Exaggerated Effect of Urbanization in the Diurnal Temperature Range Via “Observation minus Reanalysis” and the Physical Causes.

Abstract: The observation minus reanalysis (OMR) method is widely used to investigate the impact of urbanization and land use changes on regional climate. Based on homogenized temperature observations and the National Centers for Environmental Prediction/Department of Energy Atmospheric Model Intercomparison Project II reanalysis surface temperatures, we estimate the trend in diurnal temperature range (DTR) using the OMR method to be −0.324 ± 0.063 °C per decade for the period of 1980–2009 in eastern China. However, we find that reanalysis data produce a positive trend in downward surface solar radiation, while the observations do not, because the reanalysis data significantly overestimate the negative trend in total cloud coverage in the region. Moreover, the reanalysis considerably overestimates the negative trend in the local precipitation. These systematic biases cause an overestimation of the warming rate in daily maximum temperature in the reanalysis and therefore lead to the exaggerated effect of urbanization on the DTR via the OMR analysis. After adjusting the systematic bias in the trend of daily maximum temperature in the reanalysis data, the regional mean OMR trend in the DTR is estimated to be −0.065 °C per decade, which becomes much smaller than previous OMR calculations, and is more consistent with the previous estimates based on the comparison of urban and rural stations. These results not only indicate the methodological problem of the OMR method in many studies of urbanization effects but also hint at important aspects for improving the current reanalysis system. [ABSTRACT FROM AUTHOR]