Introduction
Ozone-depleting substances have been decreasing due to the Montreal
Protocol and its subsequent adjustments and amendments. As a result,
stratospheric ozone (O3) is expected to increase in the future. The last
WMO/UNEP ozone assessment concluded that increasing O3 has been observed
in the upper stratosphere around 42 km, or 2 hPa, in altitude
. Positive trends have been evaluated for both the tropics and
35–60∘ latitude bands of both hemispheres above 5 hPa levels from 2000
to 2016 . However, the trend is still not
statistically significant below 10 hPa levels. found
0.7 ± 0.9 and -0.2 ± 1.4 % per decade changes at 10 and 70 hPa,
respectively, for 35–60∘ S. The satellite measurement has an advantage
for estimating long-term trends because of its global coverage on a daily
basis. However, its drift, i.e., the long-term measurement stability, should
be quantitatively assessed with independent instruments. Ground-based ozone
lidar is a potential candidate for such purposes and can be used to estimate
drift e.g.,.
comprehensively evaluated the bias and drift of 14 limb-viewing
satellite sensors using ozonesonde and ozone lidar measurements.
They concluded that biases in the satellite sensors were within ±5 %
between 20 and 40 km and drifts were at most ±5 % per decade. They
suggested that several instruments have significant drifts; multi-instrument
comparisons are needed to derive drift. also showed a
comparison spread, which is a measure of the short-term variability, with
values of < 5–12 % for the same altitude range.
The ozone differential absorption lidar (DIAL) system was installed at the
Atmospheric Observatory of Southern Patagonia (Observatorio Atmosférico de la
Patagonia Austral, OAPA; 51.6∘ S, 69.3∘ W) in Río
Gallegos, Argentina, in 2005 . A map showing the OAPA site
is shown in Fig. . This site has been a stratospheric ozone
lidar site within the Network for the Detection of Atmospheric Compositions
Change, NDACC (http://www.ndsc.ncep.noaa.gov), since December 2008.
NDACC sites in the Southern Hemisphere (SH) are very sparse at mid- to
high latitudes (e.g., 35–60∘ S). In springtime, this site is
occasionally inside the southern polar vortex, when it has shifted off the
pole or elongated. A long persistent coverage (∼ 20 days) of the polar
vortex over the southern tip of South America occurred in 2009 for the first
time since 1979 . In the 2009 austral spring
between September and November, measurements at OAPA were performed in the
vicinity or, on some occasions, inside of the polar vortex, revealing a
variability in the potential vorticity (PV) of measured air masses inside and
outside the vortex. Accordingly, the largest variability in O3 values
would be expected in such a latitude band (35–60∘ S). Therefore,
this event was a good opportunity to assess the impact of O3 variability
on biasing behavior. To evaluate the performance of the DIAL system under
such variability, O3 data from Aura Microwave Limb Sounder (MLS) satellite
measurements were used for comparison. In addition to the
DIAL–MLS comparison, we also used O3 values from a 3-D chemical transport
model (CTM) simulation, which is based on version 3.2 of the Model for
Interdisciplinary Research on Climate (MIROC) .
Therefore, a secondary objective of this study was to examine the performance
of the model simulation. Measurement and model simulation data used here are
described in Sect. 2. The methodology used for the comparison is provided in
Sect. 3. Vertical profiles of O3 and their time series at the two selected
pressure levels are shown in Sect. 4. The results of differences that depend
on coincidence criteria are also shown in Sect. 4 and summarized for all
pressure levels (from 6 to 100 hPa). The conclusions of this study are
described in Sect. 5.
Location of the OAPA site in Río Gallegos, Argentina,
(51.6∘ S, 69.3∘ W), shown as a blue circle. Latitude ranges
from 30 to 90∘ S.
Measurements and model simulations
Stratospheric ozone lidar
DIAL is a laser-based active remote sensing system operated from the ground,
aircraft, and ship and has a robust heritage
e.g.,. O3 measurements
from DIAL have a high vertical resolution and measurements have shown
long-term stability , owing to the stratospheric
ozone lidar sites of NDACC
e.g.,. To
target the stratosphere, O3 number density is usually retrieved between 15 and 45 km in geometric altitude. The DIAL system installed at the OAPA
site in Río Gallegos, Argentina, began operating in August 2005
. The DIAL system operated at this site is fully described
in and included in a review .
The instrument is briefly described here. The DIAL technique requires two
emitted wavelengths. An excimer (XeCl) laser emitting at 308 nm with a 30 Hz
repetition rate and maximum energy per pulse of 300 mJ is used for ozone
absorption. The reference wavelength corresponds to the third harmonic of the
Nd : YAG laser emission at 355 nm with a 30 Hz repetition rate and maximum energy
per pulse of 130 mJ. The optical receiver that collects the backscattered
photons is composed of four Newtonian (f/2) telescopes defining an
array of telescopes. Each has a 50 cm diameter with parabolic aluminized
surfaces, 48 cm in diameter. This produces a total reception area of 7238 cm2. An optical fiber, 0.27 db km-1 with attenuation at 308 nm, is
placed at the focus of each telescope. The other end of the fiber is
positioned at the focus of a quartz lens placed inside a spectrometer used to
separate the received wavelengths. A mechanical chopper is positioned at the
entrance of the spectrometer. It has a rotational frequency of 300 Hz
(18 000 rpm), and its role is to block the strong lidar signals originating
from the lower part of the atmosphere, typically below 10 km.
The O3 number density profile is computed using the DIAL equation from the
difference between the signal slopes originating from Rayleigh scattering of
the emitted laser beams (nO3). Since the returned signals include
scattering and attenuation by atmospheric molecules, aerosols, and other
atmospheric components, this complementary term could be minimized with the laser
wavelength chosen in the DIAL instrument. The laser wavelength chosen in the
DIAL instrument minimizes the complementary term in the stratosphere to less
than 10 % of nO3 measured, in the presence of low aerosol loading
. Because lidar signals cover a large dynamic range, they
have to be attenuated for measurements in the lower stratosphere. Therefore,
the final O3 profile corresponds to a composite profile computed from the
low- and high-intensity Rayleigh signals which are detected
simultaneously e.g.,.
In the 2009 spring, measurements began on 6 September (UTC, Coordinated
Universal Time) during clear-sky local nighttime. Because the latitude of
OAPA is 51.6∘ S, the short night lengths with increased seasonal cloud
cover made it challenging to perform measurements after December
. In total, 23 vertical profiles of ozone were obtained
between September and November 2009, which were used for this study. Most
measurements were performed for 3–5 h to obtain a good signal-to-noise ratio
(see Table S1 in the Supplement for detailed numbers). If we assume some typical wind speed of
30 m s-1 wind speed in the lower stratosphere, a horizontal spatial resolution
becomes 300–500 km. In fact, we have evaluated horizontal distances using
air-parcel trajectory analysis at 83 hPa (Tomikawa and Sato, 2005) and the
results are summarized in Table S1. The actual vertical resolution ranged
from 0.7 to 4 km at 14 and 35 km in altitude, respectively. The total
measurement uncertainty also ranged from 3 to 15 % at the same altitudes.
For the total measurement uncertainty , we evaluated the
effect of the ozone absorption cross section, which is temperature dependent, and
found that the error is not larger than 2 %. The other source is from the correction
of aerosol contamination. The methodology uses a Fernald inversion algorithm
to evaluate the aerosol backscatter signal at 355 nm and extrapolated to 308 nm. In order to
increase the signal-to-noise ratio, the signal registered is
averaged over the full acquisition time of the measurement. The acquisition
time is typically 3–4 h, according to weather conditions.
Before processing the signal using the DIAL equation, we make two
corrections: (1) subtraction of the background signal using a linear
regression within the range of altitudes where the lidar signal is considered
negligible, typically between 80 and 150 km; and (2) dead time correction of the
detector, in order to correct the saturation of the photon-counting
signals (pile-up effect) in the lower altitude ranges .
Aura MLS
The MLS measurement covers latitudes between 82∘ N and 82∘ S
since August 2004 . It is onboard the National Aeronautics
and Space Administration (NASA) Earth Observing System (EOS) Aura satellite.
MLS measures millimeter- and submillimeter-wavelength thermal emission from
the limb of the Earth's atmosphere every 25 s, from which vertical
profiles of more than 15 chemical species are retrieved. We used the standard
O3 data product (240 GHz radiances) retrieved with the version 4.2 data
processing algorithm, which is publicly available from
http://mls.jpl.nasa.gov/. The quality of the O3 data is as follows from
. The vertical × horizontal resolutions are 3 km × 300 km at 100 hPa and 3 km × 500 km at 10 hPa. The precision
is 0.03 ppmv at 100 hPa and 0.1 ppmv at 10 hPa. The accuracy estimated from
systematic uncertainty characterization tests are 0.05 ppmv at 100 hPa and
0.3 ppmv at 10 hPa. Data screening was accomplished according to
. The former versions of the MLS O3 values are
evaluated from comparisons with DIAL . The comparisons
showed a good agreement of ∼ 5 % from 5 to 100 hPa.
Nudged chemistry–climate model based on MIROC3.2 GCM
As described in , NIES developed nudged chemistry–climate
models (CCMs) using the MIROC model. The CCM was nudged toward European
Center for Medium-Range Weather Forecasts ERA-Interim data below 1 hPa
. In these nudged CCMs, a set of model variables for
zonal wind (u), meridional wind velocities (v), and temperature
(T) were nudged. Above 1 hPa, where no ERA-Interim pressure level data
exist, the zonal means of zonal wind and temperature are nudged toward the
COSPAR International Reference Atmosphere 1986 data . The
timescale for nudging the meteorological data (u, v, and T) was set to 1 day.
The model used in this study is a spectral model with T42 horizontal
resolution (2.8∘ × 2.8∘) and 34 vertical atmospheric
layers above the surface. The top layer is located at approximately 80 km
(0.01 hPa). Hybrid sigma-pressure coordinates are used for the vertical
direction. The chemical constituents included in this model are Ox,
HOx, NOx, ClOx, BrOx, hydrocarbons for methane oxidation,
heterogeneous reactions for sulfuric-acid aerosols, supercooled ternary
solutions, nitric-acid trihydrate, and ice particles. The CCM contains
61 chemical constituents including 42 for prediction and 19 for photochemical
equilibrium, 165 gas-phase reactions, 42 photolytic processes, and
13 heterogeneous reactions on multiple aerosol types. The reaction rates and
absorption coefficients are based on JPL (Jet Propulsion
Laboratory) report report number 15-10;.
The bromine budget is increased for consistency with observations using
additions of CHBr3 and CH2Br2, which results in approximately
21 pptv total inorganic bromine, Bry, in the stratosphere around the year
2000. The volume mixing ratio of total inorganic chlorine, Cly, is
approximately 3.3 ppbv in the stratosphere over the same period. This nudged
CCM is hereafter termed the MIROC chemical transport model (MIROC-CTM).
Method for comparisons between DIAL and MLS or CTM
The O3 profiles from DIAL are used to evaluate the bias and drift, i.e.,
long-term stability of satellite measurements
e.g.,. Therefore, it is important
to show the quality of the respective ground-based DIAL performance. Although
O3 profiles at OAPA are included in , the result for
OAPA alone is not shown. also did not show coincident
O3 profiles with any limb-viewing satellite instruments. Therefore, we
revisit the quality of the DIAL O3 profiles obtained in the 2009 austral
spring.
Usually, comparisons between DIAL and limb-viewing satellite instruments are
conducted considering the differences in their vertical resolution and
retrieval strategies . MLS has covered the location of OAPA
(51.6∘ S) on a daily basis since measurements began in 2005. The
long-term stability of the MLS ozone dataset has been shown to be very good
e.g.,. For comparison between DIAL and MLS,
the DIAL profile is convolved using the following equation
:
Xcomv=Xa+AXDIAL-Xa,
where Xa is the a priori profile for each retrieval and
A is the averaging kernel functions (matrix) of MLS.
XDIAL is the DIAL ozone profile, and Xcomv is the
convolved DIAL ozone profile, which is converted to each MLS grid for
comparison. We used A for the polar winter condition from two
A's that have been provided in the MLS dataset; the other is for
the tropical condition.
Vertical profiles of O3 mixing ratios on 14 November
2009 (a) and 23 November 2009 (b) measured using DIAL
(asterisks and dotted line) and MLS (solid lines with color) over the OAPA
site (see text for additional description). A MIROC-CTM O3 profile of
the nearest grid for the OAPA site is also shown. Corresponding potential
temperatures for pressure are shown as text in the vertical axis. Differences
between DIAL and X (MLS or MIROC-CTM) (X - DIAL) are shown in the right
panel (see text). The MLS profiles are color coded based on their measurement
latitudes.
We used 500 km in distance, in the great circle distance (between 47.1 and
56.1∘ S for 69.3∘ W), and ±24 h for coincidence
criteria between DIAL and MLS measurements. Because the midpoint for the DIAL
measurement duration was usually 02:00–03:00 UTC, the time differences
(MLS - DIAL) were 0–4 or 13–17 h on the same day that correspond to
night or day paths of the EOS Aura orbit. When no MLS measurements were
available on the same day (9 cases), measurements 1 day before were used. In
those cases, the time differences were -6 to -10 or -20 to -24 h.
For the DIAL measurement on 27 October, an MLS measurement on 28 October was
used, resulting in a 26 h difference. Both DIAL measurements on 7 and
8 October used 10 MLS measurements on 7 October for matching pairs. In total,
180 matching pairs were used in this study.
For comparisons between DIAL and MIROC-CTM, we also unified the vertical
grids for comparison. The DIAL profiles were linearly interpolated onto the
pressure grids for the MLS data; the vertical increments of the DIAL profile
are as small as 150 m. The MIROC-CTM profiles on the day of each DIAL
measurement were interpolated onto the pressure grids for the MLS data using
a cubic spline. Both interpolated values were used to compute differences
(MIROC-CTM - DIAL) (see Figs. , ,
and ).
For converting the original DIAL geometric altitude and O3 number density
to pressure and O3 mixing ratio, the NCEP reanalysis data
are used. These data are registered in the NDACC database.
Possible deviations could be expected if we were to use other meteorological
data for the conversion process in DIAL. However, in this study, we used the
DIAL data that registered in the NDACC database. Another possible deviations
could also be expected if we were to use other meteorological data for the
nudging process in MIROC-CTM. The different reanalysis data may cause
different vertical and horizontal motions of air in the model, providing
different tracer correlations, hence ozone field. However, examining the performance of
the model simulation of is one of the goals of this study.
Results and discussion
Example of vertical profile comparison
Figure a shows vertical profiles of O3 measured with DIAL
compared with those of MLS on the same day (14 November 2009) as an example.
The asterisks and dotted line show the converted DIAL profile using Eq. (1)
and the original high vertical resolution DIAL profile, respectively. Each
MLS profile was color coded with its measurement latitude to observe the
latitudinal difference between DIAL and MLS. The bar in MLS O3 profiles
shows the precision reported for individual profiles. The bar in the DIAL
O3 profile shows the total uncertainty. The combined uncertainty (root sum
squared) is shown in the right
panel. In addition to the DIAL and MLS profiles, we also compared the 24 h
average O3 profiles from MIROC-CTM at 12:00 UTC. We have extracted data
from six locations between 48.8 and 54.4∘ S in latitude and 67.5 and
70.3∘ W in longitude, but the nearest grid data were plotted in
Fig. a (see Figs. and for
the variability in six model grids).
On this day, the DIAL profile above 50 hPa, i.e., pressures smaller than that
level, revealed lower O3 values, which was suggested in
due to the edge of the southern polar vortex located near
OAPA on 14 November. also suggested that an altitude
region around a potential temperature (PT) of 650 K was just inside the
vortex. Several PT levels corresponding to pressure are also shown as text in
Fig. a. This DIAL profile agrees well with MLS profiles
observed at similar latitudes – 51.7∘ S with green lines. The MLS
profiles revealed a larger latitudinal difference of ∼ 2.5 ppmv over
∼ 8∘ especially at the ∼ 50 hPa level. For reference, the
MIROC-CTM profiles also revealed latitudinal differences of ∼ 1 ppmv over
5.6∘ at the same pressure level (not shown), suggesting a weaker
latitudinal gradient in the model simulation for these conditions. In
addition, the MIROC-CTM O3 value is higher than that from DIAL around 20 hPa
levels. We discuss this feature in the MIROC-CTM in Sect. 4.2.
In the right panel of Fig. a, the differences between MLS
O3 and DIAL (MLS - DIAL) are shown. In addition, the difference between
DIAL and the nearest MIROC-CTM is shown. In general, the MLS profiles of
similar latitudes (51.7∘ S) with OAPA are in good agreement with the
DIAL profile within ±0.5 ppmv between 100 and 6 hPa. The largest
negative value is found at 46 hPa, with 2.0 ppmv for a profile of the highest
latitude measured (54.7∘ S). In contrast, the largest positive value is
found at 22 hPa, with 1.2 ppmv for a profile of the lowest latitude measured
(48.8∘ S). This indicates that lower O3 values still exist inside
the vortex – i.e., depleted ozone in the spring time has not yet recovered
– and larger O3 values are found outside the vortex at the lower latitudes
in the middle stratosphere.
Time series of DIAL O3 profiles at the OAPA site. Each profile is
shifted 5 ppmv. Data are color coded based on sPV values. Observation dates
in 2009 are shown as MMDD (e.g., 0906 is 6 September 2009).
Another example is shown in Fig. b. On this day, 23 November,
there were less latitudinal differences in the ozone field compared to the result
on 14 November as observed by MLS. Consequently, the latitudinal difference
in MIROC-CTM is also smaller on 23 November than on 14 November (not shown).
Similar to the previous result, the MLS profile at a similar latitude with OAPA
is in good agreement with the DIAL profile within ±0.5 ppmv between 83 and 6 hPa. In contrast, the MIROC-CTM O3 is
lower than DIAL by ∼ -2 ppmv between 10 and 6 hPa. This is discussed in Sect. 4.4.
Scaled PV maps from MERRA-2 on 26 September (a, e),
3 October (b, f), 14 November (c, g), and 23 November
2009 (d, h). Top and bottom rows show pressure surfaces at 20 and
50 hPa, respectively.
Time series comparison
All 23 DIAL profiles obtained in September–November 2009 were evaluated for
their variability with time. The PV values at the location and time of all
O3 profiles from DIAL, MLS, and MIROC-CTM were investigated to place the
measurements inside or outside the polar vortex. The degree of PV values at
each measurement or model grid is a robust indicator of the location relative
to the polar vortex. Here, we used meteorological data from the NASA Global
Modeling and Assimilation Office (GMAO) Modern-Era Retrospective Analysis for
Research and Applications 2 (MERRA-2) reanalysis
(http://gmao.gsfc.nasa.gov/reanalysis/MERRA-2/). We calculated the
scaled PV (sPV) for pressures between 100 and 6 hPa from the PV values from
MERRA-2 and PV / sPV ratios as a function of PT. The PV and sPV values
are provided through the MLS website as the derived meteorological products
(DMPs) . We used version 2 of DMP (GEOS-5 and MERRA-2 for
the version 4 MLS data). The sPV values (s-1) are nearly constant at
levels throughout the stratosphere
(e.g., ; ).
Figure shows all the 23 profiles of O3 obtained
by DIAL. Data are color coded based on sPV values. Ozone changes are related
to the sPV value especially above 30–40 hPa. Figure shows
sPV maps from MERRA-2 for selected days on 26 September, 3 October, and 14
and 23 November 2009. At 20 hPa, the polar vortex significantly diminishes
on 23 November compared to that on 26 September. In contrast, at 50 hPa, the
polar vortex still exists on 23 November with smaller area than that on
26 September.
An sPV value of ∼ 1.4 × 10-4 s-1 has been used to
define the Northern Hemisphere (NH) polar vortex edge center
e.g.,and references therein. In addition, values of
∼ 1.6 and ∼ 1.2 × 10-4 s-1 have been used to
define the inner and outer edges, respectively. Those vortex edge
definitions, i.e., center, inner, and outer, are according to
. We examined these values using the DMPs for the MLS
measurements for the period studied here (i.e., September to November 2009).
The results were somewhat different from those from the NH depending on time
and altitude. For example, center, inner, and outer boundaries are defined by
the absolute sPV values of 1.6 × 10-4, 1.9 × 10-4
and 1.3 × 10-4 s-1 at 68 hPa in November. The sPV values
shown in the following figures are useful guides for showing positions
relative to the vortex.
Time series of O3 mixing ratios as measured by DIAL and MLS at
18 hPa (a) and absolute and relative differences between the
two (c, e) from September to November 2009, over the OAPA site.
Panels (b) and (d, f) are the same as panels (a)
and (c, e), but for DIAL and MIROC-CTM. Data are color coded based
on sPV values. For the absolute and relative differences, sPV values for MLS
and MIROC-CTM are color coded. For MIROC-CTM, outputs from six grids are
shown (see text for additional description).
Time series of O3 mixing ratios as measured by DIAL and MLS at
56 hPa (a) and absolute and relative differences between the
two (c, e) from September to November 2009, over the OAPA site.
Panels (b) and (d, f) are the same as panels (a) and
(c, e), but for DIAL and MIROC-CTM. Data are color coded based on
sPV values. For the absolute and relative differences, sPV values for MLS and
MIROC-CTM are color coded. For MIROC-CTM, outputs from six grids are shown
(see text for additional description).
As representatives for the middle and the lower stratosphere, results at 18
and 56 hPa are shown in Figs. and ,
respectively. Figure a shows the time variation in O3
values obtained from DIAL and MLS at 18 hPa. Both O3 values are
color coded using sPV values. On several occasions, O3 values below
4 ppmv were measured by DIAL in air masses with larger sPV values, i.e.,
larger negative values indicated with blue and purple colors, in conjunction
with the polar vortex dynamics.
For both 26 September and 5 October, the polar vortex shifted toward the
South American side, covering the OAPA site. On 13–14 November, the O3
values were low again. Correspondingly, the MLS O3 values also show lower
values with higher sPV values. In general, the DIAL O3 values are within
the variations in MLS O3 values for each coincident date during all
comparison periods. To quantitatively evaluate the degree of agreement, the
differences between the two (MLS - DIAL) are shown in
Fig. c. These values are color coded using the sPV value
from each MLS measurement. We computed mean and root-mean-square (rms)
differences of O3 from all 180 data points. At 18 hPa, the mean difference
is -0.03 ppmv and the rms difference is 0.78 ppmv. Although the mean value
shows a good agreement, the variance is large especially in September.
We discuss this large variance in Sect. 4.3.
Figure b for 18 hPa also shows time variations in O3
values obtained from DIAL and those simulated with MIROC-CTM.
Figure d shows the O3 differences between DIAL and
MIROC-CTM (MIROC-CTM - DIAL). In this plot, mean and rms differences in
O3 are calculated from all data points of the nearest model grid (51.6∘ S, 70.3∘ W)
to the OAPA site (the number is 23). As a
result, the mean difference is 0.04 ppmv and rms difference is 0.72 ppmv. For
reference, Fig. e and f show the
relative differences for DIAL–MLS and DIAL–MIROC-CTM comparisons,
respectively.
Similar to the DIAL–MLS comparison, both the DIAL and MIROC-CTM O3 values
show low values with larger sPV values, which indicate that the locations are
inside the polar vortex or that the air masses originate from the polar
vortex. However, MIROC-CTM overestimates O3 values with the larger sPV
values compared to DIAL. When those higher deviations in MIROC-CTM are found,
the DIAL O3 values show smaller amounts below ∼ 4 ppmv
(Fig. b). This is also observed in the vertical profile in
Fig. a. The overestimate of MIROC-CTM may be partly due to
the relatively coarse horizontal resolution of the model with regard to a
complicated spatial structure near the boundary of the polar vortex in the
breakup season. The polar vortex begins to breakup at higher altitudes, and
then propagates downward. Another possible explanation could be due to a
weaker vertical motion of air in MIROC-CTM. Although not shown, a vertical
profile of nitrous oxide, N2O, from MIROC-CTM on 14 November 2009 is
different from that from MLS. A tight correlation between N2O and Cly
is found in the stratosphere e.g., and used to
infer the Cly value from a measured N2O value
e.g.,. At 18 hPa, the MIROC-CTM N2O
value is higher than that of MLS, resulting in a smaller value of Cly in
MIROC-CTM. Thus, a smaller active chlorine (ClOx) induces a higher O3
amount in MIROC-CTM.
Figure a and c show time variations in
O3 values from DIAL and MLS and the difference between the two at 56 hPa,
similar to Fig. a and c.
Figure b and d also show time variations
in O3 values at 56 hPa from DIAL and MIROC-CTM and the difference between
the two, similar to Fig. b and d.
Figure e and f show the relative
differences for DIAL–MLS and DIAL–MIROC-CTM comparisons, respectively. Unlike
the characteristics of the 18 hPa result, significant lower ozone values
relative to the other dates were not found inside the polar vortex on
26 September and 5 October. In contrast, on 13–14 and 23–24 November, lower O3
values inside the polar vortex were found from both of DIAL and MLS. This is
in agreement with the long-lasting polar vortex dynamics in the 2009 spring
. The mean differences between DIAL and
MLS–MIROC-CTM are as small as 0.06 and 0.16 ppmv, respectively. The rms
differences are 0.46 and 0.36 ppmv for DIAL–MLS and DIAL–MIROC-CTM
comparisons, respectively, which are smaller values than those at 18 hPa. The
overestimate of MIROC-CTM with larger sPV values, as seen at 18 hPa, is not
evident at 56 hPa. One explanation may be that the polar vortex is more
stable at 56 hPa than at 18 hPa, even on 23–24 November.
Dependency in distance and sPV difference
The good correlation between sPV and O3 values near the vortex boundary in
austral spring has been previously shown in satellite measurements
(e.g., ). Therefore, a horizontal
gradient in O3 should have been present at the vortex boundary in the 2009
spring. A previous study suggested that a better agreement is found when the
comparison is performed with matching meteorological conditions using
parameters such as sPV and equivalent latitude . Therefore,
we further examined the larger variability between DIAL and MLS at 18 hPa,
from the perspective of different sPV values. Figure a shows
the O3 difference (MLS - DIAL) versus sPV difference between DIAL and
MLS (MLS - DIAL). Similar to Fig. b, the data points are
color coded based on the sPV values of the MLS measurements. A positive
correlation between O3 and sPV differences is found, suggesting lower
O3 values in MLS (negative in the y axis) with a more poleward MLS
profile, i.e., negative in the x axis. Conversely, higher O3 values in
MLS, i.e., positive in the y axis, with the lower latitude side profile in
MLS, i.e., positive in the x axis, are also seen, although the correlation
is weaker than in the negative value area. After filtering out matching pairs
over a certain sPV difference, e.g., below or above
±0.3 × 10-4 s-1, the rms difference between DIAL and
MLS at this pressure level decreases significantly. Such an sPV criterion is
useful for suppressing the large rms difference found in O3 measurements
affected by the motion of the polar vortex. Applying such an sPV criterion
to screen the result did not change the mean difference much.
This is consistent with the result from , who
showed differences in CH4 values observed in the northern high latitudes,
and the sPV criterion with a value of 0.2 × 10-4 s-1 has
little effect below 25 km in altitude.
O3 difference versus sPV difference for DIAL and MLS at
18 hPa (a) and 56 hPa (b). Data are color coded based on
the sPV for the MLS measurements.
We also examined results from 56 hPa in Fig. b. Similar to
the results from 18 hPa, larger O3 differences are found with larger sPV
differences. Applying certain sPV criterion to these data, the mean
difference changes only slightly, but the rms difference decreases, similar
to the results from 18 hPa. The results for other pressure levels are
summarized in Sect. 4.4.
Since the MERRA-2 dataset also provides the O3 value ,
we examined those data instead of the sPV value. Figure
shows the O3 difference versus MERRA-2 O3 difference between DIAL and
MLS (MLS - DIAL). The mean difference is computed from the horizontal
axis, resulting in -0.12 ppmv at 18 hPa and -0.02 ppmv at 56 hPa. The
measured O3 difference is well reproduced by the MERRA-2 O3 that
assimilates the Aura MLS as well. At 56 hPa, a compact correlation is found
between the two differences with a slope of 1 : 1. A similar positive
correlation is also found at 18 hPa.
O3 difference versus MERRA-2 O3 difference for DIAL and MLS at
18 hPa (a) and 56 hPa (b). Data are color coded based on
the sPV for the MLS measurements.
In addition to the sPV differences examined, we evaluated the correlation
between the O3 difference and distance in the DIAL–MLS measurements
(Fig. ). In these figures (Fig. a for
18 hPa and Fig. b for 56 hPa), data points are
color coded based on the sPV difference between DIAL and MLS (MLS - DIAL).
Clearly, larger O3 differences, especially those with negative values in
Fig. a, have large sPV differences, i.e., below -0.5 × 10-4 s-1.
As shown in the figures, the O3 difference
does not depend critically on the distance between the two measurements.
O3 difference versus distance for DIAL and MLS at
18 hPa (a) and 56 hPa (b). Data are color coded based on
sPV differences between DIAL and MLS measurements.
Vertical profiles of mean and rms differences of O3 values for
DIAL and X (MLS or MIROC-CTM) (a) and those of relative
values (b). Each value is computed from each pressure level in the
time series as shown in Figs. and . The
numbers outside the plot are values of the mean (rms in parentheses)
difference at 83 and 100 hPa.
In summary, the O3 differences between DIAL and MLS can be partly
attributed to differences in the measurement points. Furthermore, the O3
difference is more correlated with sPV differences than with the difference
in distance. Therefore, it is important to analyze O3 values with sPV (or
PV) values near the polar vortex boundary, which has been suggested
previously e.g.,.
Comparison at other levels: summary
The mean and rms differences computed from the time series comparisons in
Sect. were extended for other pressure levels to
summarize the degree of agreement between DIAL and MLS or MIROC-CTM. These
results are plotted versus pressure in Fig. . Absolute
differences are shown in the left panel. Relative differences, the absolute
differences divided by their mean values of O3, are shown in the right
panel. In the left panel, mean differences (open circle and cross) for both
DIAL–MLS and DIAL–MIROC-CTM comparisons along with rms differences (dotted
lines) are shown. The mean differences of the DIAL–MLS comparison are almost
within ±0.1 ppmv between 6 and 56 hPa with 180 data points for each
level. This corresponds to the relative values, in the right panel, of
±3 %. Figure shows differences between DIAL and
MLS using the sPV criterion. The mean and rms differences shown in this
figure as blue lines are identical to Fig. . The mean
and rms differences after filtering with the sPV criteria (±0.3 × 10-4 s-1) are shown as green lines. Clearly, the rms differences
decrease 21–47 % between 10 and 56 hPa; the number of data points was
reduced from 146–180 to 107–144. However, the mean differences only change
slightly for all pressure levels, except for the 6 hPa level.
Vertical profiles of mean and rms differences with and without
scaled PV criterion screening for the DIAL–MLS O3 comparison.
Time series of differences in O3 mixing ratios as measured by
DIAL and computed by MIROC-CTM at 8 hPa from September to November 2009,
over the OAPA site. Data are color coded based on sPV values for MIROC-CTM.
For the DIAL–MIROC-CTM comparison, the mean differences are almost within
±0.3 ppmv between 10 hPa and 56 hPa, with 23 data points for each level.
This corresponds to relative values of ±8 %. Above 8 hPa, the absolute
differences increase to -0.6 ppmv, which corresponds to relative values of
-8 %. To examine the low bias in MIROC-CTM, the time series in the O3
difference between DIAL and MIROC-CTM at 8 hPa is shown in
Fig. . Larger negative deviations in MIROC-CTM are
found in October and November, especially for data with sPV values between
-1.0 and -1.5 × 10-4 s-1. Similar results are also found
from 6 and 7 hPa levels. The peak altitude of ozone in MIROC-CTM is lower
than that of DIAL, as shown in Fig. . Both the vertical and
horizontal motions of air in the model are responsible for this different
feature, but the cause is not known. As was shown in
Fig. , the vertical gradient of O3 from DIAL
above 15–20 hPa is shown to be rather weak inside the polar vortex but occasionally
strong outside or at the edge of the polar vortex. Thus, the vertical gradient of
O3 may affect the result for such occasions with the steeper gradient. The
feature presented here suggests a difficulty in the reproduced ozone field
for those pressure levels (6–8 hPa) in these latitudes and this season using this
version of MIROC-CTM. As discussed in Sect. 4.2, the polar vortex breakup
process may cause a highly variable spatial structure. This may be partly
responsible for the difference because of the insufficient spatial resolution
of the model to distinguish this process.
Both the DIAL–MLS and DIAL–MIROC-CTM comparisons show increasing rms
differences with increasing altitudes above the 20–30 hPa levels, reaching
more than 1 ppmv. This is partly due to the O3 value increasing with
increasing altitudes. Thus, relative values of the rms difference
(Fig. b) do not show strong vertical gradients compared to
the absolute values (Fig. a).
Both comparisons also show larger absolute differences below 68 hPa, reaching
0.5 ppmv (116 %) for DIAL–MLS and 0.9 ppmv (292 %) for DIAL–MIROC-CTM. This
suggests a lower bias in the DIAL measurement at these lower altitudes
(∼ 80–100 hPa) of some magnitude. As discussed in ,
this DIAL system has some difficulty in measuring around 100 hPa and below
due to saturation from backscattered photons in the low-energy channels.
Since the O3 mixing ratio from DIAL is very small below about 70 hPa, the
sensitivity might be degraded along with the saturation effect. Therefore,
DIAL data at this altitude range should be used with caution.
Another possible reason is the difference in measured ozone associated with
the difference in the original vertical resolution (∼ 1 km for DIAL
versus 3 km for MLS). In this period, lamina structures in O3 profiles
are often observed from ozonesonde measurements, especially below 20 km.
DIAL may capture lower values of O3 in these lamina structures while
collecting measurements over 3–5 h, compared to MLS, which measures
instantaneously along the orbit, in the forward direction from the spacecraft (see
Supplement). This may facilitate O3 differences, to a certain extent, even
while both measurements are accurate. In the other geophysical regions of the
Asian monsoon anticyclone, difficulties in MLS retrievals within the strong
vertical gradient of O3 have been discussed . The largest
O3 difference between DIAL and MLS at 83 hPa was found on 3 October 2009;
this case was studied using air mass trajectory analysis
and the O3 field from MIROC-CTM (see Supplement).
Conclusions
Ground-based DIAL measurements were performed at OAPA in Río Gallegos
(51.6∘ S, 69.3∘ W), Argentina, from September to November
2009, when a long-lasting southern polar vortex, and accompanying ozone
depletion, occurred over the area for the first time since 1979
. This site is one of the few NDACC DIAL sites
in the SH. Focusing on this period of large dynamical variability in measured
air masses during the movement of the polar vortex, it is possible to analyze
the effects of the polar vortex on O3 variability. Twenty-three O3
profiles were obtained by DIAL during the period. These profiles were
compared with coincident MLS O3 profiles with 180 matching pairs, based on
time and space criteria.
The mean differences between DIAL and MLS are within ±0.1 ppmv (±3 %)
from 6 to 56 hPa, showing good agreement regardless of the large sPV
variability between each matching pair. The DIAL data are also compared with
outputs from the MIROC-CTM model simulation. The mean differences between
DIAL and MIROC-CTM are within ±0.3 ppmv (±8 %) from 10 to 56 hPa.
Above 8 hPa, the mean differences increase to -0.6 ppmv (-8 %). To measure
variability in the comparison, rms differences between DIAL and MLS or
MIROC-CTM are also evaluated. For both DIAL–MLS and DIAL–MIROC-CTM
comparisons, the rms differences are nearly 0.5 ppmv for pressure levels
between 30 and 100 hPa and increase with increasing altitudes up to 6 hPa,
reaching 1.1–1.2 ppmv. From the DIAL–MLS comparison, the O3
differences depend on sPV differences at 18 hPa. Therefore, another criterion
for comparison is proposed: pairs with absolute sPV differences that exceed
0.3 × 10-4 s-1 are discarded. As a result, the rms
differences decreased significantly between 10 and 56 hPa, but the mean
differences only slightly change for all pressure levels, except for 6 hPa.
The comparison between DIAL and MLS indicates that the O3 difference is
partly due to sPV differences between measurement locations, however, as yet
unknown factors create additional differences. The comparison between DIAL
and MIROC-CTM indicates that an insufficient model spatial resolution may be
partly responsible for the O3 differences above 18 hPa during polar
vortex breakup. An insufficient model vertical motion may also be partly
responsible for the O3 differences, especially inside the polar vortex.
Both the DIAL–MLS and DIAL–MIROC-CTM comparisons also show larger mean
differences below 68 hPa, reaching 0.5 ppmv (116 %) and 0.9 ppmv
(292 %) at 100 hPa, respectively. One possible cause may be a low bias
in the DIAL O3 measurement, but this hypothesis was not confirmed in this
study. Nevertheless, finding good agreement between DIAL and MLS O3
measurements between 6 and 56 hPa is a necessary step for studies
evaluating the bias and long-term stability of satellite sensors in the future.
Because of very sparse observations from SH ground-based stations,
the continuation of long-term measurements there for NDACC is highly
recommended. This study provides an outlook for continuing measurements at
the OAPA site. The DIAL measurements at the OAPA site are available for all
years since 2005, except 2016 when no measurements were collected. The result
of the DIAL–MLS comparison using these long-term data will be published
elsewhere.