Abstract/Summary

Many cities have expanded during the 20th century, and consequently some weather stations are currently located closer to cities than before. Due to the urban heat island (UHI) effect, those weather stations may show a positive bias in their 2-m temperature record. In this study, we estimate the impact of urbanization on the temperature record of WMO station De Bilt (The Netherlands). This station has a long historical record, but the nearby city of Utrecht and its suburbs expanded during the 20th century. The temperature rise due to urbanization is estimated by conducting representative mesoscale model simulations for the land-use situation for the years 1900 and 2000. This is performed for 14 different episodes of a week, each representing a typical large-scale flow regime (Grosswettertypes) in both the winter and the summer season. Frequency distributions of these flow regimes are used to estimate an average temperature rise. We find that the model results with two rather different atmospheric boundary-layer schemes, robustly indicate that the urbanization during the 20th century has resulted in a temperature rise of 0.22 ± 0.06 K. This is more than a factor of 2 higher than a previously estimated temperature trend by using observed temperature records of stations close to De Bilt.