Lessons from a new 200 year reanalysis of the global atmospheric circulation
Abstract: The new historical reanalysis dataset generated by the Physical Sciences Division of NOAA’s Earth System Research Laboratory and the University of Colorado CIRES, the Twentieth Century Reanalysis version 3 (20CRv3), is a comprehensive global atmospheric circulation dataset spanning 1806 to 2015, assimilating only surface pressure and using monthly Hadley Centre sea ice distributions (HadISST2.3) and an ensemble of 5-day average Simple Ocean Data Assimilation with Sparse Input (SODAsi.3) and Hadley Centre (HadISST2.2) sea surface temperatures. SODAsi.3 was forced with a previous version of 20CR that itself was forced with a previous SODAsi, allowing these “iteratively-coupled” boundary conditions to be more consistent with the atmospheric reanalysis. 20CRv3 has been made possible with supercomputing resources of the U.S. Department of Energy and a collaboration with GCOS, WCRP, and the ACRE initiative. It is chiefly motivated by a need to provide an observational validation dataset, with quantified uncertainties, for assessments of climate model simulations of the 19th to 21st centuries, with emphasis on the statistics of daily weather. It uses, together with the NCEP global forecast system (GFS) numerical weather prediction (NWP) land/atmosphere model to provide background "first guess" fields, an Ensemble Kalman Filter (EnKF) data assimilation method. This yields a global analysis every 3 hours as the most likely state of the atmosphere, and also yields the uncertainty of that analysis.
20CRv3 has several improvements compared to the previous version 2c. The analysis and the 80 member ensemble are generated with the NCEP GFS at T254 resolution (about 0.75 degrees latitude by longitude) with 64 levels in the vertical, compared to T62 (about 2 degrees latitude by longitude) and 28 vertical levels in the 20CRv2c 56 member ensemble. This gives an improved representation of extreme events, such as hurricanes. Implementation of a “relaxation to prior” covariance inflation algorithm, combined with stochastic parameterizations in the GFS, provides quantitatively better uncertainty estimates than the previous multiplicative inflation of 20CRv2c. An adaptive localization helps to keep the analysis from over-fitting the observations. A variational quality control system retains more observations. An incremental analysis update procedure produces a temporally smoother analysis without spurious spin-up trends seen in 20CRv2c. Millions of additional pressure observations contained in the new International Surface Pressure Databank version 4.7, such as from the citizen science Oldweather.org project, also improve the analyses. These improvements result in 20CR version “3” having comparable or better analyses to version 2c, as suggested by improved 6 hour forecast skill, more realistic uncertainty in near-surface air temperature, and a reduction in spurious centennial trends in the tropical and polar regions.