Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Downslope Wind StormsCommon names for downslope winds include: Bora, Foehn, ChinookOccur 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, ChinookOccur 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 StormsLocal 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 StormsThe 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 CarloniThe 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. 2004(Ivančan-Picek et al. 2005)Bora- ObservationsSimulations of Bora near Zadar, Croatia (Ivančan-Picek et al. 2005).Bora- ObservationsSimulations of Bora near Zadar, Croatia (Ivančan-Picek et al. 2005).Bora- ObservationsSimulations 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 MarchThe 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)(Whiteman 2000)Downslope Wind Storms(Whiteman 2000)Windstorms at Boulder, COTypical Downslope Winds that occur in west US(Whiteman 2000)Santa Ana WindsSanta Ana WindsThree Mechanisms for Production of Severe Downslope Winds1. Long (1953) proposed a fundamental similarity between downslope windstorms and hydraulic jumps.0xhxDgxuu0xuDxhxhDFr)()1(2gDuFr22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)gDuFr22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 FlowWhen Fr > 1, (supercritical