate on both sides of zero.
In order to get a more comprehensive view of WACCM–GOMOS differences for all
latitudes we now consider ozone profiles from all eligible GOMOS occulted
stars. Profiles flagged by the ozone flags are not included, but all others
are included for those pressure levels that pass the t value and the
distribution positivity criteria discussed in Sect. . Both WACCM
and GOMOS main ozone maxima are at the Equator at 10.3 hPa. GOMOS maximum is
9.7 ppm and WACCM 9.4 ppm (difference 3 %). In the mesosphere–thermosphere
the second mixing value maximum is at the Equator where the GOMOS mixing ratio is
10.5 ppm at 0.0005 hPa (94 km) and WACCM 4 ppm at 0.0009 hPa (91 km). The
ozone minimum is located at 0.009–0.015 hPa with minimum values above 0.1 ppm
(notice that WACCM's coarse pressure grid makes altitude estimates
uncertain in the mesosphere–thermosphere). The altitude–latitude relative
difference distribution between GOMOS and WACCM as a median average of 5-day
time series from 2002 to 2011 is shown in Fig. for
the stratosphere and in Fig. for the mesosphere–lower
thermosphere. The validity range that applies to all latitudes is from
0.00012 to 85 hPa (about 16–105 km). The lower limit in Fig.
is taken as 52 hPa (about 20 km) in order to
eliminate the GOMOS positive bias below 20 km in the tropics mentioned in
Sect. . In both figures the differences are mostly statistically
significant, points where the WACCM–GOMOS difference is insignificant are
marked by crosses.
The median relative difference (WACCM–GOMOS)/median(GOMOS) of the
ozone mixing ratio (in %) in the stratosphere over 2002–2011. Latitudes are
from -90 to +90∘ with 10∘ resolution. A crossed cell
marks a point where the difference does not deviate from zero in a
statistically significant way. A cell with a dot marks a point where there
are no collocated profiles.
In the stratosphere outside the polar latitudes WACCM–GOMOS differences are
generally small, WACCM values being 0–6 % lower
than GOMOS. This slightly exceeds the ±3 % uncertainty estimates of GOMOS ozone. Larger
differences are seen in the tropical lower stratosphere and in the Arctic. In
the tropics in the lower stratosphere we see that WACCM values are larger, up
to 20 %, than GOMOS. In the Arctic between 1 and 6 hPa WACCM–GOMOS differences
are small, between 6 and 50 hPa WACCM is clearly larger than GOMOS, up to 20 %
difference at 15 hPa. In the Antarctic the differences are between -4 and +6 %.
The median relative difference (WACCM–GOMOS)/median(GOMOS) of the
ozone mixing ratio (in %) in the mesosphere over 2002–2011. Latitudes are
from -90 to +90∘ with 10∘ resolution. A crossed cell
marks a point where the difference does not deviate from zero in a
statistically significant way. A cell with a dot marks a point where there
are no collocated profiles.
Figure shows differences in the mesospheric–lower
thermosphere, which are moderate up to 0.05 hPa altitude or even up to
0.005 hPa outside the polar latitudes. Around 0.1 hPa in the
polar areas WACCM and GOMOS agree within ±5 %. During wintertime a
so-called tertiary ozone peak appears in this region see
e.g.. In the upper mesosphere
differences grow strongly and WACCM values are about 60 % smaller than GOMOS
around the second ozone peak. This result is in agreement with earlier
comparisons , where WACCM was compared with
MIPAS and SABER measurements. A similar model–measurement difference has been
seen in a HAMMONIA model study (see ). The GOMOS retrieval is
very straightforward in the mesosphere–lower thermosphere and we have not
been able to identify any potential sources of uncertainty that could lead to
such a large error in the GOMOS retrieval or data. Notice that GOMOS data
uncertainty is large at the ozone minimum and the relative difference varies
from positive to negative.
The 10 year mission averaged bias is, of course, a narrow measure of the
compatibility of WACCM and GOMOS. We now investigate how WACCM and GOMOS
ozone values develop in time. Figure shows the correlation
coefficient of WACCM and GOMOS from 5-day time series as a function of the
altitude and latitude. In the stratosphere the correlation is very high,
typically 0.85–0.95. At altitudes between the stratopause at 1 hPa and the
ozone minimum at 0.01 hPa the correlation almost vanishes. High values are
seen again between 0.01 and 0.001 hPa, but the final decrease starts just below
the second ozone peak.
WACCM and GOMOS ozone mixing ratio correlation over 2002–2011. The
correlation is calculated from 5-day time series. Latitudes are from -90 to
90∘ with 10∘ resolution. A crossed cell marks a point where
the correlation does not deviate from zero in a statistically significant
way. A cell with a dot marks a point where there are no collocated profiles.
WACCM and GOMOS ozone 5-day time series
1 August 2002–31 January 2005. Three latitude belts are shown:
50–30∘ S (red lines), 10∘ S–10∘ N (green) and
30–50∘ N (blue). GOMOS values are shown by solid lines, WACCM by
dashed lines. The 5-day time series are smoothed by a moving average of
45 days. Note that in the top panel the y axis is logarithmic.
WACCM and GOMOS ozone mixing ratio 5-day time series from 2002 to 2011
in the Arctic 60–90∘ N and in the Antarctic 60–90∘ S. In
the top panel the y axis is logarithmic. The colour coding symbols: W/S and
W/N are WACCM in Antarctic, Arctic; G/S and G/N are GOMOS in Antarctic,
Arctic.
WACCM and GOMOS ozone mixing ratio difference from
Fig. in the Arctic 60–90∘ N and in the
Antarctic 60–90∘ S. The colour coding symbols: S is Antarctic and
N is Arctic.
Figure shows the comparison of the WACCM and GOMOS ozone
mixing ratio 5-day time series from three latitude bands and at two pressure
levels from August 2002 to January 2005. The top panel shows the second
maximum where a large bias between the WACCM and GOMOS is evident. Ozone in
all three latitude bands shows semi-annual oscillations. WACCM and GOMOS
correlation is highest 0.74 at the Equator, 0.54 at
50–30∘ S and 0.35 at 30–50∘ N. At the
lowest altitude in the bottom panel we can see that WACCM values in the
tropics are consistently higher than GOMOS, resulting in the positive tropical
bias in Fig. , whereas at the mid-latitudes there is a
good agreement. Correlations are high, 0.83 at the Equator, 0.94 in the south
and 0.95 in the north.
NO2 yearly median mixing ratio profiles and median relative
differences from GOMOS Sirius occultations and from paired WACCM profiles
from 2002 to 2011 in the 40–60∘ S latitude band. Occultations
take place during late August to mid-September. The vertical axis is
pressure. (a, b) GOMOS profiles (solid lines) and WACCM profiles
(dashed lines). The colour coding in the legend shows the
measurement year and the number of measurements is in parentheses.
(c) Relative median difference WACCM–GOMOS/median(GOMOS). The colour
coding for (c) follows (a, b), but 2007–2011 lines are
dotted.
In Fig. we show the 5-day ozone mixing ratio time
series in both polar regions at the same altitudes as in Fig. .
The Arctic and Antarctic time series can be shown in the
same plot because GOMOS nighttime coverage in these regions is almost
complementary in time. Differences are shown in
Fig. . The highest altitude in Fig.
(top panel) again shows the large differences of the
second peak values (in both cases WACCM is on average 62 % smaller than
GOMOS). WACCM–GOMOS correlation is 0.59 in the Antarctic and only 0.35 in the
Arctic. The bottom panel shows results at the lower end of the valid ozone
range. The average WACCM–GOMOS difference is 2.8 % in the Antarctic and
8.3 % in the Arctic and correlations are 0.89 and 0.62, respectively. In the
Antarctic both WACCM and GOMOS show strong ozone reductions, but GOMOS
reductions are generally larger. In the Arctic WACCM ozone values are, as a
rule, considerably larger than GOMOS. This tendency continues to higher
altitudes and “explains” the positive peak found in Fig. .
The exceptionally large ozone loss in 2011
see is clearly seen in GOMOS data, but not so
clearly by WACCM. A similar even larger difference can be seen in 2010 but
in this instance without a large reduction of ozone.
Nitrogen dioxide
In Fig. we begin again with GOMOS profiles from the
Sirius occultations in the 40–60∘ S latitude band. The
average uncertainty of the WACCM and GOMOS median profiles is better than 5 %
at 40–0.5 hPa, whilst the relative WACCM–GOMOS difference is -10–+20 % at the same altitude. Around the maximum 5 hPa the difference is within ±3 %. The yearly
variation in profiles and differences is large. The reason for this variation
is the location of Sirius occultations near the Antarctic vortex where
sporadic NO2 enhancements are not totally contained in the polar region.
The median relative NO2 difference (WACCM–GOMOS)/median(GOMOS)
(in %) over 2002–2011. Latitudes are from -90 to 90∘ with
10∘ resolution. A cross marks a point where the difference does not
deviate from zero in a statistically significant way. A cell with a dot marks
a point where there are no collocated profiles.
In Fig. we show the median relative difference between
WACCM and GOMOS as a function of latitude and altitude during 2002–2011. The
most conspicuous feature of the figure is the variation of the upper valid
altitude limit. In the polar regions GOMOS measurements reach up to near
0.05 hPa (about 65 km), whereas elsewhere the highest altitude is about 0.4 hPa
(about 55 km). The all-latitude lower limit is 37 hPa (about 21 km). The
variation of the upper validity limit is a consequence of data
screening using t values and the positivity condition of the distribution
(see Sect. ). It is important to keep in mind that the high
altitude results from the polar regions come solely from the few short-lived NO2 enhancement events, whereas NO2 at the lower polar
altitudes is measured by GOMOS during the whole winter season. In the polar
areas at high altitudes WACCM values are smaller, by 50–90 %, than GOMOS.
High GOMOS NO2 values are related to extraordinary events that will be
discussed later. Outside the polar areas in the stratosphere WACCM–GOMOS
difference varies between -5 and +25 %. Excluding the polar regions however, the differences
are inside the uncertainty estimates of GOMOS NO2. The mission average
of the NO2 mixing ratio maximum is at 1.9 hPa for WACCM and at 2.9 hPa for
GOMOS. Maximum values are both around 16 ppb and situated at the Equator. The
average values in the polar regions are still much higher: in the Arctic
86 ppb and in the Antarctic 40 ppb, but these are only averages over the winter seasons.
In Fig. we show the WACCM–GOMOS NO2 correlation
coefficient's altitude–latitude distribution. In the stratosphere the
correlation is high, 0.7–0.95, except in the upper stratosphere at the
southern latitudes where the correlation vanishes. In the mesosphere at the
polar latitudes the correlation varies between 0.3 and 0.9.
WACCM and GOMOS NO2 mixing ratio correlation over 2002–2011.
Latitudes are from -90 to 90∘ with 10∘ resolution. A
crossed cell marks a point where the correlation does not deviate from zero
in a statistically significant way. A cell with a dot marks a point where
there are no collocated profiles.
NO2 mixing ratio 5-day time series at two pressure levels from
the Arctic 60–90∘ N and the Antarctic 60–90∘ S. The
colour coding symbols: W/S and W/N are WACCM in Antarctic Arctic; G/S and
G/N are GOMOS in Antarctic, Arctic.
WACCM and GOMOS NO2 mixing ratio difference 5-day time
series 2002–2011 in the Arctic 60–90∘ N and in the Antarctic
60–90∘ S. The colour coding symbols: S is Antarctic and
N is Arctic.
Figure shows WACCM and GOMOS NO2 time
series at two pressure levels in the Arctic and Antarctic from 2002 to 2011.
The differences are shown in Fig. . The upper
panel in Fig. shows that in both polar regions
almost every winter high NO2 event is detected at an altitude much
higher than the normal NO2 maximum. Most eminent peaks take place
during the 2003 Antarctic winter and during the 2003–2004 Arctic winter.
Elevated NO2 amounts, observed during the winter periods, are known to
be generated by particle precipitation events see
e.g. and enhanced downward
transport of NOx from the lower thermosphere e.g.. The lower pressure level (the
bottom panel) shows the opposite tendency. The annual oscillation of NO2
has its minimum during the mid-winter. In the Antarctic WACCM NO2
acquires exceptionally low values (in this plot the minimum is 0.0017 ppb)
due to denitrification of the lower stratosphere (see e.g.
). The corresponding GOMOS minimum value is much larger,
0.29 ppb, due to the positivity constraint imposed on GOMOS data.
NO2 mixing ratio from 5-day time series during
15 September 2003 – 31 April 2004 from GOMOS (a), from
WACCM (b) and WACCM–GOMOS difference (c) in the Arctic
60–90∘ N. All in ppb units. Notice the difference in the colour
scales.
NO3 yearly median mixing ratio profiles and median relative
differences from GOMOS Sirius occultations and from paired WACCM profiles
from 2002 to 2011 in the latitude band 40–60∘ S. Occultations
take place during late August to mid-September. The vertical axis is
pressure. (a, b) GOMOS profiles (solid lines) and WACCM profiles
(dashed lines). The colour coding in the legend shows the
measurement year and the number of measurements is in parentheses.
(c) Relative median difference WACCM–GOMOS/median(GOMOS). The colour
coding for (c) follows (a, b), but 2007–2011 lines are
dotted.
During the 2003 Antarctic winter a strong increase in NO2 values started
at the beginning of June and lasted until mid-September. This event has been
meticulously studied in using satellite measurements
from MIPAS/Envisat. The origin of the enhancement is the increase of the
NOx population in the thermosphere by electron precipitation and the
subsequent downward transport by meridional transport. In GOMOS data the
maximum 5-day median value 134 ppb (at 0.07 hPa) is achieved during 15–19 July;
the corresponding WACCM value is 24 ppb. The Antarctic NO2
enhancement during 2003 is important for two of our earlier results. In
Fig. we showed high yearly variation of Sirius NO2
profiles. WACCM 2002 and 2004 profiles around 2 hPa are considerably larger
than the corresponding GOMOS profiles, whereas during 2003 WACCM and GOMOS
profiles agree. This agreement is due to the Antarctic NO2 enhancement
during June–September 2003 that peaked before the Sirius measurements
took place. This extra NO2 lifted GOMOS values until they were on par with WACCM. In
Fig. we showed how the WACCM–GOMOS correlation around
1 hPa in the 50–80∘ S latitudinal range is much lower than
elsewhere. This correlation (mission average) is dominated by the different
temporal development of WACCM and GOMOS during June–August 2003 in this
latitudinal region. Around 1 hPa GOMOS values are dominated by the NO2
enhancement, whereas WACCM shows the usual annual cycle with the mid-winter
minimum. Therefore, a strong anti-correlation emerges between WACCM and GOMOS
during the peak of the enhancement event. This anti-correlation is repeated
during most of the Antarctic winters, but with smaller amplitude. The
correlation over all times averages to a vanishing correlation. During 2003
the NO2 enhancement and the WACCM–GOMOS anti-correlation extends to
non-polar latitudes 50–60∘ S.
Very strong NO2 increases in the Arctic took place between the end of
October 2003 and the end of March 2004. This period covers strong proton
events on 28–29 October and 2–3 November 2003 (the so-called
“Halloween” event) and a strong descent period that started in mid-January 2004.
The complexity of events is illustrated in Fig.
where we show WACCM and GOMOS NO2 mixing
ratios and their difference as a function time and pressure. The peculiar
ridge form of the distributions is a result from our dynamic GOMOS data
selection. Before the Halloween there was not enough NO2 above 1 hPa for
GOMOS to retrieve it. During April this “normal” level is restored. The
elevated NO2 amounts propagate with diminishing peak values down to 3.6 hPa (about 35 km).
It is evident that during the period shown, at altitudes above 5 hPa GOMOS
NO2 values are much larger than the those from WACCM most of the time.
Figure show how both WACCM and GOMOS capture the
enhanced NO2 values around 0.5 hPa, produced by the SPEs at the end of
October, and the descent until mid-December. WACCM seems to overestimate the
magnitude of this enhancement by 5–20 ppb, which is in agreement with
earlier results on NOyFig. 15. The maximum
difference is 39 ppb on 30 October at a pressure level 0.19 hPa. WACCM
reproduces only a fraction of the larger increase observed at 0.05 hPa at the
beginning of December. This is also true for the strong descent from the
mesosphere to upper stratosphere observed in January–April. The maximum
GOMOS value during these events is 450 ppb at 0.245 hPa as an average over
15–19 February 2004. The corresponding WACCM value is 18 ppb, meaning that the
difference is 432 ppb. Mesospheric NO2, and NOx in general, have
been underestimated in WACCM during this period due to a combination of (1) incomplete representation of in situ production by EEP and (2) incomplete recovery
from a sudden stratospheric warming in early January, resulting in
insufficient descent (see ).
Nitrogen trioxide
In Fig. we show NO3 profiles from the Sirius
occultations in the 40–60∘ S latitude band. The relative
uncertainty is better than 10 % and the relative difference from -20 to
+5 % in 1–40 hPa. Near the peak density ∼ 2 hPa (40 km) WACCM and GOMOS
values are within ±2 % but at lower altitudes WACCM values are
consistently about 20 % lower
than GOMOS.
The mission averages shows that the general valid altitude region is from
0.7 to 37 hPa (approximately 22–48 km). In the polar regions NO3 values
can be retrieved up to 0.3 hPa. GOMOS and WACCM NO3 peaks at 2.35 hPa with
270 ppt and in the 40–50∘ S latitude band. The average
NO3 values in the polar regions are below 160 ppt. In Fig.
we show the median relative differences from 2002 to 2011
between WACCM and GOMOS as a function of latitude and altitude. Around the
peak of the NO3 profile the difference between WACCM and GOMOS is
typically within ±5 %. This is much better than uncertainty estimates of
GOMOS NO3 from validation. In the polar regions, the maximum region
excluded, WACCM NO3 is up to 60 % lower than GOMOS.
The relative NO3 difference (WACCM–GOMOS)/median(GOMOS) (in %)
during 2002–2011. Latitudes are from -90 to 90∘ with 10∘
resolution. A cross marks a point where the difference does not deviate from
zero in a statistically significant way. A cell with a dot marks a point
where there are no collocated profiles.
WACCM and GOMOS NO3 mixing ratio correlation for 2002–2011.
Latitudes are from -90 to 90∘ with 10∘ resolution. A
crossed cell marks a point where the correlation does not deviate from zero
in a statistically significant way. A cell with a dot marks a point where
there are no collocated profiles.
NO3 mixing ratio 5-day time series at 3.7 hPa from WACCM and
GOMOS from 2002 to 2011 in the Arctic 60–90∘ N (a) and in
the Antarctic 60–90∘ S (b). The colour coding symbols:
W/S and W/N are WACCM in Antarctic, Arctic; G/S and G/N are GOMOS in Antarctic,
Arctic. In the both panels the y axis is logarithmic. In (c) the
mixing ratio difference is shown for the Arctic and the Antarctic in the
mixing ratio unit. The colour coding symbols: S is Antarctic,
N is Arctic.
In Fig. we show the WACCM–GOMOS NO3 correlation
coefficient as a function of the altitude and latitude. Around the NO3
maximum all latitudes show very high correlations 0.95. The reason for
this high correlation is the fact that the mixing ratio of NO3 is very
sensitive to temperature see. When we calculate the correlation of
WACCM's NO3 with the model temperature (in the stratosphere MERRA), we
get values ranging from 0.7 to 0.99 in the altitude range 2–50 hPa. Similar positive
correlation values are seen between GOMOS NO3 and MERRA temperature
between 2 and 5 hPa. Temperature-related issues are a probable cause for the
observed NO3 differences in the polar regions evident in
Fig. . It is plausible that in these particular regions MERRA
underestimates real temperatures except in the neighbourhood of the NO3
maximum. The temporal cycle is correct but the absolute values differ.
Dramatic examples of the temperature dependence of NO3 can be seen in
the polar time series of Fig. at 3.7 hPa (this
altitude seems to be most sensitive to temperature). In the Arctic, the
strongest peaks in mixing ratio are caused by the large changes in
temperature during SSW events
e.g.. In the Antarctic the NO3
cycle during the study period follows the normal annual cycle of the temperature with one exception:
during the 5-day period around 28 July 2010 NO3 values show a major jump
(for analysis of this case, see ). Note that the famous
2002 SSW in Antarctica was not captured by GOMOS measurements. It seems
that at the sudden warmings (with the Antarctic case excluded) WACCM values
considerably exceed the corresponding GOMOS values and we can speculate that
MERRA overestimates the real temperature. A detailed evolution of the strong
Arctic event in December 2003–January 2004 is shown in
Fig. . WACCM and GOMOS values show similar temporal
development, but the actual values differ.
(a) WACCM (red) and GOMOS (blue) NO3 5-day time series
7 December 2003–18 January 2004 in the Arctic 60–90∘ N at
3.7 hPa. (b) MERRA temperature for the same period and
altitude.
NO3–temperature scatter plot at 3.7 hPa. (a) The
Antarctic 60–90∘ S. (b) The Equator
10∘ S–10∘ N. (c) The Arctic 60–90∘ N.
Red dots are from WACCM and blue dots from GOMOS. Exponential fits are applied
to temperature gridded WACCM data. Data for all latitudes are from 5-day time
series from 2002 to 2011.
In order to further study the temperature dependence of NO3, we have plotted WACCM and GOMOS mixing ratio values as a function of MERRA temperature at 3.7 hPa in Fig. . The dependence on temperature is
nearly exponential from both sources in the polar regions. The coefficients
of the exponential are 0.069 1/K for the Antarctic and 0.079 1/K for the Arctic.
The fitting of the equatorial values is more prone to errors as the
temperature variation is more limited than in the polar regions. The two
polar coefficients decrease below and above the selected altitude level 3.7 hPa.
In and a formula for the ratio of
NO3 to O3 densities is derived assuming nighttime chemical
equilibrium. In Fig. we show how this theoretical ratio
and the ratio calculated from the WACCM simulated data compare with the ratio
determined from GOMOS data. The theory values are calculated using
temperature from WACCM. WACCM, GOMOS and the theoretical values show good
agreement inside the maximum region of the NO3 mixing ratio excluding
polar latitudes. Theoretical values start strongly increasing compared to
GOMOS above 1.5 hPa, whereas WACCM slightly decrease in the same region. Both
WACCM and theoretical values are lower with respect to GOMOS below 10 hPa.
Conclusions
In this work we compared the state-of-the-art chemistry–climate model
WACCM to measurements from the satellite instrument GOMOS. Measurements cover
years from 2002 to 2011 and were made at nighttime. We compared
O3, NO2 and NO3 mixing ratios using 5-day time series. We
also calculated the correlation of GOMOS and WACCM time series. The
comparisons are done with collocated profiles, which eliminate differences
from natural variability and sampling patterns.
This comparison required considerable effort to ensure the quality of
the observational data. GOMOS nighttime observations collect photons from
138 different stars which vary widely in their luminosity and effective
temperature. This variation causes large differences in the quality of trace
gas profiles. For ozone we used three GOMOS ozone data flags to remove
low-quality profiles, for NO2 and NO3 there are no such quality
flags available. In order to form reliable average profiles from individual
GOMOS trace gas profiles it was necessary to determine the altitude
limits of valid data in profiles. In the present work we determined the
limits for all time steps, all latitude bands and for all stars using two
criteria. First, we demanded that for valid altitudes the t value
(average density/uncertainty) was larger than 2. Second, we demanded that the
distribution of GOMOS values was located mainly on positive density values.
This approach has produced altitude limits of valid data that were previously estimated using a priori knowledge.
The NO3/ O3 ratio from WACCM and from the equilibrium
chemistry theory (see ) compared to the
corresponding ratio from GOMOS. Relative differences. Data are from 5-day
time series from 2002 to 2011. A cell with a dot marks a point where there are
no collocated profiles.
Our comparisons show that in the stratosphere (1–50 hPa) outside the polar
regions WACCM ozone values are 0–6 % smaller than GOMOS values, which
slightly exceeds the uncertainty estimates of GOMOS measurements. The
difference patterns are consistent in time during 2002–2011. In the tropical
region in the lower stratosphere WACCM measurements show consistently larger
values (up to 20 %) than GOMOS. In the Arctic WACCM is also larger (up to 20 %) than GOMOS. In the Antarctic the ozone hole
evolution is in better agreement. In the mesosphere above the ozone minimum
at 0.01 hPa (or 80 km) large differences are found between WACCM and GOMOS.
Differences exist in the values of the mixing ratio and also in the
correlation of time series at the second ozone maximum. Differences may be
connected to WACCM's temperatures in the mesosphere or to specific parameter
values that control the gravity wave dissipation in WACCM (see
). The correlation of GOMOS and WACCM time series is high
except in the non-polar region in the mesosphere just below the ozone minimum
and at the altitudes from the second ozone maximum and above.
Outside the polar areas and in the validity region 0.4–37 hPa WACCM and
GOMOS NO2 values agree reasonably well. In the polar areas, where solar
particle precipitation and downward transport from the thermosphere enhance
NO2 abundances, GOMOS values are much larger than WACCM. The correlation
of time series is moderate in the stratosphere except in the upper
stratosphere at southern latitudes where NO2 downdraft events cause
anti-correlation between WACCM and GOMOS. GOMOS measurements and simulation
by the new version of WACCM are in better agreement for the direct particle
initiated NO2 increases, but for the downdraft cases GOMOS values are
much larger than those from WACCM. The overall correlation of the polar
5-day time series is still quite high in the middle atmosphere.
For NO3, we find that WACCM values largely agree with GOMOS. In the validity
region 1.2–5 hPa the correlation is very high. Because the NO3
abundance is controlled by temperature, the WACCM–GOMOS NO3 difference
can be used as an indicator of the accuracy of MERRA temperature
information. We found that NO3 temperature dependence can be fitted
reasonably well by an exponential function in the polar regions. The
NO3/ O3 ratio agrees with the equilibrium chemical theory quite accurately.
The relative difference of WACCM and GOMOS vertical columns of
ozone, NO2 and NO3. The vertical extent of the column is
0.0002–50 hPa for ozone, 0.4–37 hPa for NO2 and 1.1–26 hPa for
NO3.
The differences in trace gas profiles can also be studied by comparing
vertical column densities. The vertical columns can be calculated from number
densities at geometric heights of the pressure levels. In Fig.
we show the relative difference of WACCM and GOMOS columns.
The vertical extent of the column is 0.0002–50 hPa for ozone, 0.4–37 hPa
for NO2 and 1.1–26 hPa for NO3. These limits avoid all missing
data cases and include the number density maxima of the gases. The vertical
ozone column is 208 Dobson units at the Equator (the full vertical column is
about 300 Dobson units) and about 145 Dobson units at the poles. The total
column for NO2 varies between 0.05 and 0.17 Dobson units and between
0.0003 and 0.001 Dobson units for NO3. We can see that GOMOS and WACCM total
ozone columns agree within ±2 % except in the Arctic where the WACCM
column is 10 % larger than GOMOS. WACCM NO2 column is up to 15 % larger
than GOMOS except at the southernmost latitudes where enhanced NO2
events have deeper penetration than in north. WACCM NO3 columns are -5 %
smaller outside the polar areas, whereas in the polar areas the difference is
around 30 %.
In this work we have attempted to expose agreements and differences between the
WACCM model and the GOMOS measurements. To understand underlying reasons for
differences a detailed and presumably difficult analysis of the model physics
and chemistry is necessary. Perhaps the only exception is temperature from
the external meteorological model that we think is the reason for NO3
differences in the polar regions. On the GOMOS data side, there is still room
for better algorithms and more extensive validation especially in the polar
regions. A wider comparison including additional relevant constituents from
other satellite instruments would help to vindicate our results and to pinpoint the underlying reasons for differences.
All data can be requested form the first author of this paper (see correspondence information). Data will be placed on a publicly accessible server in due time. The size of the GOMOS-paired
WACCM data set is 2.2 Gb. The GOMOS data used in this work is a MATLAB version
of the so-called user friendly (UFP) GOMOS data. These UFP data (in netCDF-4
format) are available from the ESA data portal . The
collocated MATLAB data sets include WACCM-data and the paired satellite data and is 4.8 Gb.
The SD-WACCM-D model will be available from NCAR. All the
WACCM and satellite data have been processed using MATLAB software. The
specific routines used in this work can be requested from the first author.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Quadrennial Ozone
Symposium 2016 – Status and trends of atmospheric ozone (ACP/AMT inter-journal SI)”.
It is a result of the Quadrennial Ozone Symposium 2016, Edinburgh, United Kingdom,
4–9 September 2016.
Acknowledgements
The authors want to thank anonymous reviewers for useful comments and
corrections. The work of Erkki Kyrölä was partly supported by ESA's ALGOM project.
The work of Monika E. Andersson and Pekka T. Verronen was supported by the Academy of Finland through
the project no. 276926 (SECTIC: Sun-Earth Connection Through Ion Chemistry).
Daniel R. Marsh was supported in part by NASA grant NNX12AD04G. The National Center
for Atmospheric Research is operated by the University Corporation for
Atmospheric Research under sponsorship of the National Science Foundation. Edited by: Stefan Reis
Reviewed by: two anonymous referees
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