Towards a more reliable historical reanalysis: Improvements for version 3 of the Twentieth Century Reanalysis system

Abstract

Historical reanalyses that span more than a century are needed for a wide range of studies, from understanding large‐scale climate trends to diagnosing the impacts of individual historical extreme weather events. The Twentieth Century Reanalysis (20CR) Project is an effort to fill this need. It is supported by the National Oceanic and Atmospheric Administration (NOAA), the Cooperative Institute for Research in Environmental Sciences (CIRES), and the U.S. Department of Energy (DOE), and is facilitated by collaboration with the international Atmospheric Circulation Reconstructions over the Earth initiative. 20CR is the first ensemble of sub‐daily global atmospheric conditions spanning over 100 years. This provides a best estimate of the weather at any given place and time as well as an estimate of its confidence and uncertainty. While extremely useful, version 2c of this dataset (20CRv2c) has several significant issues, including inaccurate estimates of confidence and a global sea level pressure bias in the mid‐19th century. These and other issues can reduce its effectiveness for studies at many spatial and temporal scales. Therefore, the 20CR system underwent a series of developments to generate a significant new version of the reanalysis. The version 3 system (NOAA‐CIRES‐DOE 20CRv3) uses upgraded data assimilation methods including an adaptive inflation algorithm; has a newer, higher‐resolution forecast model that specifies dry air mass; and assimilates a larger set of pressure observations. These changes have improved the ensemble‐based estimates of confidence, removed spin‐up effects in the precipitation fields, and diminished the sea‐level pressure bias. Other improvements include more accurate representations of storm intensity, smaller errors, and large‐scale reductions in model bias. The 20CRv3 system is comprehensively reviewed, focusing on the aspects that have ameliorated issues in 20CRv2c. Despite the many improvements, some challenges remain, including a systematic bias in tropical precipitation and time‐varying biases in southern high‐latitude pressure fields.

Hourly weather observations from the Scottish Highlands (1883–1904) rescued by volunteer citizen scientists

“…Assimilating these newly rescued pressures into long centennial reanalyses (e.g. Compo et al, 2011;Slivinski et al, 2019) will improve the dynamical reconstruction of this event and many similar storms and interesting weather events.…”

Assimilating monthly precipitation data in a paleoclimate data assimilation framework

“…We looked how this drought event is captured in an independent reconstruction, using the Twentieth Century Reanalysis version 3 (20CRv3; Slivinski et al, 2019). In 20CRv3 only pressure measurements are assimilated and in 1842 globally 39 observations data were used.…”

Rescue and quality control of sub-daily meteorological data collected at Montevergine Observatory (Southern Apennines), 1884–1963

“…A significant effort to rescue historical subdaily data has been made by the Twentieth Century Reanalysis (20CR) and the Uncertainties in Ensembles of Regional ReAnalyses (UERRA) projects. The 20CR reanalysis product is the first ensemble of sub-daily global atmospheric conditions (Compo et al, 2011;Cram et al, 2015;Slivinski et al, 2019) spanning from 1851 to 2014 (20CR version 3). This project supported national and regional initiatives focused on the recovery of sub-daily data extending back to the 19 th century.…”

“…This project supported national and regional initiatives focused on the recovery of sub-daily data extending back to the 19 th century. The 20CR reanalysis aimed exclusively at the assimilation of atmospheric pressure observations (Slivinski et al, 2019), thus the resulting observational database is limited to this variable.…”

“…In this figure are shown the 26 historical stations extracted from the International Surface pressure Databank version 4.7 (ISPDv4) and from Ashcroft et al (2018) datasets, where sub-daily meteorological data are already available in digital format for the period 1884-1963. The ISPDv4 is an international repository in which converge several national and international historical data collections (Compo et al, 2019). This database incorporates all the observations assimilated by 20CR version 3 (Slivinski et al, 2019) and constitutes the world's largest database of atmospheric pressure data.…”