The application of this extratropical transition technique
begins with the decision that a system is in the process of becoming extratropical.
This technique provides quantifiable criteria for making the determination
of transition. The single most important determining factor for deciding
if a system is becoming extratropical in nature is the decrease or
loss of persistent central convection. Take note of the energy associated
with the system as reflected in the curvature of the organized convective
bands and the cloud top temperatures of the deep convection. Cloud
system size may increase as the system begins to lose its tropical characteristics
with the convection displaced further eastward and poleward away from the
low-level center. These transitioning systems usually maintain their
forward motion or accelerate. If this doesn’t happen, and dissipation
begins, continue to apply the Dvorak technique. Interaction with
other synoptic systems may occur as a tropical system recurves. This
interaction usually indicates that extratropical transition is occurring.
Observational experience has shown that during the extratropical
transition an increase in the subsidence generates an irregular shaped
"warm hole" near the low-level circulation center as seen on the IR imagery
using the BD enhancement. This feature will remain through the completion
of the transition and it is possible to use this conservative feature for
positioning and as an indication of transitioning. Using these criteria,
a satellite analyst can identify the transition from tropical "T" numbers
to extratropical "XT" numbers. The factors listed below are used to determine
if a system is transitioning "XT", or still tropical "T", in nature.
1. Apply Dvorak ("T" numbers) technique if
It is important to apply the "XT" technique as soon as appropriate
in order to avoid an artificial decrease and then an increase in the intensity
of the system as the techniques are switched. It is reasonable that
in some cases there will be a slight increase in reported intensity as
the switch is made between the two techniques. Practice with this
technique should alleviate much of these artificial minima when changing
a. Decrease or partial loss
of persistent central convection and system slows and dissipation begins.
b. System dissipates in shear
type pattern and does not accelerate.
2. Apply ( "XT" numbers) technique if:
a. Loss of 1/2 or more of the
persistent central convection that surrounds the circulation center and
system maintains forward motion or accelerates.
b. System begins to interact
with other synoptic features such as a shear line or baroclinic zone.
This technique should be used until the satellite picture
indicates that the system has completed the transition to extratropical.
At that point it is recommended that the Smigielski and Mogil (1992) technique
is an appropriate technique to use if continued analyses is warranted.
The following criteria should be used to make the determination that a
system has transitioned to extratropical and the use of this technique
should be ended:
1. The system has progressed north of the polar
jet maximum and/or,
2. A well-defined cyclonic vortex, indicated by
a spiral pattern on water vapor imagery, exists over the low level circulation
center. This feature will appear as the system becomes cold-core.
Figure 1: Cyclonic vortex that has completed extratropical
Technical Questions? Contact Co-Authors Dr.
Mark Lander and MSgt Dennis
Webpage Suggestions? Contact Capt
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