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Cheng-Shang Lee
Roger Edson
, and
William M. Gray


This paper discusses the meteorological conditions associated with tropical cyclone formation in the north Indian Ocean during the 1979 FGGE year. Seven developing systems are composited together using FUGE Ill-b analyses to show the common circulation features surrounding developing cloud clusters. Three systems are further discussed to show different environmental influences on the low-level buildup of circulation during formation. The characteristics of these three disturbances’ 200 mb outflow patterns and a general discussion of north Indian Ocean tropical cyclone activity are also given.

Results show that tropical cyclone formation generally follows the initial increase of strong low-level winds on one side (either equatorial or polar) of a precyclone disturbance. This early buildup of wind appears to result from environmentally forced asymmetric wind surge action. Some of this increase appears to result from inward advection of velocity, but part of the increase seems to occur in situ. These initial strong azimuthal wind asymmetries are gradually reduced as the winds spread more evenly around the disturbance. A basic cyclone development process is the evolution of the low tropospheric flow from initial asymmetrical now (shear vorticity) to a more symmetrical circulation (curvature vorticity).

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