Interesting Paper “The Twentieth Century Reanalysis Project” By Compo Et Al 2011

I was alerted to a new paper

Compo GP,Whitaker JS, Sardeshmukh PD, Matsui N, Allan RJ, Yin X, Gleason Jr BE, Vose RS, Rutledge G, Bessemoulin P, Br¨onnimann S, Brunet M, Crouthamel RI, Grant AN, Groisman PY, Jones PD, Kruk MC, Kruger AC, Marshall GJ, Maugeri M, Mok HY, Nordli Ø, Ross TF, Trigo RM, Wang XL, Woodruff SD,Worley SJ. 2011. The Twentieth Century Reanalysis Project. Q. J. R. Meteorol. Soc. 137: 1–28. DOI:10.1002/qj.776

The abstract reads [highlight added]

The Twentieth Century Reanalysis (20CR) project is an international effort to produce a comprehensive global atmospheric circulation dataset spanning the twentieth century, assimilating only surface pressure reports and using observed monthly sea-surface temperature and sea-ice distributions as boundary conditions. It is chiefly motivated by a need to provide an observational dataset with quantified uncertainties for validations of climate model simulations of the twentieth century on all time-scales,with emphasis on the statistics of daily weather. It uses an Ensemble Kalman Filter data assimilation method with background ‘first guess’ fields supplied by an ensemble of forecasts from a global numerical weather prediction model. This directly yields a global analysis every 6 hours as the most likely state of the atmosphere, and also an uncertainty estimate of that analysis.

The 20CR dataset provides the first estimates of global tropospheric variability, and of the dataset’s time-varying quality, from 1871 to the present at 6-hourly temporal and 2◦ spatial resolutions. Intercomparisons with independent radiosonde data indicate that the reanalyses are generally of high quality. The quality in the extratropical Northern Hemisphere throughout the century is similar to that of current three-day operational NWP forecasts. Intercomparisons over the second half-century of these surface-based reanalyses with other reanalyses that also make use of upper-air and satellite data are equally encouraging.

It is anticipated that the 20CR dataset will be a valuable resource to the climate research community for both model validations and diagnostic studies. Some surprising results are already evident. For instance, the long-term trends of indices representing the North Atlantic Oscillation, the tropical Pacific Walker Circulation, and the Pacific–North American pattern are weak or non-existent over the full period of record. The long-term trends of zonally averaged precipitation minus evaporation also differ in character from those in climate model simulations of the twentieth century.”

Excerpts from the paper include

“The results in Figures 12–16 demonstrate that the 20CRv2 reanalysis has successfully incorporated the information in synoptic surface pressure observations and its beneficial impact on estimates of the global tropospheric circulation, not only on the synoptic but also much longer time-scales. We end this section with a tantalizing look at perhaps the hardest test for such a surface-pressure-based reanalysis system: its ability to represent the mean hydroclimate and its variability. Figure 17(a) compares the 1980 to 2000 mean of zonally averaged precipitation P in the 20CRv2 and the Global Precipitation Climatology Project (GPCP, Adler et al., 2003) v.2 datasets. The comparison is generally favourable, and within the uncertainties estimated from intercomparisons among other observational precipitation datasets (e.g. Adler et al., 2003). Figure 17(b) shows the 1980–2000 mean of zonally averaged precipitation minus evaporation, P–E, in the 20CRv2, and also its change (P–E) from that during the first 20-year period (1871–1891) of the reanalysis. The surprise here is that the meridional structure of (P–E) does not resemble that of P–E itself. Such a resemblance might have been anticipated from simple arguments and climate model simulations (Held and Soden, 2006) as a ‘robust’ feature of the global hydrological response to global warming. Indeed at 10◦N the sign of (P–E) is opposite to that of P–E. Assessing the realism of such aspects of the 20CRv2 and other historical reanalysis datasets will clearly continue to be of interest.”

“The overall quality of the 20CRv2 dataset may surprise some readers. While the relevance for weather studies appears to be consistent with that anticipated from advanced data assimilation algorithms applied in observing system experiments using only surface observations (Whitaker et al., 2004, 2009; Anderson et al., 2005; Thepaut, 2006; Compo et al., 2006), the relevance for climate studies, e.g. as suggested by the high correlations of monthly-mean anomalies in Figure 15 and climate indices in Figure 16, could not have been anticipated from those short feasibility experiments. The ability to generate skilful 24-hour forecasts of surface pressure (relative to persistence forecasts) even in years as data-poor as 1871 was another pleasant surprise.”

 The temporal inhomogeneity of the input fields that Compo et al used [the surface pressure sea surface temperatures and sea ice] raise questions on the robustness of their results.  Nevertheless, this is an interesting study and the results so far are quite provocative, as they raise further questions on the skill of the IPCC models to replicate (i.e. hindcast) the evolution of the climate system in the last century.

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