A Love Letter to the Twentieth Century Reanalysis

Ben Hatchett

Cooperative Institute for Research in the Atmosphere at CSU / NOAA Global Systems Laboratory

Tuesday, May 05, 2026, 2:00 pm MT

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Abstract

Reanalysis products enable us to understand circulations preceding and accompanying environmental outcomes over decadal to centennial timescales. Given limitations in computational resources, tradeoffs are necessary to balance the spatial and temporal resolution, the quantification of uncertainty through multi-member ensembles, and the length of record of reanalysis datasets. This poses a limitation when working to understand hydroclimatic processes that respond over multi-decadal timescales and when trying to develop understandings of the characteristics of extreme events. Here, I share two examples of how the Twentieth Century Reanalysis (20CR) enabled understanding of the drives of two components influencing western U.S. water resources: 1) pluvials and megadroughts, and 2) abrupt snowmelt events. For the pluvial and megadrought example, we used a semi-distributed hydrologic model to develop an understanding of how terminal lake levels in the Walker Lake Basin of California-Nevada responded more strongly to moisture transport than to storm-track variability. We also identified an asymmetric lake level response to wet years versus dry years implying a simple constraint on the frequency of wet years amidst megadrought. This indicates recent multiyear droughts are similar to centennial-scale Medieval droughts in terms of precipitation anomalies. For the snowmelt example, we share a methodology to identify extreme spring snowmelt events, termed 'snow-eater heatwaves,' by leveraging 20CR’s 80-member ensemble to calibrate a statistical melt-potential model. This allowed us to characterize the frequency, duration, and extent of these events from 1851-2015. We found that snowmelt rates double during 3–5 day-long snow-eater heatwaves and are linked to seven of the 11 western U.S. superfloods. Both projects highlight the value of long-record, best-estimates of atmospheric conditions during extreme hydrometeorologic and hydroclimatic events to inform process-based understanding and guide adaptive strategies for water resource management.

Ben Hatchett is a fire meteorologist on the User Needs Assessment Team for the NOAA Fire Weather Testbed based at the Global Systems Laboratory in Boulder, CO. He received a B.S. in Geography (minor in Hydrogeology) in 2008, a M.S. in Atmospheric Science in 2012, and a Ph.D. in Geography in 2016 from the University of Nevada, Reno. Hatchett works remotely out of California.