Mountain Lee Waves
When a cold front approaches California from the northwest and the westerly airflow increases in speed over the Sierra crest, spectacular "stationary" clouds are usually seen over the leeward valleys. These are manifestations of a mountain lee wave, as it is known (Fig. 1.2). If the ridges and troughs of the horizontal airflow pattern are likened to the bends or meanders of a stream, the lee wave phenomena are analogous to the falls and ripples. Figure 1.3 shows a typical pattern of airflow and cloud forms in a strong lee wave. Air flowing over the Sierra Nevada plunges downward, then upward, and then downward again in a series of crests and troughs. The wavelength depends on the airflow characteristics, mainly the variation of air temperature with height (lapse rate) and the increase of wind speed with height (wind shear). The amplitude is greatest in strong waves and in cases where the vertical flow pattern is in resonance with the terrain, as, for example, when the second wave crest lies over the next mountain range downwind, such as the White-Inyo mountains, east of Owens Valley.
Updrafts and downdrafts in a strong lee wave often have speeds of 2,000 ft (600 m) per minute, sometimes exceeding 4,000 ft (1,200 m) per minute. Where the air descends, it warms, and the relative humidity decreases. The warm, dry winds, which may reach speeds of 60 mph (30 m/s) or greater at the surface, are known as foehn winds. The stratocumulus cloud deck over the Sierra Nevada is called a foehn-wall or cap cloud, and its downslope extension is known as a cloudfall. After it evaporates, the invisible moisture is cooled again in the ascending current and forms the turbulent cumuliform roll cloud. Looking at the roll cloud, an observer has the impression that it is rotating, but this is an illusion caused by the wind shear. However, below the roll cloud there is commonly a true rotor circulation, which brings easterly winds at
Figure 1.2
Sailplane in 4,000 ft/min (20 m/s) updraft en route to 40,000 ft (12,200 m) over Owens
Valley. The view is from the towplane at 15,000 ft (4,570 m) on 18 December 1951 at
14:47 PST. The windward sides of the roll cloud and high lenticular clouds are seen in the
center. At the lower right, a "cloud fall" descends the steep leeward slope of the High
Sierra, and at the lower left can be seen blowing dust from Owens (dry) Lake. At about this
time a Greyhound bus was blown off Highway 395 near Lone Pine.
the surface. On rare occasions a very strong lee wave will be in resonance with the terrain so that the first wave crest lies over the White-Inyo Range.
Above the roll cloud, there may be one layer or several decks of smooth, lens-shaped altocumulus clouds which appear stationary but through which the air is passing at 50–100 mph (22–45 m/s); the cloud droplets form at the windward edge and evaporate at the leeward edge. Soaring pilots make use of a mountain lee wave by flying into the wind and ascending in the updraft zone. In Figure 1.3 the dotted line shows a typical path of a sailplane: a line of flight under the roll cloud during airplane tow, release point at "x," and then a line of flight upward. Several flights above altitudes of 40,000 ft (12,000 m) have been made in lee waves by pilots equipped with oxygen to survive the low pressure (200 to 150 mb) and with warm clothing to withstand the cold temperatures (-94°F or -70°C).
Wave clouds may appear in any month of the year, but they are most often seen in late winter and in spring. They usually reach their maximum development in
Figure 1.3
Vertical cross section of Sierra lee wave showing airflow
pattern and cloud forms.
midafternoon and are most beautiful at sunset, when the highest clouds, at 30,000–40,000 ft (9,000–12,000 m), remain colorful long after the sunlight has left the leeward valley.
A local wind of another sort may sometimes be observed by motorists in the winter season in the Owens Valley near Olancha. During strong lee wave conditions or the passage of a cold front, a great horizontal cyclonic eddy may develop about Owens (dry) Lake; there will be a northerly (i.e., from the north) wind along the route of Highway 395 but, as evidenced by blowing dust, a southerly wind on the east side toward Keeler. When the surface flow is southerly in Owens Valley, the dust from Owens Lake may be carried far north of Bishop, lowering visibility so that the mountains are nearly obscured, and the alkali dust is sometimes tasted by pilots flying at 12,000 ft (3,600 m)! Similar phenomena occur in other arid basins of California and Nevada.