Thermal Upslope Circulations

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Thermal Upslope Circulations

• Determines the climatology of regions near
  mountain ranges
• Wind
• Temperature
• humidity
• precipitation
• Initiation mechanism for thunderstorms
• Initiation mechanism for cumulus systems

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Daytime Thermal Upslope Winds

• Daytime heating along slope
• Warm over slope compared to equivalent elevation
  over free air
• Hypsometric integration of pressure down from
  above, produces relatively low pressure along
  slope compared to free air at same elevation
• Air accelerates toward slope, forming a upslope
  movement of air

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Nocturnal Katabatic Winds

• Nocturnal cooling along slope
• Cold air over slope compared to equivalent
  elevation over free air
• Hypsometric integration of pressure down from
  above, produces relatively high pressure along
  slope compared to free air at same elevation
• Air accelerates away from slope, forming a
  down-slope movement of air
• Air may also flow downslope as a gravity current

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Steady State Thermal Upslope Flow (Defant, 1951)
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Terminology of Slope Flows

• Upslope Flow refers to thermally-driven flow up
  the side-walls of a valley
• Katabatic Flow refers to the thermally-driven
  flow down the side-walls in the early evening
• Valley Breeze refers to the regional
  pressure-gradient-driven flow up the valley
  lagged to a time after the upslope regime
  evacuates mass from the upper mountains and
  creates regional thermal low pressure at high
  elevations
• Mountain Breeze Opposite of valley breeze, ie
  regional high pressure forms in the upper valley
  as it is filled with cold dense air and gives
  rise to a regional wind down the valley, again
  lagged from the onset of the katabatic winds

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Circulations in the mountain Valley

• Consider local flow in a deep mountain valley,
  such as near Vail Colorado
• Think of air having a flow viscosity resembling
  honey in a valley of similar geometry
• Then a deep valley can effectively dam up the
  flow, and trap cold air
• How does the cold air ever get out?
• How does the boundary layer evolve?

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• This is a subject of considerable interest
  because of potential development in mountain
  valleys
• Coal-Fired Power Plants
• Oil Shale development
• Ski-resorts
• Fireplaces
• cars

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Valley Wind System

• Cold air in valley forms an inversion at the top
  of cold air pool
• Inversion remains horizontal, even though slope
  flow and katabatic flow is often assymetric
  across valley due to gravitational stabilization,
  ie it is like a basin full of a heavy fluid
• Therefore the pollution trapped in the valley
• must flow up along the surface to escape
• May remain trapped for a long period of time

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Cooling along valley sidewalls causes cold air to
sink into the valley along the valley side walls,
filling the valley with cold air. Actually
upward motion in the center of the valley!
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Throughout the evening, the cold air deepens in
the valley. The inversion at the top of the cold
air remains horizontal, although occasional
gravity waves may move along the inversion,
especially if there is a mean wind blowing.
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As the sun rises, and shines asymmetrically
across the valley the air is evacuated up the
heated sidewall. In the valley, subsidence
lowers the inversion which maintains its
horizontal orientation.
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The inversion lowers throughout the day. During
winter months, when solar isolation is relatively
low, the inversion typically will not be totally
removed and so pollution can be trapped for long
periods of time.
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Pollution Problem in Mountain Valleys

• Major problem for communities in mountain valleys
• Ski resorts such as Vail, Aspen, Snowmass and so
  on have major problem with wood burning and auto
  exhaust. Many are banning wood burning as a
  result.
• Much of the Nations coal supply is in the
  Rockies, and it is convenient to build power
  plants near the coal and in the valley where
  there is needed water necessary for the coal
  fired plant. But smoke is trapped hurting the
  environment. An issue How tall must the smoke
  stacks be?

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Slope Flow Effects

• The upward motion of slope flows produces
  distinct wind regimes leading to
• Diurnal wind regimes
• Convergence zones
• Dry Lines
• Initiation of Cumulus convection
• Modulation of Cumulus convection
• Geostrophic adjustment and the low-level jet

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Chimney Effect

• When thermal upslope moves up from two sides of a
  ridge, it will converge at the peak (assuming no
  mean flow) forming an upward jet of flow
• This can initiate clouds that subsequently help
  drive and intensify the upslope

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Influence of a Mean Wind

• Add a mountain-wave effect on lee side
• Form a convergence between mountain-wave forcing
  and thermal upslope, enhancing chimney effect on
  lee-side forming a mountain wave convergence
  zone

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Effect of a mean wind is to displace the chimney
effect downwind. We will explore this more.
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Hot Spots

• Depending on wind direction, there are favorable
  spots for the initiation of cumulus clouds by
  slope flows
• Next are some results of an extensive study by
  Banta, Wurman and others of cloud initiation
  climatology in Clorado

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Relationship Between Convection over the Rocky
Mountains and Convection over the Plains to the
East

• The relevance of the mountains plains circulation
  to the precipitation climatology over the Central
  US has been studied since the 1940s
• The daily progression of storms from the
  mountains eastward provides the evidence

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Convective Systems forming over the Rockies can
be Tracked Eastward all the way to Europe!
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South Park Area Cumulus Experiment

• Study Daily evolution of slope flow regimes
• Dry days, i.e. no clouds
• Moist Days, i.e. convection
• Study how deep cumulus formation over mountains
  relates to convection over the plains

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Close-up of the South Park Area Cumulus Experiment
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Study of the Dry Boundary Layer in South Park

• Observations
• Tether balloon (like those little blimps they use
  to promote a sale, but with an instrument package
  attached)
• Aircraft
• Surface mesonet
• radar
• Models
• CSU Rams

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