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Global warming precipitation accumulation increases above the current-climate cutoff scale

  1. Diana N. Bernsteina,3
  1. aDepartment of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095-1565;
  2. bDepartment of Mathematics, University of Wisconsin–Madison, Madison, WI 53706;
  3. cDepartment of Atmospheric and Oceanic Sciences, University of Wisconsin–Madison, Madison, WI 53706

  1. Edited by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, and approved November 28, 2016 (received for review September 14, 2016)

Significance

Large accumulations of rainfall over a precipitation event can impact human infrastructure. Unlike precipitation intensity distributions, probability distributions for accumulations at first drop slowly with increasing size. At a certain size—the cutoff scale—the behavior regime changes, and the probabilities drop rapidly. In current climate, every region is protected from excessively large accumulations by this cutoff scale, and human activities are adapted to this. An analysis of how accumulations will change under global warming gives a natural physical interpretation for the atmospheric processes producing this cutoff, but, more importantly, yields a prediction that this cutoff scale will extend in a warmer climate, leading to vastly disproportionate increases in the probabilities of the very largest events.

Abstract

Precipitation accumulations, integrated over rainfall events, can be affected by both intensity and duration of the storm event. Thus, although precipitation intensity is widely projected to increase under global warming, a clear framework for predicting accumulation changes has been lacking, despite the importance of accumulations for societal impacts. Theory for changes in the probability density function (pdf) of precipitation accumulations is presented with an evaluation of these changes in global climate model simulations. We show that a simple set of conditions implies roughly exponential increases in the frequency of the very largest accumulations above a physical cutoff scale, increasing with event size. The pdf exhibits an approximately power-law range where probability density drops slowly with each order of magnitude size increase, up to a cutoff at large accumulations that limits the largest events experienced in current climate. The theory predicts that the cutoff scale, controlled by the interplay of moisture convergence variance and precipitation loss, tends to increase under global warming. Thus, precisely the large accumulations above the cutoff that are currently rare will exhibit increases in the warmer climate as this cutoff is extended. This indeed occurs in the full climate model, with a 3 °C end-of-century global-average warming yielding regional increases of hundreds of percent to >1,000% in the probability density of the largest accumulations that have historical precedents. The probabilities of unprecedented accumulations are also consistent with the extension of the cutoff.

Footnotes

  • ↵1To whom correspondence should be addressed. Email: neelin{at}atmos.ucla.edu.

  • ↵2Present address: Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, New Delhi 110016, India.

  • ↵3Present address: Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853-1901.

  • Author contributions: J.D.N. designed research; J.D.N., S.S., S.N.S., and D.N.B. performed research; S.S. carried out climate model analysis; S.N.S. collaborated on theory; D.N.B. ran the Community Earth System Model simulations; J.D.N., S.S., and D.N.B. analyzed data; and J.D.N. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1615333114/-/DCSupplemental.

Freely available online through the PNAS open access option.

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