On the uses of a new linear scheme for stratospheric methane in global models: water source, transport tracer and radiative forcing

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.