Mountain Waves and
Down-Slope Windstorms
Mesoscale
M. D. Eastin
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:
• Temperature of the downslope flowing
air is colder than the air it replaces
• Cooling winds → evaporational cooling
• No wind speed or temperature criteria
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?
θ+6Δθ
 Potential temperature increases with height
• Atmosphere is “stable” → No instant convection
• The atmosphere is stably stratified 99.9% of the time
θ+3Δθ
θ+2Δθ
θ+Δθ
Can you think of examples when and where the
atmosphere is not stably stratified?
Mesoscale
θ
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)
θ+2Δθ
θ+Δθ
A
B
E
C
θ EL
D
Damped Oscillation
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
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
Case #1
• 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)
Mesoscale
θ+Δθ
θ EL
Case #2
θ+Δθ
θ EL
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
Case #3
• 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
Mesoscale
θ+2Δθ
θ+Δθ
θ EL
Case #4
θ+2Δθ
θ+Δθ
θ EL
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
Weak Flow
Magnitude of Cross-Mountain Flow
θ+Δθ
• 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
Mesoscale
θ+2Δθ
θ EL
Strong Flow
θ+2Δθ
θ+Δθ
θ EL
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
Lenticular Clouds
θ+Δθ
θ
EL
• 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
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
• 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
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
Later
Time
Prior
Inversion
Early
Time
• During the later period,
upper-level waves exhibit
very large amplitudes
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)
Later
Time
• 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
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
Strong Inversion
 Produces strong winds on the lee slope
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
Down-Slope Windstorms
Critical Role of the Mid-level Inversion:
Numerical Simulation
with Mid-Level Inversion
Mesoscale
Numerical Simulation
without Mid-Level Inversion
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
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
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 waveoverturning 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