Introduction
Biases in satellite-based ozone trend analysis due to measurements at
different local time and drifting satellite orbits renewed the
interest in diurnal variations of stratospheric ozone
. Model projections indicate a recovery of the ozone
layer of about 1 % per decade
while the diurnal
variation in stratospheric ozone typically has an amplitude of
2–4 % which may induce a serious bias in trend estimates
from satellite ozone measurements.
The diurnal variation in stratospheric ozone was researched by new
studies based on chemistry–climate model simulations, ground-based
microwave radiometry and satellite observations
e.g.. investigated
the global, seasonal and regional behaviour of diurnal variation in
stratospheric ozone by means of the free-running chemistry–climate
model WACCM. The study explained the basic underlying physical
processes as temperature-dependent photochemical reactions within the
Chapman cycle and the catalytic NO cycle which are the main
contributors to the diurnal variation in stratospheric ozone. The
strong connection to photochemistry in the stratosphere leads to
a seasonality in diurnal ozone variation especially at high
latitudes. The maximum ozone variation during a day is up to
0.8 ppmv (15 %) at the polar circle in summer in
WACCM simulation . This surprisingly strong amplitude is
confirmed by ground-based microwave radiometer at Ny-Ålesund,
Svalbard and indicates that a correction of diurnal
sampling effects in stratospheric ozone data sets is more needed than
previously expected.
compared the diurnal variation in stratospheric ozone
from nudged chemistry–climate model simulations (SD-WACCM where SD
stands for specified dynamics) to observations from the
Superconducting Submillimeter-Wave Limb-Emission Sounder
SMILES,. The SMILES observations showed a good
agreement in the tropics to SD-WACCM data where
dynamics are nudged in the lower atmosphere.
derived the diurnal variation in stratospheric ozone
using 18 years of microwave radiometer measurements at Mauna
Loa (Hawaii). They compared the observed results to simulations of the
Goddard Earth Observing System Chemistry Climate Model
GEOSCCM, with two different
implementations of atmospheric chemical processes. The observed and
the simulated diurnal variation in stratospheric ozone agreed mostly
within 1 % (2σ) of the estimated statistical
errors. derived a climatology of the diurnal ozone
variation using a 17 years series of stratospheric ozone
profiles measured by a microwave radiometer at Bern, Switzerland. They
found indications for an interannual variability of the diurnal ozone
variation.
The good agreement of model data and observations may indicate that
a model-assisted correction of diurnal sampling effects in satellite
ozone measurements could be feasible. Alternatively to
a model-assisted correction of satellite data, the assimilation of
satellite ozone measurements into an advanced chemistry–climate model
with two-way interactions between dynamics and atmospheric composition
may be considered. The assimilating model system can incorporate the
diurnal ozone variation in a correct manner. The Earth observation
programme Copernicus of the EU develops such a chemistry–climate
model system called Monitoring Atmospheric Composition and Climate
(MACC). MACC assimilates satellite data of atmospheric composition,
including ozone, into a global atmosphere model to provide
a reanalysis of atmospheric composition for the years 2003–2012. Such
a model system might help to correct diurnally sampled ozone data from
biased satellite measurements and finally improve the quality of ozone
trend estimates.
The present study follows on and scrutinizes the WACCM
results with the help of reanalysis data. The diurnal ozone variation
from the MACC reanalysis, the ERA-Interim reanalysis and the WACCM
model is intercompared and results are confirmed at 5 hPa by
selected ground-based observations of the the Network for Detection of
Atmospheric Composition Change (NDACC) and satellite-based
observations of the Superconducting Submillimeter-Wave Limb-Emission
Sounder (SMILES). Further, the study presents a remarkably strong
diurnal variation based on advection at the polar regions in winter
which is even stronger than the known effects based on
photochemistry. This novel feature is discussed in a separated section
by means of the potential vorticity distribution and an Arctic NDACC
site.
The article is organized as follows: in Sect. the
different data sets from model systems and instruments are
described. Section intercompares the diurnal ozone
variation derived from the MACC reanalysis, the ERA-Interim
reanalysis, WACCM and NDACC instruments. Section
gives a brief summary of the results and concluding remarks.
Model systems and observations
MACC reanalysis system
The EU project Monitoring Atmospheric Composition and Climate (MACC)
fosters a chemical weather forecast system which will be fed by the
observations of the upcoming Sentinel satellites of the Copernicus
Earth Observation programme. The global model and data assimilation
system of MACC is based on the European Centre for
Medium-Range Weather Forecast's (ECMWF) integrated forecast system
IFS,. The representation of the atmospheric
chemical system is held by a chemical transport model (CTM) which is
coupled to the IFS via the OASIS4 coupler . That means
the MACC reanalysis considers two-way interaction of dynamics and
composition.
The coupled CTM is called Model of OZone And Related chemical Tracers
MOZART v3.5, and calculates chemical
production and loss rates of the atmospheric gases. The implementation
of used by the MACC reanalysis from 2009 onwards
comprises 115 species, 71 photolysis reactions, 223 gas phase
reactions and 21 heterogeneous reactions. The MOZART model simulates
tropospheric and stratospheric chemistry on a 1.125∘ by
1.125∘ horizontal grid. The vertical model domain is
divided into 60 layers on hybrid–pressure (σ-p) coordinates
with a model top at 0.1 hPa.
Satellite retrievals of reactive gases, aerosols and greenhouse gases
are assimilated into the MACC reanalysis system by a four-dimensional
variational (4-D-VAR) data assimilation system
. Stratospheric ozone data
are assimilated from different satellite-based instruments e.g. Global
Ozone Monitoring Experiment (GOME), Michelson Interferometer for
Passive Atmospheric Sounding (MIPAS), Microwave Limb Sounder (MLS),
Ozone Monitoring Instrument (OMI), Solar Backscatter UltraViolet
Instrument (SBUV/2), Scanning Imaging Absorption Spectrometer for
Atmospheric CHartographY (SCIAMACHY). For further information on the
assimilation of ozone data we refer to . Aside the
meteorological variables the MACC system calculates forecasts of
reactive gases including ozone (O3), carbon monoxide
(CO), nitrogen oxides (NOx) and formaldehyde
(HCHO), aerosols and greenhouse gases.
Data records, monitored present state, forecasts and reanalysis of
atmospheric composition are provided by the MACC project
(atmosphere.copernicus.eu). The MACC reanalysis ozone product contains
6 hourly analysis data (forecast data is available every 3 h)
at 00:00, 06:00, 12:00 and 18:00 UT and is available from 2003
to 2012. found that stratospheric ozone from the MACC
reanalysis agrees to within ±10 % in most seasons and
regions which is considerably better compared to the free-running CTM
MOZART.
ERA-Interim reanalysis
The ERA-Interim reanalysis is a global atmospheric reanalysis produced
by the ECMWF. The ERA-Interim project aimed at establishing an
improved reanalysis by approaching the existing problems of ERA-40's
hydrological cycle, stratospheric circulation and temporal consistence
of atmospheric fields .
The prognostic ozone system of the ERA-Interim reanalysis is
a simplified, built-in chemistry routine. Ozone follows a scheme of
linear relaxation to a local photochemical equilibrium which is
calculated by a two-dimensional photochemical model. The coefficients
of the ozone parametrisation are given as a function of latitude,
model level, and month, hence there is no diurnal variation or
longitudinal variation . That means the ERA-Interim
reanalysis does not represent a fully two-way interactive coupling of
dynamics and composition and it can not model the diurnal ozone cycle
in the upper stratosphere that is due to photochemistry. The
prognostic ozone system was upgraded following
who improved the representation of polar
ozone destruction by taking into account local stratospheric
temperature and the total chlorine content. The upgraded ozone system
reproduced well the inter-annual variability related to temperature in
polar vortices. For more details on the ozone system of the
ERA-Interim reanalysis we refer to and
.
The ERA-Interim reanalysis uses a 4-D-VAR data assimilation system
. The atmospheric model performs simulations on
hybrid-pressure (σ-p) coordinates with a model
top at 0.1 hPa. Reanalysis data of ERA-Interim are available
in 6 hourly data at 00:00, 06:00, 12:00 and 18:00 UT from 1979
onwards. Ozone data from the ERA-Interim reanalysis are often used in
atmospheric research e.g.. The
quality of ERA-Interim ozone data was assessed by who
found that ERA-Interim reanalysis is in better agreement than the
ERA-40 equivalent compared to a total column ozone reference. In the
stratosphere the ERA-Interim reanalysis shows mean residuals of about
±10 % compared to satellite observations .
WACCM
The Whole Atmosphere Community Climate Model (WACCM) is a fully
coupled chemistry–climate model which simulates the entirety of the
Earth's atmosphere. The WACCM model was developed by the National
Center of Atmospheric Research (NCAR) and
is embedded into the software framework of the Community Earth System
Model (CESM) which comprises a land, ice, ocean and an atmosphere
model. The atmosphere is simulated from the Earth's surface up to the
lower thermosphere at 5.1×10-6 hPa
(∼150 km).
In the present article, WACCM version 4 was utilized with the
preconfigured, free-running F 2000 scenario which reflects a perpetual
year with atmospheric conditions corresponding to the year
2000. Free-running means that the model is not influenced by effects
of data assimilation or nudging.
The chemical representation of the atmosphere is based on the
stratospheric chemistry of the CTM MOZART v3,
which comprises the main production and loss processes of 59
atmospheric species. In addition, the WACCM model simulates chemical
heating, gravity wave drag, molecular diffusion and ionization. The
ozone distribution calculated by the model feeds back to the model
dynamics.
The simulations are carried out on a horizontal resolution of
1.9∘ latitude by 2.5∘ longitude and
a vertical fragmentation of 66 layers on hybrid-pressure (σ-p)
coordinates which are terrain-following below the
100 hPa level and isobar above. The resolution of the vertical
coordinates ranges from 1.1 to 2.0 km in the middle
atmosphere. The time steps of the atmosphere and land model were
reduced to 15 min in order to achieve good results for the
diurnal variability of stratospheric composition. The global output
data set of WACCM has a time resolution of 1 h and is derived
from a one year simulation starting at 1 January 00:00 UT.
An overview on the model systems of the MACC reanalysis, the
ERA-Interim reanalysis and WACCM is given in
Table .
SMILES climatology
The SMILES was jointly operated by the Japan Aerospace
Exploration Agency (JAXA) and National Institute of Communication
Technology (NICT) at the Japanese Experiment Module on the
International Space Station (ISS). The SMILES experiment was launched
to space on 9 September 2009 and had been observing the atmosphere
from 12 October 2009 until 21 April 2010 when an instrument component
failed.
During seven months in operation SMILES has been observing profiles of
atmospheric minor constituents such as O3 (and isotopes),
HCl, ClO, HO2, BrO, HNO3. The
SMILES observations cover a latitudinal range mostly within
38∘ S to 65∘ N (exceptions occur when the
ISS was turned) with a vertical resolution of 3.5–4.1 km.
The relatively low inclination of the ISS supports the study of
diurnal variations of ozone, minor constituents, ozone isotopes, rate
constants and atmospheric tides
e.g. by means of the
SMILES observations. derived a climatology of
stratospheric and mesospheric trace gases and temperature from SMILES
observations. The ozone climatology of is distributed
via the NICT SMILES website
(http://smiles.nict.go.jp/index-e.html). Due to irregular
spatial and temporal distribution of the SMILES data, the ozone
climatology was obtained by binning the ozone measurements of SMILES
within latitude bands (20–40∘ S,
20∘ S–20∘ N, 20–50∘ N and
50–65∘ N) and over bimonthly periods.
GROMOS measurements
The GROund-based Millimeter-wave Ozone Spectrometer (GROMOS) is
situated at the Bern NDACC site, Switzerland
(46∘57′ N, 7∘26′ E) and has been
operating since 1994 . In the present study ozone
profiles are used with a time resolution of 30 min which have
been measured with the Fast Fourier transform (FFT) spectrometer of
GROMOS. Ozone profiles are retrieved at fixed pressure levels from
about 0.2 to 50 hPa with a vertical resolution of
approximately 10 km. A climatology of diurnal variation in
mesospheric and stratospheric ozone was derived for the period from
1994 to 2011 by . For further details on the GROMOS
climatology we refer to the latter study.
Ozone profiles from GROMOS are regularly used for satellite
validations or for studies on middle-atmospheric dynamics, diurnal
ozone variation and sudden stratospheric warmings
.
MLO ozone measurements
The Mauna Loa Observatory (MLO, 19.5∘ N,
204.5∘ E) is a tropical NDACC site sensing ozone
profiles. The MLO microwave radiometer has been operating since 1995
at an elevation of 3400 m where the quality of sensed profiles
benefits from a low zenith tropospheric opacity . Ozone
profiles are retrieved from 20–65 km with a vertical
resolution from 6–14 km. Technical details of the MLO
microwave radiometer are described by .
The diurnal variation of ozone was studied by from
reprocessed measurements with hourly time resolution of the profiling
MLO radiometer. They compared the results to measurements of
space-based microwave limb sounders (e.g. SMILES) and the solar
backscattered instruments of SBUV/2 and found very small differences
of less than 1.5 % amongst the profiles.
OZORAM measurements
The OZOne Radiometer for Atmospheric Measurements (OZORAM) is deployed
to Ny-Ålesund, Svalbard in the high Arctic (78.9∘ N,
11.9∘ E). The instrument is operated by the Alfred
Wegener Institute (AWI) and the University of Bremen, Germany in the
frame of NDACC. Since 2008 OZORAM has been observing ozone profiles of
the middle atmosphere from 30–70 km with an altitude
resolution of 10–20 km and time intervals of 1 h.
compared stratospheric ozone observations from OZORAM to
satellite-based instruments such as EOS-MLS and TIMED and found good
agreement within 10 %. For further information on the
technical details, the ozone retrieval and the quality of ozone
measurements we refer to .
Ground-based microwave radiometers such as GROMOS, OZORAM and the MLO
microwave radiometer measure ozone profiles from approximately
25–70 km at day- and nighttime with the same quality so that
the determination of the small diurnal cycle of stratospheric ozone is
possible. For further details on the retrieval technique of the
microwave instruments we refer to .
Results and discussion
Intercomparison with respect to ground- and satellite-based measurements
Diurnal ozone variation of model systems and selected NDACC sites are
intercompared at the 5 hPa level where satellite ozone trend
analysis is most biased by sampling effects. Further, strongest
diurnal variation in ozone appears mostly from 3–5 hPa,
little above the ozone maximum (VMR) and hence at the peak of the
stratospheric ozone layer .
By sorting and binning of all ozone values of a month according to the
local time, we get monthly mean ozone as function of local time
(O3,m(LT)) showing the mean diurnal variation in
stratospheric ozone. The relative, diurnal ozone variation is
calculated with respect to monthly mean ozone at midnight
ΔO3(LT)=O3,m(LT)-O3,m(00:00)O3,m(00:00).
Figure displays mean March (2012)
diurnal variations in stratospheric ozone over NDACC sites at northern
midlatitudes (upper panel) and tropics (bottom panel) according to
Eq. (). The figure compares data from two microwave
radiometers (dashed and solid gray lines), the WACCM model (blue
line), the MACC reanalysis (red markers) and the ERA-Interim
reanalysis (black markers). In Fig. ,
diurnal ozone variation at 5 hPa features three different
characteristics during a day: almost constant ozone over nighttime
(WACCM, MLO radiometer), a morning minimum and an afternoon
maximum. Table compares the morning minima
and afternoon maxima of the different data sets from model systems and
microwave radiometers. Figure and
Table clearly show that the ERA-Interim
reanalysis only renders a small diurnal variation in ozone of
approximately 0.8 %. This result reveals that the simplified,
linear ozone representation of the ERA-Interim reanalysis does not
adequately reflect diurnal variation in ozone.
There is agreement between the microwave radiometers, WACCM and the
MACC reanalysis in spring (Fig. a and b)
though the temporal resolution of the MACC reanalysis is rather
coarse. At 19:00 LT the ozone VMR values from the MACC
reanalysis are beyond the data of the WACCM model and the ground-based
microwave radiometers over Mauna Loa and Bern. The low temporal
resolution of the MACC reanalysis might cause minor defects in
strength of the diurnal variation in ozone. Further defects might be
related to the chemical data of the CTM of the MACC reanalysis which
is coupled every hour only. The MACC project also provides 3 hourly
forecast data which we considered to be not adapted for
intercomparison to the ERA-Interim reanalysis or the WACCM
model. Nevertheless, the 6 hourly reanalysis data of the MACC project
is already valuable for the study of the diurnal ozone variation on
a global scale.
Further, Fig. includes the SMILES
climatology (orange line) for midlatitudes (zonal mean from
20 to 50∘ N) and tropics (zonal mean
from 20∘ S to 20∘ N) which is available
for the bimonthly period of March–April only. Over Bern, Switzerland,
the SMILES climatology agrees with the morning minimum and the
afternoon maximum of WACCM and the MACC reanalysis (see
Table ), whereas over Mauna Loa, Hawaii only
the WACCM model and the MLO microwave radiometer are mutually
consistent.
Over Mauna Loa, Hawaii there is only qualitative agreement of the
SMILES climatology and all data from model systems and the MLO
microwave radiometer. compiled the SMILES climatology
by sampling bimonthly zonal mean profiles from non-sun-synchronous
orbits, thus from all local times. The apparent discrepancy of SMILES
over the Mauna Loa NDACC site (Fig. b)
may be related to the irregular data sampling due to the ISS orbit
which accounts of up to 20 % (relative error) in the SMILES
climatology product . Such discrepancies and the phase
bias in the diurnal ozone variation from ground-based measurements,
WACCM, the MACC reanalysis and the SMILES climatology (see
Fig. ) remind that our present
understanding of ozone photochemistry in the stratosphere is still
incomplete .
Intercomparison of the model systems
The strength of the diurnal variation in ozone is represented by the
peak-to-valley difference DO3 which is defined by
Eq. () for each grid point where O3,max refers
to the maximum, O3,min to the minimum ozone VMR during a day
(00:00 to 24:00 UT).
DO3=O3,max-O3,min
DO3 is the interval width of the ozone values of a day
and depends on the amplitudes of the diurnal and subdiurnal variations
without any information about timing. Further, we often discuss
monthly means of relative diurnal variation
DO3,m/O3,m where DO3,m and
O3,m are the monthly means at a gridpoint.
Figure shows zonal-mean
DO3,m/O3,m for March, June, September and
December of 2012 as derived from the MACC reanalysis. The strengths of
the diurnal ozone variation in Fig. is presented
for the pressure range from 1 to 50 hPa and all
latitudes. Figure a and c displays diurnal ozone
variation of more than 6.0 % (0.6 ppmv) above the
3 hPa pressure level in the tropics and below the
20 hPa pressure level in March and September. The diurnal
variation of ozone in the upper stratosphere is based on
photochemistry (cf. Fig. ) and hence is
a function of latitude. The diurnal variation from the MACC reanalysis
in the lower, tropical stratosphere is mostly based on Ox
transport related to vertical tidal winds and the strong vertical
ozone gradient in the lower stratosphere as described by
.
A further feature in March and September
(Fig. a and c) are enhancements of
DO3,m/O3,m in the Arctic and Antarctic upper
stratosphere of more than 15 % (1.0 ppmv) which are
clearly separated from the tropical enhancement. These features are
due to dynamics at the Arctic and Antarctic and relate to advection
effects. Aside these maxima the MACC reanalysis shows also minima of
diurnal ozone variation in March and September
(Fig. a and c) of approximately 1.5 %
(0.3 ppmv) at 10 hPa near the equator.
In the MACC reanalysis data, the diurnal variation of stratospheric
ozone is enhanced in June and December (Fig. b, d)
at the Arctic and Antarctic polar circles (marked as magenta dashed
lines). For instance, in Fig. b and d, the MACC
reanalysis shows high values of diurnal ozone variation in the
respective summer hemisphere of up to 16.9 %
(1.3 ppmv) between the 2 and 5 hPa pressure levels at
the polar circle. This feature is based on the long sunshine duration
at the polar circle in summer where ozone is accumulated during
daytime to very high values. The MACC reanalysis accurately renders
this feature of the stratospheric daily ozone cycle and assures its
strength of reflecting photochemical induced diurnal ozone variation.
However, the global maximum in diurnal ozone variation in June
(Fig. b) from the MACC reanalysis appears around
2 hPa in the Antarctic polar region and is approximately
46.6 % (3.3 ppmv) which is a surprisingly high
value. The Arctic and Antarctic stratosphere in winter is
a dynamically dominated region and most of the diurnal ozone variation
relates to advection at different time scales. The MACC reanalysis
captures the diurnal ozone variation from different origins as
photochemistry and dynamics and gives an unprecedented global picture.
In a similar manner,
Figs. and show zonal-mean
DO3,m/O3,m for March, June, September and
December as derived from simulations of the WACCM model and from the
ERA-Interim reanalysis. With regard to the intercomparison to the MACC
reanalysis and the ERA-Interim reanalysis the hourly WACCM output was
down-sampled to a 6 hourly temporal resolution.
In the WACCM simulation, diurnal variation of stratospheric ozone is
maximal in June and December at the polar circle of the summer
hemisphere in between the 1 and 5 hPa pressure levels. This
strong photochemical features of up to approximately 18.6 %
(0.9 ppmv) can be clearly seen in
Fig. b and d and are consistent to the results of
the MACC reanalysis.
From the 2 to 5 hPa pressure level where photochemistry is
important, the simulated diurnal variation in ozone of WACCM is weaker
than at the polar circles in summer. For instance, in March and
September as shown in Fig. a and c the strength
of the daily ozone cycle is approximately 5 %. Exceptions are
maxima of diurnal ozone variation at 2 hPa and
80∘ latitude in the respective autumn hemisphere in
Fig. a and c. These exceptions are artifacts of
the strong diurnal variation due to photochemistry in summer.
Compared to the MACC reanalysis, the WACCM model underestimates all
dynamically induced effects such as the diurnal ozone variation at the
Arctic and Antarctic and in the lower, tropical stratosphere. The
WACCM model shows strong effects of diurnal ozone variation only above
10 hPa which mostly are based on photochemistry. This strongly
indicates that the free-running WACCM model underestimates tidal winds
in the lower stratosphere which could be also due to the low
horizontal resolution. Further, advection processes at diurnal and
shorter time scales in the polar regions in winter are not adequately
reflected in the WACCM simulation. On the other hand, WACCM and the
MACC reanalysis are mutually consistent with diurnal ozone variation
based on photochemistry.
The ERA-Interim reanalysis does not consider locally time-dependent
ozone photochemistry and hence can not render the strong diurnal ozone
variation due to photochemistry at the polar circle in summer
(Fig. b, d). The ERA-Interim reanalysis shows this
feature at 2 hPa polewards of the polar circles with only
8.4 % (0.3 ppmv). At this point, the limit of the
simplified, linear ozone system of the ERA-Interim reanalysis becomes
evident. Unsurprisingly, the ERA-Interim ozone system can not keep up
with the fully coupled CTMs of the MACC reanalysis and the WACCM
model.
In the Arctic and Antarctic winter stratosphere the ERA-Interim
reanalysis and the MACC reanalysis benefit from their strong dynamics
assimilation systems. Such systems give a good representation of the
diurnal ozone variation due to advection processes. For instance, the
ERA-Interim reanalysis shows a maximum in diurnal ozone variation in
June (Fig. b) at around 1 hPa of
approximately 30.5 % (1.0 ppmv). This feature can
also be seen to a similar extent in the MACC reanalysis
(Fig. b). The ERA-Interim reanalysis nicely
reflects the diurnal variation of ozone based on advection in the
polar region in winter but also in the lower, tropical stratosphere.
Figure gives a comprehensive overview of
the diurnal ozone variations in the MACC reanalysis, the ERA-Interim
reanalysis and WACCM at 5 hPa. It is evident that neither
WACCM nor the ERA-Interim reanalysis can match all of the important
characteristics which figure in the MACC reanalysis. WACCM fails in
the polar winter stratosphere where the ERA-Interim reanalysis agrees
well with the MACC reanalysis. The ERA-Interim reanalysis fails in the
polar summer stratosphere where WACCM agrees well with
MACC. Especially, WACCM and the MACC reanalysis show the strong
diurnal ozone variation at the polar circle of approximately
0.8 ppmv (15 %) in summer and in addition agree well
on the seasonal timing of this feature. Contours in
Fig. depict the sunshine duration (dashed
lines) and the solar zenith angle (solid lines) which govern the
seasonal pattern of the photochemically induced diurnal variation.
The three model systems show different features of the diurnal
variation in ozone based on their specific model
composition. Eventually, it has been the weaknesses in photochemistry
or short term advection of WACCM and the ERA-Interim reanalysis which
revealed the different origins of the diurnal ozone variation in the
stratosphere. The MACC reanalysis combines the strengths of both and
therefore gives an unprecedented global picture on diurnal ozone
variation in the stratosphere.
The influence of horizontal resolution on WACCM simulations
The analysis of diurnal variation of ozone in the dynamically
dominated polar region in winter by WACCM questions the influence of
horizontal resolution on the simulation results. In
Sect. results from WACCM were
presented at medium horizontal resolution of
1.9∘ latitude by 2.5∘ longitude. Here, the
term “medium” is with respect to the higher horizontal resolution of
the MACC reanalysis T255 truncation with about
0.7∘ horizontal resolution;. Diurnal
variation of the stratospheric composition in the polar winter region
depends on the quality of the dynamical representation in the
model. This in turn highly depends on the implemented interaction
processes of the atmospheric layers as tidal waves, planetary waves,
gravity waves and atmospheric instabilities such as sudden
stratospheric warmings.
In order to test a different horizontal resolution of WACCM,
a simulation was performed at the lower 4∘ latitude by
5∘ longitude resolution. Figure
exemplifies the relative difference in ozone VMR from 18:00 to
06:00 UT relative to ozone VMR at 06:00 UT at
5 hPa for the low and the medium horizontal resolution. The
figure presents a representative day of the Southern Hemisphere in
winter. From Fig. it can be deduced that
simulated diurnal variation depends on the horizontal resolution in
the following way: higher horizontal resolution results in enhanced
diurnal variation in ozone at the polar region in winter.
This result is in line with who showed that the gravity
wave parameterization at different horizontal resolutions of WACCM
results in different variability in the stratosphere. That means,
whenever dynamics at diurnal and shorter timescales in the
stratosphere are simulated with WACCM, it is important to consider the
actual potential of the model. For instance, at the polar circle the
medium resolution is approximately 113 by 86 km which can
solve many but not for all the gravity waves in the atmosphere. For
internally-generated gravity waves without a gravity wave
parameterization, simulations need to be performed at high horizontal
and vertical resolution e.g. T213 grid with about 60 km
horizontal resolution and 300 m vertical resolution,.
From the results of Sect. and
different results of the two horizontal resolutions
(Fig. ) it is inferred that neither the low nor
the medium horizontal resolution of WACCM can represent the features
of diurnal variation in the polar winter region. WACCM with its
internal gravity wave parametrization is tuned to model adequate mean
flows and jets in the middle atmosphere and the simulation of diurnal
variation in the polar region in winter overstresses the prospects of
the model.
Diurnal variation in the polar regions
The surprisingly strong diurnal variation in the polar regions is
accessible by one of the NDACC's instruments. The OZORAM microwave
radiometer located at Ny-Ålesund, Svalbard (78.9∘ N,
11.9∘ E) observes diurnally sampled ozone profiles at
such high latitudes in summer and winter.
Figure presents the mean diurnal ozone
variation observed by the OZORAM microwave radiometer in June 2011 at
5 hPa along with the corresponding data of the MACC reanalysis
and the ERA-Interim reanalysis. The figure shows the relative diurnal
variation according to Eq. (). The diurnal ozone
variation in Fig. is based on photochemistry
and is up to approximately 8 % for the WACCM model and the
OZORAM radiometer which are almost perfectly consistent. The MACC
reanalysis shows even stronger diurnal variation in ozone of up to
10 %.
Figure shows diurnal ozone variation from OZORAM
measurements during two periods: 21–26 June 2011 and 1–6 December
2012. The figure presents the ozone time series of OZORAM per mean
ozone of the two periods, respectively. The MACC reanalysis and OZORAM
measurements agree well (mostly within the error range of OZORAM)
during the summer period in June 2011 (Fig. a). Again,
the diurnal ozone variation is based on photochemistry and shows
similar characteristics as in
Fig. . Figures
and a confirm the strong diurnal ozone variations
around the polar circle in summer which form in the MACC reanalysis
and in the independent observations of the OZORAM microwave
radiometer. However, the MACC reanalysis and OZORAM microwave
radiometer show poor agreement in ozone VMR (the MACC reanalysis shows
up to 60 % more ozone VMR during the presented period).
The bottom panel of Fig. displays OZORAM measurement
in December 2012. The OZORAM measurement and the MACC reanalysis show
qualitative agreement mostly within the error range of the radiometer
data. The diurnal ozone variation is superposed by variability at
longer and sub-diurnal time scales. Thus, the dynamically dominated
polar region in winter shows no clear diurnal signature in ozone
variability at 5 hPa.
A view on the diurnal ozone variation in the Arctic and Antarctic is
presented in Fig. from WACCM, the MACC reanalysis and
ERA-Interim reanalysis. The relative difference in ozone VMR from
18:00 to 06:00 UT relative to ozone VMR at 06:00 UT is
shown at 5 hPa for 21 June 2012. WACCM is shown for the
corresponding day of the simulation. The WACCM model and the MACC
reanalysis agree well in the Northern Hemisphere (summer) where
photochemistry is dominating the diurnal ozone variation
(Fig. a and b). In the Southern Hemisphere (winter) WACCM
seems to fail in simulating vortex dynamics and related advection
compared to the MACC reanalysis (cf. Fig. d, e). On
the other hand, the ERA-Interim reanalysis agrees well with the MACC
reanalysis in the polar region in winter (Southern Hemisphere,
Fig. b, c) but does not reflect the diurnal ozone
variation based on photochemistry in the polar region in summer
(Northern Hemisphere, Fig. e, f).
Dynamics of the polar vortices and related advection cause strong,
aperiodic ozone variation in the polar region in winter. A useful
quantity for studying synoptic variability of polar vortices is
potential vorticity or shorter PV. Potential vorticity is often used
to analyze vortex dynamics e.g.. The observed
structure, the understanding of vortex dynamics and the benefits of
potential vorticity are reviewed by .
When potential vorticity is conserved, an air parcel moves along its
potential vorticity isopleth. Further, it is assumed that the change
of the ozone VMR in the air parcel is negligible over the period of
a day in the polar winter stratosphere. Thus a diurnal change in the
potential vorticity isopleth would indicate a diurnal change in the
trajectory which is associated to a diurnal change in ozone at a fixed
geographic location
. Such colocated
variations in potential vorticity and ozone indicate diurnal variation
in ozone based on vortex dynamics.
The difference in potential vorticity in Fig. is
determined as the difference from 18:00 to 06:00 UT from the
ERA-Interim reanalysis. The Antarctic polar vortex in
Fig. shows stronger changes in potential
vorticity than the midlatitudes and tropics. These strong changes in
potential vorticity correlate to ozone variability at the same time
scale in Fig. e and f. This connection confirms that
transport processes related to the dynamics of the stratospheric polar
vortex evoke the strong diurnal ozone variation in the polar winter.
The polar vortices in the stratosphere show day-to-day variability
which manifest in elongation of the vortices and displacement from the
poles . Tides and periodic gravity wave flux
perturb the stratospheric vortex. The perturbed vortex forms vortex
Rossby waves with typical spiral band structures of zonal wavenumber
2. Partly, these structures are visible in
Fig. . To confirm the interconnection of gravity
waves to diurnal ozone variation in the polar region in winter further
research based on highly resolved data is needed
e.g. ECMWF-T799,.
Conclusions
The intercomparison of diurnal variation in ozone from reanalysis and
chemistry–climate modelling shows a wide range of agreements but also
differing features. For instance, the strong diurnal variation in
ozone at the polar circle in summer as derived from the MACC
reanalysis suites the global pattern of seasonality in diurnal ozone
variation of the WACCM model. Diurnal variation in stratospheric ozone
is mostly based on ozone accumulation due to the Chapman cycle over
day time which is not entirely balanced by catalytic ozone
depletion. The ERA-Interim reanalysis with its linearized,
two-dimensional photochemical ozone model does not reflect such
diurnal variation of stratospheric ozone based on photochemistry.
Differences also appear in the winter stratosphere where the
free-running WACCM model shows less diurnal variation than the MACC
reanalysis and the ERA-Interim reanalysis. These variations depend to
some extent on horizontal resolution of the free-running WACCM model
where higher resolutions show more variability. However, WACCM at
either low and the medium resolution tends to underestimate the
diurnal variation of ozone in winter at high latitudes which are most
likely based on vortex dynamics and related advection. Such variation
occurs in the MACC reanalysis data and is up to 47 % in the
upper stratosphere. The ECMWF ERA-Interim reanalysis confirms the
large amplitudes of diurnal and subdiurnal variation in ozone at the
stratospheric polar vortex. From analysis of the potential vorticity
structure we relate these effects to diurnal and subdiurnal vortex
Rossby waves. Here, the present conception and understanding of
diurnal ozone variation in the stratosphere is widened by the novelty
of this surprisingly strong diurnal variation in the polar winter
stratosphere.
In addition, the comparison to ECMWF's ERA-Interim and WACCM
substantiates the benefits of a coupled CTM as in the MACC reanalysis
system for the representation of the diurnal variation in
stratospheric ozone. Our intercomparison study indicates the potential
of the MACC reanalysis for an accurate description of the advection
and the photochemical effects on the diurnal variation of
stratospheric ozone while the ERA Interim reanalysis and free-running
WACCM either fail for the photochemical effects or the diurnal
advection effects.
The results show how gathering and preparation of data by the
affiliated ground stations of the NDACC network yields additional
value for atmospheric research and validation of the MACC reanalysis
model system. Ground-based microwave radiometry is an important
observation method for diurnal variation of stratospheric
ozone. Partly, it was possible to validate the different model systems
by NDACC observations. Therefore, further measurements of diurnal
ozone variation in the polar regions as performed by are
desirable to confirm and study the behaviour of diurnal variation in
ozone at different seasons in Arctic and Antarctic. For instance, the
recent start-up of the campaign instrument GROMOS-C
makes polar stratospheric ozone and its diurnal variation more
accessible to ground-based microwave radiometry and could extend the
global sampling of local ozone profiles. In addition, microwave
instruments might benefit from reanalysis data with higher temporal
resolution in order to validate and improve retrievals which focus on
subdiurnal time scales.
Interesting results emerged from simulations at different horizontal
resolutions of the WACCM model. In dynamically dominated regions as
the polar night region, ozone variation at diurnal and shorter time
scales depends on an accurate representation of dynamics at short time
scales. This in turn is based on the implementation of atmospheric
processes which interact between the atmospheric layers such as
gravity waves, 2-days waves and sudden stratospheric warmings. We
suspect that also other chemistry–climate models may have similar
weaknesses and advised application to such short time scales is
recommended.
Despite a suboptimal temporal resolution, the MACC reanalysis system
impressively showed dynamical and photochemical features of diurnal
variation in ozone at all latitudes and seasons. On this account, such
a model system of chemical data integration and assimilated dynamics
shows great promise for preprocessing diurnally sampled ozone data
from space-borne instruments and correct potential biases in ozone
trends. The diurnally sampled observations might be assimilated and
reanalyzed with a coupled chemical transport model under consideration
of a higher temporal resolution.