Radar Detected Mesocyclones within
Tropical Cyclone Gordon (1994)
Scott M. Spratt and Dave W. Sharp
National Weather Service, Melbourne, Florida
During tropical cyclone situations, attention is most
often focused upon the centroid and inner rain bands. However, it
is within the favored right-front quadrant of the outer rain bands
where the threat of tornado development is greatest. With the
installation of coastal Weather Surveillance Doppler Radars (WSR-88D) now complete, recognition of, and reaction to, tropical
mesocyclones can now be achieved with a greater degree of accuracy
than ever before. While it is apparent that a spectrum of tropical
mesocyclones exist, low-topped, small diameter circulations appear
to occur with the greatest frequency. This presentation will
illustrate radar-observed characteristics associated with such
features embedded within outer rain bands, given inherent sampling
limitations.
Banded convection, well removed from the circulation center of
Tropical Cyclone Gordon, rotated across east-central Florida from
the Atlantic during the evening and early morning hours of 15-16
November, 1994. During a six hour period, five persistent
mesocyclone signatures were manually identified and tracked by
Melbourne radar operators. The first four mesocyclones developed
over the Atlantic and possessed small diameters and shallow depths,
as compared to traditional mesocyclones. While three of these
features remained offshore or weakened upon approach to the coast,
intensification of one cell was observed prior to landfall,
resulting in a short, but deadly F2 tornado just inland from the
coast. The final long-lived circulation formed inland, within a
large west-to-east rain band, which resembled a Line Echo Wave
Pattern (LEWP). The diameter and vertical depth of this mesocyclone
was more extensive than the previous marine circulations. During
the most intense period of rotation, a bow-echo feature was
evident, coincident with the brief touchdown of an F0 tornado.
Initial indications of an increasingly favorable environment
for rotational cells across east-central Florida will be shown via
a Velocity Azimuth Display (VAD) Wind Profile (VWP) time section.
The life cycle of three individual supercell-like storms will be
depicted through reflectivity and velocity trend analyses. Other
WSR-88D products which proved useful to highlight potential
severity, relative to surrounding cells, will be discussed. The
necessity of a manual shear assessment technique, as opposed to
algorithm portrayal of mesocyclone rotational velocities, will be
stressed. A new WSR-88D Volume Coverage Pattern (VCP) will be
proposed to allow improved sampling of low-topped, small-diameter,
rotational cells.
Corresponding author address:
Scott M. Spratt, National Weather Service
421 Croton Road, Melbourne, FL 32935 scott.spratt@noaa.gov