Intensity estimates begin with an analysis of the satellite picture to determine:
1. Primary Outer Cloud Band.
A transitioning system develops a primary cloud band of middle and high level clouds that is not connected to the circulation center. It moves poleward and eastward away from the circulation center as the transition progresses. It is necessary to account for this outer cloud band and its associated energy. The log 10 spiral is used as a tool to determine the extent of this feature. The log 10 spiral is used as defined by the Dvorak technique, but is not placed with regard to the center of circulation as the primary intent here is to quantify the aerial extent of the outer cloud band arc. The outer cloud band should be measured if it is within 6 degrees of latitude of the circulation center. The measurement should be along the spine of the solid cloud band of the highest middle and high level clouds.. Isolated convective elements that are at the end of the line, but not connected to the primary outer cloud band, should not be included in the measurement. On IR imagery the outer cloud band will be outlined by the use of the BD curve, and will include the area defined by the dark grey shade or colder region. This measurement is converted to wind speeds that are in correlation with the Dvorak spiral band curvature technique. Table 1 details the application of the log 10 spiral and assigns the appropriate "XT" wind speeds.
Criteria for application of log 10 spiral to primary outer cloud band.
55 - 65 KNOTS
Middle and high clouds in pronounced curved bands.
Bands may weaken and redevelop eastward of the center. A log
10 spiral applied to the spine of the solid cloud band, within 6 degrees
of the circulation center, will have a definite curve that wraps back around
towards the left rear quadrant of the circulation center with regard to
movement in the northern hemisphere and right rear quadrant of the circulation
center with regard to movement in the southern hemisphere. The log
10 spiral is not placed with regard to the center of circulation.
Curvature measurements on the spiral of .80 to .90 for 55 knots and .95
and greater for 65 knots. When using IR imagery, the dark gray shade
or colder, on the BD curve will be indicative of deep convection for all
"XT" numbers.
35 - 45 KNOTS
Middle and high clouds in pronounced curved bands.
A log 10 spiral applied to the spine of the solid cloud band, within 6
degrees of the circulation center, will have a curve that wraps back towards
the left quadrant of the circulation center with regard to movement in
the northern hemisphere and right quadrant of the circulation center with
regard to movement in the southern hemisphere. The log 10 spiral
is not placed with regard to the center of circulation. Curvature
measurements on the spiral will be .40 to .55 for 35 knots and .60 to .75
for 45 knots.
25 - 30 KNOTS
Middle and high clouds in curved bands. A
log 10 spiral applied to the spine of the solid cloud band, within 6 degrees
of the circulation center, will have a definite curve towards the forward
quadrant of the circulation center with regard to movement in both hemispheres.
The log 10 spiral is not placed with regard to the center of circulation.
Curvature measurements on the spiral will be .15 to .20
for 25 knots and .25 to .35 for 30 knots.
25 - 30 KNOTS
Middle and high clouds in curved bands. A
log 10 spiral applied to the spine of the solid cloud band, within 6 degrees
of the circulation center, will have a definite curve towards the forward
quadrant of the circulation center with regard to movement in both hemispheres.
The log 10 spiral is not placed with regard to the center of circulation.
Curvature measurements on the spiral will be .15 to .20
for 25 knots and .25 to .35 for 30 knots.
0 - 20 KNOTS
Low-level circulation center present. Outer cloud
band is not organized. Very poorly organized cloud bands without,
or with very little, curvature. The log 10 spiral is not placed with
regard to the center of circulation. Curvature measurements on the
spiral will be at least .10 to be 20 knots. If < .10 wrap than
do not add any wind speed. Figures 2 and 3 illustrate the application
of the log 10 spiral to transitioning systems in both hemispheres.
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