Using Doppler Radar to “Nowcast” Winter Storms

...A Review

MSC Winter Severe Weather Workshop

Boulder February 2002

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

Outline.. continued

• Forecasting Precipitation Phase

– Combining Bright Bands and radial velocity displays

• Winter Quantitative Precipitation Estimation using radar.

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

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.

Two Different Warm Fronts

It’s “What Lies Beneath” That Counts.

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

Bright Band Example

B.C.

13 Dec 1998

Bright Band- RHI display

Bright Bands andForecasting

• Freezing Levels

• Vertical Extent of wet snow.

• Very useful in varying terrain.

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.

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.

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.

• 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 it’s peak and then starts to weaken.

Relationship between outflow and freezing level

• Bright band gives height of freezing level and lowest elevation of any wet snow.

• Let’s look at relationship between bright band and outflow.

• Windows media player.

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.

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.

A similar Alberta example

Cold Fronts and radial velocities

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.

Cold Fronts B.C. style

Using Doppler to Calculate Upslope Snows

Montana

(Thanks to Gene Petrewsca

_Missoula MT)

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?

Topographical Effects

Outflow Winds

The Fraser Valley

British Columbia

Negative Angles and Doppler

Valley Winds

Negative Elevation Angles

The Lower St Lawrence

Snow in the Valley Moving a different direction

As seen by Doppler

Snow in valley below horizon

Sea Clutter

Winds Channeled by Sounds

Precipitation Estimates

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24Hr RFA

• Hybrid Doppler Corrected LogZ and low level Cappi

RFA Differences between using CAPPI and Corrected Log ZR

• Corrected log Z filters ground clutter.

• Calculated from lower PPI’s than CAPPI

• Would expect log Z to be better.

• However studies have identified problems with corrected log Z.

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CAPPI

24 hr accumulations, CLOGZPPI vs Conv, 12 May 20005 km avg of highest resolution base data (King)

y = 0.7539x

R2 = 0.827

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CAPPI (mm)

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Corrected log Z

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

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

Example

December 11/12 2000

Water Equivalent Amounts from Models

Observed Snow Amounts

Estimating Snow from Radar

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

Hurricanes and Doppler

• 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?

Hurricanes

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

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.

Answer

• Wind direction constant with height.

• Wind speeds decrease (slightly) with height.

• What type of system would do this?

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.