Continuously stratified flows across an Alpine crest with a pass: Shallow and deep föhn

Shallow and deep Alpine föhn flows are analyzed within the framework of continuously stratified hydraulics. A distinguishing upstream feature of these flows is the presence of stagnant, nearly well‐mixed layers above and beneath the moving stratified fluid. These layers isolate the moving stratified fluid, which has a self‐similar parabolic‐like velocity distribution, from the stratified fluid above and beneath. The shallow and deep realizations of these flows were found on 20 and 21 October 1999 during the Mesoscale Alpine Programme. The temporal evolution from shallow to deep föhn was captured with a sequence of radiosondes, automatic weather stations, an instrumented car, and research aircraft flights.