term MESO is short for mesocyclone, which is generally a radar term. A
mesocyclone is a rotation found within the tower of a strong
thunderstorm, and can sometimes lead to the formation of a tornado, or
a tornado vortex signature on radar.
When a radar detects a mesocyclone, it is good practice to watch for
time continuity between volume scans, a diameter of two to six miles,
and an extension vertically of over 10,000 feet. These characteristics
can only be found in velocity imagery. Nonetheless, the mesocyclone
algorithm is fairly good at making sure all of the needed criteria are
roughly met. One thing that can be watched from composite reflectivity
is the time period at which a mesocyclone is present. The longer the
period at which a rotation is sustained within the head of a storm, the
better chance there is a mesocyclone is active within that given cell.
One of four key words can be found within the MESO column of the storm
the algorithm which detects mesocyclones is sensitive, false
mesocyclones can at times be detected along frontal boundaries and
squall lines, or where a definite wind shift is present, especially
when the front edge of a squall line is parallel to the radar beam.
Therefore, a mesocyclone on radar is not always an indication a storm
– A mesocyclone is likely not present. Since the radar,
through several tilts, can penetrate many layers where a mesocyclone
resides in a thunderstorm, it is unlikely that a mesocyclone will go
– "Uncorrelated Shear" – A weak and insignificant
mesocyclone may be present. Given the sensitivity of the algorithm used
for detecting mesocyclones, there is a possibility that minimal
rotation within the cloud needed for a future severe thunderstorm is
not even present. Rotation is found in only one radar slice vertically.
– "3D Correlated Shear" – A moderate mesocyclone is
possible, and some inner-cloud rotation has been detected on at least
two radar slices vertically. A 3DCO needs to be watched for persistence
– Well-developed rotation has been detected on at three or
more radar slices for the full mesocyclone indication. The thunderstorm
it is attributed to may be severe given the mesocyclone is persistent.
Radars can also sometimes falsely detect a mesocyclone if a
thunderstorm is directly over a radar site. On the flip side of that, a
radar cannot detect mesocyclones well, or possibly at all, if a
thunderstorm is close to the radar's transmitter. Beams are unable to
extend into the updraft at such a degree from the radar site. A lesser,
or possibly absent, mesocyclone classification is displayed as a result.
Mesocyclones should not be considered a formation that is viewable, as
tornadoes are. Some cloud features may suggest that a mesocyclone is
present in a thunderstorm, such as curved feeder or inflow bands.