Using Doppler Radar to Nowcast Winter Storms '''A Review

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Using Doppler Radar to Nowcast Winter
Storms...A Review

• MSC Winter Severe Weather Workshop
• Boulder February 2002

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Outline

• Doppler in Synoptic Storms is More than just a
  profiler.
• We can use the Non Symmetry of radial velocity
  displays in Nowcasting.
• Frontal Structures
• Convergence, Ascending jets.
• Topographical Effects
• Valleys, Upslope Flow

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Outline.. continued

• Forecasting Precipitation Phase
• Combining Bright Bands and radial velocity
  displays
• Winter Quantitative Precipitation Estimation
  using radar.

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Precipitation Phase

• Warm Fronts
• From radial velocity patterns
• Depth of cold air
• Nowcasting of Temperature Advections
• Changes in Strength of low level flow.
• From logz and cross sections
• bright band
• Freezing level, lowest extent of melting snow

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Example 1

• February 15 1990.
• Major event in Toronto 20 cm of snow followed by
  24 hours of ZR with 2 cm accumulations.
• Models underestimated outflow and had Toronto
  changing to rain 18 hours too early.
• Doppler useful in assessing situation.

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Two Different Warm Fronts

• Its What Lies Beneath That Counts.

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Bright Bands

• Most pcpn forms through ice process rather than
  rain process
• As snow falls through FZG LVL snow melts from
  outside, thus initially looks like slow falling
  large raindrop
• Results in 5-15 dBz increase in reflectivity
• Relative max of reflectivity (Bright band) is
  just below FZG LVL. Persists til all snow has
  melted.
• Most predominant in stratiform situations

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Bright Band Example

• B.C.
• 13 Dec 1998

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Bright Band- RHI display
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Bright Bands and Forecasting

• Freezing Levels
• Vertical Extent of wet snow.
• Very useful in varying terrain.

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March 27 2001

• Synoptic storm moves through lower mainland.
• Rain and plus 9C had been forecast.
• Outflow brought colder temps (3C YVR) and wet
  snow in higher suburbs.

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Sequence of Events on Radar

• Low level flow initially southeast then outflow
  develops down Fraser Valley.
• Low level air cools with outflow and freezing
  level drops.
• Outflow weakens and freezing level starts to rise
  again.

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March 27 2001

• Interesting example of combining bright band with
  radial velocity winds.
• Bright band shows height of melting layer
  lowering corresponding to intensity of outflow.

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• Between 1900z and 2000z lowest level flow backs
  and by 2200z there is a significant outflow down
  the Fraser Valley.
• By 0030 the outflow reaches its peak and then
  starts to weaken.

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Relationship between outflow and freezing level

• Bright band gives height of freezing level and
  lowest elevation of any wet snow.
• Lets look at relationship between bright band
  and outflow.
• Windows media player.

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Synoptic Example 2

• Dec 11 2000
• Cold frontal passage at Toronto followed 12 hours
  later by major precipitation event.
• Models indicated snow/freezing rain line very
  close to Toronto.
• Use Doppler pattern to monitor accuracy of models.

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Synoptic Example 3

• March 5 2001
• Great uncertainty about snow amounts over
  Toronto.
• Rather innocuous pattern. Strong low level
  convergence on Doppler indicated significant low
  level vertical velocities going on.

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A similar Alberta example

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Cold Fronts and radial velocities

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Cold Fronts

• Usually marked by quite sharp change in wind
  direction. This will mean sharp change in radial
  component of wind
• Will show up as a discontinuity on Doppler.

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Cold Fronts B.C. style

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Using Doppler to Calculate Upslope Snows

• Montana
• (Thanks to Gene Petrewsca
• _Missoula MT)

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Upslope Flow Developing after a cold frontal
passage

• Doppler can give you
• the component of the wind that is upslope
• the depth of the cold air
• Then if you know
• Slope of terrain
• Moisture content
• You could calculate snow accumulation rates?

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Topographical Effects

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Outflow Winds

• The Fraser Valley
• British Columbia

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Negative Angles and Doppler
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Valley Winds

• Negative Elevation Angles

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The Lower St Lawrence

• Snow in the Valley Moving a different direction
• As seen by Doppler

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Snow in valley below horizon
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Sea Clutter
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Winds Channeled by Sounds

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Precipitation Estimates

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24Hr RFA

• Hybrid Doppler Corrected LogZ and low level Cappi

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RFA Differences between using CAPPI and
Corrected Log ZR

• Corrected log Z filters ground clutter.
• Calculated from lower PPIs than CAPPI
• Would expect log Z to be better.
• However studies have identified problems with
  corrected log Z.

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CAPPI
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Corrected log Z

• Problem seems to be related to very short pulse
  length used in Doppler scan.

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Adjusting Model outputs for snow/liquid ratios

• Models forecast water equivalent.
• Snow/liquid ratios can have sharp gradients
  associated with deep winter storms
• Ratios are a function of the coldness of the
  layer where snow falls.
• Can be related to 1000-700 mb thicknesses

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Example

• December 11/12 2000

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Water Equivalent Amounts from Models
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Observed Snow Amounts
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Estimating Snow from Radar

• Adjust Observed Amounts on Radar by Snow to
  liquid ratio and estimates not bad.

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Hurricanes and Doppler

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• Trick Question
• If the eye of a hurricane were directly over a
  radar what would the radial velocity display look
  like?
• At a distance from the radar?

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Hurricanes

• At a distance hurricanes have same appearance on
  Doppler as a giant mesocyclone.

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Exercise

• Look at this picture especially look at the
  outbound winds.
• Can you tell me what is unusual about the wind
  profile implied by this display.
• Compare with the inbound winds.

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Answer

• Wind direction constant with height.
• Wind speeds decrease (slightly) with height.
• What type of system would do this?

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Answer

• Tropical Storm Karen was just to the west.
• Doppler helped to confirm some structures of
  transitioning tropical storms that had been
  observed . Low level jet lower to the east than
  to the west of the system.