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SummaryThe probabilistic approach to tropical cyclogenesis is advanced here by examining the role of convection in the early stages. The development of "hot towers", that is tall cumulonimbus towers which reach or penetrate the tropopause, and their role in tropical cyclogenesis is investigated in two well-documented cases of formation, namely hurricane Daisy (1958) in the Atlantic and Tropical Cyclone Oliver (1993) in the Coral Sea. The hot towers in Daisy had been intensively studied by Malkus and Riehl three decades ago but remained mainly unpublished. The dynamics of Oliver genesis by merging mesoscale vortices has been recently reported, but much of the aircraft data remained. This paper adds the evolving contribution of cumulus-scale events and their associated electrification, which was made possible by the addition of an electric field mill, a numerical cloud model and other remote sensors.In their genesis stages, Daisy and Oliver appeared very different because Daisy resulted from a deepening tropical wave in the Atlantic and the pre-Oliver vortex emerged eastward from the Australian monsoon trough. However, the vertical profiles of 0E in the rain areas were nearly identical, with the characteristic concave shape showing substantial midlevel minima. Therefore, both required increasing upflux of high 0E 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 shown to contribute to the pressure fall. The probabilistic concept proposes that any contribution to early pressure fall raises the probability of success. 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 upperqevel subsidence is reduced. Daisy developed in an inactive, moist environment with light, variable winds throughout the troposphere while in Oliver, strong divergent upper outflow apparently outweighed strong wind shear, although the latter was responsible for a slow and messy development of a closed, circular eye.In both storms, the hot towers in the major rainband were taller and stronger than those in the early eyewall. Onedimensional time-dependent model runs 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 electrification. Airborne radar, electrification measurements and models are fitted together 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-8 aircraft experienced liquid water at temperatures colder than -40 ~ below freezing. These results show that the claims of no supercooled liquid water in ...

Cumulus dynamics is approached as an experimental science. Its development since 1947 is briefly reviewed and the current convection models described, in particular models evolved from laboratory studies, such as the 'starting plume'.A preliminary series of experiments on real tropical cumuli has been conducted as a method for testing and improving models and for assessing the range of applicability of the laboratory results. A silver iodide seeding technique was used in a joint U.S. Weather Bureau-U. S. Navy experimental program called Project Stormfury. Eleven tropical cumulus clouds were probed by instrumented aircraft on four days in August 1963. Six were seeded by dropping pyrotechnic silver iodide generators into their tops at intervals of 100 meters or less; five were studied as 'controls.' All control clouds died without further growth. Of the treated clouds, one was seeded by misunderstanding when its top was below 10,000 feet; it collapsed. The remaining five treated clouds grew. Of these, three were seeded at low enough elevations so that natural glaciation was unlikely; these are called the 'test' clouds. All three test clouds were observed to 'explode' in two phases. The first phase was a vertical growth of 10,000-20,000 ft, occupying 10-12 minutes; the second was a horizontal expansion, with the resulting giant cloud persisting more than 30 minutes. Using laboratory and theoretical results, we construct a dynamic model of the first growth phase and test it against the aircraft measurements and photographic data. The model predicts tower ascent rates, excess temperatures, and water contents, with environment and cloud base conditions as input. It predicts that unmodified clouds could not have attained the observed heights. Furthermore, the postulated effects of seeding (fusion heating, expansion, and altered precipitation fallout) are incorporated into the model and are shown able to account for the excessive growth of the test clouds following treatment. A set of extensive control calculations shows that the main deductions are insensitive to the existing uncertainties in the input data and the modeling assumptions. Improved experiments are suggested to resolve some of the basic questions in cumulus dynamics, as well as seeding effects. 1. PROGRESS AND OBSTACLES IN CUMULUS DYNAMICS Modern cumulus dynamics began seventeen years ago with the pos•ula•ion of entrainmen• by Stommel [1947] who provided a method of computing i• from the in-cloud and environmen• sounding of the Wyman-Woodcock Caribbean expedition [Bunker et al., 1949]. These small •rade cumuli were found •o incorporate outside mass comparable •o •heir own in abou• 1 km ascen•, diluting •heir buoyancy •o a few t•ent•hs •ha• of •he adiabatic parcel. 387 388 SIMPSON, SIMPSON, ANDREWS, AND EATON Entrainment was soon shown to be the main brake on cumulus development, and Stommel's steady jet concept was extended and applied successfully to selected clouds. The improving observational facility offered by instrumented aircraft made this developmen...

Observations on damage to concrete structures, due to the 1994 Northridge earthquake, are reported from a Canadian code perspective. Most of the damaged structures were older, nonductile, structures that do not conform to current design and detailing requirements. Concern is expressed about the seismic hazard of older Canadian structures having similar deficiencies. A significant number of parking structures suffered extensive damage and a number of precast concrete parking structures collapsed. Deficiencies in these structures include lack of proper diaphragm connections, a mix of gravity load columns with ductile framing, inappropriate number and distribution of shear walls, torsional effects caused by ramps, and the creation of short columns due to geometric features. This earthquake also demonstrated the deficiencies in connections of pre-1973 tilt-up structures. Key words: seismic design, earthquake, Northridge, structures, codes, concrete, precast concrete.