radar, a wide variety of weather features of great
importance can be identified. It is beyond the scope of
this manual to describe the interpretation of doppler
For a detailed explanation of the
interpretation of doppler velocity patterns refer to the
FMH-11 (part B).
MESOCYCLONE SIGNATURE DETECTION
Mesocyclones have been found to be precursors to
many tornadoes. They generally have a core diameter
of 1.5 to 5 nmi, maximum tangential velocities of 40 kts
or greater, and time and height continuity. Previous
doppler experiments have found that weak mesocyclone
signatures generally result in severe thunderstorms.
Stronger signatures generally produce tornadoes,
especially if a tornadic vortex signature is present.
If an alert is received that a mesocyclone exists, or
if there is no alert, but a mesocyclone is suspected, there
are several products that can be used to confirm its
presence. These products and how they might be used
are discussed in the following paragraphs.
Recognition of a Mesocyclone Signature
A mesocyclone typically will develop at the
midlevels of a tomadic storm and build down to lower
levels as the storm matures. It may be possible to
monitor development of storm rotation or a
mesocyclone in the velocity field. The time lapse
capability with continuous update may be useful in
monitoring this development. In the velocity field, the
feature will appear as two velocity peaks of opposite
signs, separated azimuthally.
Interrogation of different elevation scans of the
Mean Radial Velocity product (at 2000 ft intervals
between 15,000 and 19,000 ft) can aid in determining
the height continuity of a mesocyclone. The Quarter
Screen mode can be useful for this purpose.
If correctly oriented through the storm, storm
rotation (cyclonic or anticyclonic) or a well-developed
mesocyclone may be evident in the Mean Radial
Velocity Cross Section product. The cross section
should be generated perpendicular to the mean flow. If
a mesocyclone is not evident in the Mean Radial
Velocity product or the Severe Weather Analysis Mean
Radial Velocity product, the Storm Relative Mean
Radial Velocity Map or Region products may display
this feature (part C of the FMH-11).
Removing the storm motion may aid in the detection
of a mesocyclone, but the mesocyclone circulation
pattern will exist in the velocity field even if the storm
motion is not removed.
In the presence of a strong rotational signature, the
Combined Moment product can be very useful in
determining the existence of a mesocyclone. In such a
case, rotational phenomena should be clearly evident
due to the relative position of the arrows in a given area.
In addition, high reflectivity core and high spectrum
width values may be evident in the area.
In a tornadic supercell, doppler data have indicated
that a separation of the mesocyclone core from the
bounded weak echo region occurs prior to or during the
collapse of the bounded weak echo region. With the
development of severe weather possible at this stage, the
separation of the mesocyclone core from the bounded
weak echo region may be monitored by displaying an
appropriate Reflectivity product on one graphic screen
and a Storm Relative Mean Radial Velocity Map product
on the other. A time lapse of the Mean Radial Velocity
or Storm Relative Mean Radial Velocity Map product,
magnified on the storm under investigation, may prove
very useful in determining the separation of the
mesocyclone core from the bounded weak echo region.
If a mesocyclone is evident in the Mean Radial Velocity
product, this separation may be monitored using the
time lapse capability with continuous update for the
Mean Radial Velocity and Reflectivity products.
The mesocyclone algorithm provides the position
of the feature reflected onto the lowest elevation angle
in which the feature was detected. This is due to the fact
that mesocyclones are often tilted and are, therefore,
displaced at higher elevations. Mesocyclones may not
always be identified by the algorithm for high
reflectivity core storms, for storms that produce
downbursts, or for weak tornadoes produced as a result
of convergence boundaries. Since there is no tracking
algorithm for mesocyclone features, time continuity
must be established. In addition, the Storm Relative
Mean Radial Velocity Map and Region products display
the maximum velocity sampled over four 0.13 nmi
range bins. The peak velocity values, and thus the peak
shear associated with a mesocyclone feature, are more
likely to be displayed on these products than on the
Mean Radial Velocity product, which simply displays
every fourth 0.13 nmi range bin at the same resolution.
Depending upon selection of storm motion removal,
the existence of a mesocyclone may be more evident in
the Storm Relative Mean Radial Velocity Map and