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Downslope Wind Storms
Common names for downslope winds
include: Bora, Foehn, Chinook
Occur on lee side of high-relief mountain
barriers when stable air is carried across
mountains by strong winds that increase in
height (Whiteman 2000).
Downslope Wind Storms
Common names for downslope winds include:
Bora, Foehn, Chinook
Occur on lee side of high-relief mountain barriers when
stable air is carried across mountains by strong winds that
increase in height (Whiteman 2000).
Winds are very strong at surface (sometimes exceeding
100 mph) and are caused by intense surface pressure
gradients.
Pressure gradient is intensified as descending air on the
lee side produces warming and a decrease in surface
pressure.
(Whiteman 2000)
Downslope Wind Storms
Primarily occur in winter and appear to be associated with
large-amplitude lee waves.
Descending branch of the first wave reaches the ground at
the foot of the slope because the amplitude of the first
wave has been increased
by resonance,
by wave trapping (trapping of vertical energy below a
smooth horizontal flow at a given height),
by development of hydraulic flow.
(Whiteman 2000)
Downslope Wind Storms
Local topography influences the strength of windstorms at
a given location.
Winds are strong downwind of high, continuous ridgelines.
Steep lee slopes where flow separations occur under
normal conditions, can cause an acceleration of hydraulic
flows.
Downslope winds can bring either cold or warm air into the
leeward foothills.
A cold downslope wind is called a bora and a wind that
brings very cold air to eastern Adriatic coast of Croatia.
(Whiteman 2000)
(Stull 2000)
Downslope Wind Storms
The bora originates in an area in central Asia where
temperatures are so low that, despite adiabatic warming,
the wind is still cold when it reaches the Adriatic coast.
Photo (c) Andrea Carloni
(Whiteman 2000)
The bora originates in an area in central Asia where
temperatures are so low that, despite adiabatic warming,
the wind is still cold when it reaches the Adriatic coast.
Bora- Observations
Surface observations of Bora event during Dec. 2004
(Ivančan-Picek et al. 2005)
Simulations of Bora near Zadar, Croatia (Ivančan-Picek et al. 2005).
Bora- Observations
Simulations of Bora near Zadar, Croatia (Ivančan-Picek et al. 2005).
Bora- Observations
Simulations of Bora near Zadar, Croatia (Ivančan-Picek et al.
2005).
This study suggested that the “Zadar Calm” was due to:
1. Primary wave could be responsible for low-level flow
separation over the steep terrain, leading to the strong Bora
flow “lifting off” the ground.
2. Local near surface wind speed minimum and the strongest
bora flow above it are in a good agreement with the sodar
measurements.
3. The maximum Bora speeds above Zadar were observed
between 300 and 500 m MSL, while the low-level flow was
characterized by weak winds.
Foehn
Föhn or Foehn (pronounced ‘firn’) is used
internationally to designate a warm dry downslope
wind.
The warming and drying are caused by adiabatic
compression as air descends the slopes on the
leeward side of a mountain range.
In western US this is called chinook, after
Northwest Indian tribe.
(Whiteman 2000)
Chinook
The term was first applied to a warm southwest
wind that was observed at the Hudson Bay trading
post at Astoria, Oregon, since it blew from ‘over
Chinook camp” (Burrows 1901).
(Whiteman 2000)
*Whiteman (2000) notes that the wind could not have been a true
foehn since the topography was not high enough to produce significant
adiabatic warming.
Chinook
Chinooks are primarily a western US phenomena
since the relief of the Appalachians is generally
insufficient to produce strong downslope winds.
In the Rocky Mountians, chinooks blow most
frequently from Nov. to March
The gusty warm winds rapidly melt wintertime snow
cover, called ‘snoweaters.’
(Whiteman 2000)
Chinook
Four factors contribute to the warmth and dryness of
chinook winds:
1. Air that descends the lee slope is armed and dried
by compressional heating at the dry adiabatic rate
of 9.8 °C km-1 as air is brought to lower altitudes
and, thus, higher pressures at base of lee slope.
2. When a deep flow causes air at low levels upwind
of mountain barrier to be lifted up the barrier,
latent heating occurs as clouds form and
precipitation falls on windward side, warming air
before it descends lee.
(Whiteman 2000)
Chinook
Four factors contribute to the warmth and dryness of
chinook winds:
3. Warm air descending the lee slopes can displace
a cold, moist air, thus enhancing the temperature
increase and humidity decrease associated with
the winds.
4. The turbulent foehn flow can prevent nocturnal
inversions from forming on the lee side, allowing
nighttime temperatures to remain warmer.
(Whiteman 2000)
Four Factors causing warming and drying
of downslope winds
(Whiteman 2000)
Downslope Wind Storms
Can start and stop suddenly at a given location.
This is due to changes in cross-barrier flow
component or stability of approaching flow that
cause the wavelength of the orographic waves to
change.
An abrupt cessation of downslope winds is called a
foehn pause or chinook pause. Alternating
strong wind break-ins and foehn pauses can
cause temperatures to oscillate greatly.
(Whiteman 2000)
Downslope Wind Storms
(Whiteman 2000)
Windstorms at Boulder, CO
(Whiteman 2000)
Typical Downslope Winds that occur in west US
(Whiteman 2000)
Santa Ana Winds
Santa Ana Winds
Three Mechanisms for Production of Severe
Downslope Winds
1. Long (1953) proposed a fundamental similarity
between downslope windstorms and hydraulic jumps.
Assume flow is in hydrostatic balance and bounded by a
free surface, no variations in coordinate direction parallel to
ridge axis, steady state behavior of system is governed by
shallow-water momentum and continuity equations.
u
D h
u
g

0
x
x x
uD
0
x
 ( D  h) h
(1  Fr )

x
x
2
Fr
2
2
u

gD
(Durran 1990)
Fr
2
2
u

gD
Fr is the ratio of the fluid velocity to speed of propagation of
linear shallow-water gravity waves.
The free surface can either rise of fall as the fluid encounters
a rising bottom topographic surface. This depends on
magnitude of Fr.
Hydraulic Flow
When Fr > 1, (supercritical flow) the fluid thickens and
slows down as it crosses the top of the obstacle, reaching
its minimum speed at the crest.
The accelerations experienced by the fluid are qualitatively
similar to those experienced by a hockey puck traversing a
frictionless mound of ice.
(Durran 1990)
The case Fr < 1 (subcritical flow) seems counterintuitive,
the flow thins and accelerates as it crosses the top of the
obstacle, reaching its maximum speed at the crest.
(Durran 1990)
If sufficient acceleration in stationary gravity wave, i.e., a
sufficient increase in velocity and decrease in thickness as
fluid ascends toward crest, a transition from subcritical flow
to supercritical occurs at top of mountain.
(Durran 1990)
Since flow along the lee slope is supercritical, fluid
continues to accelerate as it falls down mountain.
Eventually recovers to ambient downstream conditions in
turbulent hydraulic jump.
(Durran 1990)
Very high velocities are produced along the lee slope
because PE is converted to KE during the entire time fluid
parcel traverses mountain.
Deceleration that would otherwise occur in lee, is disrupted
when flow becomes supercritical.
(Durran 1990)
Hydraulic Flow: Wave Breaking
(Whiteman 2000)
Hydraulic Flow
Hydraulic flow produces a distinctive flow pattern in the
lee of a mountain barrier that is characterized by a
region of wave-breaking aloft and a sudden jump in
the streamline pattern downwind of the barrier.
Downslope windstorms may occur during hydraulic
flow.
(Whiteman 2000)
Hydraulic Flow
Three Mechanisms for Production of Severe
Downslope Winds
2. Vertical energy transport: downslope winds are
produced by large-amplitude vertically propagating
mountain waves.
Eliassen and Palm (1960) showed that when an
upward propagating wave encounters a region where
the Scorer parameter changes rapidly, part of its energy
can be reflected back into a downward propagating
wave.
Klemp and Lilly (1975) suggest that downslope
windstorms occur when the atmosphere is tuned so
that the partial reflections at each interface produce an
optimal superposition of upward and downward
(Durran 1990)
propagating waves.
Three Mechanisms for Production of Severe
Downslope Winds
2. Klemp and Lilly found that one of the most important
tuning requirements is that the tropopause be located
½ vertical wavelength AGL.
(Durran 1990)
¼ wavelength
(Durran 1990)
½ wavelength
Three Mechanisms for Production of Severe
Downslope Winds
3. Explanation of downslope windstorms: strong leeslope surface winds occurred after vertically
propagating waves became statically unstable and
broke.
The wave-breaking region is characterized by strong
mixing and local reversal of cross-mountain flow.
Proposed that the “wave-induced critical layer” acts as
a boundary, reflecting upward propagating waves back
toward the mountain.
(Durran 1990)
Forecasting Downslope Wind Events
Conditions favorable for downslope winds occur when:
• Wind is directed across mountain (within 30 degrees
perpendicular) and wind speed at mountaintop
exceeds terrain dependent value of 7 – 15 m/s.
• Upstream temperature profile exhibits an inversion
layer of strong stability near mountaintop level.
(Barry 2008; Durran 1990)
Composite Soundings / Boulder Windstorms
Upwind Sounding
(Barry 2008)
Downwind Sounding
11 Jan. 1972 Boulder Windstorm
(Barry 2008)
Three circumstances when atmosphere can
undergo a transition from subcritical to
supercritical flow:
1. Wave breaking forced by high mountain barrier
2. A two-layer atmosphere in terms of Scorer parameter
for mountains too small to force wave breaking
3. An atmosphere capped by a mean-state critical layer
above mountain top-bora-forcing wave breaking.
(Barry 2008; Durran 1990)
 u   D   u    gD u 
2
u   g
  u  
  Fr
 x   x   x   u x 