Lee Waves Generation over Complex Topography

The Hohe Tauern is a quasi-two-dimensional, east-west oriented range in the eastern Alps that includes the highest peak in Austria, Grossglockner at 3797 m. On 20 September 1999, during an extended period of deep-south foehn, a quasi-periodic wave train was observed in the lee of Grossglockner by a research aircraft equipped with a suite of instrumentation including downlooking Lidar and GPS dropsondes. The foehn was particularly intense on this day with a surface wind speed maximum of 50 m/s observed at Patscherkofel, near Innsbruck. It is noteworthy that in this case and a number of other situations during MAP, real-time predictions by the available mesoscale models forecasted vertically propagating large-amplitude waves rather than quasi-periodic waves in the lee, as was often observed.

Research aircraft observations, NRL's nonhydrostatic mesoscale modeling system, COAMPS, with high horizontal (grid increment of 1 km) and vertical resolution (60 levels), and the Smith linear model are used study this event. Evidence of strong descent and shooting flow in the lee of Grossglockner that transition into series of well-defined periodic lee waves are apparent in both the observations and non-linear model results. Upstream of the highest topography, flow blocking and stagnation in the lowest 2.5 km led to flow separation from the complex topography below until the air ascended the highest peak, Grossglockner. Real-data simulations of the event replicate the lee wave characteristics including the horizontal wavelength of ~15 km, similar to that observed. It is hypothesized that the non-linear processes in this case resulted in a transfer of wave energy from vertically propagating modes to quasi-periodic lee waves. However, the non-linear simulations suggest sufficient energy existed in the vertically propagating waves to result in wave overturning and breakdown in the lower-stratosphere, where reversed vertical shear was present. The non-linear and linear numerical models are used to isolate these non-linear wave dynamics. Finally, the sensitivity of the lee wave generation to upstream conditions will be explored for a variety idealized initial states using the non-linear model with a realistic boundary layer parameterization