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

2. 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

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:
air is colder than the air it replaces
Cooling winds → evaporational cooling
Mesoscale
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
Mesoscale
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
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

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
Mesoscale
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
Mesoscale
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
Mesoscale
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
Mesoscale
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 – 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

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

12. Down-Slope Windstorms
Boulder Windstorm – 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

13. Down-Slope Windstorms
Boulder Windstorm – 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

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
Mesoscale
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
Mesoscale
M. D. Eastin

16. 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

17. 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

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)
Mesoscale
M. D. Eastin

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

20. 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

21. References Mesoscale M. D. Eastin
Durran, D.R., 1986: Mountain waves. Mesoscale Meteorology and Forecasting, P. Ray Ed., American Meteorological Society, Boston,
Durran, D. R., and J. B. Klemp, 1983: A compressible model for the simulation of moist mountain waves.
Mon. Wea. Rev., 111,
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,
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,
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,
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