21st Century Changes in U.S. Regional Heavy Precipitation Frequency Based on Resolved Atmospheric Patterns

One of the struggles is, coarse climate models produce a wide range of outcomes. [Rainfall] can increase or decrease. What our method tells you is, for California, we’re very confident that [heavy precipitation] will increase by the end of the century.

Adam Schlosser, study co-author and senior research scientist in MIT’s Joint Program on the Science and Policy of Global Change

• Combines precipitation-gauge observations and atmospheric reanalysis to develop an analogue method for detecting heavy precipitation events based on prevailing large-scale atmospheric conditions
• Examines combinations of atmospheric variables for circulation (geopotential height and wind vector) and moisture (surface specific humidity, column and up to 500hPa precipitable water) to construct analogue schemes for the winter (December-January-February, DJF) of the Pacific Coast California (PCCA) and the summer (June-July-August, JJA) of the Midwestern United States (MWST)
• Calibrates the detection diagnostics of analogue schemes with 1979-2005 and validates these with 2006-2014 NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA)
• Finds that all analogue schemes significantly improve upon MERRA precipitation in characterizing the occurrence and interannual variations of observed heavy precipitation events in the MWST
• Finds that—when evaluated with the late 20th century climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5)—all analogue schemes produce model medians of heavy precipitation frequency that are more consistent with observations and have smaller inter-model discrepancies than model-based precipitation
• Finds that under Representative Concentration Pathways 4.5 and 8.5 scenarios, the CMIP5-based analogue schemes produce trends in heavy precipitation occurrence through the 21stc entury that are consistent with model-based precipitation – but with smaller inter-model disparity. The median trends in heavy precipitation frequency are positive for DJF over PCCA, but are slightly negative for JJA over the MWST
• Highlights the potential of the analogue as a powerful diagnostic tool for model deficiencies and its complementarity to an evaluation of heavy precipitation frequency based on model precipitation alone