Experimental study of mountain lee—waves by means of satellite photographs and aircraft measurements



This paper is a summary of a Ph.D. Thesis1 which was a systematic study of the influence of various meteorological factors on the occurrence and characteristics of mountain waves, more specifically of lee-waves of great horizontal extent. The data used are, beside classical meteorological informations, that given by satellite pictures completed by quasi-simultaneous measurements from planes or gliders. The analysis of many satellite pictures received at the french station of Lannion (Brittany) during a period of three years (1966–1968), gave a fairly complete description of lee-wave clouds visible over Western Europe and North Africa, and led to the following conclusions: (i) In all the cases of lee-waves that were considered, a layer with a great static stability was recorded between 1 000 and 4 000 m above sea level. (ii) The horizontal extent of the lee-waves was found to vary in inverse ratio to the thickness of this highly stable layer, which thus appears to act as a wave guide where almost all the energy of the mountain perturbation is “trapped”, in the same way as ordinary gravity waves are produced and retained at the free surface of a quiet sheet of water (so-called “surface waves”).