We extended the Linearized ozone scheme – LINOZ in the ICON (ICOsahedral Nonhydrostatic) – ART (the extension for Aerosols and Reactive Trace gases) model system to include NOy formed by auroral and medium-energy electrons in the upper mesosphere and lower thermosphere, and the corresponding ozone loss, as well as changes in the rate of ozone formation due to the variability of the solar radiation in the ultraviolet wavelength range. This extension allows us to realistically represent variable solar and geomagnetic forcing in the middle atmosphere using a very simple ozone scheme. The LINOZ scheme is computationally very cheap compared to a full middle atmosphere chemistry scheme, yet provides realistic ozone fields consistent with the stratospheric circulation and temperatures, and can thus be used in climate models instead of prescribed ozone climatologies. To include the reactive nitrogen (NOy) produced by auroral and radiation belt electron precipitation in the upper mesosphere and lower thermosphere during polar winter, the so-called energetic particle precipitation indirect effect, an upper boundary condition for NOy has been implemented into the simplified parameterization scheme of the N2O/NOy reactions. This parameterization, which uses the geomagnetic Ap index, is also recommended for chemistry-climate models in the CMIP6 experiments. With this extension, the model simulates realistic “tongues” of NOy propagating downward in polar witner from the model top in the upper mesosphere into the mid-stratosphere with an amplitude that is modulated by geomagnetic activity. We then expanded the simplified ozone description used in the model by applying LINOZ version 3. The additional ozone tendency from NOy is included by applying the corresponding terms of the version 3 of LINOZ. This NOy, coupled as an additional term in the linearized ozone chemistry, led to significant ozone losses in the polar upper stratosphere in both hemispheres which is qualitatively in good agreement with ozone observations and model simulations with EPP-NOy and full stratospheric chemistry. In a subsequent step, the tabulated coefficients forming the basis of the LINOZ scheme were provided separately for solar maximum and solar minimum conditions. These coefficients were then interpolated to ICON-ART using the F10.7 index as a proxy for daily solar spectra (UV) variability to account for solar UV forcing. This solar UV forcing in the model led to changes in ozone in the tropical and mid-latitude stratosphere consistent with observed solar signals in stratospheric ozone.
Bundesministerium für Bildung und ForschungSolcheckAgencia Estatal de InvestigaciónPID2022-141216NB-I00/AEI/10.13039/501100011033Introduction
The solar influence in the middle atmosphere involves various contributors, including the ozone response triggered by both energetic particle precipitation (EPP) and ultraviolet (UV) solar radiation . Energetic particles precipitate into the atmosphere from multiple sources: solar protons, accelerated to energies of a few hundred MeV, are associated with huge eruptions of the solar corona; galactic cosmic rays (GCRs), which include particles with energies ranging from hundreds of MeV up to GeV ; Auroral electrons, precipitated during magnetic reconnection in the magnetotail, having energies ranging from a few keV up to hundreds of keV; and radiation belt electrons, containing energies up to several MeV during geomagnetic storms . The precipitation of energetic particles into the middle atmosphere contributes to the formation of a chain of ionic reactions by ionizing and dissociating species such as N2 and O2, producing neutral reactive radicals such as H, OH, N, and NO . Both HOx (H, HO2) and NOx (N, NO, NO2) trigger catalytic chemical cycles associated with the mesospheric and stratospheric ozone loss . HOx has a shorter atmospheric lifetime compared to NOx and exhibits a higher potential for inducing ozone loss in the mesosphere . In contrast, NOx is longer-lived and can be transported downward through the stratosphere, leading to ozone loss in the stratosphere, particularly during polar winter and spring .
Electron precipitation from the magnetosphere – from the auroral and radiation belt regions – occurs nearly continuously, much more frequent than solar proton events. These particles do not penetrate as deeply into the middle atmosphere to the lower stratosphere as high-energy solar protons associated with solar proton events do, yet they can still produce larger amounts of NOx and are the main source for NOx in the high-latitude upper mesosphere and lower thermosphere . NOx variations in the mesosphere and lower thermosphere due to geomagnetic activity can be considered a proxy for electron precipitation .
The distinction between the direct and indirect effects of EPP arises from where NOx is produced and its subsequent impact on ozone. When NOx is produced in the mesosphere or lower thermosphere, it does not immediately affect stratospheric ozone. Instead, it is transported downward into the stratosphere within the polar vortex before causing ozone depletion, a process known as the EPP indirect effect (EPP IE) . In contrast, NOx produced in the lower mesosphere or stratosphere can cause ozone depletion directly in those regions. Although both processes ultimately involve ozone loss via NOx, we use the established terms “direct effect” and “indirect effect” to reflect their distinct pathways and to align with common usage in the literature. A recent publication by indicates that a direct effect on atmospheric dynamics via mesospheric HOx production and ozone loss by precipitating magnetospheric electrons in early winter might be possible as well.
As ozone plays an important role in radiative heating in the middle atmosphere, a realistic ozone field is essential in order to obtain a reasonable description of dynamical processes . Despite numerous studies on the impact of solar forcing on the climate system through the top-down effect, conclusive results have yet to be reached. The main reason is the limited statistics that can be obtained with resource-demanding full chemistry climate models. For such studies, a fast but realistic ozone scheme is essential to achieve a sufficient number of realizations.
The ozone loss in the stratosphere, induced by the downward transport of NOx during polar winter and spring, can lead to net radiative cooling due to the reduction in UV absorption. Conversely, during the polar night, ozone loss results in net radiative heating because of the reduction in IR emission . These changes subsequently alter the dynamics of the middle atmosphere, initiating a chain of dynamical shifts that contribute to top-down solar forcing during polar winter and spring. This process, driven by the EPP-NOx indirect effect, appears to impact tropospheric weather systems in the high and mid-latitudes during winter and spring .
Variable solar UV is another source of ozone variability in the stratosphere . Ozone formation is driven by photolysis of O2 in the UV spectral range at wavelengths less than 220 nm, and changes in the UV flux will affect the rate of formation of ozone particularly around the tropical stratopause . The variations of solar ultraviolet radiation depend on sunspot activity that occurs in 11-year solar cycles. During solar maximum, increased levels of UV radiation lead to higher rates of oxygen photolysis, resulting in the production of ozone .
The changes in radiative heating rates induced by both direct modulation of UV radiation at the tropical stratopause and indirect modulation through ozone changes alter temperatures and dynamics of the middle atmosphere . These radiative heating changes alter the meridional temperature gradient , thereby affecting the zonal wind. As a result, the changes in the zonal wind can modulate the behavior of planetary waves, penetrating further down to the earth's surface, eventually impacting the lower atmospheric circulation patterns such as the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) .
In this paper, we describe the implementation of variable solar UV radiation and particle precipitation by applying the UBC-NOy in the simplified NOy scheme and using the NOy tendency term in the linearized ozone chemistry scheme LINOZ. This scheme is incorporated into the chemistry-climate model ICON-ART, and the impact of solar variability due to EPP and changes in solar UV radiation on ozone in the middle atmosphere is assessed using ICON-ART-LINOZ. The results are compared with observations of NOy from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) , as well as with model outputs from the ECHAM/MESSy Atmospheric Chemistry (EMAC) model , as shown in and . Additionally, the solar signal in stratospheric ozone derived from satellite data is compared, as shown in .
Several previous parameterizations have been developed to simulate transient ozone in chemistry-climate models. The scheme introduced by provides a linear parameterization of ozone photochemistry, including a representation of polar ozone loss, which we also adopt in our setup. Another example is the SWIFT scheme discussed by and , which uses an efficient approach based on a fourth-order polynomial fit to full chemistry simulations. Although SWIFT offers high accuracy and speed, it was originally designed for use with Lagrangian transport models, making it less directly applicable to our ICON setup. In this study, we used the LINOZ scheme, which provides a computationally efficient and dynamically consistent alternative suitable for integration into global models that require interactive, yet fast ozone chemistry.
The ICON-ART-LINOZ scheme is capable, in principle, of simulating ozone under changing greenhouse gas (GHG) conditions. In the full LINOZ V3 framework, N2O and CH4 can be prescribed from evolving boundary conditions, allowing their long-term trends to influence stratospheric ozone through interactive chemistry . However, in our current implementation, N2O and CH4 are treated as fixed climatological fields and thus do not vary with changing GHG scenarios. If future studies require simulations under substantially different climate conditions or trace gas abundances, the LINOZ tables can be regenerated around a new reference state to maintain accuracy in the ozone response. This flexibility makes ICON-ART-LINOZ suitable for exploring ozone–climate interactions in future scenarios, provided that the relevant chemical inputs are updated accordingly.
The LINOZ parameterization has been shown to perform well in extreme climate scenarios, such as the CMIP 4 × CO2 case discussed by . In their study, both the Cariolle and LINOZ V1 schemes produced reasonable ozone responses to substantial temperature increases. Our implementation of LINOZ V3 builds on this by addressing a key limitation identified by which is the absence of Quasi-Biennial Oscillation-related feedback on NOy due to vertical transport in LINOZ V1. In LINOZ V3, this coupling is included, allowing for a more realistic simulation of the variability of ozone and NOy, particularly in the tropical stratosphere above 10 hPa. This confirms that the ICON-ART-LINOZ system even in its current O3–NOy-only configuration, remains applicable to study ozone in high CO2 scenarios, particularly where NOy-driven chemistry and temperature-dependent processes dominate.
The description of the ICON-ART model can be found in Sect. and , and the LINOZ is discussed in Sect. . The experimental setup is described in Sect. . Model developments including the upper boundary condition of NOy (UBC-NOy), the inclusion of the NOy-based tendency term, and the incorporation of solar UV variability, detailed in Sect. –. The quantification of the EPP and UV impact on ozone and evaluation against MIPAS observations and the EMAC model is discussed in Sect. and .
The ICON-ART modelThe ICON model description
ICON stands for ICOsahedral Nonhydrostatic model system and has been designed by a joint development between the German Weather Service (DWD), the Max Planck Institute for Meteorology (MPI-M), Deutsches Klimarechenzentrum (DKRZ), the Karlsruhe Institute of Technology (KIT), and the Center for Climate Systems Modeling (C2SM) as a unified version of numerical weather prediction (NWP) and climate configuration . Our study relies on ICON (NWP) physics package.
The horizontal discretization in ICON is based on an unstructured icosahedral-triangular C grid and It uses a hybrid vertical coordinate system that is terrain-following near the surface and transitions to constant height levels in the upper levels . Employing icosahedral-triangular C grid type is advantageous for simulating polar regions, as it eliminates the singularity issue that would otherwise be encountered when applying latitude-longitude grids .
In the ICON model, physical processes are considered by parameterization schemes that are distinct from the dynamical core which solves the governing equations of atmospheric motion. The NWP physics package, as detailed by consists of parameterizations for radiative transfer, cloud microphysics, convection, turbulent diffusion, and surface interactions. These schemes are specifically optimized for numerical weather prediction applications, which differs from the ECHAM6-based approaches used in climate modeling . The ICON physics–dynamics coupling scheme distinguishes between fast processes, such as saturation adjustment and turbulence, which are calculated with shorter time steps, and slower processes, such as radiation and convection, which are computed at longer intervals .
Chemistry and transport in ICON-ART
The extension for Aerosols and Reactive Trace Gases (ART) developed at the Karlsruhe Institute of Technology (KIT) enables the inclusion of aerosols and atmospheric chemistry into ICON . The ART model extension can be incorporated into ICON for numerical weather prediction (NWP) as well as climate configuration . Trace gases are included in ICON-ART with the ART coupler without changing the original ICON code. This setup allows for a flexible description of atmospheric trace gases using meta information within XML files, enabling a variety of simulations with different complexities . ICON-ART tracers are then transported by the ICON wind fields, and can interact with the radiative heating in ICON.
Transport of trace gases
Trace gases in ICON-ART are transported using the same nonhydrostatic dynamical core as the rest of the model, applying a finite-volume approach on an icosahedral grid . Advection of tracers is taken into account using a flux-form semi-Lagrangian method, which is mass conserving and suitable for global-scale simulations . In addition to advective transport, ICON-ART accounts for vertical diffusion in the planetary boundary layer, where turbulent mixing is parameterized following the prognostic turbulence kinetic energy (TKE) scheme developed by .
Photolysis rates
Photolysis rates in ICON-ART are handled differently depending on the chemistry scheme used:
LINOZ: this scheme uses precomputed photolysis rates stored in tabulated form, calculated using the PRATMO (Prather's Atmospheric Model) code . These rates cover the stratosphere (10–60 km) include Rayleigh scattering, and are calculated with a fixed albedo of 0.30 to account for average cloud cover. LINOZ does not calculate photolysis rates interactively; it uses these precomputed values for efficiency. It is important to note that LINOZ does not account for JO2 photolysis above 60 km, and Lyman-alpha photolysis of JH2O is not included below 70 km, where its impact is minimal.
MECCA: the full chemistry scheme (MECCA) calculates photolysis rates using CloudJ7.3 , a module that provides accurate photolysis rates based on the solar zenith angle, cloud cover, and atmospheric composition. This module is configurable and allows for accurate photolysis calculations across various atmospheric layers.
Chemistry schemes
ICON-ART supports three chemistry approaches:
Simple Lifetime Mechanism: for tracers with a fixed e-fold decay time, providing computational efficiency without complex chemical interactions .
LINOZ: a linearized ozone chemistry scheme , optimized for the stratosphere, where solar UV and EPP impact ozone.
MECCA: a comprehensive full chemistry scheme , with numerical integration managed using the Kinetic PreProcessor (KPP) , generating Fortran90 code for solving the differential equations of the chemical mechanism. The Rosenbrock solver of the third order is used for numerical stability. For the MECCA scheme, species can be calculated individually or conceptually grouped (e.g., NOy, HOy) in order to simplify chemical interactions. However, this is not automatic. Instead, each species is calculated individually, unless explicitly defined as a group in the chemical mechanism . A specific example of this is the “generic RO2” approach in MECCA, where multiple organic peroxy radicals are shown by a single generic RO2 species, reducing computational cost while maintaining chemical accuracy. The MECCA setup in ICON-ART is configured using an XML file, allowing users to define or extend chemical mechanisms without modifying the model code .
The linearized ozone scheme (LINOZ) as included in ART
For a more realistic description of ozone fields compared to a prescribed ozone climatology, we have relied on a linearized ozone scheme, LINOZ . LINOZ provides a computationally efficient alternative to a full middle atmosphere chemistry scheme, while still generating ozone fields that align well with stratospheric circulation and temperatures.
In this study, we adapted the LINOZ V3 model from to focus on the interactions between NOy and O3 under solar variable forcing. NOy is calculated following the LINOZ V3 formulation, which includes photochemical production based on fixed N2O, stratospheric and mesospheric losses, a tropospheric sink , and an upper boundary condition (UBC) that incorporates EPP-NOy input.
We employ an O3–NOy-only version of LINOZ V3. The net chemical tendency for each species is represented as a first-order Taylor expansion around climatological mean states. The production (P) and loss (L) terms are computed using precomputed coefficients that describe the sensitivity of chemical rates to the concentrations of relevant species, temperature (T), and the overhead ozone column (CO3). These coefficients are derived from the PRATMO photochemical box model , which simulates stratospheric chemistry involving O3, NOy, N2O, CH4, and H2O.
In our O3–NOy-only setup, these coefficients are simplified to capture only interactions between O3 and NOy, while N2O, CH4, and H2O are treated as fixed climatological fields. Thus, only O3 and NOy are dynamically calculated in the LINOZ scheme, whereas other species are treated as fixed climatological fields. This method ensures efficient computation and successfully captures key ozone–NOy interactions relevant to our study, while processes involving dynamically varying N2O lie outside the scope of the current implementation.
The coefficients for the production and loss terms are precomputed for 25 pressure levels (∼10–58 km), 18 latitudes, and 12 months. These values are stored in lookup tables and used to efficiently calculate the chemical tendencies for O3 and NOy during model integration.
The differential equation representing the linearized ozone version 3 method follows :
dfidt=(P-L)i0i+∑j=1j=5∂(P−L)i∂fj|0fj-fj0+∂(P−L)i∂T|0T-T0+∂(P−L)i∂CO3|0CO3-CO30.
For i=1, …, 4 and j=1, …, 5, where f1≡fO3, f2≡fN2O, f3≡fNOy, f4≡fCH4, and f5≡fH2O.
In this study, we rely on i=1 and 3 only, f1≡fO3, f3≡fNOy.
The temperature is represented by T, the overhead ozone column by CO3, and the ozone tendency term (P−L) by P for the production term and L for the loss term. Subscript “0” is used to indicate the partial derivative evaluated at the respective climatological value, and climatological values are shown with superscript “0” .
The coefficients used in the model include the reference tendency term (P-L)i0, the first-order partial derivatives with respect to each variable, temperature, and ozone column: ∂(P-L)i∂fj, ∂(P-L)i∂T, and ∂(P-L)i∂CO3.
To simplify the model for our specific focus, we made the following adjustments:
Fixed climatologies for CH4 and H2O: while this assumption may not capture long-term variations, it allows us to focus on the impacts of solar variability on ozone through NOy-related chemistry and UV photolysis.
Fixed N2O distribution: we use a climatological distribution for N2O, meaning that the production of NOy from N2O is fixed. Although this setup does not account for feedback mechanisms where changes in ozone could affect the stratospheric N2O distribution and thus NOy production, it simplifies the model to highlight the solar–ozone interaction, albeit without representing the complete solar–ozone coupling. NOy produced from N2O is assumed to follow this fixed distribution.
Previous experiments have shown that using volume mixing ratio (VMR) as the basis for the UBC provides more stable results, especially in avoiding problems related to vertical wind noise. While a flux-based UBC has its own challenges, the choice of VMR was more appropriate for this study, given the dynamics of the ICON model.
UBC for NOy: in this study, we implement a density-prescribed Upper Boundary Condition (UBC) for NOy, applied to the three uppermost model levels (the top of ICON is at 80 km). The top three levels are fixed in the vertical grid and, with the grid spacing used in this study, consistently fall within the 10−1 to 10−2 hPa range. This approach was chosen over a flux-based UBC for several reasons, as discussed in the following. In past experiments with the EMAC model, both flux-based and density-prescribed UBCs were tested. Results indicated that prescribing densities in the uppermost levels performed significantly better than the flux-based approach, particularly at 10−1 hPa, as showed in . Given the similar setup of ICON and EMAC, we expect the density-prescribed UBC to perform more reliably in our study as well. Secondly a flux-based approach depends on the accuracy of the vertical fluxes in the upper model levels. However, these levels typically form a sponge layer where vertical motions are artificially dampened, leading to unrealistic vertical fluxes. This limitation was the primary reason the flux approach did not work well in EMAC, and we anticipate similar challenges with ICON. Lastly, the UBC we apply is based on MIPAS satellite observations, which scan up to 68 km altitude. These observations implicitly include both local production of NOy in the mesosphere (due to geomagnetic storms and auroral substorms) and transport of NO from the thermosphere into the mesosphere. A flux-based approach would neglect the direct NOy production in the mesosphere, as it only accounts for the vertical transport from above. By prescribing densities in the upper model levels, we ensure that both sources – mesospheric production and thermospheric transport – are considered, just as they are in the MIPAS data.
Adjustments for solar UV variability: the LINOZ tables were recalculated for ozone to account for changes in solar UV, particularly in the J-O2 photolysis rates.
This work represents a proof of concept that studies of solar variability can be conducted using this fast, efficient model. In future studies, we plan to extend this work by implementing a full version of LINOZ V3, recalculating the NOy tendencies for solar variability, and dynamically coupling CH4, H2O, and N2O to improve the representation of chemical and dynamical processes under varying solar conditions.
Experimental setup
The ICON modelling system allows for different physics parameterizations to meet the needs of a variety of applications. In this study, we focused on a model experiment using the numerical weather prediction (NWP) configuration in the open release version April 2025 of ICON (https://www.icon-model.org/, last access: 13 October 2025). Free-running model experiments were conducted in a transient setup from 2000 to 2010, excluding the first 2.5 years to allow for model spinup. The simulations were performed on a global R2B4 grid which corresponds to a grid resolution of approximately 160 km, with a vertical resolution of 90 levels up to an altitude of around 80 km, and a model time step of 6 min for the physics and chemistry calculations. Results were output on a daily basis.
Ozone was calculated using the linearized LINOZ scheme, without coupling back to the radiation scheme to ensure the same dynamical behaviour in all model experiments. Polar spring-time stratospheric ozone loss as seen in the Antarctic ozone hole was activated using the ICON-ART-LINOZ subroutine called PolarChem described in . The experiments utilized the following forcing and boundary conditions: sea surface temperature (SST) and sea ice concentration (SIC) were taken from , solar irradiation was based on , greenhouse gases (RCP4.5) were adopted from , and tropospheric and stratospheric aerosols were based on . Ozone used for the calculation of radiative heating, as well as volcanic aerosol shortwave and longwave heating, was taken from the CMIP6 database (see https://blogs.reading.ac.uk/ccmi/forcing-databases-in-support-of-cmip6/, last access: 13 October 2025).
Three model experiments were carried out within our study: Experiment 1: without the Upper Boundary Condition of NOy (UBC-NOy), constant solar minimum (BASE). Experiment 2: with variable UBC-NOy, constant solar minimum (UBC-NOy). Experiment 3: with variable UBC-NOy, constant solar maximum (SOLMAX).
Model developmentsThe upper boundary condition of NO<sub><italic>y</italic></sub> (UBC-NO<sub><italic>y</italic></sub>)
We utilized a semi-empirical model for mesospheric and stratospheric NOy, as described by to describe the impact of auroral and radiation belt electron precipitation on NOy in the upper mesosphere. The model is characterized by the geomagnetic Ap index.
Observations of NOy (NO, NO2, NO3, HNO3, HNO4, ClONO2, and N2O5) obtained by the MIPAS Fourier transform spectrometer on board ENVISAT between 2002 and 2012 have been used to characterize the fraction of NOy produced by energetic particle precipitation (EPP-NOy) in polar winters in both hemispheres . A linear relationship with a time lag, depending on the day of the year, latitude, and altitude, was found between EPP-NOy and the geomagnetic Ap index . This relationship was used in a semi-empirical model to estimate EPP-NOy densities and their wintertime downward transport, based on the measured global distributions of NOy compounds from 2002 to 2012 .
We emphasize that the stratospheric NOy in our study is derived from both, a simplified parametrization scheme of the N2O/NOy reactions from and downward transport of UBC-NOy. In our simulations, NOy at model's top without the UBC is essentially negligible. The UBC, based on MIPAS observations, provides total NOy values that include both EPP and non-EPP components. Therefore, the difference between the reference case (without UBC-NOy) and our simulations with the UBC applied represents the additional NOy introduced through the upper boundary, which likely includes contributions from EPP but may also contain a background of non-EPP NOy.
The transport of NOy is handled by the underlying dynamics of the ICON model, where the UBC is applied at the three uppermost model levels to avoid noise from the sponge layer. In these top three levels, values are overwritten by the UBC to reflect the MIPAS-derived NOy values, while the ICON dynamics are allowed to handle transport and chemistry below this boundary. This ensures that the model properly simulates the realistic transport of NOy through the stratosphere.
The comparison of model outputs with MIPAS data validates the model’s ability to simulate the transport and chemistry of NOy as it moves through the stratosphere. While the UBC sets the boundary at the upper altitudes, the model dynamically alters NOy below this boundary, which is why this comparison remains valuable for understanding the impacts of NOy and EPP within the atmosphere.
Daily mean, area-weighted NOy in 70–90° N (a–c) and 70–90° S (d–f) from ICON-ART. (a, d) Experiment 1 (BASE), (b, e) experiment 2 (UBC-NOy), and (c, f) difference (UBC-NOy- BASE). Model runs are shown in May 2002–2010 only to allow for 2.5 years of spinup.
In Fig. , we show a comparison of ICON-ART without and with UBC-NOy. The inclusion of UBC-NOy leads to a strongly enhanced NOy at the model top, particularly during polar winter, as well as a downward-propagating “tongue” of NOy indicating transport from the upper mesosphere into the mid-stratosphere during every polar winter. Qualitatively, ICON-ART with UBC-NOy well reproduces the known behavior of EPP-NOy, with interhemispheric differences due to the differing dynamics of the high-latitude northern and southern winter middle atmosphere.
Including the NO<sub><italic>y</italic></sub>-based tendency term in ICON-ART-LINOZ
In the next step of our development, we utilized LINOZ, as described in Sect. , to incorporate an NOy-based tendency term that accounts for ozone changes in the polar stratosphere into the linearized ozone description. It is important to acknowledge that when using upper boundary NOy values, especially within the NOy tongue region, significant deviations from the climatological state occur. To enhance the reliability of the tendencies of ozone related to NOy, we have re-calculated the LINOZ tables using a climatological NOy with upper boundary values. It is important to note that ICON is free-running, so the specific upper boundary condition used does not correspond to the model's dynamics.
In this implementation, the J-NO photolysis rates were extended to cover the mesosphere. For this purpose, rates were derived from the EMAC model, ensuring that photochemical processes relevant above the stratosphere are appropriately represented. However, the NOy tendencies themselves were not recalculated for different solar conditions.
Including the solar UV variation into ICON-ART-LINOZ
In addition to particle forcing, we included solar UV variability in ICON-ART to account for induced ozone changes, primarily in the tropical stratosphere. The photochemical box model calculating the LINOZ tables applies a solar spectrum provided in 77 spectral bins. In order to implement solar spectral variations, the LINOZ tables must be recalculated using solar spectra representing solar maximum and solar minimum conditions. The spectra applied are based on two spectra taken during the ATLAS missions in November 1989 (solar maximum) and 1994 (solar minimum) and prepared as described in to comply with recent measurements of the solar constant. After transferring the spectra to the 77 spectral bins of the photochemical box model (here version 8.0) we calculated two sets of tables and used them for solar maximum and solar minimum runs.
Furthermore we calculated the values for the monthly mean 10.7 cm flux under both maximum and minimum conditions (November 1989 and November 1994) and applied a linear interpolation based on the F10.7 solar activity index between these two states within the model.
Impact of SSI changes on ozone in ICON-ART (Percentage difference between SOLMAX and UBC-NOy relative to UBC-NOy). From left to right: 70–90° S, 10° S–10° N, 70–90° N respectively.
Figure shows the impact of variable SSI as the percentage difference in ozone between solar maximum (experiment SOLMAX) and solar minimum conditions (experiment UBC-NOy), here relative to the results of the UBC-NOy experiment. larger ozone values, in the range of a few percent, align with observed solar signals in stratospheric ozone. Higher values at high latitudes could reflect the influence of the Brewer–Dobson circulation and mesospheric meridional circulation, which transport ozone from the tropical stratopause source regions to the polar mesosphere in summer and to the polar lower stratosphere in all seasons. This purely chemical impact in reality could be masked by the feedback between ozone increase and changes in radiative heating, which are not considered here.
Evaluation of the particle and solar forcing
In the following, we will evaluate the changes made to ICON-ART. ICON-ART NOy combining with UBC-NOy is compared against published model results from EMAC and against MIPAS observations in Sect. , the resulting ozone fields and ozone change due to the additional NOy and solar cycle implementation in LINOZ are discussed in Sect. .
UBC-NO<sub><italic>y</italic></sub>
As shown in Fig. , after the implementation of the UBC-NOy, we observe a high level of qualitative agreement at the top of the atmosphere between ICON-ART and a model simulation with the EMAC model also using the UBC-NOy from . The EMAC model employs MECCA stratospheric chemistry, specified dynamics relaxing towards ERA-interim reanalysis data , and variable geomagnetic forcing for 2000–2010 . Despite using the same parameterization of EPP-NOy, some differences between ICON-ART and EMAC NOy are apparent already at the top of the atmosphere due to differences in vertical transport and mixing.
Daily mean, area weighted NOy at 0.01 hPa in 70–90° S (a) and 70–90° N (b) from the chemistry-climate model ICON-ART (experiment 2, UBC-NOy) and EMAC model results from , also using the NOy upper boundary condition of .
In Fig. , NOy from ICON-ART with UBC-NOy is compared with results from the EMAC model including UBC-NOy, and with MIPAS/ENVISAT v5 NOy. All three data-sets reveal a significant agreement in temporal variation, vertical coverage, and interhemispheric differences particularly in the downward propagating “tongues” of NOy during polar winters. Small differences in the year-to-year variability particularly in the Northern hemisphere are likely due to the different middle atmosphere dynamics in the free-running ICON experiments. Stratospheric NOy is generally higher in ICON-ART than in EMAC and MIPAS. This is even true for experiment 1 (BASE), so presumably is a feature of the simplified NOy used for the stratospheric background. During the Northern Hemisphere winter of 2003/2004, NOy penetrated deeply into the stratosphere, with values of 100 ppb around 48 km per 1 hPa in ICON-ART, in good agreement with EMAC and MIPAS. Due to the stronger stratospheric polar vortex in the Southern hemisphere winter, NOy is transported further down into the stratosphere there, again in good agreement between ICON-ART with UBC-NOy, EMAC, and MIPAS.
Daily mean, area-weighted NOy in 70–90° N (a–c) and 70–90° S (d–f) from (a, d) MIPAS/ENVISAT v5, (b, e) EMAC, and (c, f) ICON-ART (UBC-NOy). EMAC and MIPAS data are from .
In Fig. , EPP-NOy in ICON-ART, shown as the differences between the UBC-NOy and BASE simulations, is compared to EMAC and MIPAS/ENVISAT v5. The result indicates that both models demonstrate a high degree of qualitative consistency with observations during winter. The EMAC model shows better agreement due to its specified dynamic mode. In both models, EPP-NOy persists into summers in a very consistent way. This is not evident in the observations and could be attributed to the sensitivity cutoff related to the NOy/CO correlation used to derive EPP-NOy from MIPAS/ENVISAT data.
Daily mean area-weighted EPP-NOy. (a, d) MIPAS/ENVISAT v5 adapted from ; (b, e) EMAC, difference from model run with UBC-NOy to base run without UBC-NOy but identical in every other respect ; (c, f) ICON UBC-NOy-BASE.
The addition of the particle forcing due to the indirect effect of EPP to the linearized ozone chemistry leads to a substantial decrease in ozone in the polar upper and mid-stratosphere in both hemispheres because of catalytic cycles that involve NOx.
Figure indicates the mixing ratio of the ozone fields after inclusion of the NOy-based tendency in ICON-ART-LINOZ version 3 in both the Northern and Southern high latitudes compared to EMAC and MIPAS/ENVISAT v5. Comparison against the EMAC model and MIPAS/ENVISAT v5 observation shows a good agreement in the absolute values, temporal coverage of ozone change, vertical coverage and variability, as well as interhemispheric differences .
Daily mean, area weighted ozone after inclusion of the NOy-based tendency in 70–90° N (a–c) and 70–90° S (d–f) from (a, d) MIPAS/ENVISAT v5, (b, e) EMAC, and (c, f) ICON UBC-NOy. The EMAC and MIPAS data are from .
The pronounced simulated low ozone values in the Southern hemisphere lower stratosphere during polar winter and spring are consistent with the Antarctic ozone hole.
Figure shows the ozone change due to EPP-NOy for high Northern latitudes (70 to 90° N) and high Southern latitudes (70 to 90° S), for ICON-ART and EMAC. The range of values, morphology, and interhemispheric differences between the two models are consistent. The slightly larger decreases in the Southern hemisphere observed in ICON may indicate stronger downwelling and a more persistent vortex, aligning with the slightly higher EPP-NOy levels. This phenomenon is less evident in the Northern hemisphere, which could be due to differences in the model dynamics.
Daily mean area-weighted ozone change due to EPP-NOy in percentage in 70–90° N (a, b) and 70–90° S (c, d) from (a, c) EMAC , and (b, d) ICON-ART. The contour intervalls are the same as in (Figs. 12 and 13).
Areas of low ozone develop in the mesosphere during the early winter months and descend to the mid-stratosphere by late winter/early spring in the Northern hemisphere. In the Southern hemisphere, they develop in the mesosphere during late winter/early spring and decline to the mid-stratosphere by early summer. This negative ozone response persists into the subsequent winter of 2004 around 1–10 hPa of the Northern hemisphere in both models (see Fig. ). The persistent early summer ozone depletion observed in the ICON model during 2003 may be linked to an Elevated Stratospheric (ES) event that occurred early in that year. EMAC does not show a similar ES event for 2003, while the 2006 ES event present in EMAC is not captured by ICON. These discrepancies highlight the variability in how the two models represent such events.
Impact of solar spectral irradiance (SSI) on ozone in ICON-ART: Percentage difference between SOLMAX and UBC-NOy relative to UBC-NOy(a). Same, but with pressure and latitude range adapted to ozone solar signal figures (two different datasets – SAGEII/SBUV) in (b).
Solar UV variation
The impact of SSI on ozone in ICON-ART (solar maximum minus solar minimum) is shown in Fig. . Differences of up to 4 % in the mid- and low-latitude stratosphere are observed in ICON-ART and are in good agreement with, and within, the large spread of observations (compared, e.g., to , their Figs. 4 and 12). Differences in structure could be attributed to missing radiative and dynamical feedback. At high latitudes, higher values of more than 3 % are shown. However, these cannot be compared directly against observations, as at high latitudes, the much larger changes due to particle precipitation mask the smaller changes caused by UV variability.
Conclusion
We have presented a new method of incorporating a top-down solar forcing into the stratospheric ozone, triggered by the EPP indirect effect, by utilizing a semi-empirical model for NOy based on the geomagnetic Ap index . This provides a more realistic representation of the stratospheric NOy densities and its wintertime downward transport. This new implementation of the nitrogen chemistry in ICON-ART will help improve the prediction of the ozone field in the model as a direct response to NOy.
The addition of geomagnetic forcing led to significant ozone losses in the polar upper stratosphere of both hemispheres due to the catalytic cycles involving NOy. Comparing to EMAC and MIPAS ICON-ART agrees well in the upper stratosphere (1 hPa), but it overestimates the transport into the stratosphere, leading to an overestimation of NOy in the mid-stratosphere (at and below 10 hPa) in many (but not all) winters. The maximum ozone loss in the mid to upper stratosphere due to the indirect effect of EPP occurs in late winter to spring.
Considering the solar UV variability in the ICON-ART model leads to the changes in ozone in the tropical stratosphere, which is in agreement with observations .
In conclusion, our study demonstrates that the inclusion of solar forcing, specifically particle precipitation and solar UV radiation, in the ICON-ART model relying on linearized ozone scheme provides realistic ozone fields.
Code and data availability
The developments described in this paper are included in the ICON model as of the ICON release in April 2025, which is available through the German Climate Computing Center (DKRZ) at https://www.wdc-climate.de/ui/entry?acronym=IconRelease2025.04 (last access: 13 October 2025). The model experiments presented in the paper were carried out using the same version (ICON release in April 2025). The ICON model is distributed under the BSD-3-Clause license. Additional details on accessing and compiling ICON can be found in the metadata and documentation provided with the release. The post-processed outputs, along with the namelist and XML configuration files for our model experiments, are available in Ramezani Ziarani (2025) (10.35097/01n9a0gccv6f2ggk, last access: 13 October 2025).
Author contributions
Conceptualization, MRZ, MS, TR; methodology, MRZ, MS, TR, BF, SB, MP; software, MRZ, TR, MS; validation, MRZ, MS, TR; formal analysis, MRZ, MS, TR; investigation, MRZ, MS, TR; writing – original draft preparation, MRZ; writing – review and editing, MRZ, MS, TR, BF, SB, MP; visualization, MRZ, MS; project administration, MS; funding acquisition, MS; All authors have read and agreed to the Submitted version of the manuscript.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Also, please note that this paper has not received English language copy-editing. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
Acknowledgements
We thank Michael Prather for granting access to Photochemical BoxModel version 8.0.
Financial support
This research has been supported by the Bundesministerium für Bildung und Forschung (BMBF; project “Solar contribution to climate change on decadal to centennial timescales”, SOLCHECK), by the Agencia Estatal de Investigación (grant no. PID2022-141216NB-I00/AEI/10.13039/501100011033), and by Severo Ochoa grant CEX2021-001131-S funded by MCIN/AEI/10.13039/501100011033 The article processing charges for this open-access publication were covered by Katholische Universität Eichstätt-Ingolstadt (KU).
Review statement
This paper was edited by Tatiana Egorova and reviewed by two anonymous referees.
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