Simulation of Mt. Pinatubo volcanic aerosol with the Hamburg climate modelECHAM4

We study the three-dimensional transport of Mt. Pinatubo volcanic cloud wit h the climate model ECHAM4. In order to obtain model results comparable wit h observations a Newtonian relaxation technique was applied, which forces p rognostic model variables towards the observations. A comparison of the sim ulated aerosol distribution with satellite data reveals good agreement for the first months after the eruption. The model, however, is unable to simul ate the tropical aerosol maximum in 1992 and also overestimates the vertica l downward and northward transport of aerosols. Substantial improvement was achieved with the introduction of reduced advective vertical transport thr ough the 380 K isentropic layer. Heating rates and top of the atmosphere fl uxes, which were calculated online for the first half year after the erupti on, are in the observed range. A comparison of Pinatubo simulations between three different vertical ECHAM4 versions (ECHAM4 L19, ECHAM4 L39, MA/ECHAM 4) indicates that a vertical resolution of approximate to 700 m in the trop opause region is sufficient to realistically reduce the vertical transport through the tropopause. Consideration of the upper branch of the Brewer Dob son circulation in the MA/ECHAM4 model improves the geographical distributi on of the volcanic cloud. The application of a relaxation technique can fur ther reduce major shortcomings of stratospheric simulations with the standa rd climate model. There remain, however some critical points in the global transport characteristics in all three models which are not fully understoo d.