AOSC Departmental Seminar
April 21, 2016

Towards an improved 20th Century reanalysis version ā€œ2cā€ dataset spanning 1850 to 2012


Gil Compo
University of Colorado/CIRES & NOAA/ESRL
Abstract:  

The historical reanalysis dataset generated by NOAA Earth System Research Laboratory 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.

The 20CRv2 is being used in a variety of climate and weather studies. As an example, we investigate whether the Pacific Walker Circulation (PWC), part of the global divergent circulation, has weakened or strengthened since 1900. Some researchers have suggested that observations of the PWC spanning the last century provide evidence that the global convective mass flux is decreasing. Global coupled climate models show a decrease in the PWC from the last century extending into the next. The debate surrounding this issue is complicated by different investigators using different indices to define the PWC, with some based on using both the rotational and divergent components of the tropical winds to diagnose what is in essence a divergent overturning circulation. The influence and effect of tropical sea surface temperatures (SST) is also a confounding issue. We find that, in contrast to coupled climate models, most observed aspects of the PWC show no trend or a strengthening over the last 120 years.

While 20CRv2 is useful, there are opportunities for improvement. A newly generated version (“2c”) includes 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.  The effect on the representation of the global divergent circulation is also illustrated. Possibilities for 200 years of reanalysis are also discussed.