Historical land use changes and mesoscale summer climate on the Swiss Plateau

[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-month-long high-resolution simulations (1.5 x 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 approximate to 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 degrees C and a nocturnal warming of the same magnitude, thereby decreasing the diurnal temperature range by 0.6 degrees 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.