Intercomparison of an improved 20th Century reanalysis version 2c dataset spanning 1850 to 2012

Abstract:

The historical reanalysis dataset generated by NOAA ESRL and the University of Colorado CIRES, the Twentieth Century Reanalysis version 2 (20CRv2), is a comprehensive global atmospheric circulation dataset spanning 1871-2012, assimilating only surface pressure and using monthly Hadley Centre SST and sea ice distributions (HadISST1.1) as boundary conditions. It has been made possible through 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 20th century, with emphasis on the statistics of daily weather. It uses, together with an NCEP global 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 6 hours as the most likely state of the atmosphere, and also yields the uncertainty of that analysis.

Improvements in the new version (“2c”) include an extension back to 1850 and the specification of new boundary conditions. These come from new fields of monthly COBE-SST2 sea ice concentrations and an ensemble of daily Simple Ocean Data Assimilation with Sparse Input (SODAsi.2c) sea surface temperatures. SODAsi.2c itself was forced with 20CR, allowing these boundary conditions to be more consistent with the atmospheric reanalysis. Millions of additional pressure observations contained in the new International Surface Pressure Databank version 3 are also included. These improvements result in 20CR version “2c” having comparable or better analyses, as suggested by improved 24 hour forecast skill, more realistic uncertainty in near-surface air temperature, and a reduction in spurious centennial trends in the tropical and polar regions. An intercomparison with ERA-Interim, MERRA, and JRA-55 reanalyses that assimilate all available upper-air and satellite observations will also be discussed.