Dynamic Modification of Airflow over Mountains
(Terrain-Forced Flows)
Mountains act as barriers to planetary-scale circulations
There are three principle processes that affect these
flows:
1. Transfer of angular momentum to the surface through
friction and form drag
2. Blocking and defection of airflow
3. Modification of energy fluxes
Dynamic Modification of Airflow over Mountains
Synoptic scale effects
Orography has three major effects on such systems:
1. Flow blocking and barrier winds upstream of
barrier
2. Modification of frontal cyclones crossing a
mountain range
3. Enhanced cyclogenesis in the lee of mountains.
Dynamic Modification of Airflow over Mountains
Blocking effects and Barrier Winds
The so-called barrier effect of mountain ranges to air
motion is more evident when considering high and
continuous mountain ranges.
Various model studies of flow blocking have shown
that the Rossby number and Froude number are
important parameters controlling the effect of
ridges on air flow.
Dynamic Modification of Airflow over Mountains
Rossby number
Ro = U / fL
Where L is the mountain half width and U the crossmountain flow component.
Dynamic Modification of Airflow over Mountains
Froude number:
Is the non-dimensional ratio of the inertial force to the
gravitational force for fluid flow
F = V2 /Lg
where V = characteristic velocity, L = length, g =
gravitational acceleration.
For airflow over a barrier,
F = V/NL
Dynamic Modification of Airflow over Mountains
Froude number:
For airflow over a barrier,
F = V/NL
V = velocity normal to barrier, N = the Brunt-Väisälä
fequency, L = width of barrier.
This represents the natural wavelength of the airflow
to the wavelength of the terrain.
Dynamic Modification of Airflow over Mountains
Baines (1995) for flow past an obstacle,
F  U / gl
where U = fluid speed, g = gravity, and l = horizontal
length or vertical height (h).
Dynamic Modification of Airflow over Mountains
Blocking effects and Barrier Winds
Air parcels that have been deflected to the left (N.H.) are at
lower pressure and so move faster than air on the right
side of flow (F-1 is large).
Coriolis force is shown to be the major factor limiting the
upstream extent of flow clocking and also the degree of
deceleration.
Air crossing the mountains at summit level is slightly
accelerated.
Dynamic Modification of Airflow over Mountains
Dynamic Modification of Airflow over Mountains
Blocking effects and Barrier Winds
Where the pressure force exerted by the wind on the
windward slope (form drag) causes the flow to become
subgeostrophic, flow moves to the left along the mountain,
setting up a barrier wind.
Simple case of an upwind barrier jet can be observed in
California along western slope of the Sierra Nevada in
winter as stable air approaching from west causes
pressure excess of 5 mb (Parish 1982).
Maximum winds reach speeds of 20-25 m/s at 1 km height
and are approximately 100 km in width.
Dynamic Modification of Airflow over Mountains
Shutts (1998) suggests that the maximum barrier wind speed
in a steady-state solution is given by:
Vb=[(Δθ/θ)gh0]0.5 [(hm/h0)-1]
where Δθ is the interface temperature difference, θ is mean
potential temperature for two layers, hm is barrier height, h0
is depth of airflow.
Example: hm = 1500 m,
h0=300m,
g=9.8ms-2,
θ= 280K
Δθ = 3K
Vb = 22 m s-1
Dynamic Modification of Airflow over Mountains
Isotachs of the
barrier jet
(Parish 1982)
(Whiteman 2000)
Dynamic Modification of Airflow over Mountains
Effect of Cold Air Damming
Cold air damming against a mountain range is often
observed. Common east of the Appalachian Mountains.
Bell and Bosart (1988) show that cold-air damming is
critically dependent on the configuration of synoptic-scale
flow.
Dynamic Modification of Airflow over Mountains
Effect of Cold Air Damming
(Whiteman 2000)
Dynamic Modification of Airflow over Mountains
Effect of Cold Air Damming
Dynamic Modification of Airflow over Mountains
Mountain barriers modify the wind field through differential
pressure effects. Observations across many ranges
indicate two characteristic types of flow disturbances:
1. Pressure differential (~10 mb) between windward (high P)
and leeward (low) slopes of the range.
2. Upstream deflection of airflow to the left (NH) in
association with orographically-disturbed pressure field.
Dynamic Modification of Airflow over Mountains
Frontal Modifications
There are several dynamic and thermodynamic mechanisms
involved in the orographic modification of fronts.
“Masking effect”
Fronts crossing a mountain system with extensive
intermontane basins may over-ride shallow cold air. This can
diminish the low-level temperature contrast across a cold
front and accentuate that across a warm front.
Forced ascent of air over the barrier leads to distortion of the
temperature structure through adiabatic processes.
Dynamic Modification of Airflow over Mountains
Effect of terrain on frontal passage
(Whiteman 2000)
Dynamic Modification of Airflow over Mountains
Frontal Modifications
Since warm fronts have a typical slope of 1:100, subparrallel
to the usually steeper (1:20) windward slope of the barrier,
air ahead of the front can become trapped, thus tending to
retard the motion of the lower section of the front.
Cold fronts have a 1:50 backward slope, also tend to be
slowed down by mountain barriers, since the wind
component normal to the front is slowed first, and to a
greater degree at lower levels.
(Barry 2008)
Frontal Modification-Snake River Valley
Steenburgh &
Blazek (2001)
Dynamic Modification of Airflow over Mountains
Effect of terrain on frontal passage
(Whiteman 2000)
Dynamic Modification of Airflow over Mountains
Effect of terrain on frontal passage
A flow that splits around a barrier converges in the lee of the barrier.
Conceptual view of a cold air mass that approaches the Alps from northwest.
(Whiteman 2000)
Dynamic Modification of Airflow over Mountains
Frontal Modifications
Simulations by Dickinson and Knight (1999) of cold front
passage over mountain ridges of varying height and width
show that:
Tall/narrow mountains and weak fronts: there is upstream
blocking and frontal propagation is discontinuous across
ridge.
Low/wide mountains: there is only weak retardation on the
windward slope and front moves continuously across a
mountain.
Regardless of mountain size and shape, front reaches the
base of the lee slope stronger, sooner, and with a decreased
cross-front scale.
A cold front slows as it
ascends a mountain
barrier and accelerates
when the cold air
behind it flows over the
barrier.
Dynamic Modification of Airflow over Mountains
Gap Winds
Strong winds are often present in gaps (major erosional
openings through mountain ranges), in channels between
mountain ranges, and in mountain passes.
Winds are usually produced by pressure-driven channeling,
caused by strong horizontal pressure gradients across the
gap, channel, or pass.
The pressure gradient may form due to synoptic-scale
systems or from differences in temperature and density
between the air masses on either side of the opening.
Dynamic Modification of Airflow over Mountains
Gap Winds
Strong winds are often present in gaps (major erosional
openings through mountain ranges), in channels between
mountain ranges, and in mountain passes.
Winds are usually produced by pressure-driven channeling,
caused by strong horizontal pressure gradients across the
gap, channel, or pass.
The pressure gradient may form due to synoptic-scale
systems or from differences in temperature and density
between the air masses on either side of the opening.
Dynamic Modification of Airflow over Mountains
Gap Winds
(Whiteman 2000)
Dynamic Modification of Airflow over Mountains
Gap Winds
Regional pressure gradients occur frequently across coastal
mountain ranges because of differing characteristics of
marine and continental air masses (Whiteman 2000).
Waterfall cloud over Mackinnon
Pass (C. D. Whiteman Photo)
Gap Flows during MAP
Mayr et al. (2004)
Dynamic Modification of Airflow over Mountains
Gap Winds
Famous gap winds include the winds of the Bay Area
(Caracena Straight) and the Columbia River Gorge. These
flows are driven by east-west pressure gradients.
These winds are enhanced by the heat low that forms due to
interior heating relative to the cooler coastal air mass.
This phenomena is found more commonly in summer as
interior basins heat. Example: Great Basin-Sierra Nevada,
air is forced over the Sierra Crest and flows into Nevada.
This phenomena is the Washoe Zephyr (mentioned by
Samuel Clemens in Roughing It).
(Clements 1999, Zhong et al. 2007)
Dynamic Modification of Airflow over Mountains
Gap Winds and the Venturi Effect
When a valley or other channel has a substantial pressure
gradient along its length and a topographic constriction at
some point along the channel, air is accelerated through the
constriction by the pressure drop across the constriction
(Whiteman 2000).
Dynamic Modification of Airflow over Mountains
Gap Winds and the Venturi Effect
Acceleration through a terrain constriction is called the
Venturi or Bernoulli effect.
The flow speed can be roughly estimated by assuming that
mass of the flow is conserved through the channel, so
speed increases when the cross section of flow narrows,
and decreases when widens.
When the PGF across the constriction is weak, total
blockage can occur with air pooling behind it.
Dynamic Modification of Airflow over Mountains
Various local flows through major mountain barriers in Europe
(Whiteman 2000)
Dynamic Modification of Airflow over Mountains
Lee Cyclogenesis
Lee Cyclogenesis is of major importance downwind of
mountain barriers to the mid-latitude westerlies.
The effect of the Rocky Mountains in causing cyclogenesis
is well known.
Cyclogenesis in Northern Hemisphere
(Barry, 2008)
Dynamic Modification of Airflow over Mountains
Lee Cyclogenesis
Buzzi et al. (1987)
1. A precursor low-pressure system over Pacific, slows and
moves northward, fills as approaches west coast.
2. The parent low disappears above the Rockies while a
pre-existing lee trough, strengthens (low-level adiabatic
warming in westerly downslope flow).
3. Cyclonic vorticity generation takes place east of
mountains and a lee cyclone develops to south of latitude
of the parent system.
4. Cold air advection offsets any warming due to adiabatic
descent. Cyclogenetic tendencies continue, as result of
upper level advection of positive vorticity, divergence,
encouraging low-level convergence and rising air.
Dynamic Modification of Airflow over Mountains
Lee Cyclogenesis
Buzzi et al. (1987)
5. Surface cold front of Pacific system may move into the lee
trough and orographic effects cease as downslope
component of flow disappears with a shift of surface wind
dir to northwest.
6. Cyclone drifts southeastward while it remains close to
mountains and accelerates northeastward and may reintensify.
Dynamic Modification of Airflow over Mountains
Lee Cyclogenesis (Steenburgh and Mass 1994)
1. Mesoscale lee trough forms in response to cross barrier
flow driven by land falling Pac cyclone.
-also, shallow baroclinic zone located east of Rockies due
to prior surge of arctic air.
2. Cyclogenesis occurs east of Rockies & lee trough
broadens in scale. Confluence associated w/ lee troughproduces frontogenesis.
3. low-level confluent frontogenesis continues
-lee trough represents a break in air mass origin
- subsided air from Rockies is confluent w/ colder air
originating over Great Plains.
Dynamic Modification of Airflow over Mountains
Lee Cyclogenesis (Steenburgh and Mass 1994)
4. As upper-level trough moves east, low center and northern
portion of trough move away from mountains.
- to the south, the lee trough remains fixed to the
topography.
- cold air advection develops behind lee trough
- arctic front begins to rotate around low.
This model does not represent Norwegian Cyclone Model:
1. no warm sector
2. there are multiple cold advection
features.
Lee Cyclogenesis (Steenburgh and Mass 1994)
5. The zone of cold advection behind lee trough overtakes it
and forms an occluded front.
6. Cyclone begins to develop a more ‘classic’ appearance
downstream of Rockies when the arctic front overtakes
the lee trough.
Lee Cyclogenesis (Steenburgh and Mass 1994)
Lee Cyclogenesis (Steenburgh and Mass 1994)
Lee Cyclogenesis (Steenburgh and Mass 1994)
Lee Cyclogenesis (Steenburgh and Mass 1994)
Lee Cyclogenesis (Steenburgh and Mass 1994)
Lee Cyclogenesis (Steenburgh and Mass 1994)