Anomalous Velocity Signatures in Low Level Super Resolution Data

Background With the implementation of RPG Build 10.0, resolution of both base

reflectivity and base velocity products available to the warning forecaster was increased.

Legacy 8-Bit reflectivity had a resolution of 1000m x 1°. Power over 4 successive 250m bins was averaged to determine reflectivity displayed at 1000m. In super-resolution, the reflectivity resolution was increased to 250m x 0.5°. Power was no longer averaged across the 4 bins. A complicated signal processing technique is used to reduce the 1° beam width into 0.5° resolution.

Legacy 8-Bit velocity had a resolution of 250m x 1°. In super resolution the velocity resolution was increased to 250m x 0.5°.

We believe that the increase in resolution of the base reflectivity products and signal processing techniques are allowing side-lobe contamination to become more visible, thereby resulting in anomalously high velocity data in certain convective situations.

Super Resolution vs. Legacy Resolution

Super Resolution 250m X 0.5° Legacy Resolution 1000m X 1.0°

Azimuthal Side Lobe Contamination

Main LobeBeam Rotation

Side Lobes

Side Lobes

Beam rotates clockwise

Side lobes hit high reflectivity core and return power back to radar

Much weaker side lobe return results in a displaced weak reflectivity signature (often a spike out of the side of a storm)

Top View

Vertical Side Lobe Contamination

Side View

Main Lobe

Side Lobes

Actual beam is cone-shaped (side lobes not only azimuthal but vertical as well)

Theory is that side lobes hit high reflectivity aloft and return power and velocity information. Data that was once masked is now visible due to new signal processing and higher resolution.

Power Calculations According to FMH-11, for any given target, the WSR-88D’s strongest

side lobes would return about 54 dB lower power than that from the main lobe.

Assuming most contamination came from the 3.4° level, the resulting reflectivity values displayed at 0.5° would range approximately from 10-17 dBZ.

Concerning Spectrum Width

0.5° Z 0.5° SRM

0.5° SW

5.3° SRM

Spectrum width is the measure of the variance of the velocity data sampled. High spectrum width can be associated with turbulent motions, but more mature circulations generally display low spectrum widths.

Examination of Data and Impacts

After one particularly strong event in North Texas in November 2008, it was determined that the anomalous data led to at least one unnecessary issuance of a tornado warning.

Examination of data in other areas of the country using super resolution data revealed the same radar phenomenon.

It is our belief that experienced warning forecasters may be able to recognize the anomalous data as being radar artifacts, but less experienced warning forecasters could misinterpret the anomaly as a strong low level circulation or TVS.

The remainder of the slides will show examples of some of the data we found and analyzed and will offer some techniques for identifying these anomalous data.

A Note About the Data There will be a small red dot used as a reference point in all of the

images. This dot may be difficult to see, so there will also be a yellow circle highlighting the area of interest.

This yellow circle is in no way related to any radar algorithm output. No algorithm data is included in the following slides.

The AZRAN listed on each image is from the radar to the center of the area of interest (red dot).

Near Springtown, TXApril 18, 2009

KFWS – April 18, 2009Near Springtown, TX

AZRAN: 327° / 30NM

0.5° Z – 0013Z

0.5° Z – 0017Z

0.5° Z – 0022Z

KFWS – April 18, 2009Near Springtown, TX

0.5° Z 0.5° V

0.5° SW

0.5° SRM

KFWS – April 18, 2009Near Springtown, TX

Reflectivity Cross Section

Velocity Cross Section

Notice the extensive echo overhang above the low reflectivity values on the 0.5° slice. Higher velocities in the mid levels of this storm could be bleeding down to low levels due to side lobe contamination.

KFWS – April 18, 2009Near Springtown, TX

Effects on quick warning decision during favorable conditions??

Near Morgan Mill, TXNovember 10, 2008

KFWS – November 10, 2008Near Morgan Mill, TX

0.5° Z 0.5° V

The environment on this particular day was favorable for the development of tornadoes, but this signature appears to be seriously misplaced. It is believed that high velocities from aloft are bleeding down onto the 0.5° V slice.

AZRAN: 258° / 42NM

KFWS – November 10, 2008Near Morgan Mill, TX

0.5° Z 0.5° V

0.5° SRM

3.4° Z

Notice the high reflectivity core above the 0.5° slice.

AZRAN: 258° / 42NM

KFWS – November 10, 2008Near Morgan Mill, TX

Reflectivity Cross Section

Velocity Cross Section

Significant high reflectivity values area located above the weak reflectivity at 0.5° . Very high velocities are noted in the same areas as the high reflectivity values. It appears that these are being shadowed onto the 0.5° V slice.

AZRAN: 258° / 42NM

KFWS – November 10, 2008Near Morgan Mill, TX

High 0.5° spectrum width values are associated with the anomalously high 0.5° velocity data.

0.5° Z 0.5° V

3.4° V 0.5° SW

AZRAN: 258° / 42NM

Near Cleo Springs, OKApril 25, 2009

KVNX – April 25, 2009Near Cleo Springs, OK

0.5° Z 0.5° V

A large area of anomalously high inbound velocities again appears in the light reflectivity area.

AZRAN: 218° / 24NM

KVNX – April 25, 2009Near Cleo Springs, OK

0.5° Z 0.5° V

0.5° SRM

8.1° Z

Notice the high reflectivity core above the 0.5° slice.

AZRAN: 218° / 24NM

KVNX – April 25, 2009Near Cleo Springs, OK

Reflectivity Cross Section

Velocity Cross Section

A large high reflectivity core is being suspended above the 0.5° slice. High velocities associated with this core appear to be bleeding down onto the 0.5° V slice.

AZRAN: 218° / 24NM

KVNX – April 25, 2009Near Cleo Springs, OK

High 0.5° spectrum width values are associated with the anomalously high 0.5° velocity data.

0.5° Z 0.5° V

8.1° V 0.5° SW

AZRAN: 218° / 24NM

Near Bowdon, GAFebruary 18, 2009

KFFC – February 18, 2009Near Bowdon, GA

0.5° Z 0.5° V

A large area of greater than 100kt inbound velocities appear in the light reflectivity area on the 0.5° slice.

AZRAN: 287° / 31NM

KFFC – February 18, 2009Near Bowdon, GA

0.5° Z 0.5° V

0.5° SRM

4.1° ZHigh velocities extend well out away from the highest 0.5° reflectivity values. Note the core of high reflectivity at 4.1° aloft is located near the low level velocity artifact.

AZRAN: 287° / 31NM

KFFC – February 18, 2009Near Bowdon, GA

Reflectivity Cross Section

Velocity Cross Section

The Bowdon storm exhibits significant reflectivity overhang on its southeast side. The very light reflectivity values on the 0.5° slice “outline” the high reflectivity values aloft. Notice the very high inbound velocities aloft. It is hypothesized that these high velocities are bleeding down to the 0.5° slice due to side lobe contamination.

AZRAN: 287° / 31NM

KFFC – February 18, 2009Near Bowdon, GA

0.5° Z 0.5° V

4.1° V 0.5° SW

High 0.5° spectrum width values are associated with the anomalously high 0.5° velocity data. Also notice the high spectrum widths associated with the 3-body scatter spike.

AZRAN: 287° / 31NM

Suggestions for Identifying Velocity ShadowsIdentify areas of light reflectivity particularly

downwind of the strongest convection (in direction of overhang). Use of an appropriate color curve will help identify these areas better.

Identify whether or not these areas of high velocity are located in a favorable area for tornado development. Many times these anomalous velocities are displaced by several miles.

Suggestions for Identifying Velocity Shadows

Do not mistake a surging RFD or descending RIJ for anomalous velocities. Many times when the RFD surges out, the reflectivity data will also show some sign of forward movement. Also, the velocity data should be higher in areas of precipitation for surging RFDs.

Artifact Velocity – Notice the misplaced location

Real Velocities – Notice how the inbounds move with the precipitation

Suggestions for Identifying Velocity Shadows

USE SPECTRUM WIDTH!In all of the cases analyzed, the spectrum width values were very high (> 16kts) in the areas of anomalous velocity data. These areas of high spectrum width were always identical in shape and size to the anomalous velocity data.

USE STORM SPOTTERS!Storm spotter information can be very valuable during times when velocity data may be unreliable.

0.5° V 0.5° SW