How the lee wave was discovered

The phenomenon of the mountain wave was discovered accidentally by a sailplane pilot soaring over a small hill in Silesia, Germany, in 1933, to the unusual altitude of ten times the height of the hill. To explore this observation further, 22 sailplanes assembled during a gliding contest were prepared – at zero extra cost – to record their position, altitude and time in case of a favorable foehn event. When luck allowed this to happen they spent a total of 65 flight hours in the vertical motion field and recorded six alternating lines of up- and downdrafts parallel to an upwind mountain range. This proved the wave character of the phenomenon. While these flights reached already considerable heights, the question remained; what the total vertical extent the mountain wave may be. This problem was explored by a special high-altitude glider flight of the author, which led to the conclusion that a mountain range as small as the Riesengebirge (20 km long, 1400 m high) was able to excite an oscillation of the whole troposphere.

These discoveries contradicted the then prevailing concept of airflow over mountains derived from the classical foehn theory, which was widely discussed in the European literature at the beginning of the 20th century. To explain the warm-dry foehn winds it was based on the somewhat simplistic model: "moist-adiabatic up" and "dry-adiabatic down".

The international discussion of the glider results was interrupted by World War II, but it led to a rush of theories which were independently developed in the warring countries (Lyra, Queney, Scorer, etc.). This was followed by an enthusiastic reunion after the war.

Since then, sailplanes have used lee waves to penetrate into the stratosphere (15 km) and powered aircraft have encountered mountain waves at over 20 km. Today the role of mountain waves in the formation of polar stratsopheric clouds (PSC) at temperatures below -80° C and their contribution to ozone depletion inside the polar vortex have become clear. Their penetration into the mesosphere is now under lively discussion. Simulations by non-hydrostatic meso-scale models of very high resolution will be discussed.