Flow Interaction with Topography

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Flow Interaction with Topography

• Fundamental concepts http//www.meted.ucar.edu/me
  soprim/flowtopo/
• Mountain waves and down slope winds
  http//www.meted.ucar.edu/mesoprim/mtnwave/
• Overview
• Hazards
• Waves
• Features
• Climatology (pg 1 only)
• Downslope winds
• Concepts
• Factors
• Interactions
• Uniform flow
• Complex flow
• Exercise
• Rotors (pgs 1-2)

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Flow Interaction with TopographyNotes
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Flow Interaction with TopographyNotes
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Flow Interaction with TopographyNotes
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Flow Interaction with TopographyNotes
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Flow Interaction with TopographyNotes
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SUMMARYMountain waves form above and downwind
of topographic barriers when strong winds blow
with a significant vector component perpendicular
to the barrier in a stable environment.
Mountain waves frequently pose a serious hazard
to mountain aviation because of strong-to-extreme
turbulence, which sometimes occurs without visual
indicators.Under some circumstances, mountain
wave activity can lead to strong and/or damaging
downslope windstorms in the lee of a mountain
barrier.A series of lenticular clouds
downstream of the barrier often indicates the
existence of trapped lee waves.Cap clouds
indicate likely wave activity downstream.Clear
air turbulence often occurs near the tropopause
due to vertically-propagating waves.Rotors are
part of a low-level turbulent zone that is
another region of potentially significant
turbulence.Foehn winds are warm, dry, gusty
downslope winds found along mountain ranges
throughout the world. Their warmth results from
adiabatic compression as winds descend lee
slopes.Bora winds are cold downslope winds that
result from a deep and very cold upstream air
mass that spills over the barrier and displaces a
warmer air mass. Downslope winds occur most
often during cold months and at night when the
atmosphere is most stable.The Froude number
expresses a ratio between the kinetic energy
(wind speed) and the potential energy (stability
times mountain height). If the Froude number is
equal to or slightly greater than 1, then there
is the likelihood of mountain wave activity If
the Froude number is less than one, then the
airflow is insufficient to carry the flow over
the mountain and the flow is blocked If Froude
number is much more than 1, airflow proceeds
right over the mountain and down the other side,
with no significant oscillations A critical
level results when the cross-barrier flow goes to
zero above mountaintop level.Critical levels do
not allow the vertically-propagating energy
associated with mountain waves to continue
upwards. Instead, that energy is deflected off
the critical layer back toward the surface.
Consequently, critical levels can contribute to
the development of and/or the strengthening of
downslope windstorms. The speed of those winds
can be 2-3 times the upwind speed at mountaintop
height.Critical levels may be self-induced by
wave breaking or result from a mean state
condition in the overall flow.NWP models can
provide valuable information on the conditions
that lead to mountain wave activity and downslope
winds. However, these NWP models require a
horizontal grid spacing of 10 km or less to
provide details of a mountain wave.Eta
coordinate models tend to have difficulties
generating vertically-propagating gravity waves
and downslope wind forecasts.Observational
platforms are valuable tools for determining the
current existence of and predicting mountain wave
activity within 12 hours. Observations of
particular interest include satellite imagery,
rawinsondes, and pilot reports.