Some Turbulence and Wind Variability Observations in the Lee of Mountain Ridges

Abstract

A field of turbulence and space and time wind variability in the lee of a ridge line is observed with double theodolite wind data. It is shown that the most turbulent areas occur above the lee slope and are associated with the sharp transition zone between the wind shadow regime just above the slope and the prevailing flow. This transition zone is quite intermittent in time and variable in height and is usually characterized by sustained vertical gusts which may extend to 3 km downwind of the ridge line. The scale of turbulence in the transition zone is quite large. Examples of pure thermodynamic upslope winds are also given. The effect of insolation and prevailing wind speed on the turbulent regime in the valley is discussed and it is shown that the zone of maximum turbulence and wind unrest tends to move to ridge line height under relatively stable temperature gradient conditions.