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 rangeof studies, from understanding large-scale climate trends to diagnosing the impactsof individual historical extreme weather events. The Twentieth Century Reanalysis(20CR) Project is an effort to fill this need. It is supported by the National Oceanicand Atmospheric Administration (NOAA), the Cooperative Institute for Research inEnvironmental Sciences (CIRES), and the U.S. Department of Energy (DOE), and isfacilitated by collaboration with the international Atmospheric Circulation Recon-structions over the Earth initiative. 20CR is the first ensemble of sub-daily globalatmospheric conditions spanning over 100 years. This provides a best estimate ofthe weather at any given place and time as well as an estimate of its confidence anduncertainty. While extremely useful, version 2c of this dataset (20CRv2c) has sev-eral significant issues, including inaccurate estimates of confidence and a global sealevel pressure bias in the mid-19th century. These and other issues can reduce itseffectiveness for studies at many spatial and temporal scales. Therefore, the 20CRsystem underwent a series of developments to generate a significant new version ofthe reanalysis. The version 3 system (NOAA-CIRES-DOE 20CRv3) uses upgradeddata assimilation methods including an adaptive inflation algorithm; has a newer,higher-resolution forecast model that specifies dry air mass; and assimilates a largerset of pressure observations. These changes have improved the ensemble-basedestimates of confidence, removed spin-up effects in the precipitation fields, anddiminished the sea-level pressure bias. Other improvements include more accuraterepresentations of storm intensity, smaller errors, and large-scale reductions in model bias. The 20CRv3 system is comprehensively reviewed, focusing on the aspects thathave ameliorated issues in 20CRv2c. Despite the many improvements, some chal-lenges remain, including a systematic bias in tropical precipitation and time-varyingbiases in southern high-latitude pressure fields. Show more