Coupled Chemistry Climate Model Research Articles

Using cloud ice flux to parametrise large-scale lightning

Abstract. Lightning is an important natural source of nitrogen oxide especially in the middle and upper troposphere. Hence, it is essential to represent lightning in chemistry transport and coupled chemistry–climate models. Using ERA-Interim meteorological reanalysis data we compare the lightning flash density distributions produced using several existing lightning parametrisations, as well as a new parametrisation developed on the basis of upward cloud ice flux at 440 hPa. The use of ice flux forms a link to the non-inductive charging mechanism of thunderstorms. Spatial and temporal distributions of lightning flash density are compared to tropical and subtropical observations for 2007–2011 from the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite. The well-used lightning flash parametrisation based on cloud-top height has large biases but the derived annual total flash density has a better spatial correlation with the LIS observations than other existing parametrisations. A comparison of flash density simulated by the different schemes shows that the cloud-top height parametrisation has many more instances of moderate flash densities and fewer low and high extremes compared to the other parametrisations. Other studies in the literature have shown that this feature of the cloud-top height parametrisation is in contrast to lightning observations over certain regions. Our new ice flux parametrisation shows a clear improvement over all the existing parametrisations with lower root mean square errors (RMSEs) and better spatial correlations with the observations for distributions of annual total, and seasonal and interannual variations. The greatest improvement with the new parametrisation is a more realistic representation of the zonal distribution with a better balance between tropical and subtropical lightning flash estimates. The new parametrisation is appropriate for testing in chemistry transport and chemistry–climate models that use a lightning parametrisation.

Low‐frequency ocean bottom pressure variations in the North Pacific in response to time‐variable surface winds

AbstractOne decade of time‐variable gravity field observations from the GRACE satellite mission reveals low‐frequency ocean bottom pressure (OBP) variability of up to 2.5 hPa centered at the northern flank of the subtropical gyre in the North Pacific. From a 145 year‐long simulation with a coupled chemistry climate model, OBP variability is found to be related to the prevailing atmospheric sea‐level pressure and surface wind conditions in the larger North Pacific area. The dominating atmospheric pressure patterns obtained from the climate model run allow in combination with ERA‐Interim sea‐level pressure and surface winds a reconstruction of the OBP variability in the North Pacific from atmospheric model data only, which correlates favorably (r=0.7) with GRACE ocean bottom pressure observations. The regression results indicate that GRACE‐based OBP observations are indeed sensitive to changes in the prevailing sea‐level pressure and thus surface wind conditions in the North Pacific, thereby opening opportunities to constrain atmospheric models from satellite gravity observations over the oceans.

Influence of projected Arctic sea ice loss on polar stratospheric ozone and circulation in spring

The impact of projected Arctic sea ice loss on the stratosphere is investigated using the Whole Atmosphere Community Climate Model (WACCM), a state-of-the-art coupled chemistry climate model. Two 91-year simulations are conducted: one with a repeating seasonal cycle of Arctic sea ice for the late twentieth-century, taken from the fully coupled WACCM historical run; the other with Arctic sea ice for the late twenty-first century, obtained from the fully coupled WACCM RCP8.5 run. In response to Arctic sea ice loss, polar cap stratospheric ozone decreases by 13 DU (34 DU at the North Pole) in spring, confirming the results of Scinocca et al (2009 Geophys. Res. Lett. 36 L24701). The ozone loss is dynamically initiated in March by a suppression of upward-propagating planetary waves, possibly related to the destructive interference between the forced wave number 1 and its climatology. The diminished upward wave propagation, in turn, weakens the Brewer–Dobson circulation at high latitudes, strengthens the polar vortex, and cools the polar stratosphere. The ozone reduction persists until the polar vortex breaks down in late spring.

Evolution of Total Atmospheric Ozone from 1900 to 2100 Estimated with Statistical Models

Abstract Statistical models that account for the separate influences on total atmospheric ozone of ozone-depleting substances, anthropogenic greenhouse gases, and natural processes are formulated from the Merged Ozone Data (MOD V8 and V8.6) and used to explore scenarios for ozone’s evolution from 1900 to 2100. The statistical models based on MOD V8 project larger growth in total ozone during the twenty-first century than do coupled chemistry–climate models globally and in the tropics where the chemistry–climate models indicate persistent ozone depletion. The statistical models based on MOD V8.6 suggest, instead, that total ozone everywhere never (or barely) recovers to 1980 levels. Since the decline in ozone-depleting substances and the increase in greenhouse gas concentrations are both expected to increase ozone in the twenty-first century, these results suggest that downward instrumental drifts may be present in MOD V8.6. Instrumental drifts, of opposite sign, may also be present in MOD V8 since it is possible to reduce the projections of the corresponding statistical models to agree with those of the chemistry–climate models by altering the long-term trends of the MOD V8 data within the estimated long-term uncertainty. Alternatively, the chemistry–climate models may project excess tropical ozone depletion by overestimating trends in the upwelling of tropical (ozone poor) air associated with global warming and the resultant decline in mean age of air. This possibility is consistent with independent observations that the age of stratospheric air has not declined during the past three decades, as the globe has warmed 0.3°C, and that model parameterizations of tropical convection may be inadequate.

Interactive ozone induces a negative feedback in CO2-driven climate change simulations

Interactively coupled climate chemistry models (CCMs) extend the number of feedback mechanisms in climate change simulations by including chemical feedback. In this study the radiative feedback from ozone changes on climate response and climate sensitivity is quantified for a series of simulations driven by CO2 increases on top of a present-day reference concentration level. Other possibly relevant feedback via atmospheric chemistry, e.g., via CH4 and N2O, is not fully quantified in the CCM setup as their concentrations are essentially fixed at the surface. In case of a CO2-doubling simulation, the ozone feedback reduces the climate sensitivity parameter by 3.4%, from 0.70 K/(W m−2) without interactive chemistry to 0.68 K/(W m−2). In case of a 4*CO2 simulation, the reduction of the climate sensitivity parameter increases to 8.4%. An analysis of feedback reveals that the negative feedback of stratospheric ozone and the associated negative feedback change in stratospheric water vapor are mainly responsible for this damping. The feedback from tropospheric ozone changes is positive but much smaller. The nonlinearity in the climate sensitivity damping with increased CO2 concentrations is shown to be due to nonlinear feedback of ozone and stratospheric water vapor.

Climate and chemistry effects of a regional scale nuclear conflict

Abstract. Previous studies have highlighted the severity of detrimental effects for life on earth after an assumed regionally limited nuclear war. These effects are caused by climatic, chemical and radiative changes persisting for up to one decade. However, so far only a very limited number of climate model simulations have been performed, giving rise to the question how realistic previous computations have been. This study uses the coupled chemistry climate model (CCM) SOCOL, which belongs to a different family of CCMs than previously used, to investigate the consequences of such a hypothetical nuclear conflict. In accordance with previous studies, the present work assumes a scenario of a nuclear conflict between India and Pakistan, each applying 50 warheads with an individual blasting power of 15 kt ("Hiroshima size") against the major population centers, resulting in the emission of tiny soot particles, which are generated in the firestorms expected in the aftermath of the detonations. Substantial uncertainties related to the calculation of likely soot emissions, particularly concerning assumptions of target fuel loading and targeting of weapons, have been addressed by simulating several scenarios, with soot emissions ranging from 1 to 12 Tg. Their high absorptivity with respect to solar radiation leads to a rapid self-lofting of the soot particles into the strato- and mesosphere within a few days after emission, where they remain for several years. Consequently, the model suggests earth's surface temperatures to drop by several degrees Celsius due to the shielding of solar irradiance by the soot, indicating a major global cooling. In addition, there is a substantial reduction of precipitation lasting 5 to 10 yr after the conflict, depending on the magnitude of the initial soot release. Extreme cold spells associated with an increase in sea ice formation are found during Northern Hemisphere winter, which expose the continental land masses of North America and Eurasia to a cooling of several degrees. In the stratosphere, the strong heating leads to an acceleration of catalytic ozone loss and, consequently, to enhancements of UV radiation at the ground. In contrast to surface temperature and precipitation changes, which show a linear dependence to the soot burden, there is a saturation effect with respect to stratospheric ozone chemistry. Soot emissions of 5 Tg lead to an ozone column reduction of almost 50% in northern high latitudes, while emitting 12 Tg only increases ozone loss by a further 10%. In summary, this study, though using a different chemistry climate model, corroborates the previous investigations with respect to the atmospheric impacts. In addition to these persistent effects, the present study draws attention to episodically cold phases, which would likely add to the severity of human harm worldwide. The best insurance against such a catastrophic development would be the delegitimization of nuclear weapons.

The SOCOL version 3.0 chemistry–climate model: description, evaluation, and implications from an advanced transport algorithm

Abstract. We present the third generation of the coupled chemistry–climate model (CCM) SOCOL (modeling tools for studies of SOlar Climate Ozone Links). The most notable modifications compared to the previous model version are (1) the dynamical core has been updated with the fifth generation of the middle-atmosphere general circulation model MA-ECHAM (European Centre/HAMburg climate model), and (2) the advection of the chemical species is now calculated by a mass-conserving and shape-preserving flux-form transport scheme instead of the previously used hybrid advection scheme. The whole chemistry code has been rewritten according to the ECHAM5 infrastructure and transferred to Fortran95. In contrast to its predecessors, SOCOLvs3 is now fully parallelized. The performance of the new SOCOL version is evaluated on the basis of transient model simulations (1975–2004) with different horizontal (T31 and T42) resolutions, following the approach of the CCMVal-1 model validation activity. The advanced advection scheme significantly reduces the artificial loss and accumulation of tracer mass in regions with strong gradients that was observed in previous model versions. Compared to its predecessors, SOCOLvs3 generally shows more realistic distributions of chemical trace species, especially of total inorganic chlorine, in terms of the mean state, but also of the annual and interannual variability. Advancements with respect to model dynamics are for example a better representation of the stratospheric mean state in spring, especially in the Southern Hemisphere, and a slowdown of the upward propagation in the tropical lower stratosphere. Despite a large number of improvements model deficiencies still remain. Examples include a too-fast vertical ascent and/or horizontal mixing in the tropical stratosphere, the cold temperature bias in the lowermost polar stratosphere, and the overestimation of polar total ozone loss during Antarctic springtime.

Stratospheric Ozone and Temperature Simulated from the Preindustrial Era to the Present Day

Abstract Results from the simulation of a coupled chemistry–climate model are presented for the period 1860 to 2005 using the observed greenhouse gas (GHG) and halocarbon concentrations. The model is coupled to a simulated ocean and uniquely includes both detailed tropospheric chemistry and detailed middle atmosphere chemistry, seamlessly from the surface to the model top layer centered at 0.02 hPa. It is found that there are only minor changes in simulated stratospheric temperature and ozone prior to the year 1960. As the halocarbon amounts increase after 1970, the model stratospheric ozone decreases approximately continuously until about 2000. The steadily increasing GHG concentrations cool the stratosphere from the beginning of the twentieth century at a rate that increases with height. During the early period the cooling leads to increased stratospheric ozone. The model results show a strong, albeit temporary, response to volcanic eruptions. While chlorofluorocarbon (CFC) concentrations remain low, the effect of eruptions is shown to increase the amount of HNO3, reducing ozone destruction by the NOx catalytic cycle. In the presence of anthropogenic chlorine, after the eruption of El Chichón and Mt. Pinatubo, chlorine radicals increased and the chlorine reservoirs decreased. The net volcanic effect on nitrogen and chlorine chemistry depends on altitude and, for these two volcanoes, leads to an ozone increase in the middle stratosphere and a decrease in the lower stratosphere. Model lower-stratospheric temperatures are also shown to increase during the last three major volcanic eruptions, by about 0.6 K in the global and annual average, consistent with observations.

Impacts of climate change on surface ozone and intercontinental ozone pollution: A multi‐model study

AbstractThe impact of climate change between 2000 and 2095 SRES A2 climates on surface ozone (O)3 and on O3 source‐receptor (S‐R) relationships is quantified using three coupled climate‐chemistry models (CCMs). The CCMs exhibit considerable variability in the spatial extent and location of surface O3 increases that occur within parts of high NOx emission source regions (up to 6 ppbv in the annual average and up to 14 ppbv in the season of maximum O3). In these source regions, all three CCMs show a positive relationship between surface O3 change and temperature change. Sensitivity simulations show that a combination of three individual chemical processes—(i) enhanced PAN decomposition, (ii) higher water vapor concentrations, and (iii) enhanced isoprene emission—largely reproduces the global spatial pattern of annual‐mean surface O3 response due to climate change (R2 = 0.52). Changes in climate are found to exert a stronger control on the annual‐mean surface O3 response through changes in climate‐sensitive O3 chemistry than through changes in transport as evaluated from idealized CO‐like tracer concentrations. All three CCMs exhibit a similar spatial pattern of annual‐mean surface O3 change to 20% regional O3 precursor emission reductions under future climate compared to the same emission reductions applied under present‐day climate. The surface O3 response to emission reductions is larger over the source region and smaller downwind in the future than under present‐day conditions. All three CCMs show areas within Europe where regional emission reductions larger than 20% are required to compensate climate change impacts on annual‐mean surface O3.

Boreal and temperate snow cover variations induced by black carbon emissions in the middle of the 21st century

Abstract. We used a coupled climate-chemistry model to quantify the impacts of aerosols on snow cover north of 30° N both for the present-day and for the middle of the 21st century. Black carbon (BC) deposition over continents induces a reduction in the mean number of days with snow at the surface (MNDWS) that ranges from 0 to 10 days over large areas of Eurasia and Northern America for the present-day relative to the pre-industrial period. This is mainly due to BC deposition during the spring, a period of the year when the remaining of snow accumulated during the winter is exposed to both strong solar radiation and a large amount of aerosol deposition induced themselves by a high level of transport of particles from polluted areas. North of 30° N, this deposition flux represents 222 Gg BC month−1 on average from April to June in our simulation. A large reduction in BC emissions is expected in the future in all of the Representative Concentration Pathway (RCP) scenarios. In particular, considering the RCP8.5 in our simulation leads to a decrease in the spring BC deposition down to 110 Gg month−1 in the 2050s. However, despite the reduction of the aerosol impact on snow, the MNDWS is strongly reduced by 2050, with a decrease ranging from 10 to 100 days from present-day values over large parts of the Northern Hemisphere. This reduction is essentially due to temperature increase, which is quite strong in the RCP8.5 scenario in the absence of climate mitigation policies. Moreover, the projected sea-ice retreat in the next decades will open new routes for shipping in the Arctic. However, a large increase in shipping emissions in the Arctic by the mid-21st century does not lead to significant changes of BC deposition over snow-covered areas in our simulation. Therefore, the MNDWS is clearly not affected through snow darkening effects associated with these Arctic ship emissions. In an experiment without nudging toward atmospheric reanalyses, we simulated however some changes of the MNDWS considering such aerosol ship emissions. These changes are generally not statistically significant in boreal continents, except in Quebec and in the West Siberian plains, where they range between −5 and −10 days. They are induced both by radiative forcings of the aerosols when they are in the snow and in the atmosphere, and by all the atmospheric feedbacks. These experiments do not take into account the feedbacks induced by the interactions between ocean and atmosphere as they were conducted with prescribed sea surface temperatures. Climate change by the mid-21st century could also cause biomass burning activity (forest fires) to become more intense and occur earlier in the season. In an idealised scenario in which forest fires are 50% stronger and occur 2 weeks earlier and later than at present, we simulated an increase in spring BC deposition of 21 Gg BC month−1 over continents located north of 30° N. This BC deposition does not impact directly the snow cover through snow darkening effects. However, in an experiment considering all the aerosol forcings and atmospheric feedbacks, except those induced by the ocean–atmosphere interactions, enhanced fire activity induces a significant decrease of the MNDWS reaching a dozen of days in Quebec and in Eastern Siberia.

A community diagnostic tool for chemistry climate model validation

Abstract. This technical note presents an overview of the Chemistry-Climate Model Validation Diagnostic (CCMVal-Diag) tool for model evaluation. The CCMVal-Diag tool is a flexible and extensible open source package that facilitates the complex evaluation of global models. Models can be compared to other models, ensemble members (simulations with the same model), and/or many types of observations. The initial construction and application is to coupled chemistry-climate models (CCMs) participating in CCMVal, but the evaluation of climate models that submitted output to the Coupled Model Intercomparison Project (CMIP) is also possible. The package has been used to assist with analysis of simulations for the 2010 WMO/UNEP Scientific Ozone Assessment and the SPARC Report on the Evaluation of CCMs. The CCMVal-Diag tool is described and examples of how it functions are presented, along with links to detailed descriptions, instructions and source code. The CCMVal-Diag tool supports model development as well as quantifies model changes, both for different versions of individual models and for different generations of community-wide collections of models used in international assessments. The code allows further extensions by different users for different applications and types, e.g. to other components of the Earth system. User modifications are encouraged and easy to perform with minimum coding.

Understanding differences in upper stratospheric ozone response to changes in chlorine and temperature as computed using CCMVal‐2 models

Projections of future ozone levels are made using models that couple a general circulation model with a representation of atmospheric photochemical processes, allowing interactions among photochemical processes, radiation, and dynamics. Such models are known as coupled chemistry‐climate models (CCMs). Although developed from common principles and subject to the same boundary conditions, simulated ozone time series vary among models for scenarios for ozone depleting substances (ODSs) and greenhouse gases. Photochemical processes control the upper stratospheric ozone level, and there is broad agreement among CCMs in that ozone increases as ODSs decrease and temperature decreases due to greenhouse gas increase. There are quantitative differences in the ozone sensitivity to chlorine and temperature. We obtain insight into differences in sensitivity by examining the relationship between the upper stratospheric seasonal cycles of ozone and temperature as produced by fourteen CCMs. All simulations conform to expectation in that ozone is less sensitive to temperature when chlorine levels are highest because chlorine catalyzed loss is nearly independent of temperature. Analysis reveals differences in simulated temperature, ozone and reactive nitrogen that lead to differences in the relative importance of ozone loss processes and are most obvious when chlorine levels are close to background. Differences in the relative importance of loss processes underlie differences in simulated sensitivity of ozone to composition change. This suggests 1) that the multimodel mean is not a best estimate of the sensitivity of upper stratospheric ozone to changes in ODSs and temperature; and 2) that the spread of values is not an appropriate measure of uncertainty.

Radiative forcing estimates of sulfate aerosol in coupled climate-chemistry models with emphasis on the role of the temporal variability

Abstract. This paper describes the impact on the sulfate aerosol radiative effects of coupling the radiative code of a global circulation model with a chemistry-aerosol module. With this coupling, temporal variations of sulfate aerosol concentrations influence the estimate of aerosol radiative impacts. Effects of this coupling have been assessed on net fluxes, radiative forcing and temperature for the direct and first indirect effects of sulfate. The direct effect respond almost linearly to rapid changes in concentrations whereas the first indirect effect shows a strong non-linearity. In particular, sulfate temporal variability causes a modification of the short wave net fluxes at the top of the atmosphere of +0.24 and +0.22 W m−2 for the present and preindustrial periods, respectively. This change is small compared to the value of the net flux at the top of the atmosphere (about 240 W m−2). The effect is more important in regions with low-level clouds and intermediate sulfate aerosol concentrations (from 0.1 to 0.8 μg (SO4) m−3 in our model). The computation of the aerosol direct radiative forcing is quite straightforward and the temporal variability has little effect on its mean value. In contrast, quantifying the first indirect radiative forcing requires tackling technical issues first. We show that the preindustrial sulfate concentrations have to be calculated with the same meteorological trajectory used for computing the present ones. If this condition is not satisfied, it introduces an error on the estimation of the first indirect radiative forcing. Solutions are proposed to assess radiative forcing properly. In the reference method, the coupling between chemistry and climate results in a global average increase of 8% in the first indirect radiative forcing. This change reaches 50% in the most sensitive regions. However, the reference method is not suited to run long climate simulations. We present other methods that are simpler to implement in a coupled chemistry/climate model and that offer the possibility to assess radiative forcing.

Effects of climate change on ozone and ­particulate matter over Central and Eastern Europe

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 50:51-68 (2011) - DOI: https://doi.org/10.3354/cr01036 Effects of climate change on ozone and ­particulate matter over Central and Eastern Europe P. Huszar1,*, K. Juda-Rezler2, T. Halenka1, H. Chervenkov3, D. Syrakov4, B. C. Krüger5, P. Zanis6, D. Melas7, E. Katragkou7, M. Reizer2, W. Trapp8, M. Belda1 1Department of Meteorology and Environment Protection, Charles University, 180 00 Prague, Czech Republic 2Faculty of Environmental Engineering, Warsaw University of Technology, 00-653 Warsaw, Poland 3National Institute of Meteorology and Hydrology − branch Plovdiv, 4000 Plovdiv, Bulgaria 4Department of Air and Water Pollution, National Institute of Meteorology and Hydrology, 1784 Sofia, Bulgaria 5Institute of Meteorology, University of Natural Resources and Applied Life Sciences, 1190 Vienna, Austria 6Department of Meteorology and Climatology, Aristotle University of Thessaloniki, 54 124 Thessaloniki, Greece 7Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, 54 124 Thessaloniki, Greece 8Air Protection Unit of Ekometria, 80-299 Gdańsk, Poland *Email: huszarpet@gmail.com ABSTRACT: We investigated the changes in exceedances of key atmospheric pollutants due to climate change in central and southeastern Europe. The RegCM3/CAMx and ­ALADIN-CLIMATE/CMAQ regional climate-chemistry modelling systems were used for high-resolution simulations (10 km). The simulations performed cover 3 decadal time slices (1991−2000, 2041−2050, 2091−2100). The future simulations were driven under the A1B IPCC scenario. Our model simulations yielded changes in ozone, particulate matter with a diameter <10 µm (PM10) and sulphur ­dioxide (SO2) resulting from climate change, without taking into consideration changes in anthropogenic emissions. We observed a shift in the average pollutant levels, and the frequency of occurrence of extreme pollution changed, especially in 2091–2100. Due to elevated air temperature, accumulated ozone exposure over the 40 ppbv (parts per billion by volume) threshold (AOT40) for crops and forests tended to increase by up to 20 to 30% in many areas of central Europe by the end of the century. Marked increases in the number of 8 h ozone threshold exceedances and ozone maxima were also observed. An increase in hourly and daily SO2 exceedances was predicted. Finally, PM10 exceedances were predicted to change due to different future climate. A small change (±1 d yr−1) was calculated for an extensive area of central Europe, and exceedances decreased around the Benelux states and Romania. The modelled SO2 and PM10 changes can be partly attributed to different horizontal and vertical mixing as evidenced by changes in the ventilation co­efficient (defined as the product of horizontal wind speed at the surface and the planetary boundary layer height), and the majority of the SO2 and PM10 exceedances occurred during winter and autumn. KEY WORDS: Future air quality · Climate change · Pollutant exceedances · Ozone · Particulate matter · Coupled climate-chemistry models Full text in pdf format PreviousNextCite this article as: Huszar P, Juda-Rezler K, Halenka T, Chervenkov H and others (2011) Effects of climate change on ozone and ­particulate matter over Central and Eastern Europe. Clim Res 50:51-68. https://doi.org/10.3354/cr01036 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 50, No. 1. Online publication date: December 01, 2011 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2011 Inter-Research.

Ozone database in support of CMIP5 simulations: results and corresponding radiative forcing

Abstract. A continuous tropospheric and stratospheric vertically resolved ozone time series, from 1850 to 2099, has been generated to be used as forcing in global climate models that do not include interactive chemistry. A multiple linear regression analysis of SAGE I+II satellite observations and polar ozonesonde measurements is used for the stratospheric zonal mean dataset during the well-observed period from 1979 to 2009. In addition to terms describing the mean annual cycle, the regression includes terms representing equivalent effective stratospheric chlorine (EESC) and the 11-yr solar cycle variability. The EESC regression fit coefficients, together with pre-1979 EESC values, are used to extrapolate the stratospheric ozone time series backward to 1850. While a similar procedure could be used to extrapolate into the future, coupled chemistry climate model (CCM) simulations indicate that future stratospheric ozone abundances are likely to be significantly affected by climate change, and capturing such effects through a regression model approach is not feasible. Therefore, the stratospheric ozone dataset is extended into the future (merged in 2009) with multi-model mean projections from 13 CCMs that performed a simulation until 2099 under the SRES (Special Report on Emission Scenarios) A1B greenhouse gas scenario and the A1 adjusted halogen scenario in the second round of the Chemistry-Climate Model Validation (CCMVal-2) Activity. The stratospheric zonal mean ozone time series is merged with a three-dimensional tropospheric data set extracted from simulations of the past by two CCMs (CAM3.5 and GISS-PUCCINI) and of the future by one CCM (CAM3.5). The future tropospheric ozone time series continues the historical CAM3.5 simulation until 2099 following the four different Representative Concentration Pathways (RCPs). Generally good agreement is found between the historical segment of the ozone database and satellite observations, although it should be noted that total column ozone is overestimated in the southern polar latitudes during spring and tropospheric column ozone is slightly underestimated. Vertical profiles of tropospheric ozone are broadly consistent with ozonesondes and in-situ measurements, with some deviations in regions of biomass burning. The tropospheric ozone radiative forcing (RF) from the 1850s to the 2000s is 0.23 W m−2, lower than previous results. The lower value is mainly due to (i) a smaller increase in biomass burning emissions; (ii) a larger influence of stratospheric ozone depletion on upper tropospheric ozone at high southern latitudes; and possibly (iii) a larger influence of clouds (which act to reduce the net forcing) compared to previous radiative forcing calculations. Over the same period, decreases in stratospheric ozone, mainly at high latitudes, produce a RF of −0.08 W m−2, which is more negative than the central Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) value of −0.05 W m−2, but which is within the stated range of −0.15 to +0.05 W m−2. The more negative value is explained by the fact that the regression model simulates significant ozone depletion prior to 1979, in line with the increase in EESC and as confirmed by CCMs, while the AR4 assumed no change in stratospheric RF prior to 1979. A negative RF of similar magnitude persists into the future, although its location shifts from high latitudes to the tropics. This shift is due to increases in polar stratospheric ozone, but decreases in tropical lower stratospheric ozone, related to a strengthening of the Brewer-Dobson circulation, particularly through the latter half of the 21st century. Differences in trends in tropospheric ozone among the four RCPs are mainly driven by different methane concentrations, resulting in a range of tropospheric ozone RFs between 0.4 and 0.1 W m−2 by 2100. The ozone dataset described here has been released for the Coupled Model Intercomparison Project (CMIP5) model simulations in netCDF Climate and Forecast (CF) Metadata Convention at the PCMDI website (http://cmip-pcmdi.llnl.gov/).

Chemistry climate model simulations of the effect of the 27 day solar rotational cycle on ozone

[1] The results from two simulations with the coupled chemistry climate model (CCM) ECHAM5/MESSy (EMAC-FUB) are analyzed for the effect of solar variability at the 27 day rotational time scale on ozone. One simulation is forced with constant spectral irradiances at the top of the atmosphere and the other one with daily varying irradiances using data of 1 year for solar maximum conditions. Consistent changes are applied to the photolysis scheme of the model. The model results show the main features of observed correlations between ozone and solar irradiance variability with a maximum positive correlation in the upper stratosphere and an anticorrelation in the mesosphere. The relative sensitivity of upper stratospheric ozone to changes in the solar ultraviolet flux is estimated to be 0.3 to 0.4% per 1% change in 205 nm flux. During periods of strong 27 day variability, a similar upper stratospheric ozone sensitivity is derived. However, when the daily solar irradiance variability is weak and dominated by the 13.5 day period, the ozone sensitivity is reduced in the subtropics. The modeled temperature response is consistent with the ozone signal. When averaged over one rotational cycle, the ozone and temperature response to a neglect of the 27 day cycle is weak and statistically insignificant in the stratosphere but of nonnegligible magnitude and statistically significant in the equatorial mesosphere. Our results suggest that ignoring daily solar flux variations on the 27 day time scale in transient CCM simulations does not lead to a significant degradation of the time mean ozone response in the stratosphere, while in the tropical mesosphere, significant errors of up to 3% may occur. This result does not exclude potential additional effects of 27 day solar cycle variability on stratospheric dynamics in winter which were, however, not the subject of this study.

A closer look at Arctic ozone loss and polar stratospheric clouds

Abstract. The empirical relationship found between column-integrated Arctic ozone loss and the potential volume of polar stratospheric clouds inferred from meteorological analyses is recalculated in a self-consistent manner using the ERA Interim reanalyses. The relationship is found to hold at different altitudes as well as in the column. The use of a PSC formation threshold based on temperature dependent cold aerosol formation makes little difference to the original, empirical relationship. Analysis of the photochemistry leading to the ozone loss shows that activation is limited by the photolysis of nitric acid. This step produces nitrogen dioxide which is converted to chlorine nitrate which in turn reacts with hydrogen chloride on any polar stratospheric clouds to form active chlorine. The rate-limiting step is the photolysis of nitric acid: this occurs at the same rate every year and so the interannual variation in the ozone loss is caused by the extent and persistence of the polar stratospheric clouds. In early spring the ozone loss rate increases as the solar insolation increases the photolysis of the chlorine monoxide dimer in the near ultraviolet. However the length of the ozone loss period is determined by the photolysis of nitric acid which also occurs in the near ultraviolet. As a result of these compensating effects, the amount of the ozone loss is principally limited by the extent of original activation rather than its timing. In addition a number of factors, including the vertical changes in pressure and total inorganic chlorine as well as denitrification and renitrification, offset each other. As a result the extent of original activation is the most important factor influencing ozone loss. These results indicate that relatively simple parameterisations of Arctic ozone loss could be developed for use in coupled chemistry climate models.

Multimodel assessment of the upper troposphere and lower stratosphere: Tropics and global trends

The performance of 18 coupled Chemistry Climate Models (CCMs) in the Tropical Tropopause Layer (TTL) is evaluated using qualitative and quantitative diagnostics. Trends in tropopause quantities in the tropics and the extratropical Upper Troposphere and Lower Stratosphere (UTLS) are analyzed. A quantitative grading methodology for evaluating CCMs is extended to include variability and used to develop four different grades for tropical tropopause temperature and pressure, water vapor and ozone. Four of the 18 models and the multi‐model mean meet quantitative and qualitative standards for reproducing key processes in the TTL. Several diagnostics are performed on a subset of the models analyzing the Tropopause Inversion Layer (TIL), Lagrangian cold point and TTL transit time. Historical decreases in tropical tropopause pressure and decreases in water vapor are simulated, lending confidence to future projections. The models simulate continued decreases in tropopause pressure in the 21st century, along with ∼1K increases per century in cold point tropopause temperature and 0.5–1 ppmv per century increases in water vapor above the tropical tropopause. TTL water vapor increases below the cold point. In two models, these trends are associated with 35% increases in TTL cloud fraction. These changes indicate significant perturbations to TTL processes, specifically to deep convective heating and humidity transport. Ozone in the extratropical lowermost stratosphere has significant and hemispheric asymmetric trends. O3 is projected to increase by nearly 30% due to ozone recovery in the Southern Hemisphere (SH) and due to enhancements in the stratospheric circulation. These UTLS ozone trends may have significant effects in the TTL and the troposphere.

Decline and recovery of total column ozone using a multimodel time series analysis

Simulations of 15 coupled chemistry climate models, for the period 1960–2100, are presented. The models include a detailed stratosphere, as well as including a realistic representation of the tropospheric climate. The simulations assume a consistent set of changing greenhouse gas concentrations, as well as temporally varying chlorofluorocarbon concentrations in accordance with observations for the past and expectations for the future. The ozone results are analyzed using a nonparametric additive statistical model. Comparisons are made with observations for the recent past, and the recovery of ozone, indicated by a return to 1960 and 1980 values, is investigated as a function of latitude. Although chlorine amounts are simulated to return to 1980 values by about 2050, with only weak latitudinal variations, column ozone amounts recover at different rates due to the influence of greenhouse gas changes. In the tropics, simulated peak ozone amounts occur by about 2050 and thereafter total ozone column declines. Consequently, simulated ozone does not recover to values which existed prior to the early 1980s. The results also show a distinct hemispheric asymmetry, with recovery to 1980 values in the Northern Hemisphere extratropics ahead of the chlorine return by about 20 years. In the Southern Hemisphere midlatitudes, ozone is simulated to return to 1980 levels only 10 years ahead of chlorine. In the Antarctic, annually averaged ozone recovers at about the same rate as chlorine in high latitudes and hence does not return to 1960s values until the last decade of the simulations.

Quantitative assessment of Southern Hemisphere ozone in chemistry-climate model simulations

Abstract. Stratospheric ozone recovery in the Southern Hemisphere is expected to drive pronounced trends in atmospheric temperature and circulation from the stratosphere to the troposphere in the 21st century; therefore ozone changes need to be accounted for in future climate simulations. Many climate models do not have interactive ozone chemistry and rely on prescribed ozone fields, which may be obtained from coupled chemistry-climate model (CCM) simulations. However CCMs vary widely in their predictions of ozone evolution, complicating the selection of ozone boundary conditions for future climate simulations. In order to assess which models might be expected to better simulate future ozone evolution, and thus provide more realistic ozone boundary conditions, we assess the ability of twelve CCMs to simulate observed ozone climatology and trends and rank the models according to their errors averaged across the individual diagnostics chosen. According to our analysis no one model performs better than the others in all the diagnostics; however, combining errors in individual diagnostics into one metric of model performance allows us to objectively rank the models. The multi-model average shows better overall agreement with the observations than any individual model. Based on this analysis we conclude that the multi-model average ozone projection presents the best estimate of future ozone evolution and recommend it for use as a boundary condition in future climate simulations. Our results also demonstrate a sensitivity of the analysis to the choice of reference data set for vertical ozone distribution over the Antarctic, highlighting the constraints that large observational uncertainty imposes on such model verification.