ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-10259-2017Comparison of ozone profiles and influences from the tertiary ozone maximum in the night-to-day ratio above SwitzerlandMoreiraLorenalorena.moreira@iap.unibe.chhttps://orcid.org/0000-0002-4791-8500HockeKlemenshttps://orcid.org/0000-0003-2178-9920KämpferNiklausInstitute of Applied Physics and Oeschger Centre for Climate Change Research, University of Bern, Bern, SwitzerlandLorena Moreira (lorena.moreira@iap.unibe.ch)1September20171717102591026823March201728March201730June20178August2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/17/10259/2017/acp-17-10259-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/10259/2017/acp-17-10259-2017.pdf
Stratospheric and middle-mesospheric ozone profiles above Bern, Switzerland
(46.95∘ N, 7.44∘ E; 577 m) have been
continually measured by the GROMOS (GROund-based Millimeter-wave Ozone
Spectrometer) microwave radiometer since 1994. GROMOS is part
of the Network for the Detection of Atmospheric Composition Change
(NDACC). A new version of the ozone profile retrievals has been
developed with the aim of improving the altitude range of retrieval
profiles. GROMOS profiles from this new retrieval version have been
compared to coincident ozone profiles obtained by the satellite limb
sounder Aura Microwave Limb Sounder (MLS). The study covers the stratosphere and middle
mesosphere from 50 to 0.05 hPa (from 21 to 70 km) and
extends over the period from July 2009 to November 2016, which results
in more than 2800 coincident profiles available for the comparison. On
average, GROMOS and MLS comparisons show agreement generally over
20 % in the lower stratosphere and within 2 % in the
middle and upper stratosphere for both daytime and nighttime, whereas
in the mesosphere the mean relative difference is below 40 %
during the daytime and below 15 % during the nighttime. In addition, we have
observed the annual variation in nighttime ozone in the middle
mesosphere, at 0.05 hPa (70 km), characterized by the
enhancement of ozone during wintertime for both ground-based and
space-based measurements. This behaviour is related to the middle-mesospheric maximum in ozone (MMM).
Introduction
Passive millimetre wave radiometry is a well-established technique to
monitor atmospheric constituents by detecting the radiation emitted by
the rotational transitions of the molecules. It makes use of the
spectral properties of the atmospheric species in order to derive
information about their distribution in the atmosphere. The main
advantages of this technique are its independence of solar irradiation
and its insensitivity to weather conditions and aerosols. Additionally
it offers a good temporal resolution of 1 h. Measurements of
ozone performed by this technique have been indispensable in
monitoring changes in the ozone layer and improving the comprehension
of the processes that control ozone abundances
e.g.. Stratospheric ozone, in spite of its
small abundance, plays a beneficial role by absorbing most of the
biologically harmful ultraviolet sunlight. The absorption of UV
radiation by ozone creates a source of heat; therefore, ozone plays
a key role in the temperature structure of the Earth's
atmosphere. Changes in the stratospheric ozone concentration alter the
radiative balance of the atmosphere, the atmospheric composition and
the dynamics of the atmosphere. Continuous long-term monitoring of
ozone is essential for the detection of long-term trends in the
stratospheric ozone layer e.g.. The ground-based
ozone radiometer GROMOS (GROund-based Millimeter-wave Ozone
Spectrometer) is part of the Network for the Detection of Atmospheric
Composition Change (NDACC). In order to satisfy the requirements of
accuracy and stability, the validation of instruments is
necessary. There have been a number of comparisons in the past,
showing that GROMOS is a reliable tool to measure stratospheric and
lower-mesospheric ozone
This paper presents a comparison between the data from the
ground-based instrument GROMOS and the space-based instrument Microwave Limb Sounder (MLS) for
the time interval from July 2009 to November 2016 covering the
stratosphere and the middle mesosphere, which corresponds to the
altitude range of 20 to 70 km (50 to 0.05 hPa). We
have also performed an analysis of the diurnal variation and its
amplitude (night-to-day ratio) of middle-mesospheric ozone at
0.05 hPa (70 km). The diurnal variation in ozone in
the lower and middle mesosphere is observed as an increase in ozone
after sunset and a decrease after sunrise. Daytime production of
atomic oxygen by photolysis of ozone (Reaction R7) and
photolysis of molecular oxygen (Reaction R5) results in
nighttime ozone production by recombination of atomic and molecular
oxygen (Reaction R6) . In addition, we
observe the annual variation in the nighttime mesospheric ozone with
a maximum in wintertime and a minimum in summertime. This maximum in
mesospheric ozone during nighttime in winter is related to the middle-mesospheric maximum in ozone (MMM)
e.g. also known as the tertiary
ozone maximum
e.g..
reported that the MMM is a phenomenon that occurs at high latitudes
close to the polar night terminator at around 72 km altitude
during nighttime in winter and extends into middle latitudes with
decreasing amplitude. interpreted the tertiary peak
by considering that in the middle mesosphere during winter, with a solar
zenith angle close to 90∘, the atmosphere becomes optically
thick to UV radiation at wavelengths below 185 nm, and, since
photolysis of water vapour (Reaction R1) is the primary source
of odd hydrogen, reduced UV radiation results in less
odd hydrogen. The lack of odd hydrogen needed for the catalytic
depletion of odd oxygen (Reactions R2, R3, and
R4), in conjunction with an unchanged rate of odd-oxygen
production (Reaction R5), leads to an increase in
odd oxygen. This results in higher ozone concentration because atomic
oxygen recombination (Reaction R6) remains as a significant
source of ozone in the mesosphere. Additionally,
extended the interpretation by considering the very slow decrease in
the ozone dissociation (Reaction R7) rate with increasing solar
zenith angle.
H2O+hν(λ<185nm)→OH+OO+OH→O2+HH+O2+M→HO2+MO+HO2→O2+OHO2+hν(λ<242nm)→O+OO+O2+M→O3+MO3+hν→O2+O
The next section describes briefly both instruments and measurement
techniques. The results of the comparison are shown in
Sect. 3. Section 4 analyses the night-to-day variability and provides
a short discussion, and the conclusions are summarized in Sect. 5.
Instruments and measurement techniquesThe ground-based microwave radiometer GROMOS
This study is based on stratospheric and mesospheric ozone volume mixing
ratio (VMR) profiles observed by GROMOS. The ground-based millimetre wave
ozone spectrometer has been operating in Bern, Switzerland
(46.95∘ N, 7.44∘ E; 577 m) since November 1994 in
the context of the Network for the Detection of Atmospheric Composition
Change (NDACC). The instrument measures the thermal microwave emission of the
pressure-broadened rotational transition of ozone at 142.175 GHz. The
vertical distribution of the ozone VMR can be retrieved from the measured
spectral line since it contains information on the altitude distribution of
the emitting molecule due to the pressure broadening. The retrieval procedure
is performed through the Atmospheric Radiative Transfer Simulator (ARTS2)
which is used as a forward model to simulate the
atmospheric radiative transfer in a modelled atmosphere and so calculate the
ozone spectrum of this modelled atmosphere. The a priori profile of
O3 VMR required for the retrieval is taken from a monthly varying
climatology from ECMWF reanalysis as far as available (70 km) and
extended above by an Aura MLS climatology (2004 to 2011). The line shape used
in the retrieval is the representation of the Voigt line profile from
. Spectroscopic parameters to calculate the ozone absorption
coefficients were taken from the JPL catalogue and the
HITRAN spectroscopic database . The atmospheric
temperature and pressure profiles are taken from the
6-hourly European
Centre for Medium-Range Weather Forecast (ECMWF) operational analysis data
and are extended above 80 km by monthly mean temperatures of the
CIRA-86 atmosphere model . The accompanying Matlab package
Qpack2 compares the modelled spectrum with the measured
spectrum and derives the best estimate of the vertical profile by using the
optimal estimation method (OEM) . The OEM also provides
a characterization and formal analysis of the uncertainties
.
Mean ozone profiles retrieved by version 2021 (red line in panel
(a)) and by version 150 (blue line in panel (a)) measured
by GROMOS during the period from July 2009 to November 2016. The blue area
(v150) and the red area (v2021) are the SDs of the ozone VMR. The mean
relative difference profile (blue line) and the SD of the differences (blue
area) are represented panel (b), using the new version as reference.
The green line delimits the ±10% area. The VMR difference
profile along with its SD are shown in panel (c).
Example of an a priori profile and a retrieved ozone profile (green
and blue lines in panel (a), respectively), averaging kernels (grey
and coloured lines in panel (b)), the measurement response (red line
in the panel), vertical resolution (cyan line in panel (c)), and
altitude peak (magenta line in panel (c)) of the GROMOS retrieval
version 150 for 15 July 2013 with an integration time of 1 h.
Mean ozone profiles recorded by GROMOS (blue line), MLS convolved
(red line), and MLS original (green line) for the time interval between July
2009 and November 2016 are shown in the left sections of both the daytime
(a) and nighttime (b) panels. The blue area (GROMOS) and
the red area (MLS) are the SDs of the coincident measurements. The middle
sections show the mean relative difference profile between data of both
instruments, with GROMOS as reference. The blue areas in the middle sections
represent the SD of the differences. The green lines in the middle sections
delimit the ±10% area. The mean VMR difference profile and its
SD (blue area) are displayed in the right-hand parts of the daytime and
nighttime panels.
Recently, we have developed a new retrieval version (version 150) with the
aim of optimizing the averaging kernels. The differences with the former
version (version 2021) are in the a priori covariance matrix, in the
measurement error, and in the integration time of the retrieval.
In version 2021 the diagonal elements of the a priori covariance matrix are
variable relative errors ranging from 35 % at 100 hPa to
28 % in the lower stratosphere and increasing with altitude from
35 % in the upper stratosphere up to 70 % in the
mesosphere. Meanwhile, in version 150 the a priori covariance matrix has
a constant value for the diagonal elements of 2 ppm. For both
retrieval versions the off-diagonal elements of the a priori covariance
matrix exponentially decrease with a correlation length of 3 km.
Regarding the measurement noise, in version 2021 it is a constant error of
0.8 K, whereas in version 150 we used a variable error depending on
the tropospheric transmission:
ΔTb′=0.5+ΔTbe-τ.
The error of the measured brightness temperature, ΔTb, is
given by the radiometer equation:
ΔTb=Tb+TrecΔf⋅tint.
The radiometer equation gives the resolution of the radiation measured, which
is determined by the bandwidth of the individual spectrometer channels
(Δf), by the integration time (tint), and by the total
power measured by the spectrometer. A constant error of 0.5 K is
considered as a systematic bias of the spectra, due to spectroscopic errors
and the water vapour continuum. The error of the brightness temperature
(ΔTb) is of the order of a few Kelvins in the line centre
and 0.5 K in the line wings of the spectrum. Therefore, the
measurement noise (ΔTb′) depends on the bandwidth of the
spectrum and on the tropospheric transmittance. This is a more realistic
approach for the retrieval than considering a constant measurement noise,
resulting in an improvement in the retrieved ozone VMR in the lower
stratosphere. The sampling time for version 150 is 1 h, and in the
case of version 2021, it is 30 min. Longer integration time improves
the retrieved ozone VMR at upper altitudes.
A comparison between version 2021 and version 150 of ozone profiles measured
by GROMOS for the time interval from July 2009 to November 2016 is displayed
in Fig. . The mean ozone profiles retrieved by version 2021, in
red, and by version 150, in blue are represented in panel (a). The
SDs of the ozone VMR are shown by the coloured areas (red in the case of
v2021 and blue for v150). The mean relative differences (blue line in panel
(b)) and the VMR differences (blue line in panel (c)) range
from 30 % (0.5 ppm) in the lowermost stratosphere to within
5 % (0.2 ppm) in the middle stratosphere and increase to
10 % (0.4 ppm) in the upper stratosphere and up to
18 % (0.05 ppm) at 0.05 hPa (70 km). The
blue areas in panels (b) and (c) represent the SDs of the
differences, relative differences, and VMR differences. We can conclude from
Fig. that the differences between version 2021 and version 150
appear in the lower stratosphere and in the mesosphere.
Figure displays an example of a GROMOS retrieval accomplished by
the new retrieval version 150. Panel (a) shows the a priori (green line)
and the retrieved profile (blue line) measured in July 2013 at noon. The
averaging kernels (AVKs) and the measurement response are represented in
panel (b). The AVKs are multiplied by 4 in order to be displayed along with
the measurement response (red line). The AVK lines are grey except for some
selected altitudes, which are shown in different colours to make
Fig. easier to interpret. AVKs are a representation of the
weighting of the information content of the retrieval parameters. The
measurement response is an estimate of the a priori contribution to the
retrieval and can be obtained by subtracting the area of the AVK from 1. It
is considered a reliable altitude range of the retrieval when the true state
dominates over the a priori information, i.e. where the measurement response
is larger than 0.8 (an a priori contribution smaller than 20 %). The
measurement response shown in Fig. is around 1 from 18 to
70 km. The magenta line in panel (c) shows the altitude peak
of the corresponding kernels and proves that the AVK peaks at its nominal
altitude for the considered altitude range. Finally, the cyan line displays
the vertical resolution, which is quantified by the full width at half
maximum in the averaging kernels. The vertical resolution of this new
retrieval version of GROMOS lies between 10 and 15 km below
40 km altitude and between 15 and 20 km below 70 km
altitude. In version 2021, the vertical resolution lies generally within
10–15 km in the stratosphere and increases with altitude to
20–25 km in the lower mesosphere. Between 20 and 52 km (50
to 0.5 hPa), the measurement response is higher than 0.8. For more
details on version 2021, we refer to . Comparing the
measurement response and the vertical resolution obtained by version 2021 and
by version 150, we can conclude that there is an improvement in the results
retrieved by version 150. We assume that the changes performed in the
a priori covariance matrix, in the measurement noise, and in the integration
time result in the improvement of the retrieval product, mainly observed in
the lowermost and in the uppermost limit of the retrieved ozone VMR profile.
For technical details and measurement principles of the instrument, see, for
example, and and references included
therein.
Time series of averaged daytime and nighttime O3 VMR
measurements of GROMOS (blue line) and MLS (red line) for the period from
July 2009 to November 2016 at different pressure levels. An averaging kernel
smoothing has been applied to the series of the MLS measurements coincident
in time and space with the GROMOS measurements. Both time series are smoothed
over seven points or ∼1 week in time by a moving average.
Scatter plots of coincident O3 VMR measurements of GROMOS
and MLS for the period from July 2009 to November 2016 at different pressure
levels. The black line is the linear fit of both time series, and m is the
slope of the linear fit. The green line indicates the case of identity,
O3(MLS) =O3(GROMOS). r values are correlation
coefficients of the MLS and GROMOS time series.
Panel (a) shows the diurnal variation in O3 VMR
measured at noon (GROMOS in red, MLS convolved in orange, and MLS original in
magenta) and at midnight (GROMOS in blue, MLS convolved in cyan, and MLS
original in black) at 0.05 hPa (70 km), and panel
(b) shows its evolution throughout the year averaged for the time
interval under assessment (July 2009–November 2016). All time series are
smoothed in time by a moving average over 15 points (∼1 week).
Panel (a) displays the night-to-day ratio (NDR) of GROMOS
(blue line) and MLS (red line) at 0.05 hPa (70 km) for the
time period from July 2009 to November 2016, and panel (b) shows its
evolution throughout the year averaged for this time period. The time series
presented in (a) are smoothed in time by a moving average over 30
data points (∼1 month), and the orange line (MLS) and the cyan line
(GROMOS) shown in (b) are averaged over seven data points (∼1
week).
The Aura Microwave Limb Sounder
The MLS is a passive microwave limb-sounding
radiometer onboard the NASA Aura satellite. The Aura spacecraft was
launched in 2004 into a near polar, sun-synchronous orbit with
a period of approximately 100 min. The satellite overpasses
the GROMOS measurement location (at northern midlatitudes) twice
a day, approximately around noon and midnight. The standard product
for ozone is derived from MLS radiance measurements near
240 GHz. The vertical resolution of the ozone profiles ranges
from 3 km in the stratosphere to 6 km in the
mesosphere . The present study has used ozone
profiles from version 4.2. A summary of the quality of version 4.2
Aura MLS Level 2 data can be found in . Details
about the Aura mission can be found in .
Comparison of MLS and GROMOS
The vertical resolution of the MLS is within 3.5 km in the
stratosphere and up to 5.5 km in the middle
mesosphere. Therefore, in order to compare ozone profiles of GROMOS
with MLS, an averaging kernel smoothing is applied to the ozone
profiles of the satellite data. The smoothed profile of MLS adjusted
to the vertical resolution of GROMOS is expressed as
xMLS,low=xa,GROMOS+AVKGROMOS⋅(xMLS,high-xa,GROMOS),
where AVKGROMOS is the averaging kernel matrix of
GROMOS, xMLS,high is the measured MLS profile, and
xa,GROMOS is the a priori profile used during the
retrieval procedure of GROMOS. The application of averaging kernel smoothing
for the comparison of profiles with different altitude resolutions has been
introduced and described by, e.g., .
Every profile utilized in the comparison between MLS and GROMOS should be
coincident in time and space. The requirement of time coincidence is
satisfied when both measurements are within 1 h in time. The selected
criterion for spatial coincidence is that horizontal distances between the
sounding volumes of the satellite and the ground station have to be smaller
than 1∘ in latitude and 8∘ in longitude.
The present study extends over the period from July 2009 to November 2016 and
covers the stratosphere and middle mesosphere from 50 to 0.05 hPa
(from 21 to 70 km), and according to the spatial and temporal
criteria, more than 2800 coincident profiles are available for the
comparison. Figure a and b show the mean ozone profiles of the
collocated and coincident measurements of GROMOS (blue line), MLS convolved
(red line), and MLS original (green line) during the daytime and nighttime,
respectively. The relative difference profile in percent given by
(xMLS,low-xGROMOS)/xGROMOS
is displayed in the middle section of both Fig. a and b along with
the SD of the differences (blue area). The green line delimits the ±10% area. The mean profile of the VMR differences is shown in the
right sections of both Fig. a and b. The mean relative differences and
the VMR differences during the daytime (nighttime) are over 20 % or
0.5 ppm (15 % or 0.4 ppm) in the lower stratosphere,
decreasing with altitude to 0.7 % or 0.02 ppm (2 %
or 0.06 ppm) at the stratopause and increasing with altitude up to
38 % or 0.085 ppm (15 % or 0.12 ppm) at
0.05 hPa (70 km). We conclude from Fig. that
during nighttime GROMOS measures more O3 VMR (ppm) than MLS except
for the lower stratosphere, where MLS measures more O3 VMR (ppm)
than GROMOS, both during the daytime and nighttime. Nevertheless in the
mesosphere GROMOS measures more O3 VMR (ppm) than MLS, both during
the daytime and nighttime.
For an overview of the differences between coincident profiles, the
average over daytime and nighttime values of the ozone VMR (ppm) time
series of GROMOS (blue line) and MLS (red line) are displayed in
Fig. for different pressure levels. All time series have
been smoothed by a moving average over seven data points (∼1
week). The agreement between both ground-based and satellite-based
instruments depends upon altitude and time. A negative deviation of
GROMOS series with respect to MLS occurs in the lower stratosphere. On
the other hand, a positive deviation of GROMOS with respect to MLS is
observed in the middle stratosphere for summers 2011, 2012, 2014, and
2015. Further, we notice a negative bias of GROMOS during summer 2016
from the stratopause towards the mesosphere. The scatter plots of averaged daytime and nighttime O3
VMR measurements of GROMOS and MLS are shown in Fig. at the same pressure levels as
Fig. . The black lines, linear regression lines of the
observations, are close to the green 1-to-1 lines (O3(MLS) =O3(GROMOS)), except in the lower
stratosphere, where we find the negative deviation of GROMOS with
respect to MLS. The linear fit deviates from the identity where there
is less ozone in the case of GROMOS during winter in the middle to
upper stratosphere as we also observe in Fig. , along with
the positive deviation of GROMOS with respect to MLS during some
summers. The calculation of the correlation coefficients also reveals
good agreement with r>0.75 for all altitude levels except for the
altitude above 50 km where r is around 0.55.
Analysis of the night-to-day ratio
The diurnal variation in mesospheric ozone is characterized by an increase at
the beginning of the nighttime and by a decrease after sunrise. This effect
is explained by the recombination of atomic and molecular oxygen
e.g.. Because the ozone distribution in the
mesosphere is mainly controlled by photochemistry, it depends strongly on the
solar zenith angle ; therefore, an annual variation is
also expected in mesospheric ozone. Figure shows both the diurnal
variation in mesospheric ozone and the annual variation in nighttime
mesospheric ozone. To analyse the variability in mesospheric ozone we have
used ozone VMR measurements coincident in space and in time recorded by
GROMOS and by MLS for the time period from July 2009 to November 2016.
Fig. a displays the O3 VMR measured at noon (GROMOS in
red, MLS convolved in orange, and MLS original in magenta) and at midnight
(GROMOS in blue, MLS convolved in cyan, and MLS original in black) at
0.05 hPa (70 km) for the time period already mentioned. The
original MLS data, i.e. those not weighted with GROMOS AVKs, are shown in
order to provide an insight into the observability of the effect of MMM at
northern midlatitudes by GROMOS. We define the average between the values
recorded within 2 h around midnight (noon) as the midnight (noon)
value. The daytime mesospheric ozone does not show any distinct annual
variation. On the other hand, the annual variation in nighttime mesospheric
ozone is characterized by a maximum in wintertime and a minimum in
summertime. Fig. b shows the evolution of the midnight mesospheric
ozone throughout the year averaged for the time interval from July 2009 to
November 2016. All time series displayed in both panels of Fig.
have been smoothed in time by a moving average over 15 data points (∼1
week). A closer observation shows that the annual variation in the nighttime
ozone exhibits a primary maximum over wintertime and a secondary maximum
around springtime. Our results on the annual variation in mesospheric ozone
at Bern (Switzerland, 46.95∘ N, 7.44∘ E) are in agreement
with the ones observed at Lindau (Germany, 51.66∘ N,
10.13∘ E) by . Disagreements appear in the
amplitudes where the maximum values of GROMOS and MLS original do not exceed
1.5, or 1.2 ppm in the case of MLS convolved, whereas at Lindau the
maximum values exceed 3 ppm at 70 km. Nevertheless, our
results are expected since this maximum in mesospheric ozone during nighttime
in winter is related to the middle-mesospheric maximum in ozone (MMM), and
according to , its effect extends into midlatitudes with
decreasing amplitude. Furthermore, we have analysed the amplitude of the
diurnal variation, the night-to-day ratio (NDR). The NDR is closely related
to the MMM, but it is also related to the change in the diurnal variation
from winter to summer . The annual variation in the NDR
is modulated by oscillations of a planetary timescale .
reported that during a sudden stratospheric warming event
the tertiary ozone maximum can decrease significantly or can even be
completely destroyed. has shown the loss of the tertiary
ozone layer in the polar mesosphere due to the solar proton event in November
2004.
Fig. a displays the NDR of GROMOS (blue line) and MLS (red line) at
0.05 hPa (70 km) for the time interval from July 2009 to
November 2016, while Fig. b shows its evolution throughout the year
averaged for the time interval under assessment. Both time series were
smoothed in time by a moving average over 30 points (∼1 month). The
orange line (MLS) and the cyan line (GROMOS) depicted in Fig. b
show a moving average over seven data points (∼1 week) with the aim of
clarifying Fig. . Both the ground-based and the satellite-based
instruments confirm the expected winter enhancement of the NDR, also observed
at Lindau by , although the data from the latter exhibit
larger amplitudes. We observe winter-to-summer values of a factor of 1–2,
whereas at Lindau, winter-to-summer values vary by a factor of 2–3 at
70 km. Thus, despite the definition of the MMM
being restricted to high latitudes, we can report its observation with
a smaller amplitude at midlatitudes.
Conclusions
Stratospheric and middle-mesospheric ozone profiles for the period
from July 2009 to November 2016 recorded by the ground-based
instrument GROMOS and by the space-based instrument MLS were used to
perform a comparison and to evaluate the diurnal variability and its
amplitude, the NDR. The agreement between
measurements coincident in space and time for both data records is
within 2 % (0.06 ppm) between 30 and 50 km
(15–0.7 hPa), increasing up to 20 % (0.5 ppm)
at 20 km (50 hPa), for both daytime and nighttime. In
the mesosphere the difference increases up to 38 %
(0.085 ppm) during the daytime and up to 15 %
(0.12 ppm) during the nighttime at 70 km
(0.05 hPa). In general terms, we report good agreement between
the new retrieval version (v150) of GROMOS and the version 4.2 of
MLS. Furthermore, we observe extensions of the middle-mesospheric
maximum in ozone (MMM) during winter towards northern
midlatitudes. This effect is smaller in amplitude at midlatitudes
compared to high latitudes. Moreover, the winter enhancement of
nighttime mesospheric ozone is observed by GROMOS and MLS above Bern.
Routines for data analysis are available upon request by
Lorena Moreira (lorena.moreira@iap.unibe.ch).
The data from the GROMOS microwave radiometer is available
via http://ftp.cpc.ncep.noaa.gov/ndacc/station/bern/hdf/mwave. MLS v4.2
data are available from the NASA Goddard Space Flight Center Earth Sciences
Data and Information Services Center (GES DISC):
http://disc.sci.gsfc.nasa.gov/Aura/data-holdings/MLS/index.shtml.
KH performed the retrieval of the GROMOS measurements. LM carried out the data analysis
and prepared the paper. NK is the principal investigator of the radiometry project. All authors have contributed to the interpretation of the results.
All authors declare that there are no conflicts of
interest.
This article is part of the special issue “Twenty-five years of
operations of the Network for the Detection of Atmospheric Composition Change
(NDACC) (AMT/ACP/ESSD inter-journal SI)”. It is not associated with a
conference.
Acknowledgements
This work was supported by the Swiss National Science Foundation under Grant
200020–160048 and MeteoSwiss GAW Project: “Fundamental GAW parameters
measured by microwave radiometry”. Edited by:
Hal Maring Reviewed by: three anonymous referees
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