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NEXRAD Storm Relative Mean Velocity
Storm Relative Velocity is useful for observing velocities within storm systems
and predicting tornado development. WSI Intellicast offers
images of Storm Relative Velocity at its website, and we link to these on our
radar page. Storm Relative Velocity images are only produced
when the station is scanning in precipitation mode. When no precipitation is
present in the radar scan, Storm Relative Velocity is identical to Radial Velocity. However, when a
precipitation event has been identitfied and is being tracked at the NEXRAD station, all velocities
are adjusted for the mean speed and direction of the storm cell (s). This can result in velocities
with much different directions and speeds than in radial velocity.
In the following velocity images, notice the area of precipitation in the SE sector
(bottom right) of the two scans. In the Radial Velocity on the left, the precipitation appears mostly
range-folded, and a mass of birds arriving on the Texas coast is moving to the north. In the Storm
Relative Mean Velocity on the right, the velocity has been twisted 90º counter-clockwise so
every reflector is moving adjusted to the storm over the Gulf of Mexico. Only a few velocity pixels
have been read from this storm, meaning that the Storm Relative Mean Velocity has been calculated
from a very small sample.
Sharp observers will notice that there is a 5 minute difference between the
radial and storm relative velocities. The corresponding 16:40 UTC 5 May, 1999 Radial Velocity scan
was not available for exact comparison with 16:40 UTC 5 May, 1999 Storm Relative Mean Velocity
scan; however, such dramatic differences in similar, though not exact, scans illustrate that,
before interpreting velocity information, some knowledge of how the data are produced is necessary.
In this case, it would appear based on Storm Relative Mean Velocity data that birds were moving in the
unlikely directions of W and WNW over the Texas coast; in fact birds are moving N as would be
expected in the peak period of spring migration!
| Radial Velocity | Storm Relative Mean Velocity | |||||||||||||||||||||||||||||||||||||
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| HGX: Houston TX (29.47N 95.08W): Radial Vel 124nm / Elev=0.5 deg / 1 km²/pixel Precip Mode / VCP 11 05/05/99 16:45 UTC - Max: -72 kts +122 kts | HGX: Houston TX (29.47N 95.08W): Storm Rel. Mean Vel 124nm / Elev=0.5 deg / 1 km²/pixel Precip Mode / VCP 11 05/05/99 16:40 UTC - Max: -103 kts +92 kts | |||||||||||||||||||||||||||||||||||||