Observation of the airflow over the Alps during a foehn event

Abstract

A detailed analysis is presented of the large-scale, mesoscale and local features of a south-foehn event in the Alps on 8 November 1982. On this day, with a substantial cross-mountain flow, instrumented aircraft made programmed flights back and forth across the Alps between southern Germany and northern Italy. Rawinsonde observations were used to complete the data set. A mesoscale double mountain wave with an amplitude of 1 km was found in the upper troposphere, In the mid troposphere above the Inn valley a rather pronounced wave with an amplitude of 2 km and a wavelength of 50 km was analysed. This wave was close to overturning. The foehn turned out to be accompanied by low wave drag at high levels and with strong wave drag at lower levels. Downward southerly (westerly) momentum flux was evaluated to be 0.3 (0.1) Pa. Mountain drag was estimated to be between 1.6 and 6.7Pa. Light to moderate turbulence was observed immediately to the lee in a low-level turbulence zone over the region of strong gusty surface winds. Finally, special attention has been devoted to the similarities and possible differences between foehn in the Alps and chinook in the Rocky Mountains.