Mesoscale Phenomena

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.

Thunderstorms

At any time from early May to early October, there may be an incursion of tropical air from the south; then thunderstorms are possible. The intense heating of the arid Southwest during the summer months creates the upper-air anticyclone and surface low-pressure area that provide the circulation necessary for the northward flow of tropical air. In eastern California and Nevada, this summer monsoon is best developed during the period from early July to late August.

At first, the moisture enters the area at the high levels, and a thundery spell is commonly heralded by the appearance of rather exotic cirrus clouds from the south quadrant. Within a day or two, if the flow persists, the air at middle and low levels is also moist, and daily thundershowers can occur. These are strongly diurnal in their development; that is, they develop as a result of the daily heating of the mountain slopes by the sun, and they decline after sunset.

The appearance of patchy, turreted altocumulus clouds at sunrise is a good indication of possible thunderstorms later in the day. Heating of the rocky mountain slopes causes the air to rise toward the crests, and soon cumulus clouds form above these upslope currents. The clouds continue to rise upward, becoming what the weather observer calls towering cumulus. Near midday their tops develop a fibrous appearance indicative of ice crystal formation, and they are said to be glaciated. Soon they develop anvil-shaped cirrus tops with streamers of ice clouds stretching downwind at those levels (30,000–40,000 ft, or 9,000–12,000 m). At this time, lightning flashes from cloud to mountain and heavy local showers of rain and, commonly, graupel , or pea-sized hail, fall on the crests and ridges of the ranges. By this time hikers and climbers should have taken shelter.

Later, as downdrafts of cool air predominate, the thunderstorm ceases, and as the sky brightens to the west, the clouds begin to thicken over the leeward valley, where the day-long heating has created rising thermal currents. Often at 5:00 P.M. or 6:00 P.M. PST a brief thundershower is experienced in valley locations. As the shower moves eastward and the lowering sun in the west shines on the dark cloud and rain shafts, a brilliant rainbow is visible. Finally, when the sky has cleared and stars have appeared, lightning might continue to flash in the east if a nocturnal storm continues over central Nevada.

During some summers there are numerous thunderstorms, as in 1955, 1956, 1967, 1976, 1983, and 1984; in other years there are very few. It is difficult to predict exactly where the storms will occur, as this depends on subtle differences in wind velocity, amount of moisture, rate of growth, and topography. In the morning hours, the eastern slopes of the mountains are heated, the warm currents rise, and an easterly upslope wind forms in the valleys. If the upper synoptic flow has an easterly component, the clouds will develop even more rapidly. In the afternoon, the western slopes of the mountains are heated more effectively by the sun, and, especially if aided by a westerly breeze, the cloud development intensifies there. Once the cloud has formed, latent heat is released, which increases the cloud's buoyancy and causes it to rise more vigorously. Because these storms are so localized, commonly affecting a single canyon, intense cloudbursts may cause flash floods. These commonly occur when cloud bases are below the mountain tops. Such events often go unobserved in the Inyo Mountains and remote Nevada ranges and are discovered some days or weeks later. The damage is usually greater there, though, because roads commonly follow the canyons and washes.

As a general rule, it does not rain on summer nights in the mountains, but there are exceptions. Occasionally, the remnants of tropical storms called "easterly waves"

are carried northward along the Sierra and over much of Nevada. The cloudiness is general, and precipitation may be widespread, continuing at night and commonly accompanied by low clouds and fog, lightning, and thundershowers. Such episodes occurred in July 1956 and in August 1965.