Numerical investigation of an extreme storm with the Canadian Regional Climate Model: the case study of windstorm VIVIAN, Switzerland, February 27, 1990

The windstorm VIVIAN that severely affected Switzerland in February 1990 ha s been investigated using the Canadian Regional Climate Model (CRCM). This winter storm was characterised by a deep cyclone in the North Atlantic and by strong geopotential and baroclinic north-south gradients in the troposph ere over Western Europe resulting in high windspeeds in Switzerland. Our pr incipal emphasis is to demonstrate the ability of the CRCM to simulate the windfield intensity and patterns. In order to simulate winds at very high r esolution we operate an optimal multiple self-nesting with the CRCM in orde r to increase the horizontal and vertical resolution. The simulation starts with downscaling NCEP-NCAR reanalyses at 60 km with 20 vertical levels, fo llowed by an intermediate 5-km simulation with 30 vertical levels nested in the former. The 5-km output is in a final phase used for initial and later al conditions for a 1-km resolution simulation with 46 vertical levels. The multiple self-nesting in the horizontal is necessary to reach sufficient r esolution to better capture the orographic forcing that modulates the atmos pheric circulation at fine scales, whereas the vertical resolution enhancem ent helps to better simulate the boundary layer that modulates the windspee d along the surface and better represents the atmospheric circulation with a complex vertical structure (low-level jets, gravity waves and frontal fea tures). It has also been found that the simulated temporal variability of t he windfield and of most variables at the finer scales increases with the i ncreasing nesting frequency. This indicates that as we progress towards fin er scales in the horizontal, the vertical and the nesting frequency enhance ment helps to simulate windspeed variability. However, the variability with in the larger domain is limited by the archival frequency of reanalysis dat a that cannot resolve disturbances with time scale shorter than 12 h. Resul ts show that while the model simulates well the synoptic-scale flow at 60-k m resolution, cascade self-nesting is necessary to capture fine-scale featu res of the topography that modulate the flow that generate localised wind e nhancement over Switzerland.