Scale interaction processes during the MAP IOP 12 south fohn event in the Rhine Valley

This paper examines the three-dimensional structure and dynamics of a south fohn flow in the Rhine Valley during its entire life cycle from 29 October 1999 until 31 October 1999. The south fohn event was documented in the framework of the Mesoscale Alpine Programme (MAP). This study investigates the synoptic-scale forcing sources, the dynamical processes driving the circulation of the fohn flow in the complex network of tributaries of the Rhine Valley, and the degree of inhomogeneity on the scale of the FORM ('FOehn in the Rhine Valley during the MAP' programme) target area at the bifurcation between the Rhine and Seez Valleys. Several important data sources were used (ground-based Doppler lidar, scintillometers, constant-volume balloons, radiosoundings and surface stations) as well as non-hydrostatic mesoscale simulations. The fohn penetrating the FORM target area on 30 October 1999 is preceded by a nocturnal shallow fohn phase which does not penetrate down to the ground due to katabatic drainage flow from the main transverse (east-west oriented) tributaries in which fohn cannot penetrate. This paper shows the contribution of the main tributaries of the Rhine Valley in directing the fohn flow towards the FORM target area during (i) the shallow fohn phase where the westerly upper-level flow is deflected in the main longitudinal (i.e. north-south oriented) tributaries; (ii) the penetrating fohn phase, where the south/south-westerly upper-level flow is nearly aligned with the longitudinal tributaries. The channelling efficiency of the main longitudinal tributaries of the Rhine Valley (particularly the Domleschg) is higher during the fohn phase when the south-westerly upper-level flow, experiencing mountain-wave-induced downward motion, penetrates these tributaries, than during the shallow fohn. In the FORM target area, the structure of the fohn flow varies on a 1-kilometre horizontal length-scale and the time evolution of the respective location of the cold air pool and the warm fohn air is investigated in detail. Also, flow splitting between the Rhine and Seez Valleys occurs during the entire fohn life cycle, but its vertical extension is maximum during shallow fohn, when lower- and upper-level flows are fully decoupled.