The Impact of Historical Land-Use Changes on the Near-Surface Atmospheric Conditions on the Swiss Plateau

The effects of historical land-use changes on the Swiss Plateau on the near-surface atmospheric conditions of typical summer days in July were investigated in a case study. A mesoscale dynamical nonhydrostatic model with a horizontal resolution of 1 km(2) was used. Two simulations of historical (around 1850) and present conditions as well as six sensitivity experiments were conducted in order to assess the importance of changes in the physiological, morphological, and soil properties. The modeled daily average temperature was 0.25 degrees C lower under present than past land-use conditions. During the day, changes in temperature showed a heterogeneous spatial pattern that depended on the type of the land-use conversion and its geographic location. In areas where afforestation took place, an average warming of more than 1.0 degrees C was observed. In contrast, deforestation resulted in a cooling of up to 2.0 degrees C. Furthermore, an average increase in the water vapor mixing ratio of 0.2 g kg(-1) was simulated during the day. During the night, the average temperature was up to 0.6 degrees C lower under present land-use conditions. This nighttime cooling is mainly restricted to low-lying areas on the formerly frequently inundated plains. The diurnal temperature range in the area of land-use changes increased by 0.1 degrees-0.3 degrees C. The sensitivity experiments showed that the daytime temperature decrease was mainly caused by morphological changes, while the altered physiological properties are the dominant factor for the nighttime cooling. The water vapor mixing ratio during the day was mainly affected by changes in the physiological properties.