Introduction
Energetic particle precipitation is a potential contributor to the solar
influence on the middle atmosphere, and has recently been recommended for the
first time as a solar forcing parameter for the upcoming CMIP-6 model studies
. Energetic particles precipitating into the atmosphere
lead to the formation of neutral radicals like, e.g., H, OH,
N, and NO by reaction chains involving ionization, excitation,
and dissociation of the most abundant species, N2 and O2, and
subsequent ion chemistry . Both HOx (H,
OH) and NOx (N, NO, NO2, NO3)
contribute to catalytic ozone loss in the middle atmosphere, HOx mainly in
the mesosphere (above ≈ 1 hPa), and NOx mainly in the
stratosphere (below ≈ 1 hPa) . Energetic particles
come from different sources, mainly from the Sun, but also from outside the
solar system .
Of particular importance for the middle atmosphere are protons from large
eruptions of the solar corona, so-called coronal mass ejections, and
electrons from high-speed solar wind streams further accelerated in the
terrestrial magnetosphere.
Solar coronal mass ejections are sporadic events related to sunspots and the
solar magnetic cycle; however, though the events are rare and mainly
restricted to the declining phase of the solar maximum, protons are
accelerated to energies of tens to hundreds of MeV, and can penetrate
directly into the mesosphere and upper stratosphere, and, in events with
particularly hard energy spectra, even down to the lower stratosphere. NOx
increases of up to 2 orders of magnitude in the upper stratosphere and
mesosphere as well as mesospheric ozone loss of more than 80 % have been
observed related to strong so-called solar proton events
.
Model studies of these events generally show a good morphological agreement,
indicating that the main processes during the solar proton events are
reasonably well understood .
Energetic electrons are accelerated towards Earth due to magnetic
reconnections in the magnetotail during auroral substorms; these electrons
then precipitate into the lowermost thermosphere down to 90 km. In
geomagnetic storms, radiation belt electrons can be accelerated to energies
high enough to precipitate into the mesosphere as well. Aurorae and
geomagnetic storms are much more frequent than solar proton events, and
though particles do not precipitate as far down into the middle atmosphere,
the amount of NOx formed due to these events likely is much larger, being
the main source of the strong increase in NO in the high-latitude
lower thermosphere. Variations in the density of NOx in the mesosphere and
lower thermosphere related to geomagnetic activity as a proxy for auroral
electron precipitation are reported based on observations, (e.g., by
). Mesospheric ozone loss
and an increase in mesospheric OH have been observed to be related directly
to increases in both electron fluxes
and geomagnetic activity
.
NOx from the high-latitude upper mesosphere and lower thermosphere can
propagate downward during polar winter in the large-scale downwelling motions
of the winter middle atmosphere. As the photochemical lifetime of NOx is
in the range of weeks to months during polar winter, NOx from the upper
mesosphere and lower thermosphere can reach down far into the stratosphere.
Enhanced values of mesospheric and stratospheric NOx attributed to auroral
production or geomagnetic activity have been observed sporadically in polar
winters for many decades
. However, these
observations were mostly limited to sunlit areas, and thus did not observe
deep into polar night. Observations from the Michelson Interferometer for
Passive Atmospheric Sounding (MIPAS, ), which also
covered the polar night region, show that NOx produced by energetic
particle precipitation (called EPP NOx in the following) reaches down to
altitudes below 30 km in both hemispheres, in nearly all winters observed
. Particularly high values of EPP NOx are observed in
Northern Hemisphere late winters after the strong sudden stratospheric
warming events in winters 2003/2004, 2005/2006, and 2008/2009
. These
warmings were followed by long-lasting downwelling in the mesosphere and
upper stratosphere enabled by a strong polar vortex re-forming after the
event. It was shown from both observations
and model results that this period of enhanced
downwelling was characterized by the formation of an elevated stratopause in
the upper mesosphere.
The so-called EPP indirect effect due to downwelling of
auroral NOx into the stratosphere leads to an increase in the catalytic
ozone loss in the upper and middle stratosphere, progressing from the upper
to lower stratosphere during polar winter. Lower values of ozone in the upper
stratosphere are observed during winters characterized by large particle
forcing or enhanced values of NOy , and
downwelling negative ozone anomalies were observed for the first time by
and . However, quantification of the
particle-induced ozone loss by observations is difficult, because (a) MIPAS
observations of EPP NOx show that EPP-induced ozone loss must occur in
every year, so only relative differences can be obtained from observations;
(b) stratospheric ozone is quite variable anyway; and (c) a much longer time
series than used by would be necessary to attribute the
observed anomalies clearly to the particle precipitation. Model studies are
more suited to studying the ozone loss, as models can do on–off experiments
with and without particle precipitation in a clearly defined way.
Such model studies were carried out in the past
;
however, in most cases, either only one particular winter or situation was
investigated, the EPP-NOx input was not well constrained, or a model
experiment with freely adaptable dynamics was carried out, making comparison
to observations more difficult.
As ozone is one of the key species in the radiative heating of the middle
atmosphere, changes in ozone even above the main ozone layer will have an
impact on temperatures and dynamics of the middle atmosphere. Analyses of
observations using either geomagnetic activity or the hemispheric power index
as proxies for particle precipitation suggest that such a coupling between
EPP and atmospheric dynamics indeed exists during polar winter, characterized
by a warming of the mid to late winter upper stratosphere at high latitudes
. Analyses of several decades of surface air
temperatures suggest that geomagnetic activity even affects tropospheric
weather systems down to the surface in mid to late winter
.
However, the supposed changes in stratospheric net radiative heating related
to EPP NOx have, to our knowledge, not been analyzed in detail, though the
general consensus so far is that a net cooling is expected during late winter
and spring due to the reduction in upper stratospheric ozone, in contrast,
and possibly contradiction, to the observed upper stratospheric warming; this
contradiction generally is explained by a coupling with wave breaking and
reflection .
In this paper we analyze results from three chemistry-climate models
considering proton and electron forcing over the period from mid-2002 to
mid-2010, e.g, covering 11 polar winters in both hemispheres.
NOy in the middle
atmosphere from all three models is compared to MIPAS observations to
evaluate the model results. The ozone loss at high latitudes in the middle
atmosphere is quantified from the difference of model runs with and without
particle impact, and changes in the net radiative heating are estimated from
these results.
The models used are described in Sect. ; MIPAS data are
described in Sect. . NOy intercomparison with MIPAS data
and the impact of energetic particle precipitation on middle atmosphere
NOy are shown in Sect. ; ozone intercomparisons with MIPAS
data and the quantification of the EPP impact on ozone and stratospheric
heating rates based on model results are discussed in Sect. .
Description of models and observations
Models
We use results from three different models in this study which have been used
to determine the impact of energetic particle precipitation in the past
(e.g., ) to analyze differences in the model
results due to the implementation of the particle impact and the model
transport schemes, and to derive a range of possible model results. The
models used are 3dCTM , KASIMA , and
EMAC . The models cover different vertical regimes: 3dCTM
and KASIMA cover the altitude region from roughly the tropopause up to the
lower thermosphere (called high-top models in the following), and the EMAC
model covers altitudes from the surface up to the mesopause (called
medium-top models in the following). All three models have a detailed
description of middle atmosphere chemistry, and use temperatures and wind
fields which are relaxed to meteorological analysis data provided by ECMWF in
the stratosphere and below. However, the models differ in the treatment of
energetic particles on the atmospheric composition, and in the treatment of
the impact of non-resolved gravity waves on atmospheric transport. 3dCTM and
KASIMA cover the source region of particle-induced NOx production, and
NOx production is driven by prescribed ionization rates; 3dCTM
additionally also considers photoionization. EMAC does not cover the source
region of NOx production, and the indirect effect is considered by
prescribing NOy at the model upper boundary.
KASIMA and EMAC are chemistry-climate models internally calculating
temperatures and wind fields, which however are nudged to meteorological
analysis data below the stratopause; 3dCTM is a chemistry-transport model
driven by prescribed temperatures and windfields. KASIMA and EMAC use
standard parameterizations of the gravity wave drag, while in 3dCTM, only
resolved gravity waves are considered, restricting the spectrum to
wavelengths ≥ 500 km.
The models are described in more detail in the following subsections. In
Sect. , the different model scenarios used are
described.
3dCTM
The three-dimensional chemistry and transport model (3dCTM) is an advanced
version of the 3dCTM described in . The model is based
on a combination of the stratospheric transport model as described in
and a chemistry and photolysis code adapted from the
SLIMCAT model . The model operates on isobaric
surfaces and reaches from the tropopause to the lower thermosphere
(317.00 hPa–5×10-6 hPa, approximately 10–140 km) with a
vertical resolution of 1–3 km. The horizontal resolution is 2.5∘×3.75∘.
Temperatures, densities and wind fields are prescribed using output data from
the Leibniz Institute Middle Atmosphere Model – LIMA .
LIMA is nudged to tropospheric and stratospheric data from ECMWF-ERA40 below
45 km which introduce realistic short-term and year-to-year variability.
LIMA applies a triangular horizontal grid structure with 41 804 grid points
in every horizontal layer (Δx≈Δy≈ 110 km).
This allows us to resolve the fraction of the large-scale internal gravity
waves with horizontal wavelengths of ≥ 500 km. Temporal LIMA data are
made available to 3dCTM every 6 h; 3dCTM uses a family approach for neutral
gas-phase constituents in the stratosphere at altitudes below the 0.33 hPa
level, but not in the mesosphere and lower thermosphere. The impact of
energetic particles is considered by prescribed ionization rates for
precipitation of electrons, proton and alphas from the AIMOS model
, version v1.6. Photoionization of N2, O2,
N and O in the EUV and NO photoionization in the
Ly-α band has been included as well. EUV photoionization rates are
calculated based on the parameterization of . Ionic
reactions are not included in the chemistry scheme, but the production of odd
nitrogen species as a function of ionization rates and atmospheric state is
calculated using the parameterization of , adapted for
photoionization by implementing a dependency on the primary ion composition.
The production of HOx is considered using the parameterization of
, an approach which is widely used and has been validated
both in comparison to observations of ozone loss, and in comparison to ion
chemistry model results; see, e.g., .
KASIMA
The Karlsruhe Simulation of the Middle Atmosphere (KASIMA) is a
three-dimensional mechanistic model of the middle atmosphere solving the
primitive equations including middle atmosphere chemistry .
For the simulations presented here, the model was run on isobaric surfaces
from 7 to 120 km with a vertical resolution of 750 m in the stratosphere,
gradually increasing to 3.8 km at the upper boundary. The horizontal
resolution in the simulation is ≈ 5.4∘×5.4∘
(T21). The model is coupled to the specific meteorological situations by
using the analyzed geopotential field at the lower boundary (7 km) and
applying analyzed vorticity, divergence and temperature fields from ECMWF
ERA-Interim below 1 hPa.
The parameterization of the gravity-wave drag is based on the formulation of
. The parameterization has been modified compared to the
version described in in order to better describe the
cross-mesopause transport often observed after sudden stratospheric warmings.
The spectral distribution of the vertical momentum flux is now described with
a Gaussian function of a centroid of 7 m s-1 and a standard deviation
of 50 m s-1 with phase speeds of 0, 20, 40, 60 and 80 m s-1.
The filter condition for critical phase speeds has been extended to be
applied when the absolute difference between the speeds is less than
10 m s-1. The latter condition effectively prevents gravity waves of
low phase speed from propagating and breaking in the lower mesosphere. Only
gravity waves of higher phase speed then break at higher altitudes, causing
an elevated stratopause to build. In addition, the numerical implementation
of the vertical diffusion has been re-formulated for better mass conservation
according to . The model includes a full middle atmosphere
chemistry scheme based on a family concept. In the mesosphere, the family
members are transported separately. To consider the impact of energetic
particle precipitation, proton and electron ionization rates from the AIMOS
model version v1.6 are prescribed; 1.25 NOx per ion
pair are produced with a partitioning between N and NO of 45
and 55 % as described in and ;
0–2 HOx per ion pair are formed following .
EMAC
The ECHAM/MESSy Atmospheric Chemistry (EMAC) model is a numerical chemistry
and climate simulation system that includes sub-models describing
tropospheric and middle atmosphere processes and their interaction with
oceans, land and human influences . It uses the second
version of the Modular Earth Submodel System (MESSy2) to link
multi-institutional computer codes. The core atmospheric model is the 5th
generation European Centre Hamburg general circulation model
ECHAM5,. For the present study we used ECHAM5 version
5.3.02 and MESSy version 2.52. The model covers the vertical range from the
surface up to 0.01 hPa on 90 (pseudo-)pressure layers with a vertical
resolution of about 1 km. The horizontal resolution is 2.8∘×2.8∘ (T42L90). The model is nudged to ERA-Interim reanalysis
data from the surface to 1 hPa with decreasing nudging strength in the
transition region in the six levels above. For gravity waves we use the GWAVE
submodel which contains the original Hines non-orographic gravity wave
routines from ECHAM5 in a modularized structure. For
parameter rmscon (root-mean-square wind speed at a bottom launch level of
642.90 hPa), which controls the dissipation of gravity waves, we use a value
of 0.92 m s-1. For gas-phase reactions the MECCA submodel is used
, and for photolysis the JVAL submodel .
The 218
gas-phase reactions and 68 photolysis reactions are included. Most of the
reaction constants are taken from . A family concept for
NOx is applied in the whole model domain. Geomagnetic forcing of NOy in
the mesosphere is considered by applying an upper boundary condition (UBC) of
NOy, parameterized by the geomagnetic Ap index (newly developed submodel
UBCNOX). This applies an online version of the upper boundary condition for
the amount of NOx described in and .
All three parts of the parameterization (background, energetic particles and
elevated stratopause events (ESEs)) are calculated directly on the model
grid. ESEs are detected online by the criteria suggested in
using a threshold value of 53 K for the temperature gradient between
0–30∘ N and 70–90∘ N at 1 hPa. For the Ap index
values recommended for CMIP6 are used. NO is
prescribed in the four highest levels of the model (pressure at midpoint
0.01–0.09 hPa) instead of NOy, and NO2 is set to 0 in those
levels to balance NOx (see ). For solar proton events
we use the SPE submodel , incorporating daily values
of precalculated ionization rates as described in which
are available updated to 2015 based on observed proton fluxes at
http://solarisheppa.geomar.de/cmip6. Full ionization rates are applied
where the geomagnetic latitude is greater than 60∘. For every ion
pair produced, 0.55 N and 0.7 NO are formed as suggested by
. Between 0 and 2 OH are formed per ion pair based on
as described in . Note that effects
from the SPE submodel in NO and NO2 are overwritten by the UBCNOX
submodel in the four highest model levels.
Model experiments
Properties of the model experiments used in this study.
Models
Experiments
Ionization rates
EPP NOx
Period
3dCTM
Base
No ionization
none
01/1999–05/2010
v1.6 phioniz
Aimos v1.6 p++ e- + photoionization
N / NO variable
01/1999–05/2010
KASIMA
Base
No ionization
none
09/2002–12/2010
v1.6
Aimos v1.6 p++ e-
1.25 NOx / IPR, N / NO constant
09/2002–12/2010
EMAC
Base
No ionization
none
01/1999–03/2012
UBC
Jackman p+
NOx upper boundary
01/1999–03/2012
The model scenarios used in the following are listed with their main
properties in Table . All models carried out model runs over
the period of ENVISAT observations, 3dCTM from January 1999 to May 2010,
KASIMA from September 2002 to December 2011, and EMAC from 1999 to
March 2012. 3dCTM runs are limited by the availability of LIMA data, which,
when the model runs were set up, were available only until mid-2010.
All models carried out one reference scenario with no particle impact, called
Base in the following. Additionally, all models carried out one scenario
including full particle forcing as available to the respective model: 3dCTM
including protons and electrons from AIMOS model v1.6 and photoionization
(v1.6 phioniz); KASIMA including protons and electrons from AIMOS model v1.6
(v1.6); and EMAC including protons from the database and
NOy upper boundary conditions as a constraint for the EPP indirect effect
(UBC, ).
MIPAS
MIPAS ) is a limb-viewing infrared spectrometer on the
Envisat research satellite. MIPAS measured atmospheric emission from which
vertical profiles of temperature and various trace species are inferred.
MIPAS provided global coverage in an altitude range from cloud top altitude
to about 68 km in its nominal observation mode. The MIPAS measurement period
was 2002 to 2012, with a major data gap due to technical problems in 2004.
After the interruption of operation the measurement was changed towards
inferior spectral but improved spatial resolution, but this technical issue
is of minor relevance to this study. Data products from the retrieval
processor built and operated by the Institute of Meteorology and Climate
Research (IMK) at the Karlsruhe Institute of Technology (KIT) in cooperation
with the Instituto de Astrofísica de Andalucía (IAA-CSIC)
are used. The following MIPAS data
obtained from the nominal observation mode are used in this study: O3,
HNO3, ClONO2, N2O4, NO2, and NO. The data versions used here
are documented in . The retrieval of NO and NO2 is
described in . Updates in the ozone retrieval
scheme since are documented in .
Characterization of the datasets used
NOy data from all model experiments as well as from MIPAS data are used.
NOy is provided by the models on a daily basis as the sum of N,
NO, NO2, NO3, 2N2O5, HONO2, HNO4, and ClONO2. KASIMA
additionally includes HONO and BrNO3. MIPAS observes the
NOy species NO, NO2, HONO2, HNO4,
N2O5 and ClONO2, and the total observed NOy is calculated
from these. Model results as well as observations are averaged over high
latitudes (70–90∘ N/S) as area-weighted daily averages. For
MIPAS, daily averages are derived from both upleg and downleg observations,
e.g., for local times of around 22:00 and 10:00. For KASIMA and 3dCTM, model
data are output once per day for the whole model domain at 12:00 UT, and
this snapshot is then averaged over the latitude bands. For EMAC, model data
are output two to three times per day (every 10 h).
Ozone volume mixing ratios in 70–90∘ N/S are provided in the same
way as the NOy data. Additionally, daily data of the hemispheric total
NOy content are derived from the model results in Sect. ,
and changes in solar heating and cooling rates and the energy absorption are
calculated from the modeled ozone fields in Sect. .
Variability of energetic particle precipitation and the dynamical state of the middle atmosphere, 2002–2010
(a) Daily sunspot number from
www.ngdc.noaa.gov/stp/spidr.html, May 2012. (b) Daily mean AE
index from the Kyoto database (http://wdc.kugi.kyoto-u.ac.jp, February
2016). Red lines denote days of strong solar proton events as given by
http://umbra.nascom.nasa.gov/SEP/, February 2017 (see text). For events
lasting more than 1 day, only the first day is marked.
(c) Temperature in the mid-stratosphere (15.216 hPa) at high
southern (black, 70–90∘ S) and high northern (red,
70–90∘ N) latitudes. Data are taken from output of the 3dCTM, but
originate from ERA-40. The tick marks here and in all the following figures
mark the first day of a year.
An overview of the particle and dynamical forcing throughout the time period
investigated here is provided in Fig. . Shown are the daily
sunspot number as a proxy for solar activity, the daily AE index as a proxy
for geomagnetic activity, and temperatures in the mid-stratosphere at high
latitudes in both hemispheres as a proxy of the dynamical state of the middle
atmosphere. Also marked are days with known solar proton events.
The time period investigated covers solar activity variations of nearly one
full 11-year solar cycle. At the beginning of the time period in 2002, solar
activity is shortly after its maximum. It then decreases from maximum to
minimum from 2003 to 2006, and reaches a deep and extended minimum with daily
sunspot numbers mostly below 10, often zero, in 2008 and 2009. From late 2009
on, sunspot numbers start to rise again, indicating the start of the next
solar cycle.
Geomagnetic activity follows solar activity insofar as activity is high at
the beginning and end of the time series, and lowest in 2008 and 2009.
However, short, sporadic events of enhanced geomagnetic activity related to
geomagnetic storms and auroral substorms occur even during the deep solar
minimum in 2008 and 2009. The frequency of sporadic events seems to be quite
high throughout the time period, though the strength of the events – denoted
by the magnitude of the disturbance of the geomagnetic field, i.e., the value
of the geomagnetic indices – is lower in the solar minimum period. During
solar maximum and the transition to solar minimum (2002–2005), the
geomagnetic AE index mostly has values above 100 nT, reaching 400–1200 nT
in strong geomagnetic storms. From 2006 on, geomagnetic activity falls below
100 nT on quiet days. From 2006 to 2008 and again in 2010, 400 nT is
exceeded during stronger geomagnetic storms; during the deepest minimum in
2009, even stronger storms fall below 400 nT.
Days of known solar proton events as provided by
http://umbra.nascom.nasa.gov/SEP/ as mean fluxes of protons with
energies larger than 50 MeV are marked as red lines in the middle panel of
Fig. . Only events where the mean flux is larger than
50 pfu are shown here. Very large
events with fluxes of ≥ 50 MeV protons larger than 1000 pfu occur on
21 April 2002, 28–29 October 2003, 2–3 November 2003, 25–26 July 2004,
16–17 January 2005, 14–15 May 2005, 8–11 September 2005, and
6–7 December 2006. The strongest event, with mean proton fluxes of
29 500 pfu, was the so-called Halloween storms in late October 2003. No
events at all are listed in 2007, 2008, and 2009; one smaller event with
proton fluxes of 14 pfu occurred in August 2010 (not shown here).
Daily mean stratospheric temperatures in high southern and northern latitudes
are shown in the bottom panel of Fig. for 15.216 hPa (about
25 km). Temperatures are based on ECMWF ERA-40 re-analysis data taken from
output of the 3dCTM model (see Sect. ). In the Southern
Hemisphere, stratospheric temperatures show a smooth annual cycle following a
more or less sinusoidal behavior with maxima during polar summer and minima
during mid-winter. Small excursions from this behavior occur in early summer,
denoted by short-term increases in stratospheric temperatures of up to 20 K.
These are the final warmings denoting the breakdown of the meridional
circulation in spring. In the Northern Hemisphere, a similar annual cycle is
observed, though the amplitude is lower – summers are colder, winters
warmer. Also, late winter and spring in the Northern Hemisphere are dominated
by strong excursions from the smooth behavior of up to 50 K. These are
mid-winter sudden stratospheric warmings, strong disturbances of the mean
circulation. Sudden stratospheric warmings throughout the time period
observed here are listed, e.g., in and
. Nine warmings are listed from early 2002 to mid-2010,
occurring between late December and late February. Sudden stratospheric
warmings lasting for more than 10 days occurred in January 2004,
January 2006, and January 2009. These warmings were followed by an elevated
stratopause and strong and long-lasting mesospheric and upper stratospheric
descent . They
will be called strong sudden stratospheric warmings in the
following. Changes in the temperature structure and dynamics of the middle
atmosphere during and after these events have been investigated in a number
of studies (e.g., ). In the daily
temperatures shown here, these three events are easily distinguished as
increases in temperatures by more than 40 K over a few days (more than 50 K
in 2009), and a slow recovery to cold winter temperatures in the weeks
afterwards. It has been shown that the onset of these events is driven by
planetary wave activity ,
while downward transport from the thermosphere after the event is driven
mainly by non-orographic gravity waves .
Modeled and observed EPP NOy
In the following, modeled EPP NOy is investigated in detail. In a first
step, model simulations in the upper mesosphere at 0.01 hPa are compared to
investigate how the implementation of particle impacts affects the model
results (Sect. ). The temporal variation is investigated and
compared to MIPAS observations for the whole vertical domain of the MIPAS
observations (≈ 10–68 km, e.g., 200–0.03 hPa) to evaluate
whether the models capture the main features of the EPP impact (see
Sect. ). Absolute differences between models and
observations are discussed to evaluate how well models reproduce the EPP
impact quantitatively (Sect. ). In a last step, the total
hemispheric amount of EPP NOy is derived from model results, and compared
to previous derivations from observations (Sect. ).
Model–model intercomparison in the upper mesosphere
Temporal evolution of the volume mixing ratio of NOy (ppb) from
all three models at high southern and northern latitudes
(70–90∘ S/N), at 0.01 hPa. Also shown are relative differences
from KASIMA and 3dCTM to EMAC results (%). Light blue shading denotes winter
in the Southern Hemisphere/Northern Hemisphere (second and third quarters of
each year/first and fourth quarters of each year). The gray shading in the
difference plots denotes the period at the start where KASIMA results were
not yet available. To highlight differences in polar winter, differences in
summer are shown as dots, while differences in winter are shown as solid
lines. KASIMA: red; 3dCTM: blue; EMAC: green.
In Fig. , model results are shown at 0.01 hPa
(≈ 80 km), respectively, for high latitudes in both hemispheres
(first and third panels); 0.01 hPa corresponds to the center of the
uppermost layer of the medium-top EMAC model, and also corresponds to an
altitude just below the mesopause where thermospheric NOy enters the
middle atmosphere.
At 0.01 hPa, all three models display a consistent behavior. All show an
annual cycle with strong maxima during winter, when NOy mixing ratios are
roughly 2 orders of magnitude larger than during polar summer. All three
models show an additional daily variability of NOy related to strong,
sporadic events in particle fluxes (3dCTM, KASIMA) or geomagnetic activity
(EMAC). This day-to-day variability is reproduced consistently by all three
models during polar winter. During polar summer it is missing in EMAC as the
Ap-dependent contribution in the UBC parameterization is much smaller than
the background NOy contribution during summer. Absolute values of NOy
at 0.01 hPa do not show systematic differences between models in the
Southern Hemisphere, but in the Northern Hemisphere, NOy from EMAC is
higher than NOy from 3dCTM and KASIMA throughout most winters.
The relative difference between NOy from EMAC and from the other two
models is also shown in Fig. (second and fourth panels).
The relative differences confirm that, on average, values during Southern
Hemisphere winter are consistent in all three models, while in Northern
Hemisphere winters, EMAC shows systematically higher values than the other
two models. Another systematic feature is observed during polar winter: the
relative differences show a kind of U-shape during most winters, with more
negative values (EMAC larger than other models) during mid-winter, and more
positive values (EMAC lower than other models) during early and late winter.
3dCTM and KASIMA on average agree very well with each other. However, in the
Southern Hemisphere, there are two winters in which NOy values in 3dCTM
and KASIMA are distinctly different, winter 2003 (lower values in 3dCTM) and
winter 2009 (lower values in KASIMA). In the Northern Hemisphere, 3dCTM and
KASIMA appear to agree on average in all winters, with the exception of
periods probably related to sudden stratospheric warmings (early 2004, early
2006, early 2008, and early 2009); during these periods, 3dCTM underestimates
NOy compared to KASIMA.
To summarize, at the source region of particle forcings in the upper
mesosphere (0.01 hPa), all three models show a reasonable, consistent
behavior. We conclude that the upper boundary condition in a medium-top
model, or prescribed ionization rates in the mesosphere and lower
thermosphere from the AIMOS model (version 1.6) in a high-top model, lead to
a generally consistent description of particle-induced NOy in the upper
mesosphere. However, small systematic differences between the model using the
upper boundary condition (EMAC) and the models using AIMOS data (3dCTM,
KASIMA) indicate possible systematic differences in NOy related to the use
of the upper boundary condition and the AIMOS rates between hemispheres, and
in the annual variation. Sporadic differences between 3dCTM and KASIMA
results are likely due to dynamical effects, in particular during the strong
sudden stratospheric warmings in the Northern Hemisphere in early 2004, early
2006, early 2008, and early 2009, when 3dCTM underestimates NOy compared
to KASIMA.
Variations in the temporal–spatial domain
Southern Hemisphere temporal evolution of the volume mixing ratio of
NOy in all three models and MIPAS measurements in the full common altitude
range. (a) MIPAS, (b) 3dCTM, (c) KASIMA, and
(d) EMAC. The white contour lines refer to the respective MIPAS
values (at 10, 20, 100, and 1000 ppb) smoothed over 7 days for clarity. In
black are the respective model contours (not smoothed). The gray shaded areas
in the first panel are periods without MIPAS data, and in the third panel,
periods before the start of the KASIMA model run.
Northern Hemisphere temporal evolution of the volume mixing ratio of
NOy in all three models and MIPAS measurements in the full common altitude
range. (a) MIPAS, (b) 3dCTM, (c) KASIMA, and
(d) EMAC. The white contour lines refer to the respective MIPAS
values (at 10, 20, 100, and 1000 ppb) smoothed over 7 days for clarity. In
black are the respective model contours (not smoothed). The gray shaded areas
in the first panel are periods without MIPAS data, and in the third panel,
the period before the start of the KASIMA model run.
MIPAS data and model results at high southern and northern latitudes are
shown in Figs. and . White contour lines are
MIPAS isolines of 10, 20, 100, and 1000 ppb smoothed over 7 days for
clarity, and black contour lines are the same (unsmoothed) isolines for the
models. In the MIPAS data, the solar proton event of October/November 2003
(the Halloween storms) is clearly visible as an enhancement in mesospheric
(≤ 1 hPa) NOy in both hemispheres. Additionally, downwelling of
NOy from above the top altitude covered by MIPAS data (above the
mid-mesosphere) is observed in every winter for which MIPAS data are
available, also in both hemispheres. The same general structures are
simulated by all three models. Additionally, the models also show responses
to three more solar proton events (January 2005 and December 2006 in the
Southern Hemisphere, and September 2005 in the Northern Hemisphere) which are
not covered by MIPAS observations due to data gaps. KASIMA and 3dCTM also
clearly show a large range of more minor, sporadic events in the upper
mesosphere in most polar summers from 2002 to 2008. These occur above the
altitude range where MIPAS observations are available during polar summers,
and are likely due to mid-energy electron precipitation as provided by the
AIMOS model. They are not predicted by EMAC because direct ionization due to
electron precipitation is not considered in this model.
While the EPP indirect effect is seen in both observations and model results
in all polar winters, some discrepancies are observed in the amount of NOy
transported down into the stratosphere, and in the downward speed and
vertical coverage of the EPP signal, both between the different models and
between (all) models and observations. These differences will be discussed in
detail in the following subsection.
Quantification of model–observation differences
Absolute difference in NOy (ppb) between models and MIPAS
observations at high southern (70–90∘ S) latitudes. Model data
have been interpolated to the vertical grid of the MIPAS data, and days and
altitudes are only shown where MIPAS data are available and fulfill the
averaging kernel criterion. From top to bottom: 3dCTM v1.6 phioniz, KASIMA
v1.6, and EMAC UBC. The black lines show the 10, 20, and 3000 ppb contours
of the MIPAS data, smoothed with a 7-day running mean for clarity. The gray
shaded areas are periods without MIPAS data; the period before the start of
the KASIMA model run is also shown in the second panel.
Absolute difference in NOy (ppb) between models and MIPAS
observations at high northern (70–90∘ N) latitudes. Model data
have been interpolated to the vertical grid of the MIPAS data, and only days
and altitudes are shown where MIPAS data are available and fulfill the
averaging kernel criterion. From top to bottom: 3dCTM v1.6 phioniz, KASIMA
v1.6, and EMAC UBC. The black lines show the 10, 20, and 3000 ppb contours
of the MIPAS data, smoothed with a 7-day running mean for clarity. The gray
shaded areas are periods without MIPAS data, and in the second panel the
period before the start of the KASIMA model run is also shown.
For a more quantitative comparison of models and observations, model results
from all three models have been interpolated onto the pressure grid of the
MIPAS observations for high latitudes (70–90∘) in both
hemispheres. Only days and altitudes where MIPAS data are available are
considered, and the absolute differences in NOy are shown in
Fig. for the Southern Hemisphere, and in
Fig. for the Northern Hemisphere. Contours of MIPAS data
smoothed over 7 days at 10, 20, and 3000 ppb are overlayed for clarity.
These contours are chosen because they envelop the EPP-NOy signal observed
by MIPAS in the upper stratosphere and lower mesosphere (10–0.1 hPa) in
many winters quite well, in particular in Southern Hemisphere winters 2003,
2007, 2008, and 2009, and in Northern Hemisphere winters 2002–2003,
2003–2004, and 2008–2009.
In the source region of the particle precipitation, the upper mesosphere
above 0.1 hPa, no clear picture emerges. 3dCTM overestimates NOy in the
Southern Hemisphere in this region in mid and late winter, but sometimes
underestimates NOy in early winter. For KASIMA and EMAC, periods of
overestimation and underestimation vary throughout most winters. In the
Northern Hemisphere, 3dCTM overestimates NOy in the upper mesosphere in
early to mid-winter 2007–2008, 2008–2009, and 2009–2010, but
underestimates NOy in most late winters, and throughout winter 2002–2003
and 2003–2004. KASIMA underestimates NOy compared to MIPAS in the
uppermost mesosphere in all Northern Hemisphere winters, while in EMAC,
periods of overestimation and underestimation are again observed in all
winters.
Despite the strong similarities of the modeled NOy in the upper mesosphere
during the winters discussed in the previous section, the indirect effect in
the upper stratosphere to mid-mesosphere (10–0.1 hPa) is captured rather
differently by the three models.
In 3dCTM, the indirect effect is overestimated throughout all polar winters in the Southern Hemisphere mesosphere. However,
the timing of the downwelling from the mesosphere to the stratosphere is
delayed compared to MIPAS observations, which leads to an underestimation of
NOy in the early to mid-winter upper stratosphere, in particular in winter
2003. In the Northern Hemisphere, the indirect effect is underestimated
strongly in winter 2003/2004 but overestimated in winter
2007–2008 and in early winter 2008–2009.
In KASIMA, the indirect effect in the upper stratosphere to mid-mesosphere is underestimated in all winters and both hemispheres.
However, the timing of the downwelling seems to be captured quite well.
In EMAC, the indirect effect in the upper stratosphere on the mid-mesosphere is overestimated in nearly all winters and both
hemispheres. The timing of the downwelling NOy suggests that downwelling
is too fast in the upper stratosphere/lower mesosphere.
3dCTM and KASIMA strongly underestimate the indirect effect after the sudden stratospheric warmings in early
2004 and early 2009 by more than 100 ppb. The speed of downward transport
after the warming is too fast in EMAC: the EPP-NOy signal reaches the
stratosphere too quickly, and proceeds to too low altitudes. However, the
amount of NOy transported down after the warming seems to be captured
reasonably well. This is consistent with results of the EMAC model for
Northern Hemisphere winter 2008/2009 as shown in .
These results suggest that in 3dCTM, transport through the winter mesosphere
is restricted particularly in the Southern Hemisphere, so that NOy
accumulates there throughout the winter. In the Northern Hemisphere, a
similar overestimation in the upper stratosphere and lower mesosphere is
observed in early and mid-winter from 2007 to 2010, but not in the early
years. In contrast, downward transport in EMAC through the lower mesosphere
in early winter is too fast, leading to too low values in the mesosphere and
too high values in the upper stratosphere. Only one strong
solar proton event is captured by MIPAS observations during this time period
(October 2003). In the Southern Hemisphere, NOy during and after this
event is overestimated by the models in the lower mesosphere (1–0.1 hPa),
indicating an overestimation of proton ionization rates there. This is shown
consistently in all three models. This might indicate a problem either of the
proton flux data used or of the photochemical lifetime of NOy in the
models after the solar proton event. However, it is probably not due to the
calculation of the ionization rates, as different models are used by 3dCTM
and KASIMA (AIMOS) or EMAC (Jackman rates). The impact of the different
assumptions in ionization models (including AIMOS and Jackman rates) on
NOy and ozone is discussed, e.g., in . This
overestimation continues well into the following polar summer. In the
Northern Hemisphere, modeled NOy is overestimated below
≈ 0.2 hPa by KASIMA and EMAC, but is underestimated above. The
underestimation in the mesosphere seems to be due to an underestimation of
the indirect effect, while the overestimation at altitudes below 0.2 hPa has
already been discussed in a detailed model–measurement intercomparison of
this event .
In the lower stratosphere below 20 hPa, positive differences occur during
mid to late winter occasionally in all models, in particular in the Southern
Hemisphere, and strongest in 3dCTM. These are likely due to the models'
representation of the formation of polar stratospheric clouds in the cold
polar vortex, and their subsequent sedimentation out of the stratosphere
(denitrification).
3dCTM underestimates NOy in the polar summer stratosphere by 3–30 ppb.
This is observed in both hemispheres, and in all polar summers. KASIMA and
EMAC agree much better with observations in the polar summer stratosphere,
though a small underestimation of NOy compared to MIPAS data is observed
in these models also during some summers. However, the underestimation of
stratospheric summer-time NOy in 3dCTM is likely connected to background
NOy, not to particle precipitation, and will not be discussed further
here.
To summarize, the EPP indirect effect on NOy in the upper stratosphere and
lower mesosphere is captured quite differently by all three models, depending
on the speed of the downward transport in the lower mesosphere and upper
stratosphere, which ultimately depends on the different treatment of gravity
wave drag in the models. 3dCTM and KASIMA strongly underestimate the indirect
effect in NOy after the strong sudden stratospheric warmings in Northern
Hemisphere winters 2003–2004 and 2008–2009. This is consistent with results
of a dedicated model–measurement intercomparison involving three high-top
and five medium-top models investigating the January 2009 SSW
. The impact of the warming is better represented in EMAC,
which however transports NOy down too fast after the warming. It should be
noted that the downward transport is even faster than in the EMAC version
shown in due to a different setting of the gravity wave
drag scheme. Only one strong solar proton event was observed by MIPAS during
this time period; the impact of this event was overestimated by all three
models in the Southern Hemisphere.
Thus, when the impact of particle precipitation on total NOy,
stratospheric ozone loss, and net radiative heating is determined from these
models, it should be kept in mind that the indirect effect during Southern
Hemisphere winters and Northern Hemisphere dynamically quiet winters (i.e.,
winters without strong sudden stratospheric warmings) will likely be
underestimated by KASIMA, but will likely be overestimated by 3dCTM and EMAC.
However, after sudden stratospheric warmings the impact of the indirect
effect will likely be underestimated by 3dCTM and KASIMA but represented
reasonably well by EMAC. The impact of large solar proton events might be
overestimated, though it should be pointed out that this assumption is based
on observations of one strong solar proton event only.
Total NOy
Total EPP NOy (Gmol) in the middle atmosphere as the difference
between model runs with and without particle forcings. From top to bottom:
3dCTM, KASIMA, and EMAC. Red: hemispheric amount in the Southern Hemisphere;
blue: hemispheric amount in the Northern Hemisphere; solid lines, dark
colors: 200–0.01 hPa (lower stratosphere to mesopause); dashed lines, light
colors: 9–0.01 hPa (mid-stratosphere to mesopause). The gray shading in the
second panel denotes the period at the start where KASIMA results were not
yet available.
The daily total amount of NOy in each hemisphere is calculated from the
model results as follows: in a first step, the total column amount is
calculated for daily zonal averages for each model run on the native latitude
grid of the respective model. This is calculated for each model run over the
vertical range from 200 hPa (roughly the lower boundary of 3dCTM) to
0.01 hPa (the upper boundary of EMAC). An additional calculation is carried
out for the vertical range from 9 to 0.01 hPa covering a vertical range not
affected by denitrification in the Antarctic ozone hole in any winter
(compare to Fig. ). The area of each latitude bin is then
calculated for each model grid, and the daily hemispheric amount is derived
by adding up total NOy amounts in each latitude bin separately for the
Northern Hemisphere and Southern Hemisphere, and for each model scenario. The
EPP-NOy amount is derived as the difference between a model run with full
particle forcing (3dCTM v1.6 phioniz, KASIMA v1.6, EMAC UBC) to the
respective Base model run without particle forcing.
Results of the total EPP-NOy amount are shown for all three models in
Fig. . All three models show a similar behavior. The main
features are the following.
In both hemispheres, EPP NOy shows values between 0.5 and 5 Gmol, with a distinct annual variability
favoring the winter periods, and sporadic, short-lived increases of up to
2 Gmol per hemisphere related to strong solar proton events.
The total amount of EPP NOy is strongest in the winters of the transition from solar maximum to minimum: 2003 to 2005
in the Southern Hemisphere, 2003–2004 and 2004–2005 in the Northern
Hemisphere. The very high values in Southern Hemisphere winter 2003 and
Northern Hemisphere winter 2004–2005 seem to be due to a combination of the
indirect effect transporting NOy down into the stratosphere, and a strong
solar proton event in mid to late winter (October 2003 and January 2005).
The impact of the October 2003 SPE is more pronounced in 3dCTM and KASIMA than in EMAC, where the maximum NOy values are
reached already before the SPE. In Northern Hemisphere winter 2003–2004, the
October 2003 solar proton event occurred in early winter. The very high
values in Northern Hemisphere winter 2003–2004 seem to be due to a
combination of the large solar proton event in early winter and the sudden
stratospheric warming in late winter. However, very different values are
predicted by the models for the impact of the sudden stratospheric warming:
an increase of about 2 Gmol in EMAC, half a Gmol in KASIMA, and no
distinctive increase in 3dCTM.
EPP NOy is enhanced over the whole model period by more than 0.5 Gmol in both hemispheres. This
indicates that once NOy has reached the mid-stratosphere, its effective atmospheric lifetime is rather longer
than 1 year. EPP NOy accumulates over the solar maximum years, and does
not drop to zero before the next maximum starts.
This accumulation effect is emphasized by the very low values displayed by KASIMA at the beginning of the KASIMA model period in mid-2002.
The highest values of EPP NOy are predicted by EMAC, and the lowest by KASIMA, in agreement with the results of the comparison to
stratospheric NOy from MIPAS shown in the previous section.
The annual variation is less pronounced in the Southern Hemisphere NOy from KASIMA. This might be due to the stronger
(and, apparently, more realistic) denitrification in KASIMA compared to the
other two models: denitrification redistributes NOy in the lowermost
stratosphere very efficiently, taking NOy out of the gas phase and
sedimenting it out of the middle atmosphere completely. This becomes evident
by comparing to the EPP NOy from altitudes above the vertical range where
denitrification occurs (9 hPa): this shows a comparable annual variation in
all three models.
To summarize, energetic particle precipitation provides a nearly constant
background of EPP NOy in both hemispheres, from a few tenths of Gmol
during solar minimum to 1–2 Gmol during solar maximum. Superposed on this
background is a distinct annual cycle with higher values during polar winter
due to the EPP indirect effect, that is, downwelling of particle-induced
NOy probably originating in the aurora during polar winter at high
latitudes. Additionally, there are sporadic increases of more than 1 Gmol
per event due to strong solar proton events, and in the Northern Hemisphere,
also due to strong sudden stratospheric warmings.
The strongest NOy increases are simulated by the models for the
October 2003 solar proton event: between 0.9 Gmol per hemisphere (3dCTM in
the Northern Hemisphere, EMAC in the Southern Hemisphere) and 1.8 Gmol per
hemisphere (KASIMA in the Southern Hemisphere). Increases of several tenths
of Gmol are predicted for the solar proton events in January 2005, July 2004,
and December 2006. This is on the same order of magnitude as previous
estimates of hemispheric NOy increases for solar proton events in
November 1960 (1.1 Gmol, ), September 1966 (0.34 Gmol,
) and the October 2003 event (0.75–2.82 Gmol,
), but lower than estimates for
the August 1972 event (2.98–3.40 Gmol, )
and the October 1989 event (5.56–6.97 gMol per hemisphere,
).
Comparison of modeled EPP NOy (Gmol) with data
derived from MIPAS . For the models, the difference between
the highest value of this winter minus the lowest value of the preceding
summer is given; in brackets, the maximal value of the winter is
given.
Winter
MIPAS
3dCTM
KASIMA
EMAC
NH 2002/2003
0.51
0.29 (1.55)
0.28 (0.28)
0.62 (2.24)
SH 2003
2.5
2.74 (3.45)
3.04 (3.05)
3.20 (4.69)
NH 2003/2004
3.17
1.41 (2.27)
2.31 (2.56)
3.74 (4.91)
SH 2004
–
0.45 (2.00)
0.33 (1.56)
1.06 (3.10)
NH 2004/2005
1.19
1.13 (2.56)
1.10 (2.82)
0.95 (2.90)
SH 2005
1.5
1.29 (2.55)
0.90 (2.09)
1.36 (3.16)
NH 2005/2006
0.45
0.17 (1.77)
0.26 (2.23)
0.40 (1.96)
SH 2006
0.69
0.35 (1.69)
0.12 (1.43)
0.76 (2.33)
NH 2006/2007
0.5
0.44 (1.34)
0.57 (2.06)
0.65 (1.77)
SH 2007
0.7
0.56 (1.77)
0.17 (1.43)
0.57 (2.01)
NH 2007/2008
0.39
0.38 (1.19)
0.18 (1.48)
0.37 (1.24)
SH 2008
0.9
0.51 (1.37)
0.24 (1.32)
0.81 (1.86)
NH 2008/2009
0.18
0.23 (0.95)
0.25 (1.20)
0.28 (1.01)
SH 2009
0.51
0.36 (1.14)
0.02 (1.007)
0.36 (1.33)
NH 2009/2010
0.11
–
The winter-time increase due to the indirect effect is in the range of a few
tenths of Gmol in the solar minimum, to up to 1.5–3.7 Gmol in the
transition from the declining phase of the solar maximum. These values are
higher than in previous studies summarized in , which
provide a range of zero to 1.5 Gmol per winter based on HALOE
observations, but are in good agreement with
studies based on MIPAS data . In
, the winter-time increase in EPP NOy due to the
indirect effect is derived from MIPAS observations for every Northern
Hemisphere and Southern Hemisphere winter covered by MIPAS observations
(mid-2002–early 2012). To make these observations directly comparable to our
model results, the increase in every Northern Hemisphere and Southern
Hemisphere winter is derived from the model results by subtracting the lowest
value in the preceding summer. Results for all winters and all model runs are
summarized together with the MIPAS observations (, their
Table 1) in Table . Observations and model results are generally
in good agreement, with values of more than 1 Gmol from Southern Hemisphere
winter 2003 to Southern Hemisphere winter 2005 (note that there are no MIPAS
observations during Southern Hemisphere winter 2004), and values lower than
1 Gmol at the beginning and end of the time period, in Northern Hemisphere
winter 2002–2003 and after Southern Hemisphere winter 2005. In years with
low EPP NOy, 3dCTM and KASIMA are more likely to underestimate EPP NOy,
while in EMAC, EPP NOy is more likely to be in agreement with, or higher
than, observed EPP NOy. The highest values of EPP NOy of more than
3.17 Gmol are observed by MIPAS in Northern Hemisphere winter 2003–2004.
EPP NOy in this winter is underestimated by 3dCTM (1.41 Gmol) and KASIMA
(2.31 Gmol) but overestimated by EMAC (3.74 Gmol).
Ozone intercomparison, quantification of ozone loss and net radiative heating change
In the following, modeled ozone is compared to MIPAS observations, and the
particle impact on stratospheric ozone and net radiative heating will be
quantified from model results by comparing the model runs with full particle
forcing (3dCTM v1.6 phioniz, KASIMA v1.6, and EMAC UBC) to the respective
Base model scenarios. Changes in ozone are discussed in
Sect. ; changes in radiative heating and cooling rates are
derived and discussed in Sect. .
Comparison of modeled and observed ozone fields
Comparison of ozone time series at high southern (70–90∘)
latitudes from MIPAS observations with model results including full particle
forcing (3dCTM v1.6 phioniz, KASIMA v1.6, EMAC UBC). From top to bottom:
MIPAS ozone mixing ratios (ppm), difference of 3dCTM, KASIMA and EMAC mixing
ratios to MIPAS mixing ratios (ppm). Solid and dotted black lines are the 1
and 0.5 ppm contours of the model fields.
Same as Fig. but for high northern
(70–90∘) latitudes.
In Figs. and , a comparison of the model
results including full particle forcing (3dCTM v1.6 phioniz, KASIMA v1.6 and
EMAC UBC) to MIPAS observations is shown at high latitudes
(70–90∘) in both hemispheres. Mixing ratios of the MIPAS ozone
fields are shown as well as the difference between the model and the
observations where MIPAS data are available. MIPAS data have been restricted
to the vertical range of the limb scans, i.e., below 68 km, though data are
retrieved above. The 0.5 and 1 ppm contour lines of the model fields are
shown as a reference to how well the main features of the temporal and
vertical variation of ozone are captured. Ozone is characterized by a
winter-time maximum in the upper mesosphere (above ≈ 0.4 hPa) and
by a local maximum in the stratosphere (100–1 hPa) which maximizes during
winter and spring. In the Southern Hemisphere, the impact of the Antarctic
ozone hole is clearly visible as a nearly complete loss of ozone in the
lowermost stratosphere (below ≈ 30 hPa) in early spring. These main
structures are represented well by all models. However, the mesospheric
winter-time maximum is overestimated by the high-top models (3dCTM and
KASIMA) above ≈ 0.1 hPa by up to 2 ppm (nearly a factor of 2),
while it is underestimated by EMAC by up to 1.5 ppm. In the lower mesosphere
and at the stratopause (2–0.1 hPa), all three models underestimate ozone by
0.5 ppm to more than 2 ppm. This underestimation is largest (more than
2 ppm) in EMAC in the Northern Hemisphere, where it also displays an annual
variation with the largest values in spring. In the region of the
stratospheric maximum (100–2 hPa), 3dCTM overestimates ozone by 0.5–2 ppm
in the Southern Hemisphere and by 0.5–1.5 ppm in the Northern Hemisphere.
KASIMA and EMAC generally reproduce the values of the stratospheric maximum
well, with differences less than ±1 ppm. The strong overestimation of
ozone in this region by 3dCTM might be due to the underestimation of NOy
discussed in the previous chapter. Below the main amount of the stratospheric
ozone layer (100–10 hPa during late winter and spring), ozone is
overestimated by all models in the Southern Hemisphere, possibly due to
underestimation of the ozone loss in the Antarctic ozone hole region. This is
most pronounced in 3dCTM (more than 2 ppm), with much lower values of
0.5–1 ppm in KASIMA and EMAC, and is much less pronounced in the Northern
Hemisphere. In summary, all models display systematic differences (biases) in
ozone compared to observations. However, it is unlikely that these biases are
related to the treatment of energetic particles in the models, as the impact
of energetic particle precipitation is usually – with the exception of
strong solar proton events – masked by the much larger dynamical variability
of ozone. In the next subsection, we will investigate this interannual
variation of ozone, and how it relates to energetic particle precipitation;
in the following subsection, Sect. , the energetic particle
impact is extracted from the model results by comparing the model runs with
full particle forcing with the Base model scenarios.
Comparison of modeled and observed ozone anomalies
(a) Relative anomalies of MIPAS O3 compared to a
daily climatology derived from the 2006–2009 mean (%; see text) for high
southern latitudes (70–90∘ S) for the period 2002–mid-2010.
(b–g) Relative anomalies for the model results, calculated in the
same way for mid-2002 to mid-2004. Left: model runs including particle
impact; right: model runs without particle impacts. From top to bottom:
3dCTM, KASIMA and EMAC.
(a) Relative anomalies of MIPAS O3 compared to a
daily climatology derived from the 2006–2009 mean (%; see text) for high
southern latitudes (70–90∘ N) for the period 2002–mid-2010.
(b–g) Relative anomalies for the model results, calculated in the
same way for mid-2002 to mid-2004. Left: model runs including particle
impact; right: model runs without particle impacts. From top to bottom:
3dCTM, KASIMA and EMAC.
Due to the large dynamical variability of ozone particularly in the
stratosphere, an impact of energetic particle precipitation on ozone can be
difficult to derive from observations. Previous studies based on observations
have compared observations of ozone in situations with and without elevated
amounts of NOy on the same day and in the same latitude range
, the evolution of ozone in years with high energetic
particle fluxes compared to years with low particle fluxes
, and the composite difference of years with high minus
years with low particle fluxes . All
methods have yielded lower values of ozone in the upper stratosphere
presumably related to the energetic particle precipitation; the composite
method also shows negative ozone anomalies proceeding downwards from the
stratopause to the mid-stratosphere during polar winter. In the following, we
will analyze ozone anomalies for the time period 2002–2010 from MIPAS
observations to investigate the interannual variation of ozone. Anomalies are
calculated as follows. First, a climatology is built for every day of the
year as the mean of the years 2006–2009; i.e., the 1 January data of all
years are averaged to give the climatological value for 1 January, and so on.
The period 2006–2009 was chosen for two reasons. MIPAS data are available
nearly continuously during this period. Also, during this time geomagnetic
activity as a proxy for particle forcing was low, so the ensuing climatology
can be used as a reference for low particle forcing. Anomalies are calculated
by subtracting the value of this climatology for every day of the year. The
resulting percentage anomalies are shown in Fig. and
Fig. for high southern and northern latitudes
(70–90∘ S/N).
The strongest anomalies, of more than ±30 %, are observed in the time
period mid-2002 to early 2004 and in 2005 in both hemispheres, while in the
period 2006–2010, anomalies are mostly in the range ±20 %
(SH) /(-30,+20) % (NH) above 10 hPa. Anomalies in the upper mesosphere
are characterized by a strong annual variation with a change in sign from
summer to winter; the highest negative anomalies of more than -50 % are
reached during Southern Hemisphere winter 2003 and Northern Hemisphere winter
2003–2004 in this altitude region. In the mid-mesosphere to mid-stratosphere
(0.2–10 hPa), anomalies are characterized by subsequent positive and
negative downwelling signals related to changes in the speed of the
downward-poleward motion during polar winter. Below 10 hPa, very strong
anomalies of more than ±40 % occur mainly during winter and spring,
particularly in the Southern Hemisphere, and possibly related to interannual
variations in the Antarctic ozone hole. A strong positive anomaly is
observed, e.g., in late 2002 in the Southern Hemisphere below 20 hPa, which
likely is related to the ozone-hole split during this Antarctic winter
. A clear negative downwelling signal of 5–30 % is
observed in Antarctic winter 2003 from the lower mesosphere
(≈ 0.3 hPa) in mid-winter to the lower stratosphere (below 10 hPa)
in spring, preceded by a weaker (5–10 %) positive signal. The structure
and strengths of this signal are similar to the downwelling anomalies derived
from global satellite observations for the Southern Hemisphere
when comparing composites of years with
high minus low geomagnetic activity, and are interpreted as particle impacts
there. In the Northern Hemisphere, a similar negative downwelling signal
starts in the lower mesosphere in early winter 2003–2004, but is interrupted
by a strong (> 40 %) positive anomaly in late 2003 and early 2004 that
is probably related to the onset of the sudden stratospheric warming. In late
winter and spring 2004, after the sudden stratospheric warming, an even
stronger negative anomaly of more than -50 % descends from the upper
mesosphere (0.02 hPa) to the stratopause (1 hPa) in spring. However, from
the observations it is not clear whether the strong anomalies during and
after the warming are related to dynamical changes related to the warming, or
are due to the strong NOy signal observed after the warming. This will be
investigated in the following by comparing anomalies obtained in the same way
from model results with and without particle forcing.
For the model data, anomalies are obtained in the same way as for the
observations. First, a daily climatology is derived for the years 2006–2009
for the model runs with full particle forcing (3dCTM v1.6 phioniz, KASIMA
v1.6, and EMAC UBC) and for the Base model runs. Percentage anomalies are
shown for the period mid-2002 to mid-2004 in the lower left panels of
Figs. and for the model runs including full
particle forcing, in the lower right panels for the Base model runs.
Model runs including energetic particle precipitation generally show a very
consistent pattern of anomalies compared to the observations. In particular,
all three models show very large negative ozone anomalies in the upper
mesosphere (≥ 0.1 hPa) in Southern Hemisphere winter 2003, with values
ranging from 20–40 % (3dCTM) to more than 40 % (KASIMA, EMAC) over a
period of several weeks. Equally, all three models show very large positive
(≥ 40 %) anomalies in late Southern Hemisphere winter 2002 in the
lowermost stratosphere (200–10 hPa). All three models show a negative
anomaly proceeding downwards from the lower mesosphere (0.1 hPa) in
mid-winter 2003 to the lower stratosphere (≤ 10 hPa) in late spring
2003 in the Southern Hemisphere, preceded by a smaller positive anomaly in
the upper stratosphere (10–1 hPa). However,
the strength of this negative anomaly as well as the apparent speed of its
downward motion vary from model to model, consistent with differences in the
descent rate in the stratosphere already discussed in
Sect. for NOy. Transport across the stratopause is
restricted in 3dCTM, leading to too high anomalies in the lower mesosphere
(1–0.1 hPa) and a too small anomaly in the stratosphere; the downwelling
speed is captured well in KASIMA, though the amplitude of the anomaly is too
small compared to observations, while the stratospheric anomaly is too large
in EMAC. In the Northern Hemisphere, all three models again show very large
negative ozone anomalies of more than -50 % in the upper mesosphere
(≥ 0.1 hPa) in winter 2003–2004. The general structure of the
anomalies from the lower mesosphere to lower stratosphere is also captured
well by all three models, with a sequence of positive anomalies in the early
winter stratosphere (≈ 10–1 hPa) followed by a negative signal
moving down from the mesosphere to the upper stratosphere, which is
interrupted due to the sudden stratospheric warming in December 2004, by a
very strong positive signal, followed again by a negative signal after the
sudden stratospheric warming. While the strengths of the positive signals
seem to be captured well by all three models, the strengths of the negative
signal preceding the warming again vary from model to model, with values
generally lower than observed in 3dCTM and higher than observed in KASIMA and
EMAC. The strong negative signal after the warming in the mid-stratosphere to
lower mesosphere is strongly underestimated by 3dCTM and KASIMA, but captured
well by EMAC, in good agreement with results shown for the downwelling signal
of NOy discussed in Sect. .
The Base model runs without energetic particle precipitation show a very
similar pattern of anomalies to the model runs with full particle forcing
discussed in the previous paragraph, indicating that most of the anomalies
observed are not due to chemical changes due to the particle forcing, but due
to changes in dynamics (temperature and large-scale transport) from year to
year. In particular, the anomalies below 10 hPa are nearly identical in
model runs with and without particle forcing, indicating that all anomalies
below this pressure level are due to dynamical changes. This includes a
negative signal which moves down from about 10 hPa in late Southern
Hemisphere winter 2003; though this appears to be connected to the negative
anomaly moving down from the mesosphere to the mid-stratosphere throughout
the winter, it is apparently not due to chemical changes due to the energetic
particle precipitation. As the specified temperatures and wind fields in all
model experiments are based on the meteorological analyses in the
stratosphere, it can not be ruled out that changes in dynamics from year to
year are related to the particle precipitation, particularly in the years of
strong particle forcing (2002–2004); however, it is not possible to assess
this with these model experiments. A negative anomaly moving down from the
mesosphere to the mid-stratosphere during Southern Hemisphere winter 2003 and
Northern Hemisphere winter 2003–2004 is also observed in all three models in
the model runs without particle forcing, indicating that these negative
anomalies are partly due to changes in the downward transport speed or
horizontal mixing. However, the strength of the negative anomalies is larger
in the model runs including particle precipitation, indicating that energetic
particle precipitation also contributes to these anomalies. In particular, in
Southern Hemisphere winter 2003 and in Northern Hemisphere winter 2003–2004
after the sudden stratospheric warming, negative anomalies are much larger in
the model runs with particle forcing than in the respective Base model runs,
indicating that during these periods the particle impact plays a significant
role. In most other winters, the differences between ozone anomalies in the
model runs with and without particle forcings are much smaller (not shown
here). The large negative anomalies in the upper mesosphere in Southern
Hemisphere winter 2003 and Northern Hemisphere winter 2003–2004 are not
observed in the model runs without particle forcing, so they are probably due
solely to particle forcing. Because of the apparent quite strong year-to-year
variations in ozone due to changes in the vertical and poleward transport
during polar winters even in the Southern Hemisphere seen here as downwelling
positive/negative anomalies, it is very difficult to extract a signal of
particle precipitation from stratospheric and mesospheric ozone by comparing
years with and without particle precipitation; as the comparison of the
analysis of model runs without particle forcing shows, downwelling negative
anomalies can be produced for dynamical reasons and falsely attributed to
particle forcing. In the following section we investigate the impact of
energetic particle precipitation on ozone in the stratosphere by comparing
model runs with and model runs without particle forcing; as the model runs
use specified dynamics, the dynamical variability of ozone from year to year
is the same in both model experiments, and the difference between the model
experiments highlights the particle impact only.
Modeled ozone anomalies due to particle precipitation
Relative ozone anomalies due to energetic particle precipitation at
high southern latitudes (70–90∘ S), calculated as the difference
of model runs with to without particle impact (%).
(a) 3dCTM (v1.6 phioniz – Base),
(b) KASIMA (v1.6 – Base), (c) EMAC (UBC – Base). The gray
shaded area in the middle figure denotes the time period before the start of
the KASIMA model runs. Dashed and dotted vertical gray lines mark zero,
one-quarter, one-half, and three-quarters of each year. The thin black
contours refer to -1.5 (solid), -1 (dashed) and -0.5 (solid) ppm.
Relative ozone anomalies due to energetic particle precipitation at
high northern latitudes (70–90∘ N), calculated as the difference
of model runs with to without particle impact (%). (a) 3dCTM (v1.6
phioniz – Base), (b) KASIMA (v1.6 – Base), (c) EMAC (UBC
– Base). The gray shaded area in the middle figure denotes the time period
before the start of the KASIMA model runs. Dashed and dotted vertical gray
lines mark zero, one-quarter, one-half, and three-quarters of each year. The
thin black contours refer to -1.5 (solid), -1 (dashed) and -0.5 (solid)
ppm.
Ozone anomalies due to energetic particle precipitation only are derived from
the model results as the difference of model runs with 3dCTM v1.6 phioniz,
KASIMA v1.6, and EMAC UBC to the respective Base model run without particle
impacts. Percentage differences are shown for all three models at high
southern latitudes (70–90∘ S, Fig. ) and at high
northern latitudes (70–90∘ N, Fig.).
At high southern latitudes, all three models show a consistent behavior with
a distinct annual/vertical variation in the ozone loss, as well as a few
sporadic events of short-lived ozone loss.
In the upper mesosphere above 0.1 hPa, all three models predict strong ozone
losses during polar winter. The vertical structure and strength of these
winter-time mesospheric ozone losses vary from year to year, and between
models from 10–30 %
in winter 2009 (KASIMA, 3dCTM) to 70–90 %/50–70 % in winter 2003 (EMAC/KASIMA).
In KASIMA and EMAC, mesospheric ozone loss is constant or increases with altitude above
0.1 hPa, while in 3dCTM, the ozone loss decreases again above ≈ 0.03 hPa,
maximizing around 0.1 hPa.
The negative ozone anomaly simulated in every winter moves downward from the mesospheric
ozone loss region in early winter to the mid-stratosphere
(≈ 10 hPa) in late winter. Again, values vary from winter to winter
and between models, with the highest values of 30–50 % reached in the
upper stratosphere (2–6 hPa) in the solar maximum winter 2003 (EMAC), and
the lowest
values of 2–5 % reached in the solar minimum winter 2009 (KASIMA). The temporal structure
and vertical extent of these downwelling negative anomalies are in good agreement with the
observations of negative stratospheric ozone anomalies relative to enhanced geomagnetic
activity; see . However, absolute values cannot be
compared directly because investigate the interannual
variation, while here, the difference
between ozone with and without particle forcing is investigated for the same year.
Absolute differences in the stratospheric winter-time ozone anomalies are largest in EMAC
(more than 0.5 ppm in all winters, more than 1 ppm in all winters but 2009,
and more than 1.5 ppm in winters 2003 and 2005 between 10 and 1 hPa), lower
in 3dCTM (0.5–1 ppm in all winters but 2009), and lowest in KASIMA (more
than 0.5 ppm only in winter 2003). Values for winter 2003 are in good
agreement with a previous model study incorporating MIPAS NOx into the
lower mesosphere and showing 1–1.5 ppm ozone loss between 30 and 40 km
compared to a model run without excess NO2 .
Strong solar proton events in October 2003, January 2005 and December 2006 lead to an instantaneous
loss of ozone from the upper stratosphere to the mesopause (10–0.01 hPa).
In the mesosphere, this impact is short-lived and restricted to a few days
only, while in the stratosphere below 1 hPa, ozone loss continues throughout
the summer.
After winters with a strong stratospheric ozone loss signal, ozone loss of 2–5 % continues through
Antarctic spring and summer in the mid and upper stratosphere (20–1 hPa)
until the next winter. In a few
summers, this mid-stratospheric summer ozone loss can reach values of 5–10 %, in particular in early 2004
and early 2005. In these summers, the continuing ozone loss from the indirect effect seems to be strengthened
by strong solar proton events occurring in early spring (October 2003) or
during summer (January 2005).
Small regions of a positive ozone change are observed in the lower mesosphere in early and late winter below
the regions of strong mesospheric winter-time ozone loss (3dCTM, EMAC); these are likely due to self-healing, i.e.,
stronger ozone formation below regions of ozone loss because of the stronger UV radiation.
Mesospheric ozone loss has been observed to be related to strong energetic
electron precipitation events and also to be related
to the 27-day cycle of the geomagnetic activity . It is
also predicted by model studies
, and is likely related to
the increase in HOx during electron precipitation events. It is restricted
mainly to polar winter because during summer, the background in HOx is
higher, and therefore the relative increase in HOx due to the electron
precipitation is rather smaller . The
values simulated here are in the range of the observations, which show ozone
losses of about 10 % averaged over one winter, and up to 90 % in
individual strong events . The mesospheric ozone loss
also agrees well with model results by the CMAM model driven with prescribed
auroral and medium-electron ionization for the period 1979–2006
. They show a multi-annual mean of 10–30 % (Northern
Hemisphere) or 30–80 % (Southern Hemisphere) of ozone loss compared to
model runs without particle impacts in mid-winter (DJF or JJA) in the upper
mesosphere above ≈ 65 km (about 0.1 hPa). Particle-induced ozone
loss in the mesosphere is mainly due to catalytic cycles involving HOx,
which is released from positive water cluster ions formed by incorporating
water vapor, and thus depends on the availability of water vapor
. It has been shown recently that during
polar winter, mesospheric ozone loss can also be initiated by NOx
indirectly by changing the partitioning of HOx from HO2 to OH
. Differences in the vertical structure of the ozone loss
between KASIMA and EMAC might therefore denote different gradients in the
mesospheric water vapor and NOy content in the models. However, the strong
mesospheric ozone loss predicted by EMAC is more likely due to the
implementation of the upper boundary condition: in every time step, NO
in EMAC is overwritten by the upper boundary NOy. To balance NOx, other
NOx species, e.g., NO2, are set to zero. This leads to realistic
values of NOy as shown in Sect. ; however, in every
chemistry time step, the reactions NO + O3 ⟶ NO2 +
O and NO + HO2 ⟶ NO2 + OH are also processed, changing
the partitioning of HOx and destroying larger amounts of ozone than in the
Base run.
Ozone loss in the mid-stratosphere is mainly due to catalytic cycles
involving NOx, and the stratospheric ozone loss predicted by the models
can be directly related to the EPP NOy brought into the stratosphere by
the indirect effect and by large solar proton events. The winter-time ozone
loss seems to be mainly due to the indirect effect, and this continues well
into summer, in agreement with the long-term accumulation of EPP NOy
discussed in the previous chapter. Strong summer-time ozone loss, e.g., in
summer 2003–2004 and 2004–2005, nevertheless seems to be due mainly to
strong solar proton events.
In the Northern Hemisphere, mesospheric ozone loss throughout the winter is
predicted by all models similar to the Southern Hemisphere. Solar proton
events in October 2003, January 2005 and September 2005 lead to instantaneous
ozone loss from the mid-stratosphere to the upper mesosphere in all models.
However, the annual variation of the stratospheric ozone loss due to the
indirect effect looks distinctly different. In the Southern Hemisphere, a
continuous downwelling negative anomaly reaching down to the mid-stratosphere
(10 hPa) is observed in every winter. In the Northern Hemisphere, this is
not the case. In the solar minimum winters (2006–2007 to 2009–2010),
stratospheric ozone loss is significantly lower than in the Southern
Hemisphere winters, and the signal does not reach down to 10 hPa in most of
these winters. In the solar maximum winters, a strong stratospheric ozone
loss of more than 50 % (late winter 2003–2004 in EMAC), 30–50 % (winter
2003–2004 in KASIMA and early winter 2003–2004 in EMAC) or 10–30 %
(winters 2003–2004 in 3dCTM, 2004–2005 in 3dCTM and EMAC) is predicted, and
this ozone loss continues well into summer. However, the structure of the
downwelling signals in these winters is distinctly different to the structure
in Southern Hemisphere winters, with two distinct peaks of ozone loss, one in
early winter, one in late winter. The second peak in late winter 2003–2004
is due to the strong downwelling of NOy from the mesosphere after the
strong sudden stratospheric warming. It is stronger in EMAC than in KASIMA,
and weakest in 3dCTM, following the differences in NOy in the models. In
EMAC, downwelling ozone losses of 10–30 % are also observed outside solar
maximum that are related to the sudden stratospheric warmings in early 2006
and early 2009; these are weaker in KASIMA and not observed in 3dCTM. After
the sudden stratospheric warmings in early 2004, early 2006, and early 2009,
stratospheric ozone loss of 5–30 % continues throughout Arctic spring and
summer in EMAC. However, in other summers, continuing stratospheric ozone
loss from the EPP indirect effect is lower than in the Southern Hemisphere,
and less than 1 % in some summers (2006–2010 in 3dCTM, 2008 and 2010 in
KASIMA). Absolute differences during winter again range from less than
0.5 ppm (all but 2003–2004 and 2004–2005 in 3dCTM, all but 2003–2004 and
2005–2006 in KASIMA, 2002–2003, 2007–2008, and 2009–2010 in EMAC) to more
than 1 ppm (KASIMA) and more than 1.5 ppm (EMAC) after the sudden
stratospheric warming in winter 2003–2004. These values are in good
agreement with observations of ozone variations of 1–2 ppm in longitudes
with enhanced NOy compared to air parcels without enhanced NOy in early
April 2004 at 2 hPa as well as with observed
differences of about 1 ppm of ozone mixing ratios in March and April 2004
compared to previous winters at ≈ 40 km . The
structure of the downwelling signal appears to be consistent with a previous
model study incorporating MIPAS NOx into the lower mesosphere and showing
2–3 ppm ozone loss between 35 and 40 km compared to a model run without
excess NO2 in Northern Hemisphere winter 2003–2004
. These results agree well with EMAC results which also
reach more than 2 ppm, but are higher than KASIMA or 3dCTM results. On
average, ozone loss in the mid to upper stratosphere during mid-winter agrees
well with results from the CMAM model (5–30 % during JJA in 30–40 km in
a 1979–2006 multi-year average in the Southern Hemisphere, 0.5–5 % in the
Northern Hemisphere).
To summarize, stratospheric ozone loss due to energetic particle
precipitation is predicted by all three models in most winters in both
hemispheres, but the vertical range, temporal structure, and strength of the
ozone loss vary from year to year, between models, and between hemispheres.
Stratospheric ozone loss often continues into polar summer. In the Southern
Hemisphere, strong summer-time ozone losses are related mainly to strong
solar proton events, while in the Northern Hemisphere they are related mainly
to strong sudden stratospheric warmings. In the Northern Hemisphere,
winter-time stratospheric ozone loss seems to be dominated by sudden
stratospheric warmings as well, and is small in winters without strong
warmings. In contrast, in the Southern Hemisphere, the winter-time
stratospheric ozone loss is dominated by continuous downwelling of NOy
from the mesosphere, and is predicted by all models to occur in every winter.
Changes in net radiative heating
In the following, changes in the net radiative heating due to changes in
ozone are derived from the modeled ozone changes discussed in the previous
paragraph. All three models calculate radiative heating and cooling rates,
but use different spectral resolutions and parameterizations. To obtain
results for all three models independent of differences in the
parameterizations, the shortwave heating rate in the Hartley bands of ozone
(λ≤320 nm) and the longwave radiative cooling in the ν=001→ν=000 transition of ozone at 1042 cm-1
(9.6 µm) are estimated using the daily zonally averaged ozone and
temperature fields of the respective model as follows.
In a first step, the change in radiative flux in each model box is calculated
depending on the temperature in the box center, and the column density of
ozone between the upper and lower box boundaries for each model scenario.
For the shortwave radiation in the Hartley bands of ozone, the change in
radiative flux is derived from the amount of downwelling solar radiation
absorbed in the box:
ΔFHartley=JO3O3,colΔE,
where JO3 is the daily mean Hartley band photolysis rate,
O3,col is the column density of ozone between upper and lower box
boundaries, ΔE is the energy transferred into heat, and ΔF is
the change in radiative flux in W m-2. The amount of energy transferred
into heat is estimated as the mean energy of a photon in the Hartley bands,
at 260 nm (7.64×10-19 J).
The daily mean photolysis rate is calculated using the photolysis scheme of
3dCTM for the respective latitude and day of year using fixed ozone, density,
and temperature profiles by calculating photolysis rates every five minutes
and averaging over a full day. The Huggins and Chappuis bands have not been
taken into account here because they contribute to solar heating only in the
lower stratosphere and below .
For the longwave radiation, in a first step the upwelling flux is calculated
from 200 hPa up to 0.01 hPa, and the downwelling flux from 0.01 hPa down
to 200 hPa. The limits are chosen to make results from all three models
comparable: from the upper limit of EMAC down to the lower limit of 3dCTM. As
we are interested only in the mid-to upper stratosphere, the emission of
thermal radiation at the surface is not considered, and the upward and
downward fluxes can be calculated as
ΔF001,out(ν)=(ΔF001,in(ν)+πB(ν,T)σ001(ν)O3,col)e-σ001(ν)O3,colcos(53∘)
where ΔF001,out(ν) is the flux out of the box at
wavenumber ν in W m-2 cm-1,
ΔF001,in is the flux into the box, B(ν,T) is the
Planck function at wavenumber ν and temperature T, and
σ001(ν) is the absorption cross section at wavenumber ν,
considering only the v=001→v=000 transition. Line-by-line
absorption cross sections have been taken from the HITRAN database
(http://hitran.org, February 2017, ), and binned
into 1.5 cm-1 intervals. A mean air-mass factor of
1cos(53∘) is used as an approximation of the integral
over the whole sphere. The radiative flux of the whole band is obtained by
integrating over wavenumber from 950 to 1098.5 cm-1. Temperatures of
the Base model scenario are used. The change of flux in each box is then
obtained by the differences of ingoing and outgoing upward and downward flux.
It should be stressed that for a more exact calculation of the longwave
cooling due to ozone, a line-by-line calculation of the v=100→v=000, v=010→v=000, and v=100→v=000 transitions
should be carried out; however, results obtained here agree mostly to about
±20 % with calculations performed with the GRANADA non-LTE model using
climatological profiles as described in (not shown here).
Heating or cooling rates are obtained from the fluxes as
∂T∂t=gΔFΔpcp
where cp is the specific heat capacity of air, g the acceleration of
gravity taken to be constant as 9.81 m s-1, Δp the pressure
width of the box, and ∂T∂t the heating rate in
K s-1. It should be pointed out that above about 60 km altitude,
non-LTE effects as well as the diurnal variability of ozone become
increasingly important both for the shortwave heating and for the longwave
cooling terms. These are not considered here, and only results below
≈ 0.1 hPa are considered in the following.
Changes in daily net radiative (shortwave and longwave) heating
rates (K day-1) due to particle-induced ozone changes derived from all
three models (a: 3dCTM; b: KASIMA; c: EMAC) at
high southern latitudes (70–90∘ S). (d) The net
radiative heating rate (sum of the shortwave and longwave contributions) in
the EMAC Base scenario (K day-1). Black dashed lines are the -0.1 and
-0.5 K day-1 contours of the net radiative heating rate change in
EMAC; black solid lines are the +0.1 and +0.5 K day-1 contours.
Changes in daily net radiative (shortwave and longwave) heating
rates (K day-1) due to particle-induced ozone changes derived from all
three models (a: 3dCTM; b: KASIMA; c: EMAC) at
high northern latitudes (70–90∘ N). (d) The net
radiative heating rate (sum of the shortwave and longwave contributions) in
the EMAC Base scenario (K day-1). Black dashed lines are the -0.1 and
-0.5 K day-1 contours of the net radiative heating rate change in
EMAC; black solid lines are the +0.1 and +0.5 K day-1 contours.
The changes in net radiative heating and cooling rates due to particle
precipitation are calculated as the difference between model runs with to
without particle impacts, and the net heating rate change is the sum of
changes in heating and cooling rates.
The net changes in heating rates due to the particle-induced ozone loss are
shown for high southern (70–90∘ S) and high northern
(70–90∘ N) latitudes in Figs. and .
For all three models and in both hemispheres, changes in the net heating are
confined to the upper stratosphere and above (above 20 hPa), and peak around
the stratopause (around 1 hPa). Net heating rate changes show a clear annual
variation with positive values (a net heating) during winter, and negative
values (a net cooling) during spring. The net heating during winter is due to
a decrease in radiative (longwave) cooling due to the ozone reduction, while
the net cooling during spring is due mainly due to a decrease in radiative
(shortwave) heating, which is balanced to some extent by the decrease in
radiative (longwave) cooling. In some years, the net cooling of the spring
upper stratosphere continues well into summer. In the Southern Hemisphere,
these are years with strong solar proton events in spring or summer (2004,
2005, 2007); in the Northern Hemisphere, these are mainly years with strong
sudden stratospheric warmings (2004, 2006, 2009), but there is also 1 year
with a solar proton event in late winter (2005). The strongest and
longest-lasting impacts of more than 0.6 K day-1 (EMAC) or more than
0.1 K day-1 (KASIMA) for several months are predicted for the sudden
stratospheric warmings in Northern Hemisphere winters 2003–2004, 2005–2006,
and 2008–2009. The solar proton event of October 2003 has an impact
comparable with the sudden stratospheric warmings as predicted by EMAC on net
heating rates of more than 0.5 K day-1, but lasts a few days only, and
is predicted by all three models. Values of 0.1–0.2 K day-1 are then
predicted to continue throughout the summer. In the lowermost panels of
Figs. and , the daily averaged sums of all
shortwave and longwave contributions to radiative heating and cooling (called
net radiative heating rate in the following) are shown for comparison for the
EMAC Base scenario. The strongest changes in the net radiative heating rate
due to energetic particle precipitation during mid-winter occur in the upper
stratosphere and lower mesosphere (10–0.1 hPa), at the lower edge of a
region of strong radiative cooling in the lower mesosphere. The net radiative
heating rates increase from 1 K day-1 at 10 hPa to more than
10 K day-1 at 1 hPa. Changes due to particle precipitation are on the
order of magnitude of 0.1 K day-1 in this altitude range, reaching
more than 0.5 K day-1 in Southern Hemisphere winter 2003 (EMAC). This
amounts to relative changes of 1–10 % in most winters, up to 50 % in
winter 2003 (EMAC). In contrast, negative changes to the net radiative
heating during spring after strong solar proton events or sudden
stratospheric warmings occur in a region of low net radiative heating rates
ranging from less than 0.5 to 1–2.5 K day-1; changes after the strong
solar proton event in October 2003 in the Southern Hemisphere exceed 20 %,
and changes after the strong sudden stratospheric warming in Northern
Hemisphere winters 2003–2004 and 2005–2006 approach 100 %. The continuing
cooling of ≈ 0.1 K day-1 in the upper stratosphere after the
sudden stratospheric warmings throughout the summer are in the range of
4–10 % of the net radiative rate of 1–2.5 K day-1 at this time.
During spring the upper stratosphere approaches radiative equilibrium, so
small changes in the net radiation budget – as after sudden stratospheric
warmings or winter-time solar proton events – could potentially have a large
impact.
Daily changes in the net amount of energy absorbed or transmitted
due to particle-induced ozone destruction in the stratosphere (W m-2),
derived from the difference of model runs with model runs without particle
forcing, as a function of time and latitude.
To investigate the temporal–spatial structure of the particle impact onto
the net energy absorbed or emitted, the changes in radiative flux derived
above for both the shortwave and longwave components are added up over the
vertical region from 200 to 0.2 hPa. The upper limit was chosen because
above this region, the simplified approach used here is no longer valid; see
above. The resulting changes in the net flux are shown in Fig. .
Results confirm many of the findings already discussed for the heating rates:
the largest changes to the net energy absorbed are related to strong solar
proton events in the Southern Hemisphere, and to sudden stratospheric
warmings in the Northern Hemisphere. Positive changes (net heating) are
predicted for winter-time, but are restricted to high latitudes poleward of
60∘. Negative changes (net cooling) are predicted mainly for spring
and summer, and for latitudes equatorwards of 60∘. The net cooling
reaches well into mid-latitudes (30∘) in some years (2003 and 2006
in the Southern Hemisphere, 2004 and 2005 in the Northern Hemisphere). This
latitudinal extent of the particle impact well into mid-latitudes (up to
30∘) is consistent with results of EPP NOy derived from MIPAS
observations as shown in (their Fig. 12).