Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 126

across the ridge, the same airstream could produce either large-amplitude lee waves or none at
all, depending on its direction. (After Scorer 1967: 76.)
LEE WAVES
The behavior of airflow over an obstacle depends largely on the vertical wind profile, the
stability structure, the shape of the obstacle, and the surface roughness (Barry 2008;
Doyle et al. 2011). When wind passes over an obstacle, its normal flow is disrupted, and
a train of waves may be created that extends downwind for considerable distances. The
major mountain ranges produce large-amplitude waves that extend around the globe
(Vosper and Parker 2002). On a smaller scale, these waves take on a regional signific-
ance reflected in their relationship to the foehn, in distinctive cloud forms, in upper-air
turbulence, and in downwind climate (Scorer 1961, 1967; Durran 1990; Reinking et al.
2000; Brady and Waldstreicher 2001; Doyle et al. 2011). The amplitude and spacing of
lee waves depend on the wind speed and the shape and height of the mountains, among
other factors (Epifanio and Rotunno 2005). An average wavelength is between 2 and 40
km (1-25 mi), the vertical amplitude is usually between 1 and 5 km (0.6-3 mi), and they
occur at altitudes of 300-7,600 m (1,000-25,000 ft) (Durran 1990). Wind speeds with-
in lee waves are quite strong, frequently exceeding 160 km (100 mi) per hour (Scorer
1961; Doyle et al. 2011).
The most distinctive visible features of lee waves are the lenticular (lens-shaped)
or lee-wave clouds that form at the crests of waves. These are created when the air
reaches dew point and condensation occurs as the air moves upward in the wave (Lud-
lam 1980). The clouds do not form in the troughs of the waves, since the air is des-
cending and warming slightly (Fig. 3.29). The relatively flat cloud bottoms represent the
level of condensation, and the smoothly curved top follows the outline of the wave crest.
The clouds are restricted in vertical extent by overlying stable air (Vosper and Park-
er 2002). Lenticular clouds are relatively stationary (hence the name “standing-wave
clouds”), although the wind may be passing through them at high speeds. Lee-wave
clouds frequently develop above one another, as well as in horizontal rows. They typic-
ally consist of one to five clouds and extend only a few kilometers or miles downwind,
but satellite photography has revealed series of 30 to 40 clouds extending for several
hundred kilometers (Baderet al. 1995).
Much of the early knowledge about lee waves was acquired by glider pilots who
found, to their surprise, that there was often greater lift to the lee of a hill than on the
windward side. The pilots had long made use of upslope and valley winds, but by this
method could never achieve a height of more than a couple of hundred meters above
the ridges. In southern England, the members of the London Gliding Club had soared
for years in the lift of a modest 70 m (230 ft) hill, never achieving more than 240 m (800
ft). After discovering the up-currents in the lee wave, however, one member soared to
a height of 900 m (3,000 ft), 13 times higher than the hill producing the wave (Scorer
1961). German pilots were the first to explore and exploit lee waves fully. In 1940, one
pilot soared to 11,300 m (37,400 ft) in the lee of the Alps. The world's altitude record of
15,460 m (49,213 ft) was set in 2006 in the lee of the Andes Mountains of Argentina.
Another aspect of lee waves is the development of rotors, awesome roll-like circula-
tions that develop to the immediate lee of mountains, usually forming beneath the wave