Tropical Cyclone Genesis and Intensification

This paper discusses the likely physical processes which are responsible for distinguishing between tropical disturbances which develop into tropical cyclones and disturbances which do not.The pre-typhoon cloud cluster disturbance is differentiated from the prominent non-developing cloud cluster disturbance primarily by its surrounding 2–8o radius tropospheric wind and vertical wind shear patterns. Other parameters such as mean vertical motion, moisture, convergence, vertical stability, surface wind speed, etc. are not significantly different. The favorable wind and vertical wind shear patterns are initially established by general circulation features.A paradox to the tropical cyclone development problem is the dual role of the tropical disturbance’s transverse circulation. The in-up-out mean radial circulation needed in a mechanical sense to spin-up the tangential flow also acts in an opposite sense to export moist static energy from the disturbance. Outflow air has higher energy than inflow air. Weather system maintenance and development can occur only if the disturbance’s surface energy flux is large enough to overcome the energy losses from the mean radial circulation and radiation. This requires extra energy flux from the ocean beyond that given by the bulk formula from space and time averaged wind speed and moisture values. Such extra surface energy fluxes have been calculated from budget analysis of our rawinsonde data as a residual and are observed to be related to the product of the weather system’s surrounding vertical wind shear and upward vertical motion.It is hypothesized that the pre-storm weather system’s larger surrounding region tropospheric vertical wind shear patterns lead to the development of more frequent and more intense rainband convection. The downdrafts developed by these rainbands tap the ocean for more energy by simultaneous increasing surface wind speed and decreasing moisture and temperature. This results in a nonlinear increase of surface energy flux along the rainbands. These extra rainband energy gains cause a significantly larger surface energy flux than that specified by the broadscale bulk formula using composited data. This surface energy increase is diffused throughout the vortex and allows the energy depleting mean radial circulation to be balanced. Smaller residual energy amounts are often available to intensify the vortex. Recent numerical modeling experiments by W. Fingerhut (1981) are verifying that such enhanced surface energy fluxes and vertical wind shear relationships can differentiate developing weather systems from those which do not develop.