Impact of climate model resolution on soil moisture projections in central-western Europe

Abstract. Global climate modelsproject widespread decreases in soil moisture over large parts of Europe. This paper investigates the impact of model resolution on the magnitude and seasonality of future soil drying in central-western Europe. We use the general circulation model EC-Earth to study two 30-year periods representative of the start and end of the 21st century under low-to-moderate greenhouse gas forcing (RCP4.5). In our study area, central-western Europe, at high spatial resolution ( ∼25 km) soil drying is more severe and starts earlier in the season than at standard resolution ( ∼112 km). Here, changes in the large-scale atmospheric circulationand local soil moisture feedbacks lead to enhanced evapotranspirationin spring and reduced precipitation in summer. A more realistic position of the storm trackat high model resolution leads to reduced biases in precipitation and temperature in the present-day climatology, which act to amplify future changes in evapotranspirationin spring. Furthermore, in the high-resolution model a stronger anticyclonicanomaly over the British Isles extends over central-western Europe and supports soil drying. The resulting drier future land induces stronger soil moisture feedbacks that amplify drying conditions in summer. In addition, soil-moisture-limited evapotranspirationin summer promotes sensible heatingof the boundary layer, which leads to a lower relative humidity with less cloudy conditions, an increase in dry summer days, and more incoming solar radiation. As a result a series of consecutive hot and dry summers appears in the future high-resolution climate. The enhanced drying at high spatial resolution suggests that future projections of central-western European soil drying by CMIP5 models have been potentially underestimated. Whether these results are robust has to be tested with other global climate modelswith similar high spatial resolutions.