Emanuel proposes a three-part study of tropical cyclone dynamics: a) A field experiment, to test a simple hypothesis that tropical cloud clusters are "triggered" to "ignition" (cyclogen- esis) only if the mid-tropospheric source air for small-scale convective downdrafts in the vicinity of the disturbance's center becomes warm/moist enough to set up a downdraft-surface layer-cloud updraft cycle that results in a continually warmer and/or moister downdraft source layer. The field experiment is required because neither conventional past observations nor numerical models resolve the spatial or temporal detail needed to test the hypothesis. b) A set of analyses of past hurricanes to see whether cyclone initiation is also accompanied by the approach of an upper-level flow system with a locally high measure of combined cyclonic rotation and thermal instability, so that it lines up over a similar low-level area generated by clusters of tropical rain showers c) Generalization of the current barotropic (one-layer) theory of steering of mature hurricanes by the ambient large scale flow, by constructing a simple two-layer (baroclinic) model. Although it has consistently been observed that the vertical shear of the ambient flow of tropical cyclones is weak at best, Emanuel argues that the cyclones themselves have pronounced vertical structure, with the lower tropospheric cyclone overlain by a more anticyclonic vorti- city center. The third study will improve our understanding of the factors governing hurricane movement. The first two should provide more information on the perennial question as to why so few tropical oceanic rain systems eventually develop into mature tropical cyclones.
