Turbulence on the lee side of a mountain range: Aircraft observations during PYREX
Résumé
This study presents an analysis of turbulence data from aircraft measurements made during the PYREX experiment. The data were gathered in a central region of the Pyrénées, a mountain-range with a more or less west-east orientation which constitutes a major barrier for northerly and southerly flows. The data used come principally from a Merlin IV aircraft which performed about 100 straight-and-level 20 km runs perpendicular or parallel to the main mountain-range axis. The basic turbulence moments are presented. The data from mountain flows with similar upstream conditions were gathered together in order to construct composite two-dimensional turbulence fields in the vertical plane perpendicular to the axis of the range. These fields clearly show the location of the turbulence areas on the lee side of the mountain range. The turbulent kinetic energy field shows that turbulence is principally produced by a wake effect. On the lee side the profiles of the various turbulence parameters indicate a maximum at an altitude close to the average height of the mountain in a region having both a strong wind shear and a weak lapse rate. Finally, we show that the turbulence observed in this study obeys the universal law σ-3ωϵλ≈︁ constant, where s̀w is the standard deviation of the vertical velocity, λ the spectral length-scale and ϵ the dissipation rate of turbulent kinetic energy. The average value of 2.3 found for this constant is comparable to the values found by other authors under various conditions of turbulence, in particular homogeneous turbulence.
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