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 −0.93°C, −2.78°C, and −2.27°C, 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.