INTERPRETATION
DISCUSSION
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Regional and Mesoscale Meteorology Team Cooperative Institute for Research in the Atmosphere (CIRA) Colorado State University Fort Collins, Colorado |
January 25, 2002
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Most of the tropical storms and hurricanes that occur in the Atlantic, Carribean and Gulf of Mexico, originate from pre-existing tropical disturbances that track east to west from continental Africa across the tropical Atlantic. The disturbances are called easterly waves, and nearly all of the August-September intense hurricanes that hit the U.S. form and intensify in association with easterly waves. These type hurricanes have been referred to as “Cape Verde” hurricanes, named after the islands in the tropical eastern Atlantic.
The cyclonic circulation associated with easterly waves is strongest
near the 700 hPa level. However, Atlantic hurricanes can also develop
in association with upper lows that have strong cyclonic circulations near
the 200 hPa level that extend to lower levels but weaken going downward.
Upper lows that are cut off from the mid-latitude westerlies are called
subtropical lows. At times, the subtropical lows intensify and have
strong surface circulations and high winds. They are designated subtropical
storms if surface winds attain gale force. Subtropical storms are
assigned intensities based on satellite images, using a method analagous
to the familiar Dvorak technique for tropical cyclones. Click
on images to enlarge.
Figure 1 |
Figure 2 |
Figure 3 |
Figure 4 |
During the 2001 hurricane season, three hurricanes (Karen, Noel, and
Olga) and a tropical storm (Lorenzo) transitioned from subtropical storms
to tropical cyclones. During this transition, persistent deep convection
develops near the surface center and the radius of maximum winds decreases.
Water vapor images are useful for identifying and tracking subtropical
lows, and also when observing the transition of a subtropical storm to
a tropical storm. Figures 1 through 4 are water vapor images near the time
of first designation as a tropical storm, showing the four tropical cyclones
that originated from subtropical storms. Note that the upper subtropical
low pattern can be seen along with a much smaller cloud pattern indicative
of a tropical storm. For comparison, a very different appearance in the
water vapor images is observed with four examples at initial tropical storm
stage for “Cape Verde” systems (Chantal, Dean, Erin, and Iris) in
Figures 5 through 8.
Figure 5 |
Figure 6 |
Figure 7 |
Figure 8 |