Evaluation of ERA-40, NCEP-1, and NCEP-2 reanalysis air temperatures with ground-based measurements in China
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We assess the correspondence of reanalysis air temperatures from ERA-40, NCEP-1, and NCEP-2 with homogenized observational data from China for 1958-2001 and 1979-2001. Results indicate that climatologies for annual ERA-40, NCEP-1, and NCEP-2 air temperatures are lower than observations by -0.93C, -2.78C, and -2.27C, respectively. Large negative differences for most of western China primarily contribute to this cool bias. Error analysis indicates that the internal coherence of ERA-40 data is better than NCEP-1 or NCEP-2. Although NCEP-2 air temperatures represent an improvement over NCEP-1, biases of NCEP-1 and NCEP-2 data relative to observations are still much larger than for ERA-40. Areas with positive/negative air temperature differences (dT) between reanalysis and observational data correspond to negative/positive elevation differences (dH). The high correlation coefficients of -0.94, -0.88, and -0.85 between dT and dH for ERA-40, NCEP-1, NCEP-2, and observations, respectively, illustrate that the air temperature differences between reanalysis data and observations are primarily related to elevation differences. Furthermore, a spatial and temporal comparison of trends also indicates that ERA-40 temperature changes correspond most closely to observed trends in China. In general, our comprehensive analysis of the three global reanalysis products indicates that, both on a seasonal and annual basis, ERA-40 temperatures correspond most closely to observations, and biases are due mainly to the elevation differences. Copyright 2008 by the American Geophysical Union.
