S P A R C
Stratospheric Processes And their Role in Climate
Overview of the New CCMVal Reference and Sensitivity Simulations in Support of Upcoming Ozone and Climate Assessments and the Planned SPARC CCMVal Report
Veronika Eyring, DLR-Institut für Physik der Atmosphäre, Germany (Veronika.Eyring@dlr.de),
Martyn P. Chipperfield, University of Leeds, UK (email@example.com),
Marco A. Giorgetta, Max Planck Institute for Meteorology, Germany (firstname.lastname@example.org),
Douglas E. Kinnison, National Center for Atmospheric Research, USA (email@example.com),
Elisa Manzini, Centro Euro-Mediterraneo per i Cambiamenti Climatici and Istituto Nazionale di Geofisica e Vulcanologia, Italy (firstname.lastname@example.org),
Katja Matthes, Freie Universität Berlin, Germany (email@example.com)
Paul A. Newman, NASA Goddard Space Flight Center, USA (Paul.A.Newman@nasa.gov),
Steven Pawson, NASA Goddard Space Flight Center, USA (firstname.lastname@example.org),
Theodore G. Shepherd, University of Toronto, Canada (email@example.com),
Darryn W. Waugh, Johns Hopkins University, Baltimore, USA (firstname.lastname@example.org).
On behalf of the CCM Validation Activity for SPARC (CCMVal)
The CCMVal community has defined new reference and sensitivity simulations to be carried out in support of upcoming ozone and climate assessments and which are tailored to the planned SPARC CCMVal report on the evaluation of coupled Chemistry-Climate Models (CCMs), see http://www.pa.op.dlr.de/CCMVal/SPARC_CCMValReport/SPARC_CCMValReport.html. The three reference simulations that should be run by the various modelling groups with highest priority are described in Section 1. Depending on the computing capacity of the individual groups, it is recommended that in addition to the reference simulations the sensitivity simulations described in Section 2 should be performed by as many groups as possible. However, it is most important that groups simulate the full time period specified, to allow a reliable comparison between the different models.
The overriding concept behind the choice of these reference and sensitivity simulations is to achieve the best possible scientific results. Accordingly, the first requirement is to evaluate the models against observations. That is the rationale behind REF0, a time-slice experiment performed under conditions equivalent to 2000, for the period for which the highest density in observations is available. A long reference run will provide good statistics for the model comparison. The second requirement is to see how well the models can reproduce the past behaviour of stratospheric ozone. That is the rationale behind the 'past' transient reference simulation REF1, which is forced by observations. Experience in Eyring et al.  showed that it is important to establish a good baseline from which to identify the effects of halogens on ozone, and to avoid spin-up problems. Based on this experience, REF1 requires around 10-years spin-up prior to a 1960 start. The third requirement is to see what the models predict for the future evolution of stratospheric ozone. That is the rationale behind the ‘future’ transient reference simulation REF2, which is forced by trace gas projections and modeled sea surface temperatures (SSTs). Experience in Eyring et al.  showed that it is important to have a continuous time series from the models covering both past and future, in order to avoid inhomogeneity in the datasets (in terms of both absolute values and variability), and also that the simulations extend to 2100 in order to fully capture the process of ozone recovery from the effects of ozone-depleting substances (ODSs). Based on this experience, REF2 also requires around 10-years spin-up prior to a 1960 start, and extension to 2100. To provide continuity with Eyring et al. , and track any changes in the models, REF2 is based on the same GHG scenario (SRES A1B, medium) as used in Eyring et al. . For both REF1 and REF2, it is recommended that groups perform at least a small ensemble (e.g. three simulations) so that an uncertainty range for the model results can be established.
The sensitivity simulations are designed to augment, in various ways, the science that can be obtained from the reference simulations. To rigorously assess the effects of perturbations on a climate simulation, and to quantify internal model variability, it is necessary to have a control run with constant forcings. That is the rationale behind the time-slice experiment CTL0 under 1960 conditions. While REF0 has constant forcings, it is in a strongly perturbed time period, and the 20-year period of REF0 is not sufficient to fully define multi-decadal variability. SCN1 is a sensitivity simulation that is consistent with REF1 with the exception that an additional source of stratospheric inorganic bromine (Bry) from very short-lived substances (VSLS) is included, in light of the fact that observations derived from the breakdown of long-lived organic source gases underestimate the Bry abundance in the stratosphere by about 5 ppt. In SCN2a, the GHG scenario is changed from A1B (medium) to A2 (or a new IPCC scenario to be defined in mid-2008). An A2-like scenario has been suggested by the Working Group on Coupled Modelling (WGCM) as one of the benchmark concentration scenarios for the next round of coordinated Atmospheric-Ocean Global Circulation Model (AOGCM) and Earth System Model (ESM) simulations. Thus SCN2a will allow us to ‘map’ the CCMVal REF2 results onto the A2 scenario. SCN2b (fixed halogens) is designed to address the science question of what is the effect of halogens on stratospheric ozone (and climate) in a changing climate (by comparison with REF2). SCN2c (no greenhouse-gas induced climate change) is designed to address the nonlinearity of ozone depletion/recovery and climate change (by comparison with REF2 and SCN2b). SCN2d is designed to address the impact of ‘realistic’ natural variability on the REF2 simulations, for which the natural variability is underestimated.
1. CCMVal Reference Simulations
This section gives an overview of the main characteristics of the new CCMVal reference (REF) simulations. The key characteristics are also summarised in Table 1.
1.1 Time-Slice Experiment (REF0)
REF0 is a time-slice experiment for conditions equivalent to the year 2000, proposed to facilitate the comparison of model output against constituent datasets from various high-quality observational data sources and meteorological analyses under a period of high chlorine loading and peak ozone losses. Each simulation is integrated over 20 annual cycles following adequate spin-up (10 years is recommended). The model data of these 20 years are evaluated against contemporary observations (i.e., during the satellite measurement period of UARS, Aura, ENVISAT, Odin, SAGE, SBUV, TOMS, ACE, etc.) and compared to results of other CCMs. The 20 years of output are necessary in order to compare mean quantities with large variability (e.g., polar temperatures). It should be possible to start the analysis of runs based on REF0 much earlier than the other scenarios and to collect extended output, which will be useful for developing the diagnostics as well as providing a preliminary evaluation.
1.2 Reproduce the past: Reference Simulation 1 (REF1), Core Time Period 1960 to 2006
REF1 (1960-2006) is defined as a transient run from 1960 to the present and is designed to reproduce the well-observed period of the past 35 years during which ozone depletion is well recorded. It allows a more detailed investigation of the role of natural variability and other atmospheric changes important for ozone balance and trends. All forcings in this simulation are taken from observations.The set-up and forcings are very similar to the REF1 simulations that were evaluated in Eyring et al. . A re-assessment of temperatures, trace species and ozone in the CCM simulations will allow documenting progress of individual models and overall progress on the representation of key processes compared to the last CCM assessment. The comparison of CCM results with observations will also allow some groups to identify and correct previously unrecognised model errors and will help to indicate a range of model uncertainties. This transient simulation includes all anthropogenic and natural forcings based on changes in trace gases, solar variability, volcanic eruptions, quasi-biennial oscillation (QBO), and SSTs/SICs. REF1 covers the time period from at least 1960 to 2006 (with around 10 years spin-up prior to 1960) to examine model variability and to replicate as closely as possible the atmospheric state in this period.
Greenhouse Gases (N2O, CH4, and CO2) between 1950 and 1996 are taken from IPCC  and merged with the NOAA observations forward through 2006. NOAA CO2, CH4, and N2O were scaled to agree on January 1996 with the historical IPCC data.Surface mixing ratios of Ozone Depleting Substances (CFC-11, CFC-12, CFC-113, CFC-114, CFC-115, CCl4, CH3CCl3, HCFC-22, HCFC-141b, HCFC-142b, Halon1211, Halon1202, Halon1301, and Halon2402) in REF1 are taken from Table 8-5 of WMO . The mixing ratios are calculated by a box model using yearly emissions and are given for the middle of the month. The time series does not contain a yearly variation in mixing ratios. Through 2004 the values are maximally forced to equal global estimates calculated from observations (for details see Chapter 8 of WMO ). For models that do not wish to represent all the brominated and chlorinated species in Table 8-5 of WMO , the halogen content of species that are considered should be adjusted such that model inputs for total chlorine and total bromine match the time series of total chlorine and bromine given in this table. Sea surface temperatures and sea ice concentrations in REF1 are prescribed as monthly mean boundary conditions following the global sea ice concentration and sea surface temperature (HadISST1) dataset provided by the UK Met Office Hadley Centre [Rayner et al., 2003]. This dataset is based on combined satellite and in situ observations. To prepare the data for use in forcing a model, and in particular to correct for the loss of variance due to time-interpolation of monthly mean data, it is recommended that each group follows the procedures described on the C20C project website (see http://grads.iges.org/c20c/c20c_forcing/karling_instruct.html). This describes how to apply the AMIP II variance correction method (see http://www-pcmdi.llnl.gov/projects/amip/AMIP2EXPDSN/BCS/amip2bcs.php for details) to the HadISST1 data.
1.3 Making predictions: Reference simulation 2 (REF2), Core time period 1960 to 2100
REF2 is an internally consistent simulation from the past into the future. The objective of REF2 is to produce best estimates of the future ozone-climate change up to 2100 under specific assumptions about GHG increases (Scenario SRES A1B) and decreases in halogen emissions (adjusted Scenario A1) in this period. REF2 only includes anthropogenic forcings. External natural forcings such as solar variability and volcanic eruptions are not considered, as they cannot be known in advance, and the QBO is not externally forced (also as it cannot be known in advance; furthermore it represents the internal dynamics of the model). To avoid introducing inhomogeneity into the time series, these natural forcings are not applied in the past either.
2. CCMVal Sensitivity and Control Simulations
The following CCMVal sensitivity and control experiments are proposed:
SCN1 (1960-2006, REF1 with additional organic bromine):Observations suggest that stratospheric inorganic bromine (Bry) derived from the breakdown of long-lived (>3 years) organic source gases (i.e., CH3Br, halon-1211, halon-1301, and halon-2402) underestimate the Bry abundance in the stratosphere by about 5 ppt, with estimates ranging from 3 to 8 pptv. Observations also suggest that very short-lived substances (VSLS) with atmospheric lifetimes of less than 0.5 years make up the missing stratospheric Bry [Chapter 2 of WMO 2003, 2007]. The supply of bromine from VSLS can result in a substantial fractional increase to the amount of bromine in the lowermost stratosphere, with important consequences for ozone trends and the photochemical budget of ozone, particularly during times of high aerosol loading. SCN1 was developed to quantify the effect of bromine on ozone from VSLS. This scenario is consistent with REF1 with the exception that an additional source of 5 pptv of Bry from VSLS is included. In SCN1, we are proposing to add the species dibromomethane (CH2Br2) to the chemical mechanism of participating CCMs. The lifetime of CH2Br2 is approximately 120 days at 5 km [Table 2.3, WMO 2007] and the reaction with OH is the dominant loss process [Table 2.4, WMO 2003]. The estimated fraction of CH2Br2 mixing ratio in the tropical upper troposphere relative to the abundance in the marine boundary layer is approximately 0.8 [Table 2.2, WMO 2007]. Therefore, if the surface abundance of CH2Br2 is set to 3 pptv, the stratospheric Bry abundance should increase by approximately 5 pptv (i.e., 5 pptv total Bry / 2 Br per CH2Br2 molecule / 0.8 is equal to ~3.0 pptv CH2Br2). If modelling groups prefer not to add a new species to their CCM, we propose adding 5 pptv of total bromine to the shortest-lived organic bromine source gas currently included in the chemical mechanism.
SCN2a(2000-2100, REF2 with GHG scenario different than SRES A1B) is a transient simulation similar to REF2, but with the GHG and ozone precursor scenario changed from SRES A1B (medium) to A2 (or a new IPCC scenario to be defined in mid-2008). Accordingly, if the model does not include an interactive ocean, SSTs and SICs are prescribed from an AOGCM simulation that is consistent with the GHGs scenario. SCN2a is designed to be consistent with one of the new coordinated Climate Change Stabilization Experiments proposed for AOGCMs and ESMs [Meehl et al., 2007]. Ideally AOGCMs and ESMs will include their own atmospheric chemistry schemes, but many models do not have this option. For this category of models ozone fields have to be prescribed in the simulations. There are therefore two motivations for this run. One is to assess the future evolution of the ozone-climate change under a different GHG scenario than the A1B scenario used in REF2, and the second is to compute a best estimate of ozone fields consistent with the GHG scenario for community use in IPCC AR5 models. Ozone precursors in SCN2a are similar to REF1 and REF2 until 2000, and use the adjusted IIASA A2 scenario through 2100 [M. Amann and P. Rafai, pers. communication, 2007] or a new IPCC scenario to be defined in mid-2008.
SCN2b (1960-2100, REF2 with halogens fixed at 1960 levels) is a transient simulation similar to REF2, but withhalogens fixed at levels corresponding to 1960 throughout the simulation, whereas GHGs and SSTs/SICs are the same as in REF2. It is designed to address the science question of what are the effects of halogens on stratospheric ozone and climate, in the presence of climate change. By comparing SCN2b with REF2, the impact of halogens can be identified and it can be assessed at what point in the future the halogen impact is undetectable, i.e. within climate variability. This was the definition of full recovery of stratospheric ozone from the effects of ODSs that was advanced in WMO .
SCN2c (1960-2100, REF2 with GHGs fixed at 1960 levels) is a transient simulation similar to REF2, but withGHGs fixed at levels corresponding to 1960 throughout the simulation, whereas the adjusted scenario A1 halogens are the same as in REF2. It is designed to address the science question of how nonlinear are the atmospheric responses to ozone depletion/recovery and climate change. To that end, GHGs are fixed at 1960 levels throughout the simulation. SSTs/SICs will be a 1955-1964 average of the values used in REF2. By comparing the sum of SCN2b and SCN2c (each relative to the 1960 baseline) with REF2, the nonlinearity of the responses can be assessed. SCN2c also addresses the policy-relevant (if academic) question of what would be the impact of halogens on the atmosphere in the absence of climate change.
SCN2d(1960-2100, REF2 with natural forcings and QBO) is designed to address the impact of ‘realistic’ natural variability on the REF2 simulations, for which the natural variability is underestimated. This sensitivity simulation is defined similar to REF1, with the inclusion of solar variability, volcanic activity, and the QBO in the past. Future forcings include a repeating solar cycle and QBO, under volcanically clean aerosol conditions. SSTs/SICs are simulated or prescribed as in REF2. GHGs and halogens will be the same as in REF2. We recommend using a repeating solar cycle based on the observed daily spectra described in Lean et al. . It is proposed to repeat the solar cycles 20 to 23 (1962-2004) and therefore neglect the extreme solar cycle 19 (peaking in 1957/58).
CTL0 (mininum 20 years, REF0 but for 1960 conditions) is a time-slice simulation under 1960 conditions designed to establish a baseline control simulation for the reference and sensitivity simulations. The objective is to provide a statistical characterisation of the internal variability of the CCMs prior to major perturbations of the ozone layer. The control 1960 simulation has ODSs, GHGs, and solar irradiance held fixed. SSTs/SICs in this simulation are (analogous to REF0) prescribed from observations by using a climatological mean derived from years 1955 to 1964 of the HadISST1 dataset, repeating every year. Given these design constraints, the only source of variability is the internal dynamics of the atmosphere (and land properties like snow cover and soil moisture), while natural variability arising from solar variability and volcanic eruptions is excluded. Moreover, there are no secular changes in greenhouse gases and halogens, hence no long-term trends, which will allow a statistical characterisation of random short-term trends. This is important for assessing the statistical significance of trends in the reference and sensitivity simulations. After a spin-up period of about 10 years, each simulation is integrated over at least 20 annual cycles for analysis. However, the goal of a 46-year control simulation is strongly encouraged, 46 years being the length of the REF1 simulation. Some of the reference and sensitivity simulations could branch off from CTL0, thereby reducing their respective spin-up periods to a few years.
3. Summary and Outlook
CCM groups are encouraged to run the proposed reference simulations with the specified forcings. In order to facilitate the set-up of the reference simulations, CCMVal has established a website where the forcings for the simulations can be downloaded (http://www.pa.op.dlr.de/CCMVal/Forcings/CCMVal_Forcings.html). This web site was developed to serve the needs of the CCM community, and encourage consistency of anthropogenic and natural forcings in future model/model and model/observation inter-comparisons. Any updates as well as detailed explanation and further discussion will be placed on this website. In addition to the reference runs, the groups are encouraged to run as many sensitivity simulations as possible. The hope is that these additional runs will be available in time to provide useful input for the anticipated UNEP/WMO Ozone Assessment in 2010, so that the ozone projections from the CCMs can be assessed for different halogen and GHG scenarios, and not just from one scenario as in WMO [2003, 2007].
The proposed simulations will be evaluated as part of the planned SPARC CCMVal Report by 2009 in time for consideration in the anticipated UNEP/WMO Ozone Assessment in 2010. The SPARC CCMVal report itself has two major aims: 1) provide valuable base material for that assessment, and 2) improve the understanding of the strengths and weaknesses of CCMs and thus increase their integrity and credibility. Regarding mechanisms for model evaluation, a set of standard diagnostics has been agreed at the first CCMVal workshop (Grainau, Germany, November 2003) and further refined at the second workshop (NCAR, Boulder, USA, October 2005). Output for these standard diagnostics [Eyring et al., 2005] and possible additional diagnostics needed for the individual chapters of the SPARC CCMVal report will be collected in Climate and Forecast (CF) standard compliant netCDF format from all models in the central database at the British Atmospheric Data Centre (BADC). In addition it is anticipated to obtain observational datasets for the core diagnostics. The specified forcings for the new reference simulations and the new data request will be made available for download at the CCMVal website. The proposed timeline for the SPARC CCMVal report can be found at http://www.pa.op.dlr.de/CCMVal/SPARC_CCMValReport/SPARC_CCMValReport_Timeline.html.
We wish to thank the CCMVal community for a lively and fruitful discussion and for the excellent cooperation. Thanks go to Georgiy Stenchikov for providing the pre-calculated aerosol heating rates, Neal Butchart and the UK Met Office for providing observed sea surface temperatures and sea ice concentrations, Larry Thomason and Simone Tilmes for providing observed sulfate aerosol surface area densities, as well as to Tom Conway, Ed Dlugokencky, Jim Elkins, Geoffrey Dutton and Stephen Montzka for supplying CO2, CH4 and N2O mixing ratios from the NOAA Cooperative Global Air Sampling Network. We also thank Guus Velders and John Daniel for providing surface mixing ratios of ozone depleting substances, Ross Salawitch for helpful discussions regarding bromine, Judith Lean for the specification of the solar cycle, as well as Markus Amann, Andreas Baumgärtner, Christoph Brühl, Peter Rafai, Martin Schultz, David Stevenson, and Michiel van Weele for their help with ozone and aerosol precursor datasets.
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