Winter weather

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Winter weatherLake weather

• Thanks to COMET program and Greg Byrd for many
  graphics

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Review precipitation growth

• Recall that until -40C, some H2O in the
  atmosphere will still be in liquid form

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Snow crystal types
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Precipitation growth mechanisms
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Cloud types

• Optimal conditions for ice growth are saturated
  air with respect to water (supersaturated for
  ice) and a temperature of -10 to -18C

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Aviation and icing
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Critical rain/snow thicknesses

• 1000-500mb 5400 m
• 1000-700mb 2840 m
• 1000-850mb 1290 m
• 850-700mb 1540 m
• For freezing rain, on which side of these should
  you be on?

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Why 850 mb?

• Closest level to the surface historically
  analyzed.
• Not confounded by surface features.

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Combining levels
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Critical thickness values
RED 1000-700 2840m THK CYAN 850-700 1540m
THK YELLOW 1000-850 1300m THK MAGENTA
700-500 2560m THK GREEN 850-500 4100m
THK WHITE 1000-500 5400m THK BLUE 850mb 0
degree Isotherm
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Top-down method

• Start where the precipitation forms, and then
  follow to the ground.

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Elevated warm layer

• How does this arise?
• Significant ice in cloud if below -10C

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Precipitation profiles
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Near surface cold layer

• Depth?
• Surface vs. air temperature?
• Wet bulb temperature at surface
• If 1.5C, unlikely to be snow

15
Evaporation and melting
If Wet Bulb Zero is 1500m up, rain If Wet Bulb
Zero is uncertain
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Sample skew-t
What would we forecast here?
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Model output statistics

• POZ Probability of freezing rain/sleet
• POS Probability of snow
• TYP Most likely type of precipitation (R/S/Z)

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Model problems

• 1. The procedure used may not accurately reflect
  the vertical profile
• 2. There may be errors in fields such as surface
  temperature
• 3. The effects of unresolved terrain and other
  unresolved surface features can not be accounted
  for
• 4. Relatively large interpolation distances can
  result in greater errors
• 5. Model terrain representation in the mountains
  can cause problems because of inaccurate model
  terrain height.

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Lake weather

• Due to time considerations, were only going to
  talk lake-effect precipitation
• Lake-effect vs. lake-enhanced what is the
  difference?
• Though Great Lakes most common, many other areas
• Great Salt Lake, Finger Lakes, Chesapeake Bays,
  Sea of Japan, Hudson Bay

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Lake weather

• We understand lake weather quite well
• We have trouble predicting it
• Models are inadequate
• Radar coverage can be inadequate

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Lake weather
Heat, moisture, convergence, upslope movement,
and instability
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2004-2005 Lake Erie snowbelt
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Important considerations

• Instability
• Fetch
• Wind shear
• Upstream moisture
• Synoptic (large)-scale forcing
• Orography/topography
• Snow/ice cover on the lake

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Instability

• How unstable?
• A Lake T 850mb T difference of 13C implies
  absolutely unstable conditions
• How deep is the instability?
• Less than 5000 ft (1.5 km) and there may not be
  enough vertical motion
• Why does instability end vertically?

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Fetch

• How long air is over water
• Small differences in wind direction can lead to
  huge differences in fetch
• 250- 225 mile fetch
• 230 - 80 mile fetch

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Favorable Fetches for Lake-Effect Snow

• from LaDue (1996)

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Wind and wind shear

• Wind speed
• Too strong not enough time over lakes
• Too weak bands may not make it inland
• Wind shear
• Most organized bands happen with very little
  directional shear
• Above 60 (surface 700 mb) difference and
  lake-effect doesnt really occur

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Upstream moisture

• Higher relative humidities upstream increase the
  likelihood of lake-effect
• Bands can reform (Lake Huron bands redevelop over
  Erie)

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Synoptic forcing

• Cold advection aloft destabilizes
• PVA may lift inversion

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Topography
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Types of snow

• Single band
• Multiple band
• Multiple lake band

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Inter-season variability
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Inter-season variability
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Climatology
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Climatology
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Single Band Development

• Usually occurs with moderate to strong, deep
  instability - few strong bands
• Usually occur when flow parallel to long axis of
  the lakes (long fetch)
• 20 km wide but 200 km in length
• Move towards shore if axis of wind is slightly
  off parallel

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Single Bands
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Single Bands
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Multiple Bands

• from NWS Marquette (1996)
• Weaker than single bands,shallower mixed layer
• Horizontal roll convection
• Occur when mean boundary layer wind is more
  normal to the long axis of the lake
• Oriented parallel to the mean wind direction
• 10 km wide, 50 km long

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Multiple bands
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Multiple-Lake Bands

• Bands develop over one lake and then may
  reorganize or intensify over the next.

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Lake Huron fetch