COLD AIR DAMMING

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COLD AIR DAMMING

• Seminar Presentation

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Presentation Outline

• What is cold air damming
• What processes lead to this phenomenon
• How does damming effect weather systems
• Past and present modeling capabilities
• Potential research topic
• Atmospheric models in a GIS

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Cold Air Damming

• Occurs when low-level flow from the southern
  portion of a cold anticyclone becomes trapped
  along the eastern slopes of a mountain range
• This easterly wind in the anticyclone is
  deflected southward by the terrain creating a
  northerly component to the wind over time along
  the mountains
• This advects cold air from the north into the
  trapped low-level stable air mass and creates a
  region of extremely cold temperatures between the
  mountains and the coast

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Locations where Damming Occurs
Geography of Cold Air Damming

• Along the eastern slopes of the Appalachians in
  central Virginia North Carolina and South
  Carolina
• Along a portion of the eastern slopes of the
  Rockies in north central Colorado

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Presentation Outline

• What is cold air damming
• What processes lead to this phenomenon
• How does damming effect weather systems
• Past and present modeling capabilities
• Potential research topic
• Atmospheric models in a GIS

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Orientation of Pressure Field

• Focus in on cold air damming along the
  Appalachians
• In order for this to occur in the winter and
  early spring you need a high pressure center to
  the north
• This will bring winds in toward the eastern
  slopes of the Appalachians

H
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Diagram of Wind Trajectory Shift
H
Coriolis Force
Pressure Gradient Force
Parcel Trajectory
- Air slows down as it approaches the stable
eastern slopes- Causes the coriolis force to
weaken in magnitude - Pressure gradient force
begins to dominate- This force imbalance
deflects the wind trajectory southward
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January 2004 Case Surface Data
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The Damming Process
Terrain Blocking

• Flow comes to a screeching halt along the eastern
  slopes
• Over time an existing low-level stable layer
  intensifies and the cold air mass develops
• Along with this comes a deflection of the parcel
  trajectory causing the cold air mass to surge
  southward

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Diagram of Generated Cold Pool
Cold AdvectionFrom North
Cold Pool Surges South Over Time
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January 2004 Case - Temperatures
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January 2004 Case 24 hour Temp.
23Z 6pm
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Presentation Outline

• What is cold air damming
• What processes lead to this phenomenon
• How does damming effect weather systems
• Past and present modeling capabilities
• Potential research topic
• Atmospheric models in a GIS

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Precipitation

• When temperatures drop below the freezing mark
  (0C or 32F) liquid precipitation can morph into
  a frozen state
• Cold air damming can generate layers of
  sub-freezing temperatures in the low-levels
• Precipitation type depends on the thickness of
  this layer

Effects of Precipitation Type
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January 2004 Case Radar
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Presentation Outline

• What is cold air damming
• What processes lead to this phenomenon
• How does damming effect weather systems
• Past and present modeling capabilities
• Potential research topic
• Atmospheric models in a GIS

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Resolution of Elevation Data
USGS 3 Second Digital Elevation Map
Eta-32 Model Terrain
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(2)
Early Eta-48 48-km grid/38 vertical levels
Eta-32 32-km grid/45 vertical levels
90-meter grid
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Mountain Topography of Models

• There have been improvements in the ability of
  these models to pick up cold air damming events
• Some research can be done to gain a more detailed
  knowledge of the damming process and any
  advancements that could arise in forecasting the
  event

Eta Step-Mountain Topography
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- Green Shaded Region represents the actual
slope and topography of the mountain - Brown
Blocks represent what the atmospheric model
factors into its equations.
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Presentation Outline

• What is cold air damming
• What processes lead to this phenomenon
• How does damming effect weather systems
• Past and present modeling capabilities
• Potential research topic
• Atmospheric models in a GIS

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Research Ideas

• Compare terrain slope to
• deflections of the wind
• width of the stable pressure ridge that develops
• degree of southward acceleration
• Improve resolution of atmospheric models to match
  better with available terrain models
• These topics will help to advance our ability to
  determine how far and fast this cold pool will
  travel south
• This would make it easier to gauge what
  precipitation type will fall along the east coast
  through the duration of an event

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Presentation Outline

• What is cold air damming
• What processes lead to this phenomenon
• How does damming effect weather systems
• Past and present modeling capabilities
• Potential research topic
• Atmospheric models in a GIS

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Atmospheric Models

• These are 4-dimensional models
• X Y Z Time
• Have many vertical layers associated with them
  above the surface (designated pressure surfaces)
• Are not on localized scales right now making
  them tough to integrate with detailed topographic
  data

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Complex Models
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Conclusion

• The cold air damming research questions that I
  discussed can and should be done because they
  will help to make it much easier to forecast the
  magnitude of these events and make precipitation
  estimation more accurate
• Before they can be pursued though advancements
  in the conversion from atmospheric modeling to
  GIS modeling need to occur
• Until then winter weather will continue to be
  interesting to forecast when cold air damming
  surges in

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References

• (1) Cold Air Damming Causes Examples and
  Forecast Parameters
• http//meted.ucar.edu/mesoprim/cad/webcast/print.
  htm
• (2) Eta Model Characteristics Background
  Information http//www.comet.ucar.edu/nwplessons/e
  talesson2/characteristicsbackground.htm
• (3) Vertical Resolution and Coordinates
• http//www.met.tamu.edu/class/metr452/models/2001
  /vertres.html
• (4) USGS 3 Second Digitial Elevation Maps
• http//weather.unisys.com/usgs/3sec/