UPPER-LEVEL STRUCTURE OF THE FORMATIVE TROPICAL CYCLONE

ROBERT W. FETT USAF, AWS Member, National Weather Satellite Center, Weather Bureau, Washington, D.C.

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Abstract

In a survey of examples of formative tropical cyclones viewed by the TIROS satellite, and a comparison of many of these views with surface, upper-air, and time and space-section analyses, it is found that, the upper-level structure of the formative cyclone of moderate intensity is grossly similar to that of the storm at maturity. Common features in most examples studied include: (1) an upper-level trough or shear line which preceded the low-level trough and which moved with the storm system; and (2) an upper-level ridge or anticyclone which was generally superimposed over the convective area east of the lower-level trough or vortex. As has been found with the mature storm, strong subsidence occurred under the upper trough and coincided with a cloudiness minimum area in advance of the major convective cloudiness and west of the trough axis. Upward vertical motion with heavy convective activity was suggested under the upper-level ridge or anticyclone to the rear of the lower-level trough. Several examples depict these features and demonstrate changes that occurred as the storms passed individual stations. The TIROS photos were invaluable in this study as a source of additional data for confirming the distribution and type of cloudiness, for implying flow patterns at upper and lower levels, and for providing the basis for reasonable estimates of the stage of development of the entire system. It is concluded that an easterly wave model incorporating an upper-level trough and ridge or anticyclone as integral features of the moving system is one which describes most, adequately the majority of observations concerning formative tropical cyclones of moderate intensity and is one which is especially satisfying in terms of fulfilling the various theoretical and physical considerations.

*Now on assignment with Detachment 2, 30th Weather Squadron, Air Weather Service.

Abstract

In a survey of examples of formative tropical cyclones viewed by the TIROS satellite, and a comparison of many of these views with surface, upper-air, and time and space-section analyses, it is found that, the upper-level structure of the formative cyclone of moderate intensity is grossly similar to that of the storm at maturity. Common features in most examples studied include: (1) an upper-level trough or shear line which preceded the low-level trough and which moved with the storm system; and (2) an upper-level ridge or anticyclone which was generally superimposed over the convective area east of the lower-level trough or vortex. As has been found with the mature storm, strong subsidence occurred under the upper trough and coincided with a cloudiness minimum area in advance of the major convective cloudiness and west of the trough axis. Upward vertical motion with heavy convective activity was suggested under the upper-level ridge or anticyclone to the rear of the lower-level trough. Several examples depict these features and demonstrate changes that occurred as the storms passed individual stations. The TIROS photos were invaluable in this study as a source of additional data for confirming the distribution and type of cloudiness, for implying flow patterns at upper and lower levels, and for providing the basis for reasonable estimates of the stage of development of the entire system. It is concluded that an easterly wave model incorporating an upper-level trough and ridge or anticyclone as integral features of the moving system is one which describes most, adequately the majority of observations concerning formative tropical cyclones of moderate intensity and is one which is especially satisfying in terms of fulfilling the various theoretical and physical considerations.

*Now on assignment with Detachment 2, 30th Weather Squadron, Air Weather Service.

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