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

[1] The impacts of historical land use changes on the Swiss Plateau on the summer climate were investigated. The land use changes consist primarily of a conversion of wetlands with extended peat soils into highly productive agricultural lands. We used the ''Lokal-Modell'' of the Consortium for Small-Scale Modeling to conduct several 1-monthlong high-resolution simulations (1.5 Â 1.5 km 2 ) with present and past landscape conditions. The past land use was reconstructed from historical maps (1800-1850) over a relatively small perturbed area of %400 km 2 embedded in an otherwise unchanged present-day landscape. The documented changes in land use and land cover lead to a daytime cooling of up to 0.3°C and a nocturnal warming of the same magnitude, thereby decreasing the diurnal temperature range by 0.6°C. The alteration of the radiation budget and the surface energy balance leads to a shallower daytime mixed layer under present compared to past conditions, with a reduction in the convective upward transport of moisture. The cloud coverage over the study area decreased by up to 5% on average under present conditions. The regional precipitation is most strongly affected in the mean downwind areas and not so much over the area of land use changes. Because of orographic effects in the downwind area a decrease in precipitation in lower-elevation areas was modeled, while precipitation in the adjacent higher-elevation downwind areas increased. In contrast to our expectations the soil water status did not play a dominant role in this area because of the high water table. Even under present conditions the availability of soil moisture and the accessibility of the groundwater to plant roots is, on average, still not a limiting factor for plant transpiration. Citation: Schneider, N., and W. Eugster (2005), Historical land use changes and mesoscale summer climate on the Swiss Plateau,

The effects of historical land-use and land-cover changes on the climate of the Swiss Plateau in the different seasons were investigated. In the 19th century, a civil engineering project was initiated to reshape the lake and river system on the Swiss Plateau in order to ban the frequent flooding during extreme weather events. The landscape modifications consisted primarily of a conversion of wetlands with extended peat soils into a highly productive agricultural landscape. Historical maps (1800-1850) served as a basis for the reconstruction of the past land use. The "Lokal-Modell" of the Consortium for Small-Scale Modelling was used to conduct eight one-month long high-resolution simulations (1.5 × 1.5 km 2 ) with present and past landscape conditions. The modified soil and surface properties led to distinctly altered energy and moisture exchanges at the surface and as a consequence affected the local and regional climate. The climatic changes show different characteristics and magnitudes in the cold (October -March) as compared to the warm season (April -September). The landscape modifications led to an average daytime cooling between −0.12• C (January) and −0.61• C (April) and a night-time warming of 0.19 • C−0.34 • C. The differences in the mean monthly temperatures show a warming of 0.1• C−0.2 • C in the cold season and a cooling of similar magnitude in most of the study area in the warm season. The modification of the radiation budget and the surface energy balance distinctly affected the convective activity in the study area in the warm season, but had only a weak effect on convectivity in the cold season. The cloud coverage in the warm season is therefore distinctly reduced compared to the past.