On the small-scale dynamics of flow splitting in the Rhine valley during ashallow foehn event

Flow splitting in the Rhine valley has been observed with a transportable w ind lidar (TWL) during a shallow-foehn event in the framework of the Mesosc ale Alpine Programme (MAP). The Doppler lidar recorded in detail flow split ting, foehn wind gusts, and flow reversal. Such structures have not previou sly been observed with comparable detail by conventional in-situ instrument s. In addition to the TWL, boundary-layer processes have been documented by means of rawinsondes and surface stations. This paper presents an analysis of the processes giving birth to flow splitting between the Seez and Rhine valleys during Intensive Observation Period (IOP) 5 (1-3 October 1999) by combining the collected data with hydraulic theory. The study shows that th e splitting of the channelled flow is associated with (1) the existence of a stagnation point at the intersection of the Seez and Rhine valleys, and ( 2) the deflection of the flow by the lateral sidewalls of the valleys.