Abstract: Rain is a critical part of the hydrologic cycle and a key process in understanding the evolution of climate. This talk will provide a background on why and how rain is a critical part of the climate system. The methods for representing rain formation at the scale of micrometer (10-6m) in models which represent the earth’s general circulation (at the scale of 10+6m) are approximate, and this is a critical problem in weather and climate. To approach this problem, we take detailed numerical approaches to the rain process typically used for small scale models, and put them in a global model. This results in simulations that have higher fidelity with respect to observations and reduce longstanding biases in the global model, but at prohibitive computational cost. We then train neural networks on the detailed treatments, and put those back into the full model. This reproduces all the major features of the detailed simulations from weather to climate, with no impact in computational cost. Diagnostics on the neural networks illustrate the challenges in representing these processes with machine learning such as the potential for overfitting and problems with extrapolation, as well as how physical processes important for climate challenge machine learning methods and provide opportunities for extending them.

Bio: Dr. Andrew Gettelman is a senior scientist at the National Center for Atmospheric Research, specializing in global climate modeling. His work concerns development and analysis of global model simulations of the climate system with a focus on development of physical parameterizations of clouds in global climate models, and the physics and chemistry of the Upper Troposphere and Lower Stratosphere. In addition to developing and managing community earth system models at NCAR, Dr. Gettelman is involved in model evaluation data analysis with satellites and field programs, having served on several satellite science teams and on the science teams for field projects. Dr. Gettelman has been a visiting professor in Physics in Oxford, an Erskine visiting fellow at the University of Canterbury in Christchurch, New Zealand, and a visiting professor in the Institute for Atmospheric and Climate Science at the Swiss Federal Institute of Technology (ETH) in Zürich, Switzerland. Dr. Gettelman is a member of the atmospheric model development team for the Community Earth System Model. Dr. Gettelman is a contributing author and reviewer for international scientific assessments of climate change and ozone depletion. He has served on the COMET advisory board, and the NCAR Earth Observing Laboratory advisory board. Dr. Gettelman is author or co-author on over 200 peer reviewed publications, and a textbook (Demystifying Climate Models, Springer). Dr. Gettelman has a PhD in Atmospheric Sciences from the University of Washington and a Bachelors of Science in Civil Engineering from Princeton University.