2.1 Mountain-related weather in the Rocky Mountains

The Rockies and other mountains in the western USA induce a variety of local weather phenomena. In response to the diurnal radiative cycle, slope winds develop that represent major local departures from the synoptic-scale flow (e.g., Pielke and Segal, 1986). During the daytime on sunny days, upslope flows develop along the heated slopes. The rising buoyant plumes are significant sources of turbulence for general aviation, and become the sites for convective development. Depending upon the large-scale wind direction and speed, convection can be initiated over the crests or on the lee slopes relative to the prevailing flow. Thus, convective precipitation patterns and temporal evolution can exhibit considerable day-to-day variation.

In stable situations, precipitation is enhanced on the windward slopes through a complicated process involving microphysical and blocking effects (e.g., Smith, 1989a). Where there is a prevailing downslope flow, precipitation can be totally suppressed.

At night, cool air descends the slopes forming drainage flows that accumulate cool air in valleys. These circulations influence local fog formation and, since cities have historically arisen in valleys, there is a significant impact on ground and air transportation. There is also the problem of air quality and human health. Because power plants and heavy industries tend to be sited in valleys, orographic circulations of this type may require use of relatively expensive low-sulfur coal and restrict certain industrial activities. Understanding the mesoscale climatology of these flows can be valuable in siting of new businesses, industries and transportation facilities.

In stronger prevailing winds, the influence of mountains becomes largely one of deflecting the airflow, resulting in mesoscale barrier jets on the flanks of the mountain, and gap winds where flow accelerates through valleys or other breaks in the high terrain. A good example is the Wyoming wind corridor where westerly flow, diverted by the Front Range, shoots through to the Great Plains (Marwitz and Dawson, 1984). Depending upon the direction and speed of the prevailing flow, a convergence line or a series of eddies form in the lee of the terrain. A notable example is the Denver cyclone, generated by the Palmer Ridge (e.g., Wilczak and Christian, 1990). In convective situations, the convergence line or eddy can become a preferred site for initiation of thunderstorms and tornadoes (e.g., Brady and Szoke, 1989; Wakimoto and Wilson, 1989).

In the lee of the Front Range of the Rockies, from Colorado northward, a complex interaction of several orographic processes can lead to damaging downslope windstorms (e.g., Lilly and Zipser, 1972; Clark et al., 1994). Similar events can occur in the Wasatch Mountains of Utah and the Sierras of California. Upon occasion, wind speeds in these events can produce damage measured in tens of millions of dollars (Storm data, 1982). In some communities, building codes have been elevated to decrease the frequency and amount of property damage resulting from downslope windstorms. However, they continue to pose a variety of problems to ground transportation because vehicles can be blown off the road, and because of blowing snow or dust raised by the winds. These storms can also pose hazards to aviation because they are often affiliated with strong cross-runway winds, large wave motions, wind shear, vortices and turbulence (Carney et al. 1996). Interaction of the small-scale terrain features with the wind can also induce the formation and shedding of whirlwind-like eddies that can cause localized speed increases in orographic windstorms (Idso, 1975).

On the regional scale, a lee trough frequently sets up immediately east of the Front Range of the Rocky Mountains (e.g., Steenburgh and Mass, 1994). Though the mechanism of this formation may be more complicated on some occasions, in many instances the trough development is augmented hydrostatically by adiabatic warming as strong cross-mountain flow near mountain top descends along the terrain or mixes to the surface. Dramatic temperature changes of 10oC or more can occur near the leading edge of these "Chinook" winds, where a shallow entrenched cold dome is suddenly displaced. When jet stream disturbances reach the lee trough, cyclogenesis is favored. In situations where low-level anticyclones push southward toward the Rockies, a type of blocking phenomenon sometimes referred to as "cold-air damming" can ensue (Dunn, 1987). On the western flank of the anticyclone, where the gradient wind has a component from the east, the mountains block the westward progress of the cold air mass. A cold dome can then become entrenched along the east slopes, and a southward surge of cold air may develop (Young and Johnson, 1984; Shapiro, 1984; Colle and Mass, 1995). On occasion, a very intense cold surge of this type may reach Texas (yielding a "blue norther"; Mogil, 1985), Mexico, or even Central America. The surge of the cold air there sometimes produces local gap winds referred to as "Tehuantepecers" in the Gulf of Tehuantepec (e.g., Donn, 1965).

During the development of these cold surges, blizzard conditions can occur through eastern Colorado, Nebraska and Kansas. Near the Front Range of the Rockies where the easterly winds ascend and pile up against the terrain, warmer, moister air originally over the region is lifted and can result in mesoscale snowstorms (Boatman and Reinking, 1984). In association with the blocked flow, mesoscale northerly "barrier jets" can develop along the terrain (Dunn, 1992). Local anticyclones caused by ridges can focus ascent and snowfall (Wesley et al. 1995). The details of the complicated evolution of the wind and precipitation patterns are not generally predicted accurately by operational numerical models. Forecast failures of this type can cause aviation accidents and major problems with ground transportation.

Forecasting the lift-out of the cold dome can also be difficult (Mayr and McKee, 1995). Its presence or absence is crucial in determining whether or not gusty downslope windstorms will remain aloft or reach the surface (Lee et al., 1989). If the cold dome remains entrenched, warm fronts are unable to invade the region when the next cyclone system approaches from the west or southwest. As a consequence, cyclones may weaken or disappear, reducing the amount of precipitation that might otherwise have occurred. Often a cyclone center re-appears near the Mississippi River valley east of the damming region, but the forecasts of precipitation and storm track are usually poorer than average in these situations (e.g., Junker et al., 1989). In this way, orographically induced damming can influence forecast accuracy 1000 km to the east.

A nocturnal low-level jet is a common feature over the sloping terrain between the Mississippi River valley and the Rockies (Bonner, 1968; Mitchell et al., 1995). The nocturnal jet is thought to contribute to the development of nocturnal thunderstorms, heavy precipitation, mesoscale convective complexes and flash floods (Wallace, 1975). The formation of the nocturnal jet is the result of the superposition on the large-scale geostrophic flow and ageostrophic winds arising from boundary layer evolution over sloping terrain, (e.g., Fast and McCorcle, 1990; Savijarvi, 1991).

In the summer season when there is a net positive radiative balance, a deep, hot, dry boundary layer can develop over the arid mountain plateaus of Mexico and the desert southwest. When strong westerly or southwesterly winds develop near mountaintop (~700 mb) over these regions, often in association with the approach of a mid-latitude trough, the elevated boundary layer air can be swept off the mountains and carried to the Midwest (e.g., Carlson et al., 1983; Arritt et al., 1992). Because of higher moisture availability over the richly vegetated Midwest, boundary layer potential temperatures are much lower than those over the mountains. Thus, a "capping inversion" develops in these situations, often strong enough to put a thundershower-suppressing "lid" over the entire Midwest. This can suppress precipitation and when the lid conditions persist, may contribute to the formation of Midwestern drought. In the course of a Midwestern drought, the influence of the "lid", in turn, may extend even to the East Coast (Lanicci and Warner, 1991; Farrell and Carlson, 1989). In these types of weather patterns, significant precipitation only occurs near the weak edges of a lid, where strong vertical motions due to approaching frontal systems raise and "break" the lid. When the lid is weaker, daytime heating plus convergence and ascent from surface mesoscale boundaries are sufficient to break through (Graziano and Carlson, 1987). The dry line is one such boundary, and is a surface manifestation of the processes that also create the elevated mixed layer (e.g., Ziegler et al., 1995).