On the role of "hot towers" in tropical cyclone formation

The probabilistic approach to tropical cyclogenesis is advanced here by exa mining the role of convection in the early stages. The development of "hot towers", that is tall cumulonimbus towers which reach or penetrate the trop opause, and their role in tropical cyclogenesis is investigated in two well -documented cases of formation, namely hurricane Daisy (1958) in the Atlant ic and Tropical Cyclone Oliver (1993) in the Coral Sea. The hot towers in D aisy had been intensively studied by Malkus and Riehl three decades ago but remained mainly unpublished. The dynamics of Oliver genesis by merging mes oscale vortices has been recently reported, but much of the aircraft data r emained. This paper adds the evolving contribution of cumulus-scale events and their associated electrification, which was made possible by the additi on of an electric field mill, a numerical cloud model and other remote sens ors. In their genesis stages, Daisy and Oliver appeared very different because D aisy resulted from a deepening tropical wave in the Atlantic and the pre-Ol iver vortex emerged eastward from the Australian monsoon trough. However, t he vertical profiles of theta(E) in the rain areas were nearly identical, w ith the characteristic concave shape showing substantial midlevel minima. T herefore, both required increasing upflux of high theta(E) subcloud air in order to accomplish the formation stage, with about two hot towers each in the nascent eyewall. In both cases, partial eyewalls developed at the edge of the convection, permitting subsidence in the forming eye, which was show n to contribute to the pressure fall. The probabilistic concept proposes th at any contribution to early pressure fall raises the probability of succes s. When the incipient storm goes through those fragile phases more rapidly, the risk of death by the onset of unfavorable large-scale factors such as wind shear or upper-level subsidence is reduced. Daisy developed in an inac tive, moist environment with light, Variable winds throughout the troposphe re while in Oliver, strong divergent upper outflow apparently outweighed st rong wind shear, although the latter was responsible for a slow and messy d evelopment of a closed, circular eye. In both storms, the hot towers in the major rainband were taller and strong er than those in the early eyewall. One-dimensional time-dependent model ru ns were used to simulate both in Oliver with two important results: 1) the taller rainband clouds permitted greater high level heating, if it could be retained; and 2) greater electrification and more lighting occurred in the rainband although the partial eyewall clouds also showed strong electrific ation. Airborne radar, electrification measurements and models are fitted t ogether to understand their relationship. An important result is the clear inference that fairly deep mixed phase regions existed in both eyewall and rainband, in which the DC-X aircraft experienced liquid water at temperatur es colder than -40 degrees C below freezing. These results show that the cl aims of no supercooled liquid water in tropical cyclones require re-examina tion with the proper measurements of electricification that are now feasibl e.