Aircraft measurements and numerical simulations of mountain waves over the central Alps: A pre-MAP test case

Abstract

In preparation for the field phase of the Mesoscale Alpine Project (MAP), in situ research-aircraft observations from the DLR Falcon, and three-dimensional high-resolution numerical simulations are used to investigate mountain waves generated during a south föhn event on 10 October 1987. The model simulation qualitatively compares favourably with the aircraft observations in terms of identifying the horizontal and vertical locations of maximum gravity-wave amplitude and rapid decay of wave amplitudes in the lower stratosphere. However, even with reasonably specified large-scale conditions, the vertical velocity and temperature perturbations are not in quantitative agreement with the aircraft measurements. The results highlight some deficiencies that still exist in our predictive capability to simulate gravity-wave generation and propagation explicitly over three-dimensional topography for non-steady conditions. The complexity of the gravity-wave response points toward the necessity of a straightforward aircraft observing strategy that features linear flight segments oriented along the cross-mountain wind direction, a rapid repetition of these segments in order to assess the significance of transients, and a combination of in situ measurements with remote-sensing data (e.g. from lidar systems and dropsondes), an approach that was successfully applied during the MAP.