Frontal modification and lee cyclogenesis in the Alps: A case study using the ALPEX reanalysis data set

The interaction of a cold front with the Alps is studied by means of real-c ase numerical simulations for a case occurring at the end of the Alpine Exp eriment (ALPEX) on 28 April-2 May, 1982. Simulations are per-formed with th e numerical weather prediction model chain Europa-Modell (EM) and its one-w ay nested high-resolution model (HM) of the German and Swiss Weather Servic es. The outer EM simulation (56 km horizontal resolution) uses initial and lateral boundary fields taken from the ALPEX-IIIb reanalysis data set. This reanalysis data set is based on the operational EM analysis scheme, but ta kes into consideration a wide range of field phase data taken during the AL PEX field campaign. A comparison of simulations driven by the ALPEX and ECM WF reanalysis (ERA) data is performed, The simulation driven by the former captures the intensity and vertical depth of the developing lee cyclone sub stantially better than the corresponding run driven by the ERA. The transient development of the impingement of a cold front on the Alps in duces a wide range of mesoscale phenomenon such as flow splitting, mistral, north foehn, cyclogenesis, anticyclonic vortex and bora. These flow evolut ions are analyzed using HM simulations with a horizontal resolution of 14 k m and visualized performing comprehensive trajectory calculations.