East Asia and Western Pacific METEOROLOGY AND CLIMATE

536
TYPHOON FORMATION AND DEVELOPMENT - AN OBSERVATIONAL POINT OF VIEW
Cheng-Shang Lee
Department of Atmospheric Sciences,
National.Taiwan University
The physical processes which lead to the formation and
development of tropical cyclones are still not well-understood. The
current observational analysis attempts to advance our knowledge
towards fully understanding these complicated processes by using the
rawinsonde composite and individual case analyses. Results have
indicated that the large scale momentum surges, which can provide
inward eddy vorticity flux, are conducive to the development of a
cloud cluster to a tropical cyclone. The cumulus heating efficiency is
very low due to the weak vorticity field associated with the system
during this formation stage. The most common large scale forcing
includes the cross-equatorial surges due to strong cold out-break flow
in the counter-hemisphere and the trade wind surges.
After the formation of a tropical cyclone, the heating efficiency
increases due to the increased vorticity associated with the system. A
tropical cyclone can develop much faster if the spiral rain bands are
active, because they can provide the needed inward moisture and
angular momentum flux for cyclone to develop. However, the cyclone can
also increase its developing trend through the CISK mechanism. At this
stage, it might be more realistic to pay attention to the processes
which can hinder the cyclone developing trend, such as the strong
middle to upper level wind shear or the land masses influence.
The intensity limit which a typhoon can reach is often posed by
underneath sea surface temperature. However, the existence of TUTT can
often redirect the upper level outflow to form outflow channel (or the
vacumn effect) which is often favorable for typhoon development.
In addition, results using satellite data analysis also show that
a convection burst often occurs before or during the major intensity
increase. The maximum inner region convection often leads the maximum
intensity of typhoon.