Assessment of ECMWF reanalysis data in complex terrain: Can the CERA-20C and ERA-Interim data sets replicate the variation in surface air temperatures over Sichuan, China?

Surface air temperatures (SATs) derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim and CERA-20C reanalysis data sets are compared with data from 43 observation stations in Sichuan for 1979-2010. The results show (a) the temperatures from the ERA-Interim and CERA-20C data sets are strongly correlated with those from the observation stations, although significant cold biases are seen on both annual and seasonal timescales. (b) The biases in SATs are predominately influenced by the differences between the actual topography and the topography used in the reanalysis models. Larger differences in temperature are observed in the plateau and mountainous regions of Sichuan. We confirmed larger SAT biases at high altitudes by categorizing the elevation into four bands, each with a spacing of 1,000m. (c) We reduced the biases resulting from elevation by using an elevation correction method with internal lapse rates derived from different reanalysis pressure levels. The annual mean bias was reduced from -2.86 to -0.75 degrees C for the ERA-Interim data set and from -5.27 to -2.21 degrees C for the CERA-20C data set. After calibration, the correlation coefficients between the difference in SAT (observed minus reanalysis data) and the difference in elevation (station elevation minus model elevation) decreased from -0.97 and -0.91 to -0.29 and -0.30 for the ERA-Interim and CERA-20C data sets, respectively. These significant differences should not be ignored in the application of reanalysis data sets to climate research. The evaluation and calibration of reanalysis data sets are essential before making assessments of regional climate change, especially over regions with complex topography.