Historical Reanalysis: What, How, and Why?

Page from old ship's log
Page from old ship's log

Reanalysis datasets combine past weather observations with estimates from weather models, to provide consistent, global, subdaily estimates of the weather. In an upcoming issue of the Journal of Advances in Modeling Earth Systems, a CIRES researcher from ESRL’s Physical Sciences Division (PSD) provides a commentary about a recent paper introducing a new dataset from the European Centre for Medium-Range Weather Forecasts (ECMWF), CERA-20C, which is the first 100-year reanalysis that utilizes a coupled ocean-atmosphere system. NOAA and CIRES have their own 150-year reanalysis, the Twentieth Century Reanalysis (20CR), that is uncoupled and only assimilates surface pressure observations into an atmospheric model. Conversely, CERA-20C assimilates both ocean and atmosphere observations. This commentary broadly discusses what historical reanalyses are, how they are created, and why this latest coupled reanalysis is important.

CERA-20C is an improvement over the previous historical reanalysis from ECMWF (ERA-20C) due to its coupled model and its improved quantification of uncertainty. It adds to the relatively small set of historical surface-input reanalyses, providing PSD scientists another dataset to compare to the 20CR datasets. Additionally, this commentary emphasizes the importance of creating historical reanalyses to the cycle of improving the historical observation network and weather models.

While modern reanalyses that assimilate satellite and upper-air data are used quite frequently, historical sparse-input reanalyses are often not as well-known. These historical reanalyses have the benefit of extending further back in time than modern reanalyses (100 years or more). A consistent century-long history of weather can be used to study specific historical storms in a global context, to investigate extreme events such as hurricanes and droughts, and for more general climate studies. The goal of this paper is to highlight the achievements of CERA-20C in particular while discussing the importance of historical reanalyses in general.

Posted: September 13, 2018

The author of "Historical Reanalysis: What, How, and Why?" is Laura Slivinski of the ESRL Physical Sciences Division.