Abstract
Abstract. A detailed full-chemistry 3-D chemistry and transport model (CTM) is used to evaluate the current stratospheric O3 parameterisation in the European Centre for Medium-Range Weather Forecasts (ECMWF) model and to obtain an alternative version of the ozone scheme implicitly including heterogeneous chemistry. The approach avoids the inaccurate treatment currently given to heterogeneous ozone chemistry in the ECMWF model, as well as the uncertainties of a cold-tracer. The new O3 scheme (COPCAT) is evaluated within the same CTM used to calculate it. It is the first time such a comparison has been possible, providing direct information on the validity of the linear parameterisation approach for stratospheric ozone. Simulated total column and O3 profiles are compared against Total Ozone Mapping Spectrometer (TOMS) and Halogen Occultation Experiment (HALOE) observations. COPCAT successfully simulates polar loss and reproduces a realistic Antarctic O3 hole. The new scheme is comparable to the full-chemistry in many regions for multiannual runs. The parameterisation produces less ozone over the tropics around 10 hPa, compared to full-chemistry and observations. However, this problem can be ameliorated by choosing a different ozone climatology for the scheme. The new scheme is compared to the current ECMWF scheme in the same CTM runs. The Antarctic O3 hole with the current ECMWF scheme is weaker and disappears earlier than with the new COPCAT scheme. Differences between the current ECMWF scheme and COPCAT are difficult to explain due to the different approach used for heterogeneous chemistry and differences in the photochemical models used to calculate the scheme coefficients. Results with the new COPCAT scheme presented here show that heterogeneous and homogeneous ozone chemistry can be included in a consistent way in a linear ozone parameterisation, without any additional tunable parameters, providing a parameterisation scheme in better agreement with the current knowledge of stratospheric O3 chemistry than previous approaches.
Highlights
Numerical weather prediction (NWP) models cannot yet afford to include detailed chemistry schemes, a realistic representation of radiatively active species is essential for the correct assimilation of satellite radiances into these models
The corresponding chemistry and transport model (CTM) runs were completely equivalent to those used to obtain the results shown in Figs. 5a and b, i.e. the CTM was driven by ERA-40 for year 2000 and the only difference is in the version of the coefficients used in the runs
compared coefficients derived under year meteorology (COPCAT) is in good overall agreement with SLIMCAT full-chemistry, a significant reduction in the tropical maximum O3 at 10 hPa occurs when using the parameterisation
Summary
Numerical weather prediction (NWP) models cannot yet afford to include detailed chemistry schemes, a realistic representation of radiatively active species is essential for the correct assimilation of satellite radiances into these models. To avoid the artificial split between gas-phase and heterogeneous processes, a better conceptual approach can be explored, which represents a good compromise between the realistic heterogeneous full-chemistry and a simple one-tracer parameterisation Such approach is the use of a scheme of the form described in Eq (1) but, instead of using the additional 5th term, using an alternative set of coefficients (c0, c1, c2 and c3) that implicitly includes heterogeneous ozone chemistry. In the new scheme longterm changes in stratospheric chlorine (or odd nitrogen, water vapour etc.) can be taken into account in a consistent way by deriving coefficients from the CTM for different periods This new COPCAT approach is a more natural way to include both gas-phase and heterogeneous processes, and is expected to improve the representation of polar ozone, but has the advantage of including mid-latitude heterogeneous processes that have been so far ignored by most O3 linear schemes.
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