Assessing Hurricane Intensity Using Satellites - Satellite-based Applications on Climate Change - page 148

FOUR PRIMARY PATTERNS
AND TYPICAL T-NO.'S

CURVED BAND

SHEAR

CDO

EYE

BANDING EYE

PRE-
STORM

TD TS HURRICANE

0 1 2 3 4 5 6 7 8

Fig. 10.1 The primary Dvorak cloud patterns in relation to T-number and tropical cyclone
intensity ranges

10.2.1 Operational Dvorak Technique

The Dvorak technique (Dvorak 1984 ) primarily uses satellite observed cloud
patterns and infrared (IR) cloud top temperatures to estimate intensity, with inde-
pendent methods for visible and IR satellite imagery. It uses an intensity unit called
a T-number in increments of ½ ranging from T1 to T8. The Dvorak T-number
intensity scale is normalized according to typical observed daily changes in inten-
sity (one T-number per day). T2.5 is the minimal tropical storm intensity (18.0 m/s),
T4.0 is minimal hurricane intensity (33.4 m/s), T6.0 has a wind maximum of
59.1 m/s, and T8.0 approximates a record maximum intensity (87.4 m/s).
The cloud patterns in the Dvorak technique are divided into the four basic
patterns in Fig. 10.1 (curved band, shear, central dense overcast, and eye), with a
fifth sub-pattern called a banded eye. With weaker intensities, the analysis is usually
based on either the curved band pattern or the shear pattern. Using the curved band
analysis, the extent to which a spiral shaped band of deep convective clouds
surrounds the tropical cyclone center determines the intensity. The shear pattern
refers to the cloud pattern observed when broadscale vertical wind shear induces a
distinctly asymmetric cloud pattern with respect to the tropical cyclone low-level
circulation center. The degree of deep convective cloud displacement due to the
vertical shear decreases with intensification.
As a TC intensifies, the cloud pattern typically evolves into what is called a
central dense overcast (CDO), which describes the deep convective clouds that
surround the center. As intensification continues, an eye is observed within this
central dense overcast. The eye is the familiar cloud-free or cloud minimum area