dynamic modification of airflow over mountains (terrain-forced flows)

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.


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.


Rossby number

Ro = U / fL

Where L is the mountain half width and U the cross-mountain flow component.


Froude number:

Is the non-dimensional ratio of the inertial force to the gravitational force for fluid flow

F = V2 /Lgwhere V = characteristic velocity, L = length, g =

gravitational acceleration.

For airflow over a barrier,

F = V/NL


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.


Baines (1995) for flow past an obstacle,

glUF /

where U = fluid speed, g = gravity, and l = horizontal length or vertical height (h).



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.



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 MountainsShutts (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= 280KΔθ = 3K

Vb = 22 m s-1


Isotachs of the barrier jet

(Parish 1982)

(Whiteman 2000)


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.



(Whiteman 2000)


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.


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.


Effect of terrain on frontal passage

(Whiteman 2000)



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)



(Whiteman 2000)



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)



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.


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.


Gap Winds

(Whiteman 2000)


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 MackinnonPass (C. D. Whiteman Photo)

Gap Flows during MAP

Mayr et al. (2004)


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)


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).


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.


(Whiteman 2000)

Various local flows through major mountain barriers in Europe


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)


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.


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 re-intensify.


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 trough-produces 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.



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 sector2. there are multiple cold advection

features.


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.

dynamic modification of airflow over mountains (terrain-forced flows)

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