TROPICAL CYCLONE STRUCTURE AND INTENSITY CHANGE

This paper is concerned with two basic areas in the study of tropical cyclones: (a)structure and (b)genesis and intensity change.; Utilizing the compositing approach, fourteen years (1961-1974) of northwest Atlantic rawinsonde soundings are analyzed to obtain the basic thermodynamic and wind fields of the hurricane. A comparison is undertaken of the basic structure of the composited west Atlantic hurricane and the west Pacific typhoon as reported by Frank (1977a, b). Similarities and differences are discussed.; In order to investigate differences which lead to tropical cyclone genesis and intensification, eighteen Atlantic and Pacific data sets are also composited and divided into two groups: (a)deepening and (b)filling/steady systems. Deepening systems are found to have supergradient winds in the lower troposphere and less subgradient winds in the upper levels than filling/steady disturbances. The thermal wind equation indicates that an imbalance exists such that deepening systems have larger vertical tangential wind shear (WS) than baroclinicity. The opposite is true of filling systems.; A genesis and intensification mechanism is proposed based on the adjustment of the baroclinicity to the imbalance in the vertical shear of the tangential wind. This mechanism agrees with the work of Silva Dias and Schubert (1979) and Schubert et al. (1980) which shows that in the tropics--where the Rossby radius of deformation is large--the temperature field adjusts to the wind field. It is suggested that the initial imbalance results from alterations of the disturbance's rotational part of the wind caused by the large-scale flow. Cumulus clouds and vertical mass recycling occurring in this favorable initial vertical shear are thought to maintain and increase the original imbalance. The system tries to reach a balanced state by increasing the baroclinicity (B) in order to balance the larger vertical wind shear (WS). Intensification is produced as the inner area of the disturbance warms. Intensification continues as long as the imbalance is maintained. Under unfavorable vertical shears, cumulus activity acts to reduce vertical wind shear causing a decrease in the baroclinicity and weakening.; A primary finding of this research is the documentation of the important role of cumulus convection in the rearrangement of horizontal momentum. This rearrangement of momentum in the radial and tangential direction is estimated to be of fundamental importance to an understanding of tropical cyclone structure, genesis, and intensification. Qualitative arguments are employed to show how cumulus updraft and downdraft action can produce such momentum alteration.