Abstract
Abstract. This study evaluates effects and applications of a new linear parameterisation for stratospheric methane and water vapour. The new scheme (CoMeCAT) is derived from a 3-D full-chemistry-transport model (CTM). It is suitable for any global model, and is shown here to produce realistic profiles in the TOMCAT/SLIMCAT 3-D CTM and the ECMWF (European Centre for Medium-Range Weather Forecasts) general circulation model (GCM). Results from the new scheme are in good agreement with the full-chemistry CTM CH4 field and with observations from the Halogen Occultation Experiment (HALOE). The scheme is also used to derive stratospheric water increments, which in the CTM produce vertical and latitudinal H2O variations in fair agreement with satellite observations. Stratospheric H2O distributions in the ECMWF GCM show realistic overall features, although concentrations are smaller than in the CTM run (up to 0.5 ppmv smaller above 10 hPa). The potential of the new CoMeCAT tracer for evaluating stratospheric transport is exploited to assess the impacts of nudging the free-running GCM to ERA-40 and ERA-Interim reanalyses. The nudged GCM shows similar transport patterns to the offline CTM forced by the corresponding reanalysis data. The new scheme also impacts radiation and temperature in the model. Compared to the default CH4 climatology and H2O used by the ECMWF radiation scheme, the main effect on ECMWF temperatures when considering both CH4 and H2O from CoMeCAT is a decrease of up to 1.0 K over the tropical mid/low stratosphere. The effect of using the CoMeCAT scheme for radiative forcing (RF) calculations is investigated using the offline Edwards–Slingo radiative transfer model. Compared to the default model option of a tropospheric global 3-D CH4 value, the CoMeCAT distribution produces an overall change in the annual mean net RF of up to −30 mW m−2.
Highlights
The stratosphere is being increasingly acknowledged as one of the keys to adding more skill to numerical models in a wide range of timescales and applications, from weather forecasts to climate studies, as well as a potential source of seasonal meteorological predictability (Solomon et al, 2010; Maycock et al, 2011; Scaife et al, 2012)
In past versions of the Integrated Forecast System (IFS) general circulation model (GCM), a global CH4 value of 1.72 ppmv was used by the European Centre for Medium-Range Weather Forecasts (ECMWF) radiation scheme (Bechtold et al, 2009); such a value is typical of tropospheric levels, and was shown by Monge-Sanz (2008) to cause temperature biases in the upper stratosphere compared to the use of the CoMeCAT tracer coupled to the ECMWF radiation scheme
The largest differences with observations are found at high latitudes, especially in the Southern Hemisphere (SH), where CoMeCAT runs with ERA-40 underestimate CH4
Summary
The stratosphere is being increasingly acknowledged as one of the keys to adding more skill to numerical models in a wide range of timescales and applications, from weather forecasts to climate studies, as well as a potential source of seasonal meteorological predictability (Solomon et al, 2010; Maycock et al, 2011; Scaife et al, 2012). In the past more attention was paid to the description of stratospheric O3 in models; for example, the European Centre for Medium-Range Weather Forecasts (ECMWF) first included their current O3 parameterisation (Cariolle and Déqué, 1986; Cariolle and Teyssèdre, 2007) to improve the use of satellite radiance data by providing the radiance observation operators with accurate 3-D ozone fields instead of a climatology (Dethof and Hólm, 2004)
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