Downslope Wind Storms Common names for downslope winds include: Bora, Foehn, Chinook Occur on lee side of high-relief mountain barriers when stable air is carried across mountains by strong winds that increase in height (Whiteman 2000).

Common names for downslope winds include: Bora, Foehn, Chinook Occur on lee side of high-relief mountain barriers when stable air is carried across mountains by strong winds that increase in height (Whiteman 2000). Winds are very strong at surface (sometimes exceeding 100 mph) and are caused by intense surface pressure gradients. Pressure gradient is intensified as descending air on the lee side produces warming and a decrease in surface pressure. Downslope Wind Storms (Whiteman 2000)

Primarily occur in winter and appear to be associated with large-amplitude lee waves. Descending branch of the first wave reaches the ground at the foot of the slope because the amplitude of the first wave has been increased by resonance, by wave trapping (trapping of vertical energy below a smooth horizontal flow at a given height), by development of hydraulic flow. Downslope Wind Storms (Whiteman 2000)

Downslope Wind Storms Local topography influences the strength of windstorms at a given location. Winds are strong downwind of high, continuous ridgelines. Steep lee slopes where flow separations occur under normal conditions, can cause an acceleration of hydraulic flows. Downslope winds can bring either cold or warm air into the leeward foothills. A cold downslope wind is called a bora and a wind that brings very cold air to eastern Adriatic coast of Croatia. (Whiteman 2000)

(Stull 2000)

Downslope Wind Storms The bora originates in an area in central Asia where temperatures are so low that, despite adiabatic warming, the wind is still cold when it reaches the Adriatic coast. (Whiteman 2000) Photo (c) Andrea Carloni

The bora originates in an area in central Asia where temperatures are so low that, despite adiabatic warming, the wind is still cold when it reaches the Adriatic coast.

Surface observations of Bora event during Dec (Ivančan-Picek et al. 2005) Bora- Observations

Simulations of Bora near Zadar, Croatia (Ivančan-Picek et al. 2005).

Bora- Observations Simulations of Bora near Zadar, Croatia (Ivančan-Picek et al. 2005).

Bora- Observations Simulations of Bora near Zadar, Croatia (Ivančan-Picek et al. 2005). This study suggested that the “Zadar Calm” was due to: 1.Primary wave could be responsible for low-level flow separation over the steep terrain, leading to the strong Bora flow “lifting off” the ground. 2.Local near surface wind speed minimum and the strongest bora flow above it are in a good agreement with the sodar measurements. 3.The maximum Bora speeds above Zadar were observed between 300 and 500 m MSL, while the low-level flow was characterized by weak winds.

Föhn or Foehn (pronounced ‘firn’) is used internationally to designate a warm dry downslope wind. The warming and drying are caused by adiabatic compression as air descends the slopes on the leeward side of a mountain range. In western US this is called chinook, after Northwest Indian tribe. Foehn (Whiteman 2000)

The term was first applied to a warm southwest wind that was observed at the Hudson Bay trading post at Astoria, Oregon, since it blew from ‘over Chinook camp” (Burrows 1901). Chinook (Whiteman 2000) *Whiteman (2000) notes that the wind could not have been a true foehn since the topography was not high enough to produce significant adiabatic warming.

Chinooks are primarily a western US phenomena since the relief of the Appalachians is generally insufficient to produce strong downslope winds. In the Rocky Mountians, chinooks blow most frequently from Nov. to March The gusty warm winds rapidly melt wintertime snow cover, called ‘snoweaters.’ Chinook (Whiteman 2000)

Four factors contribute to the warmth and dryness of chinook winds: 1.Air that descends the lee slope is armed and dried by compressional heating at the dry adiabatic rate of 9.8 °C km -1 as air is brought to lower altitudes and, thus, higher pressures at base of lee slope. 2.When a deep flow causes air at low levels upwind of mountain barrier to be lifted up the barrier, latent heating occurs as clouds form and precipitation falls on windward side, warming air before it descends lee. Chinook (Whiteman 2000)

Four factors contribute to the warmth and dryness of chinook winds: 3. Warm air descending the lee slopes can displace a cold, moist air, thus enhancing the temperature increase and humidity decrease associated with the winds. 4. The turbulent foehn flow can prevent nocturnal inversions from forming on the lee side, allowing nighttime temperatures to remain warmer. Chinook (Whiteman 2000)

Four Factors causing warming and drying of downslope winds (Whiteman 2000)

Can start and stop suddenly at a given location. This is due to changes in cross-barrier flow component or stability of approaching flow that cause the wavelength of the orographic waves to change. An abrupt cessation of downslope winds is called a foehn pause or chinook pause. Alternating strong wind break-ins and foehn pauses can cause temperatures to oscillate greatly. Downslope Wind Storms (Whiteman 2000)

Downslope Wind Storms

(Whiteman 2000) Windstorms at Boulder, CO

Typical Downslope Winds that occur in west US (Whiteman 2000)

Santa Ana Winds

Three Mechanisms for Production of Severe Downslope Winds 1.Long (1953) proposed a fundamental similarity between downslope windstorms and hydraulic jumps.

Assume flow is in hydrostatic balance and bounded by a free surface, no variations in coordinate direction parallel to ridge axis, steady state behavior of system is governed by shallow-water momentum and continuity equations. (Durran 1990)

Fr is the ratio of the fluid velocity to speed of propagation of linear shallow-water gravity waves. The free surface can either rise of fall as the fluid encounters a rising bottom topographic surface. This depends on magnitude of Fr.

Hydraulic Flow

When Fr > 1, (supercritical flow) the fluid thickens and slows down as it crosses the top of the obstacle, reaching its minimum speed at the crest. The accelerations experienced by the fluid are qualitatively similar to those experienced by a hockey puck traversing a frictionless mound of ice. (Durran 1990)

The case Fr < 1 (subcritical flow) seems counterintuitive, the flow thins and accelerates as it crosses the top of the obstacle, reaching its maximum speed at the crest. (Durran 1990)

If sufficient acceleration in stationary gravity wave, i.e., a sufficient increase in velocity and decrease in thickness as fluid ascends toward crest, a transition from subcritical flow to supercritical occurs at top of mountain.

(Durran 1990) Since flow along the lee slope is supercritical, fluid continues to accelerate as it falls down mountain. Eventually recovers to ambient downstream conditions in turbulent hydraulic jump.

(Durran 1990) Very high velocities are produced along the lee slope because PE is converted to KE during the entire time fluid parcel traverses mountain. Deceleration that would otherwise occur in lee, is disrupted when flow becomes supercritical.

Hydraulic Flow: Wave Breaking (Whiteman 2000)

Hydraulic Flow (Whiteman 2000) Hydraulic flow produces a distinctive flow pattern in the lee of a mountain barrier that is characterized by a region of wave-breaking aloft and a sudden jump in the streamline pattern downwind of the barrier. Downslope windstorms may occur during hydraulic flow.

Hydraulic Flow

2. Vertical energy transport: downslope winds are produced by large-amplitude vertically propagating mountain waves. Eliassen and Palm (1960) showed that when an upward propagating wave encounters a region where the Scorer parameter changes rapidly, part of its energy can be reflected back into a downward propagating wave. Klemp and Lilly (1975) suggest that downslope windstorms occur when the atmosphere is tuned so that the partial reflections at each interface produce an optimal superposition of upward and downward propagating waves. Three Mechanisms for Production of Severe Downslope Winds (Durran 1990)

2. Klemp and Lilly found that one of the most important tuning requirements is that the tropopause be located ½ vertical wavelength AGL. Three Mechanisms for Production of Severe Downslope Winds (Durran 1990)

¼ wavelength ½ wavelength (Durran 1990)

3. Explanation of downslope windstorms: strong lee- slope surface winds occurred after vertically propagating waves became statically unstable and broke. The wave-breaking region is characterized by strong mixing and local reversal of cross-mountain flow. Proposed that the “wave-induced critical layer” acts as a boundary, reflecting upward propagating waves back toward the mountain. Three Mechanisms for Production of Severe Downslope Winds (Durran 1990)

Forecasting Downslope Wind Events (Barry 2008; Durran 1990) Conditions favorable for downslope winds occur when: Wind is directed across mountain (within 30 degrees perpendicular) and wind speed at mountaintop exceeds terrain dependent value of 7 – 15 m/s. Upstream temperature profile exhibits an inversion layer of strong stability near mountaintop level.

Upwind SoundingDownwind Sounding (Barry 2008) Composite Soundings / Boulder Windstorms

11 Jan Boulder Windstorm (Barry 2008)

Three circumstances when atmosphere can undergo a transition from subcritical to supercritical flow: 1.Wave breaking forced by high mountain barrier 2.A two-layer atmosphere in terms of Scorer parameter for mountains too small to force wave breaking 3. An atmosphere capped by a mean-state critical layer above mountain top-bora-forcing wave breaking. (Barry 2008; Durran 1990)