Global Atmospheric Chemistry Modeling with EC-Earth: Understanding past and predicting future tropospheric ozone in a changing climate

Principal Investigator

Dr. T.C.P. van Noije
Royal Netherlands Meteorological Institute (KNMI)
PO Box 2013730 AE
De Bilt
The Netherlands

noije@knmi.nl

Other researchers: Dr. M. van Weele, Dr. J.E. Williams, vacancy

Project description

Ozone is a key component of the atmosphere and is in many ways of great importance to ecosystems and mankind. In the troposphere ozone is considered to be both an air pollutant and an important driver of climate change. Exposure to elevated levels of ground level ozone may be harmful to human health and cause damage to crops and natural vegetation. After carbon dioxide and methane, tropospheric ozone is the third most important contributor to anthropogenic global warming. Recent model simulations have indicated that the direct forcing by tropospheric ozone has contributed substantially to global warming in the Northern Hemisphere, especially at high latitudes during winter and spring and over polluted areas during summer. Moreover, the coupling between surface ozone and vegetation provides an additional mechanism by which ozone increases contribute to global warming, where increasing ambient ozone concentrations tend to reduce plant productivity and suppress the rate at which carbon dioxide is removed from the atmosphere. It has recently been suggested that the resulting indirect radiative forcing could contribute more to global warming than the direct radiative forcing of tropospheric ozone.

Given the rapid increase of anthropogenic emissions of ozone precursor gases in China, India and other developing economies, background tropospheric ozone levels have increased significantly over recent years and will most likely continue to do so in the coming decades. Recent studies indicate that ozone increases in the lower stratosphere, driven by changes in the stratospheric circulation, have also contributed to the increases in tropospheric ozone that have been observed at northern midlatitudes and in the Arctic since the mid-1990s.

One of the aims of this special project is to understand the upward trends in tropospheric ozone that have been observed at northern midlatitudes and in the Arctic since the mid-1990s by analyzing the different factors that have contributed to these trends, including the impact of changes in emissions, tropospheric transport patterns and the stratospheric circulation. Once we have attributed these recent changes in ozone, we will explore the range of changes that can be expected in the first half of the 21 st century because of projected changes in emissions and climate.

To simulate the observed past and expected future changes in tropospheric ozone we will use the new chemistry-climate model EC-Earth, which is based on the ECMWF seasonal forecast model. The first version of EC-Earth (www.ecearth.knmi.nl) consists of the ECMWF IFS (Integrated Forecasting System) atmosphere model, the OPA ocean and sea-ice model, the TM5 atmospheric chemistry transport model (Krol et al., 2005), the TESSEL land component, and the OASIS coupler. The model is currently being further developed into an Earth System Model by a consortium of member states of ECMWF, lead by KNMI. Different than current climate models, the model contains parameterizations developed for weather forecast models. Also, it has data-assimilation capacities which makes it ideally suited for decadal prediction experiments. The current EC-Earth system offers excellent opportunities for studying the impact of dynamical variability and climate change on atmospheric chemistry.

The special project will improve our understanding of past and future changes in tropospheric ozone, and is therefore relevant for both global and regional air quality as well as for climate change. The project is also essential for the further development of EC-Earth and thus for the development of the next generation climate scenarios. The research planned in relation to this project in particular focuses on improving the atmospheric chemistry in EC-Earth, which is relevant both for the EU MACC project, led by ECMWF, as well as for the further development of the ECMWF seasonal forecasting model. Furthermore, the project will provide diagnostic information on various aspects of the ERA-Interim reanalysis.

Reanalysis simulations (1995-2004)

For reconstructing the observed changes in tropospheric ozone we propose to use the atmosphere-only version of EC-Earth and constrain the dynamics with the new ECMWF meteorological reanalysis ERA-Interim, by application of a relaxation scheme for winds, temperature and surface pressure. Because EC-Earth has the same dynamical core as the assimilation system used in the reanalysis, this combination has a unique advantage over chemistry-transport models driven by offline meteorological fields as well as over other chemistry-climate models.

The use of the ERA-Interim reanalysis also constitutes a significant improvement over earlier chemical reanalysis simulations, such as those performed within the EU project RETRO ( www.retro.enes.org ), in which we had active participation. Tests to date indicate that many of the problems experienced in ERA-40 (e.g. Uppala et al., 2005; Noije et al., 2004, 2006) have been eliminated or reduced in the new reanalysis (Simmons et al., 2007). Of particular importance for the work proposed here is that the stratospheric circulation in ERA-Interim is much improved, even compared to ECMWF operational analyses (Monge-Sanz et al., 2007; Kinnison et al., 2007). ERA-Interim thus provides the unique opportunity to consistently analyze the different factors that have contributed to the ozone changes since the 1990s.

The reanalysis simulation will focus on the 10-year period 1995-2004, well after the eruption of Mt. Pinatubo in 1991. The year 1994 will be used for spin-up.

Attribution of past changes

The processes contributing to the simulated changes in tropospheric ozone will be analysed in a series of sensitivity simulations. These simulations will be setup according to the baseline reanalysis simulation, but differ in one of the following aspects: (1) To study the impact of dynamical variability and changes in lower stratospheric ozone, the dynamical fields will be relaxed towards the ERA-Interim winds, temperature and surface pressure fields for the year 1995; (2) To study the impact of global changes in anthropogenic emissions, the anthropogenic emissions of precursors of ozone and aerosol will be fixed to their 1995 values; (3) To study the impact of the rapid increase in anthropogenic emissions in Asia, all non-Asian anthropogenic emissions of ozone precursors and aerosol will be fixed to their 1995 values.

Impacts of climate change on future ozone

The use of EC-Earth will also allow us to quantify how projected changes in climate will affect ozone in the future. In many respects this is still an open question, which involves a variety of complex feedback mechanisms. So far two dominant mechanisms have been identified in the literature (e.g. Stevenson et al., 2006). First, increasing levels of water vapour will lead to higher ozone destruction rates, especially in the tropical lower troposphere. Second, most climate model simulations show an enhancement of the stratospheric overturning circulation and, consequently, an enhanced flux of stratospheric ozone into the troposphere.

To study these effects we will use the atmosphere-only version of EC-Earth constrained only by the sea surface temperatures (SSTs) and by the sea ice (SI) extent. The baseline will be a simulation with SSTs, SI, and emissions for the year 2000. For estimating how climate change and changes in emissions will affect the future evolution of ozone in the troposphere and lower stratosphere, another simulation will be performed with SSTs, SI and projected emissions for a year in the future, e.g. 2030 or 2050. Finally, to separate the two factors, in a third simulation emissions will be assumed to change, while SSTs and SI will be retained at their 2000 levels. In each simulation the model will be run for a period of 11 years, of which the first year is used for spin-up. With these simulations we intend to contribute to Atmospheric Chemistry and Climate (AC&C), a joint initiative of IGBP-IGAC and WCRP-SPARC (see http://www.igac.noaa.gov/ACandC.php ).

Timetable

2009: Perform reanalysis simulation and sensitivity runs

2010: Perform first set of future climate simulations, described above

2011: Perform coupled chemistry-climate simulations with EC-Earth, including the feedback of the simulated concentrations of greenhouse gases and aerosols to the cloud and radiation scheme

Explanation of the requested budget

A 10-year run of EC-Earth without atmospheric chemistry costs about 15.000 units. A similar amount is needed for the atmospheric chemistry module at the present default resolution. Thus the requested budget corresponds to at most 100 years of online atmospheric chemistry simulations in each of the years 2009, 2010 and 2011. This should allow us to perform the proposed simulations together with a reasonable amount of runs required for code development and testing.

References

Krol, M., S. Houweling, B. Bregman, M. van den Broek, A. Segers, P. van Velthoven, W. Peters, F. Dentener, and P. Bergamaschi (2005), The two-way nested global chemistry-transport zoom model TM5: algorithm and applications, Atmos. Chem. Phys., 5, 417-432.

Monge-Sanz, B.M., M.P. Chipperfield, A.J. Simmons, and S.M. Uppala (2007), Mean age of air and transport in a CTM: Comparison of different ECMWF analyses, Geophys. Res. Lett., 34, L04801, doi:10.1029/2006GL028515.

Kinnison, D.E., et al. (2007), Sensitivity of chemical tracers to meteorological parameters in the MOZART-3 chemical transport model, J. Geophys. Res., 112, D20302, doi:10.1029/2006JD007879.

Simmons, A., S. Uppala, D. Dee, and S. Kobayashi (2007), ERA-Interim: New ECMWF reanalysis products from 1989 onwards, ECMWF Newsletter No. 110 – Winter 2006/2007, 25-35.

Stevenson, D.S., F.J. Dentener, M.G. Schultz, K. Ellingsen, T.P.C. van Noije, et al. (2006), Multimodel ensemble simulations of present-day and near-future tropospheric ozone, J. Geophys. Res., 111, D08301, doi:10.1029/2005JD006338.

van Noije, T.P.C., H.J. Eskes, M. van Weele and P.F.J. van Velthoven (2004), Implications of the enhanced Brewer-Dobson circulation in European Centre for Medium-Range Weather Forecasts reanalysis ERA-40 for the stratosphere-troposphere exchange of ozone in global chemistry transport models, J. Geophys. Res., 109, D19308, doi:10.1029/2004JD004586.

van Noije, T.P.C., A.J. Segers and P.F.J. van Velthoven (2006), Time series of the stratosphere-troposphere exchange of ozone simulated with reanalyzed and operational forecast data, J. Geophys. Res., 111, D03301, doi:10.1029/2005JD006081.

Uppala, S.M., et al. (2005), The ERA-40 re-analysis, Q. J. R. Meteorol. Soc., 131, 2961-3012.

Additional information

New Project for 2009

Would not accept support for 1 year only.