Circulations on radarThe
radar depicts velocities on a radar screen with negative numbers (
green)
implying flow toward the radar and positive numbers (
red)
indicating flow away from the radar. For each of the figures below assume
the radar is at the bottom of the box. Assume north is up.
| A) To the right is an example
of how an east wind would look on the radar. Even
though there are inbound and outbound velocities shown this is
not a circulation. Instead, the radar is observing an east
wind. Due north of the radar, an east wind would
have zero radial velocity because the wind is
perpendicular to the beam which is moving from south to
north. Northeast of the radar, a component of the wind is
moving toward the radar so the radar shows a negative
radial velocity. Northwest of the radar, a
component of the wind is moving away from the radar so the radar
shows a positive radial velocity. |
B) To the left is an example of a
weak circulation. Many thunderstorms will have a weak
circulation at some point in their life cycle. This is
because the thunderstorm will prevent air from flowing through
it. Instead, like a rock in the middle of a stream, air
has to flow around the thunderstorm. With air moving
faster around the thunderstorm than through the thunderstorm, it
can appear as weak circulation when viewed on radar. |
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| C) To the right is an example of a
strong mesocyclone. Meteorologists rate mesocyclones
by adding the maximum inbound velocity to the absolute
outbound velocity and dividing by two. The result is the
rotational velocity (or how fast the thunderstorm is
rotating). Generally, the distance between the maximum
inbound and maximum outbound should be less than 5 miles.
In this case, the mesocyclone has a rotational velocity of at
least 35 kts which would be moderate to strong. A
persistent mesocyclone is indicative of a storm which can
product large hail, damaging winds, and even tornadoes. |
D) While the radar cannot detect an
individual tornado, it can detect the parent circulation in the
thunderstorm which produces the tornado. This is called a tornado
vortex signature (TVS). A TVS is calculated by
adding up the maximum inbound and outbound velocities which are
adjacent to each. If the sum is at least 70 kts, it is
considered a TVS. To the left is an example of how a TVS
will look on radar. |
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