Typhoon Paka which recently hit Guam (13N, 145E) had very strong wind gusts reported which
have been a topic of much debate in the past week. During landfall , Anderson Air Force
Base, which is located on the northern tip of Guam reported a wind gust of 205 knots. A
wind gust this strong had never before been reported, thus the debate. But in fact,
things could have been worse for Guam. As Paka approached Guam from the east it underwent
weakening. Below is a loop of six hourly average images starting on 6Z December 15
continuing through 00Z on December 17. The average images were calculated by first
creating storm relative images (or subtracting the storm motion out of the images) using
the SR command. These storm relative images were then averaged over six hour periods,
resulting in the pictures shown in the loop. These average images were then reloaded and
magnified by a factor of 2. In addition, these average images were used to create a time
series of the Dvorak digital inferred intensity estimates (Fig 1) of this storm ( using
the command DVMET) -- note the weakening prior to Paka's landfall on Guam. Actual
intensities for these six hour periods were 140, 140, 135, 120, 125, 125, 125, and 120
knots, respectfully. At the time this storm affected Guam the maximum sustained winds
were estimated at 125 knots.
This type analysis clearly shows two factors involved with this weakening of Typhoon Paka.
The first weakening influence is vertical wind shear. In the first image, there appears
to be very little vertical shear, in that the storm has nearly symmetrical convection
around its center. As time progresses, the deep convection becomes increasingly asymmetric
with the largest concentrations of convection on the east side of the center, indicating
westerly wind shear with height (see frames 2-5). A second factor involved with Paka's
weakening is that the storm went through a concentric eyewall cycle. This occurs when a
second eyewall begins to form at a greater radius than the original eyewall, and the inner
eyewall weakens with time (see frames 5-7). When this occurs the storm can dramatically
weaken. At the end of this cycle the inner eyewall is gone, and a new outer eyewall,
typically at greater radius is present and becomes better organized (see frames 7-8).
As the new eyewall establishes itself at smaller radii, new intensification often occurs.
Here's the loop !
John Knaff
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