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

Abstract

Abstract. A new linear parameterisation for stratospheric methane (CoMeCAT) has been developed and tested. The scheme is derived from a 3-D full chemistry transport model (CTM) and tested within the same chemistry model itself, as well as in an independent general circulation model (GCM). The new CH4/H2O scheme is suitable for any global model and here is shown to provide realistic profiles in the 3-D TOMCAT/SLIMCAT CTM and in the ECMWF (European Centre for Medium-Range Weather Forecasts) GCM. Simulation results from the new stratospheric scheme are in good agreement with the full-chemistry CTM CH4 field and with observations from the Halogen Occultation Experiment (HALOE). The CH4 scheme has also been used to derive a source for stratospheric water. Stratospheric water increments obtained in this way within the CTM produce vertical and latitudinal H2O variation in fair agreement with satellite observations. Stratospheric H2O distributions in the ECMWF GCM present realistic overall features although concentrations are lower than in the CTM run (up to 0.5 ppmv lower above 10 hPa). The potential of the new CoMeCAT scheme for evaluating long-term transport within the ECMWF model is exploited to assess the impacts of nudging the free running GCM to ERA-40 and ERA-Interim reanalyses. In this case, the nudged GCM shows similar transport patterns to the CTM forced by the corresponding reanalysis data, ERA-Interim producing better results than ERA-40. The impact that the new methane description has in the GCM radiation scheme is also explored. Compared to the default CH4 climatology used by the ECMWF model, CoMeCAT produces up to 2 K cooling in the tropical lower stratosphere. The effect of using the CoMeCAT scheme for radiative forcing (RF) calculations has been investigated using the off-line Edwards-Slingo (E-S) radiative transfer model. Compared to the use of a tropospheric global 3-D CH4 value, the CoMeCAT distributions produce an overall decrease in the annual mean net RF, with the largest decrease found over the Southern Hemisphere high latitudes. The effect of the new CH4 stratospheric distribution on these RF calculations is of up to 30 mW m−2, i.e. the same order of magnitude, and opposite sign, as the inclusion of aircraft contrails formation in the radiative model.