Atmosphere Ocean Science Colloquium

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Changes in the Distribution of Rain in Response to Warming

Speaker: Angie Pendergrass, University of Washington

Location: Warren Weaver Hall 1302

Date: Wednesday, March 5, 2014, 3:30 p.m.

Synopsis:

We develop a methodology to calculate the frequency and amount of rainfall as functions of the rain rate. We define two modes of response, one in which the distribution of rainfall increases in equal fraction at all rain rates and one in which the rainfall shifts to higher or lower rain rates without a change in mean rainfall. We apply these modes to simulations of global warming from CMIP5. The response to CO2 doubling in the multi-model mean of CMIP5 is characterized by an increase of 1 % K-1 and a shift to higher rain rates of 3.3 % K-1. In addition to these increase and shift modes of change, some models also show a substantial increase in rainfall at the highest rain rates, which we call the extreme mode of response to warming. In some models this extreme mode can be shown to be associated with increases in grid-scale condensation, or grid point storms. We also calculate the response of the tropical rainfall distribution to ENSO phases in models and observations. The CMIP5 multi-model mean agrees with observations in showing a very large shift of 14-15 % K-1, indicating large increases in the heaviest rain rates associated with El Nino.