Hans-F Graf significantly contributed to progress in three fields: Volcanic impact on climate, global scale atmospheric dynamics, and clouds and aerosols. His most influential work is in volcanic effects on climate and atmospheric composition. Early analysis and simulations predicted “continental winter warming” as a consequence of big tropical volcanic eruptions that was later confirmed by many other authors. Investigations of processes within volcanic plumes were performed with ATHAM, the very high resolving model developed in
HFG's group at MPI for Meteorology, also covering biomass burning. The papers on the effect of silent and weakly eruptive volcanic emissions had a strong impact on volcanologists and atmospheric chemists alike. In 1990 he started working with Judith Perlwitz on the dynamical coupling of troposphere and stratosphere and its relevance for the North Atlantic Oscillation. He also very early investigated, using coupled climate models, the individual and combined effects of El Niño and volcanic eruptions. Currently he is working on different effects of two types of El Nino on mainly European weather and climate and the underlyiing mechanisms. The current end-product of his research on improved cloud modelling capacities is the Convective Cloud Field Model, a new convection parametrization based on self-organization principles.