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National Weather Service Steve Davis - Lead Forecaster. With help from Paul Sirvatka Professor of Meteorology College of DuPage. Email : Max Power at center of beam. Beam Power Structure. Side Lobe Energy. ½ Power Point. ½ Power Point.

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National weather service steve davis lead forecaster l.jpg
National Weather ServiceSteve Davis - Lead Forecaster

With help from Paul Sirvatka Professor of MeteorologyCollege of DuPage

Email :

Beam power structure l.jpg

Max Power at center of beam

Beam Power Structure

Side Lobe Energy

½ Power Point

½ Power Point

Side Lobes cause most of the clutter in close proximity to the radar

The Radar Beam is defined by the half power points

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Beam Power Structure

  • The side lobes can interfere with the signal and lead to ground clutter.

  • The beam width indicates why the beam must be elevated at least 1/2 degree so signal does not go straight down to the ground.

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.96 Degree Beam Resolution

D = Beam Width

Radar resolution with respect to beam width / range


If R = 60 NM 120 NM 180 NM 240 NM

D = 1 NM 2 NM 3 NM 4 NM

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Pulse Repetition Frequency- PRF

PRF controls the Max Radar Range and Max Unambiguous Velocities

PRF is the number of pulses per second transmitted by a radar

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The Doppler Dilemma

Rmax and Vmax depend on PRF

Rmax= The range to which a transmitted pulse can travel and return to the radar before the next pulse is transmitted.

Vmax= The maximum mean radial velocity that the radar can unambiguously measure (before dealiasing).

* Rmaxis inversely related to PRF

* Vmaxis directly related to PRF

As PRF increases, Rmax decreases and Vmax Increases!

The Doppler Dilemma: There is no single PRF that maximizes both Rmax and Vmax

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Defeating the Doppler Dilemma

The WSR-88D employs a dual PRF scanning strategy to help defeat the “Doppler Dilemma”

The 88D performs redundant sampling on the lowest 2 elevation slices and interlaced sampling on the “middle” slices to maximize range/velocity data, and minimize ground clutter.

In this example of Volume Coverage Pattern (VCP) 21, the lowest two elevation slices are sampled twice. Once using a low PRF (CS) to maximize range data and then again using a high PRF (CD) to maximize velocity data. The middle slices (blue) are sampled once but use an alternating, or interlaced, high and low PRF (B) on each radial. The upper elevation slices use only a high PRF (CDX) to maximize velocity data. Range issues are not a problem in the higher elevations, precluding the use of a low PRF.

CS = Contiguous Surveillance B = Batch

CD = Contiguous Doppler CDX = Contiguous Doppler X

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Reflectivity Images

Base Reflectivity and Composite Reflectivity

Base Reflectivity

Composite Reflectivity

  • Displays the maximum returned signal from all of the elevation scans

  • Better summary of precipitation intensity

  • Much less deceiving than Base Reflectivity

  • Subtle 3-D storm structure hidden

  • 0.5° elevation slice

  • Shows only the precipitation at the lowest tilt level

  • May underestimate intensity of elevated convection or storm cores

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Reflectivity Images

Composite Reflectivity

  • Displays the maximum returned signal from all of the elevation scans to form a single image

  • Can often mask some Base Reflectivity signatures such as a hook echo

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Base vs Composite Reflectivity

Which is which?

Composite Reflectivity

Base Reflectivity

  • Notice the heavier returns and more coverage

  • Notice the lighter returns

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Hail Detection

  • Returns > 55 dBz usually indicate hail.

  • However, the probability of hail reaching the ground depends on the freezing altitude.

  • Usually, a freezing level above 14,000 feet will not support much hail.

  • This is because the hail melts before reaching the ground.

  • Freezing level can be determined from an upper air sounding.

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Max return of 60 dbZ

Max return of 65 dbZ

Freezing level was 7,000 feet Freezing level was 17,000 feet

Produced golf ball sized hail Produced no hail

hail production depends directly on freezing level.

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Vertically Integrated Liquid (VIL)

  • Take a vertical column of the atmosphere: estimate the amount of liquid water in it.

  • High VIL values are a good indication of hail.

  • The white pixel indicates a VIL of 70.

  • This storm produced golfball size hail.

  • Trouble with VIL is that the operator has to wait for the scan to complete before getting the product.

  • Assumptions are problematic!

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The Hail Spike

Also called Three-Body Scattering

  • A dense core of wet hail will reflect part of the beam to the ground, which then scatters back into the cloud, and is bounced back to the antenna.

  • The delayed returns trick the radar into displaying a spike past the core.

  • Usually, will only result from hail 1 inch in diameter or larger (quarter size).

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Echo Tops

Fairly accurate at depicting height of storm tops

Inaccurate data close to radar because there is no beam angle high enough to see tops.

Often has stair-stepped appearance due to uneven sampling of data between elevation scans.

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Doppler Velocity Interpretation

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Azimuth Resolution Considerations

Rotational couplet identification can be affected by azimuth resolution.

As the diagram shows, the closer a rotation is to the radar the more likely it will be identified correctly. If the rotation is smaller than the 10 beam width (possible at long ranges) then the rotation will be diluted or averaged by all the velocities in that sample volume. This may cause the couplet to go unidentified until it gets closer to the radar.

Enlarged image along a radial. Individual “blocks”

represent one sample volume. This graphically shows the radar resolution.

Azimuth 3

Weak inbound, weak outbound

Rotation too small to be resolved

Azimuth 2

Strong inbound, strong outbound

Azimuth 1

Stronger inbound than outbound

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The Zero Isodop “Problem”

When the radial is perpendicular to the the wind, the radar displays zero velocity - This “zero zone” is called the “Zero Isodop”.

What percentage

of actual wind

will the radar detect?

00 = 100% - Parallel

150 = 97%

300 = 87%

450 = 71%

600 = 50%

750 = 26%

900 = 0% - Perpendicular

When the wind velocity is parallel to the radial, the full component of the wind is measured

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Large Scale Winds

Use the Zero Isodop to assess

the vertical wind profile.

The combination shape of the zero isodop indicates both veering and backing winds with height.


Backward “S” Shape

Backward “S” shape of the zero isodop indicates backing winds with height. Backing may imply cold air advection.

“S” Shape

“S” shape of the zero isodop indicates veering winds with height. Veering may imply warm air advection.

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Large Scale Winds

Uniform Flow with Jet Core

Straight Zero Isodop indicates uniform direction at all levels. The inbound/outbound max’s show a jetcore aloft with weaker winds above and below.

Uniform Flow

Straight Zero Isodop indicates uniform direction at all levels.

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Example from KMKX 88D

Low level

jet max

January 5, 1994

Steady snowfall

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The VAD Wind Profile(Velocity Azimuth Display)

Plots out winds with respect to height as time increases from left to right

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Small Scale Winds- Divergence/Convergence -

Divergent Signature

Often seen at storm top level or near the ground at close range to a pulse type storm

In all of the following slides, note the position of the radar relative to the velocity signatures. This is critical for proper interpretation of the small scale velocity data.

Convergence would show colors reversed

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Small Scale Winds- Cyclonic Convergence/Divergence -

Anticyclonic convergence/ divergence would show colors reversed in each panel.

Cyclonic Convergence

Cyclonic Divergence

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Small Scale Winds- Pure Cyclonic Rotation -


rotation would show colors reversed

Pure Cyclonic Rotation


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Small Scale Velocity Example

Rotation seen with the Big Flats Tornado. August 27, 1994 ~ 9 PM.

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Storm Relative Velocity - SRVvs.Base Velocity

In General:

When diagnosing rotational characteristics, use SRV

SRV subtracts out the motion of a storm to display pure rotational characteristics of that storm. Often, the motion of the storm will mask or “dilute” the rotational information. This is especially true when rotations are subtle.

When diagnosing Straight Line Winds (bow echo, microbursts), use Base Velocity

The strength of an advancing line of storms producing straight line winds is a sum of the winds produced by the storms, plus the movement of the storms. Using SRV would take one component away.


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SRV vs. Base Velocity- Strong Rotation -

Storm Relative Velocity

Base Velocity

Persistent rotation from Tornadic Thunderstorm

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SRV vs Base Velocity- Subtle Rotation -

Base Velocity

Storm Relative Velocity

Janesville F2 tornado. June 25th, 1998 ~ 700 PM

Interesting note: These scans are at 3.40 elevation. The 0.50 elevation showed little rotational information.

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SRV vs Base Velocity- Subtle Rotation -


Little/no rotation seen at lowest elevation

Base Velocity

Storm Relative


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SRV vs Base Velocity- Oakfield -

Base Velocity

Storm Relative Velocity

Oakfield F5 tornado. July 18, 1996. Although the rotation was intense, the low precip (LP) nature of the storm at this time, limited the amount of energy returned back to the 88D by precipitation targets. In this case, though the rotation was strong, the SRV clearly was the better tool for diagnosing the strength of the rotation.

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SRV vs Base Velocity- Straight Line Winds -

Base velocity shows max inbound winds of 55 to 60 kts.

SRV shows max inbound winds of 30 to 40 kts.

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Questions to Answer:

  • If velocity is toward the radar directly, the velocity indicated is ____%.

  • If the velocity is perpendicular to the radar beam, the velocity indicated is ____%.

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Questions to Answer

  • What happens to resolution the further an object is from the radar? See page 6 of 7 from the Radar notes.

  • Which is better for resolution, a 2 degree beam width or a 1 degree beam width? (See page 4 of 7.) Why?

  • How wide is the beam at 100 nautical miles from the radar?

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Something to Do:

  • Define the following and indicate what they would look like on a Doppler radar display:

    • Veering winds with height

    • Backing winds with height

    • Convergence

    • Divergence

    • Cyclonic rotation

    • Anticyclonic rotation