Comparison of trends and variability in CRU, ERA-40 and NCEP/NCAR analyses of monthly-mean surface air temperature.

Comparison of trends and variability in CRU, ERA-40 and NCEP/NCAR analyses of monthly-mean surface air temperature.
Date Published
ERA-40 Project Report Series
Document Number
P. Jones
da Bechtold
Anton Beljaars
P.W. Kållberg
S. Saarinen
S. Uppala
P. Viterbo
Event Series/Collection
ERA Report
Abstract Monthly-mean anomalies in surface air temperature from the ERA-40 and NCEP/NCAR reanalyses are compared with corresponding values from the CRUTEM2v dataset, which is derived directly from monthly station data. There is mostly very good agreement as regards short-term variability, particularly between ERA-40 and CRUTEM2v. Least-square linear trends are significantly lower forthe two reanalyses when computed over the full period studied, from 1958 to 2001, but ERA-40 trends are within 10% of CRUTEM2v values for the northern hemisphere when computed over the period from 1979 onwards. A small number of erroneous station values that entered the CRUTEM2v analysis and an even smaller number of highly suspect values in the reanalyses have been identified by the three-way comparison. Gaps in the availability of synoptic surface data contribute to relatively poor performance of ERA-40 prior to 1967. Trends and variability are quite similar in the ERA-40 background and analysis fields, although the increments made to the background by ERA-40's analysis of synoptic screen-level temperature observations bring closer agreement with CRUTEM2v. The increments reduce a warm model bias prevalent at middle and high latitudes and a cold bias at low latitudes. Trends and variability in ERA-40 temperature analyses throughout the boundary layer are generally similar to those at the surface from the late 1970s onwards. Evidence points to a cold bias early in the period at 500hPa over the data-sparse southern extratropics and at the surface over Antarctica. One indicator of this comes from comparing the ERA-40 analyses with a simulation produced using the same model and same distributions of sea-surface temperature and sea-ice as used in the ERA-40 data assimilation. The simulation itself reproduces quite well the trends in surface air temperature over land seen in CRUTEM2v, and captures some of the variability.