Impact of trends in historical surface roughness over Europe on extra-tropical windstorms in CMIP6

Extratropical windstorms are amongst the highest rated perils for the European continent. Extreme wind speeds of these synoptic scale systems occur primarily in the winter season and often cause damage to buildings, forests and infrastructure, and thus can have large socio-economic impacts.In our studies of extratropical windstorms in the CMIP6 model ensemble, we found remarkable trends of opposite sign in the wind speed during the historical period. More specifically, we found a continuous increase in the surface wind speed in the early historical period between 1850 and 1920, and an even stronger decrease thereafter until the present.In a case study with one of the models (MPI-ESM) we found that the trends in the wind speed relate to a trend of opposite sign in the roughness length, thus the wind speed increases in eras with a decrease in the surface roughness (and tree fraction) and vice versa. While this relationship is expected and physically reasonable, it appears that the interaction of surface parameters with the atmosphere was different in CMIP5 climate models, as there is no comparable reaction of surface wind speeds to the trends in surface parameters (e.g. tree fraction).Since the historical era serves as the reference for any derived climate change signal, these trends might affect the amplitude of the changes in a future climate and the derived conclusions. Also, state of the art climate change signals regarding storminess might need to be reconsidered with this newly represented land-atmosphere interaction in the models.We further explore this phenomenon by eliminating the influence of the roughness on the wind speed and investigate the effect that this correction has on the appearance of climate change signals of extratropical windstorms.