This paper presents an analysis of the dynamics of gap flow within the Wipp valley on October 20 and 21, 1999. Key components of the analysis are the radial wind data collected by the NOAA/ETL scanning Doppler Lidar located within the valley, in situ and Eldora data collected by the NOAA P3 flying above and within the valley, and idealized numerical simulations.

The in-situ P3 observations collected within the valley on 20 October were interpolated to a two-dimensional cross-section along the axis of the valley. Throughout this cross-section, the Lidar-radial winds interpolated from the P3 flight tracks are in very good agreement with the radial winds observed by the Lidar itself. Based on this good agreement, we create a synthetic data set consisting of three-dimensional volume scans from the Lidar, which contain only data about the radial wind component, and the vertical cross-section of P3 observations which contains thermodynamic data and information about both components of the horizontal wind. The fundamental dynamical signatures in this synthetic data set are then compared with semi-idealized numerical simulations and with the predictions of shallow-water hydraulic theory.

The relatively shallow foehn flow on October 20 is contrasted with the deep foehn case on October 21. Volume data collected by the Lidar and the Eldora Doppler radar on the NOAA P3 are again compared with semi-idealized numerical simulations in order to elucidate the underlying flow dynamics.