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Mountain Waves and Down-Slope Windstorms

Mountain Waves and Down-Slope Windstorms. Mountain Waves and Down-Slope Windstorms. Down-Slope Winds Conceptual Model of Mountain Waves Cloud Formations Down-Slope Windstorms Definition Past Events Development Mechanisms Forecasting Climatology for Southern Appalachians.

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Mountain Waves and Down-Slope Windstorms

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  1. Mountain Waves and Down-Slope Windstorms M. D. Eastin

  2. Mountain Waves and Down-Slope Windstorms • Down-Slope Winds • Conceptual Model of Mountain Waves • Cloud Formations • Down-Slope Windstorms • Definition • Past Events • Development Mechanisms • Forecasting • Climatology for Southern Appalachians M. D. Eastin

  3. Down-Slope Winds • Definitions: • Chinook Winds: • Temperature of the downslope flowing • air is warmer than the air it replaces • Warming winds → dry adiabatic descent • No wind speed or temperature criteria • Santa Ana (CA) • Sundowner (Santa Barbara, CA) • Föhn (Alps) • zonda and pulche (Andes) • kachachan (Sri Lanka) • Bora Winds: • Temperature of the downslope flowing • air is colder than the air it replaces • Cooling winds → evaporational cooling • No wind speed or temperature criteria M. D. Eastin

  4. Conceptual Model • Mountain Waves: • Air parcels are displaced vertically as flow is • forced over a ridge or mountain range • If the atmosphere is stably stratified, then the • air parcels will descend on the other side and • begin to oscillate about their equilibrium level • Also called “internal gravity waves” • Stably Stratified? • Potential temperature increases with height • Atmosphere is “stable” → No instant convection • The atmosphere is stably stratified 99.9% of the time • Can you think of examples when and where the • atmosphere is not stably stratified? θ+6Δθ θ+3Δθ θ+2Δθ θ+Δθ θ M. D. Eastin

  5. Conceptual Model Mountain Waves: Oscillate about their Equilibrium Level? A When a low-level air parcel (with low θ) is forced aloft it enters a local environment characterized by higher-θ air B The air parcel will be negatively buoyant and begin to accelerate downward → will continue until the parcel and environmental θ are equal (the parcel’s “equilibrium level”, or EL) C Downward momentum will carry the parcel into an environment characterized by lower-θ air (the parcel “overshoots” its EL) D The air parcel will be positively buoyant and begin to accelerate upward → will continue until the parcel and environmental θ are equal E Upward momentum will again carry the parcel into a higher-θ environment Return to B → Damped oscillation develops θ+2Δθ θ+Δθ A B E θ EL C D Damped Oscillation M. D. Eastin

  6. Conceptual Model • Mountain Waves: • The amplitude of mountain waves depends • primarily on three parameters: • Height of the mountain • Magnitude of the stable stratification • Magnitude of the cross-mountain flow • Case 1: Short Mountain – Weak Stratification • Small initial vertical displacement • Small resulting negatively buoyancy • Small “overshoot” of EL • Weak oscillation (quickly damped) • Case 2: Tall Mountain – Weak Stratification • Large initial vertical displacement • Moderate resulting negatively buoyancy • Moderate “overshoot” of EL • Moderate oscillation(but damped) Case #1 θ+Δθ θ EL Case #2 θ+Δθ θ EL M. D. Eastin

  7. Conceptual Model • Mountain Waves: • The amplitude of mountain waves depends • primarily on three parameters: • Height of the mountain • Magnitude of the stable stratification • Magnitude of the cross-mountain flow • Case 3: Short Mountain – Strong Stratification • Small initial vertical displacement • Moderate resulting negatively buoyancy • Moderate “overshoot” of EL • Moderate oscillation (but damped) • Could produce downslope windstorm • Case 4: Tall Mountain – Strong Stratification • Large initial vertical displacement • Large resulting negatively buoyancy • Large “overshoot” of EL • Large oscillation(slowly damped or breaks) • Good chance of downslope wind storm Case #3 θ+2Δθ θ+Δθ θ EL Case #4 θ+2Δθ θ+Δθ θ EL M. D. Eastin

  8. Conceptual Model • Mountain Waves: • The amplitude of mountain waves depends • primarily on three parameters: • Height of the mountain • Magnitude of the stable stratification • Magnitude of the cross-mountain flow • Magnitude of Cross-Mountain Flow • Assume the height of the mountain and • the stable stratification are held constant • The stronger the flow, the larger the initial • vertical displacement and amplitude of • the resulting downstream oscillation • Strong flow could produce a downslope • windstorm for even a short mountain • or a weak stratification • Strong flow will very likely produce a • windstorm when both a tall mountain • and strong stratification are present Weak Flow θ+2Δθ θ+Δθ θ EL Strong Flow θ+2Δθ θ+Δθ θ EL M. D. Eastin

  9. Cloud Formations • Mountain Wave Clouds: • If the air parcel forced aloft is moist enough • to achieve saturation (i.e. reach it’s LCL) • then a cloud will form • Referred to as “lenticular” clouds • Multiple rows of clouds can form • downstream of a mountain range • if the air is moist and the oscillation • amplitude is large • The cloud rows are often oriented • parallel to the mountain range Lenticular Clouds θ+Δθ θ EL M. D. Eastin

  10. Down-Slope Windstorms • Definition • Strong winds that blow down the lee slope • of a mountain for a sustained period • Gusts often exceed 50 m/s (100 mph) • Typical Past Events: • Boulder, CO – 11-12 January 1972 • Chinook wind • 135 mph gust • 20 gusts above 120 mph in 45 minutes • $20 million damage • 40% of structures damaged • Knoxville, TN – 26 January 1996 • Chinook wind • 34 mph gusts • Minimal damage to a few houses • Los Angeles, CA – 14 October 1997 • Santa Ana winds • 87 mph gusts • Large fires in Orange County M. D. Eastin

  11. Down-Slope Windstorms • Boulder Windstorm – 11-12 January 1972 • Synoptic Pattern before the Event: • Strong winds (>25 kts) at mountain top (~680mb) • and at mid-levels (600-400mb) • Primarily zonal flow (no synoptic waves) • Strong stable stratification • Mid-level inversion (near ~615mb) M. D. Eastin

  12. Down-Slope Windstorms • Boulder Windstorm – 11-12 January 1972 • Aircraft Observations during the Event: • Aircraft observations divided • into two periods: • Early (lower-levels) • Later (upper-levels) • During the early period, • large amplitude waves • observed beneath the • inversion show evidence • of air descending to the • surface near Boulder • before ascending again • During the later period, • upper-level waves exhibit • very large amplitudes Later Time Prior Inversion Early Time From Lilly (1978) M. D. Eastin

  13. Down-Slope Windstorms • Boulder Windstorm – 11-12 January 1972 • Aircraft Observations during the Event: • Aircraft observations divided • into two periods: • Early (lower-levels) • Later (upper-levels) • During the early period, • strong near-surface winds • associated with descending • branch of a wave observed • along lee slope • During the later period, • upper-level waves also • exhibit strong winds in • conjunction with the • descending branch Later Time Prior Inversion Early Time From Lilly (1978) M. D. Eastin

  14. Down-Slope Windstorms • Development Mechanism #1: Reflection of Waves • Assumes there is a mid-tropospheric layer • of enhanced stability (a mid-level inversion) • Assumes winds are strong at mountain top • and increase in magnitude with height • When an upward propagating wave encounters • the enhanced stability, part of its energy is • reflected downward • Over time, as more air parcels are forced • aloft, multiple waves have part of their • energy reflected downward • The net effect is a downward transport of • high momentum air from aloft to the surface • Produces strong winds on the lee slope Strong Inversion M. D. Eastin

  15. Down-Slope Windstorms • Development Mechanism #2: Self-Induced Critical Layer • Assumes winds are strong at mountain top • and increase in magnitude with height • Assumes mountain is tall • Large amplitude waves are generated • Waves become unstable and “break” • (like the big waves that surfers ride) • The resulting overturning circulation • creates a “wave breaking region” that • behaves like a mid-level inversion layer • Subsequent waves begin to reflect off the • the inversion, producing a net downward • transport of high momentum air from aloft • down toward the surface • Produces strong winds on the lee slope M. D. Eastin

  16. Down-Slope Windstorms Critical Role of the Mid-level Inversion: Numerical Simulation with Mid-Level Inversion Numerical Simulation without Mid-Level Inversion M. D. Eastin

  17. Down-Slope Windstorms Numerical Simulation Movie #1 (Short Mountain with Mid-Level Inversion) Numerical Simulation Movie #2 (Tall Mountain with Mid-Level Inversion) M. D. Eastin

  18. Down-Slope Windstorms • Forecasting: • Conditions Favorable for Development: • Wind speed at mountain top level is greater than 20 knots • Wind direction is within 30º of perpendicular to ridgeline • Upstream temperature profile exhibits an inversion or layer of strong • stability near mountain top level • Ideal terrain includes long ridges with gentle windward slopes and • steep lee slopes (Colorado Front Range and Smokey Mountains) • Low mid-level humidity • Night time or early morning • No lee side cold pool (no cold air damning) M. D. Eastin

  19. Down-Slope Windstorms Climatology for the Southern Appalachians: M. D. Eastin

  20. Mountain Waves and Down-Slope Windstorms • Summary: • Down-Slope Winds • Conceptual Model of Mountain Waves • Physical processes • Critical factors • Cloud Formations • Down-Slope Windstorms • Definition • Past Events • Development Mechanisms • Forecasting • Climatology for Southern Appalachians M. D. Eastin

  21. References Durran, D.R., 1986: Mountain waves. Mesoscale Meteorology and Forecasting, P. Ray Ed., American Meteorological Society, Boston, 472-492. Durran, D. R., and J. B. Klemp, 1983: A compressible model for the simulation of moist mountain waves. Mon. Wea. Rev., 111, 2341-2361. Durran, D.R., 1986: Another look at downslope windstorms. Part I: On the development of analogs to supercritical flow in an infinitely deep continuously stratified fluid. J. Atmos. Sci., 93, 2527-2543. Durran, D.R., and J.B. Klemp, 1987: Another look at downslope winds. Part II: Nonlinear amplification beneath wave- overturning layers. J. Atmos. Sci., 44, 3402-3412. Klemp, J. B. and D. K. Lilly, 1975: The dynamics of wave-induced downslope winds. J. Atmos. Sci., 32, 320–339. Klemp, J. B. and D. K. Lilly, 1978: Numerical simulation of hydrostatic mountain waves. J. Atmos. Sci., 35, 78–107. Lilly, D. K., 1978: A severe downslope windstorm and aircraft turbulence event induced by a mountain wave. J. Atmos. Sci., 35, 59-77. Lilly, D. K. and E. J. Zipser, 1972: The Front Range windstorm of 11 January 1972 – a meteorological narrative. Weatherwise, 25, 56–63. M. D. Eastin

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