Director of the Max Planck Institute for Meteorology (Hamburg, Germany), where he heads the Atmosphere in the Earth System Department. He holds a professorship at the University of Hamburg and was formerly Professor of Atmospheric Sciences at the University of California, Los Angeles (UCLA).
His research has made fundamental contributions to understanding of atmospheric convection, aerosols and clouds and their influence on the climate system. His basic research on clouds attempts to answer questions such as what determines clouds’ aggregate behavior, and how do they help set large-scale circulations and mediate interactions among other components of the Earth System.
He holds the Clarence Leroy Meisinger Award of the American Meteorological Society and from 2009-2013 served as a lead author of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).