Comparison of trends and variability in CRU, ERA-40 and NCEP/NCAR analyses of monthly-mean surface air temperature.
| Title | Comparison of trends and variability in CRU, ERA-40 and NCEP/NCAR analyses of monthly-mean surface air temperature.
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Report
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| Date Published |
04/2004
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| Series/Collection |
ERA-40 Project Report Series
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| Document Number |
18
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| Author | |
| Event Series/Collection |
ERA Report
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| 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. |
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