mesoscalem. d. eastin mountain waves and down-slope windstorms

Mesoscale M. D. Eastin

Mountain Waves andDown-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



Down-Slope WindsDefinitions:

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


Conceptual ModelMountain 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

θ+Δθ

θ+2Δθ

θ+6Δθ

θ+3Δθ



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

θ+Δθ

θ+2Δθ

A B

C D

EEL

Damped Oscillation



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)

θ

θ+Δθ

EL

θ

θ+Δθ

EL

Case #2

Case #1





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

θ

θ+2Δθ

EL

θ

θ+2Δθ

EL

θ+Δθ

θ+Δθ

Case #4

Case #3





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

θ

θ+2Δθ

EL

θ+Δθ

θ

θ+2Δθ

EL

θ+Δθ

Weak Flow

Strong Flow


Cloud FormationsMountain 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

θ EL

θ+Δθ

Lenticular Clouds


Down-Slope WindstormsDefinition

• 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


Down-Slope WindstormsBoulder 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)



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

From Lilly (1978)

EarlyTime

LaterTime

PriorInversion



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

From Lilly (1978)

EarlyTime

LaterTime

PriorInversion


Down-Slope WindstormsDevelopment 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


Down-Slope WindstormsDevelopment 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


Down-Slope WindstormsCritical Role of the Mid-level Inversion:

Numerical Simulationwith Mid-Level Inversion

Numerical Simulationwithout Mid-Level Inversion



Numerical Simulation Movie #1

(Short Mountain with Mid-Level Inversion)

Numerical Simulation Movie #2

(Tall Mountain with Mid-Level Inversion)


Down-Slope WindstormsForecasting:

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)


Down-Slope WindstormsClimatology for the Southern Appalachians:


Summary:

Down-Slope Winds

Conceptual Model of Mountain Waves• Physical processes• Critical factors

Cloud Formations


• Definition• Past Events• Development Mechanisms• Forecasting• Climatology for Southern Appalachians



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.

mesoscalem. d. eastin mountain waves and down-slope windstorms

Documents

mountain magnitude

amplitude of mountain

mountain range

air parcels

crossmountain flowcase

moderate oscillation

lower air

downslope flowing air