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

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 Mesoscale M. D. Eastin

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: air is colder than the air it replaces Cooling winds → evaporational cooling Mesoscale M. D. Eastin

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 Mesoscale M. D. Eastin

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 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 Mesoscale M. D. Eastin

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 Mesoscale M. D. Eastin

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 Mesoscale M. D. Eastin

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 Mesoscale M. D. Eastin

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 Mesoscale M. D. Eastin

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 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 Mesoscale M. D. Eastin

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) Mesoscale M. D. Eastin

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) Mesoscale M. D. Eastin

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) Mesoscale M. D. Eastin

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 Mesoscale M. D. Eastin

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 Mesoscale M. D. Eastin

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

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

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) Mesoscale M. D. Eastin

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

Down-Slope Windstorms 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 Mesoscale M. D. Eastin

References Mesoscale M. D. Eastin 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. Mesoscale M. D. Eastin