When, where, and how weakly rotating convective regions develop into full-blown tropical cyclones remains an essentially unsolved problem in atmospheric dynamics, albeit a problem with unusually significant implications for forecasting high-impact weather. In this project Montomgery and his group will explore his "marsupial hypothesis," which relates tropical cyclogenesis to the critical layers of tropical waves. The "marsupial hypothesis" comprises three sub-hypotheses: 1) The weakly rotating regions of thunderstorms that develop into tropical cyclones form at the critical level, or steering level, of tropical traveling waves. At this level the wave speed equals that of the background wind, so that fluid, including the the weakly rotating "proto-vortex," moves with the wave. 2) For stronger waves, the critical level evolves into a critical layer, which forms a pocket of closed streamlines. In this pocket, the developing vortex is partially isolated or protected (thus "marsupial" theory) from the entrainment of dry air and the deleterious affects of vertical shear. 3) Tropical waves are amplified by the latent heat release associated with these proto-vortices.
These hypotheses will be tested in a program of observational analyses and modeling. Preliminary analyses of atmospheric data indicates that critical layers of African easterly waves are, as hypothesized, preferred regions for tropical cyclone development, and that convection within the wave trough helps to maintain the wave. These analyses will be extended to monsoon trough cases, and cyclogenesis cases will be balanced with null cases, in which cyclogenesis does not occur. Problems with reanalyses in the tropics - that the results are strongly conditioned by the cumulus parameterization scheme - will be circumvented by analyzing data from other sources, including the Tropical Rainfall Measurement Mission (TRMM) satellite data, and in-situ data obtained during the summer-fall 2008 T-PARC (THe Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign) field campaigns.
Model experiments will be carried out using the Weather Research and Forecasting (WRF) model. Dry experiments will examine the vortex shedding from tropical waves that produces proto-vortices. Moist experiments with standard treatments of cumulus convection will examine wave-amplification, and cloud-scale modeling will consider the growth of proto-vortices.
Broader impacts of this project derive from the societal benefits of improved understanding of tropical cyclogenesis and the potential impact of such understanding on prediction. Montgomery's group will collaborate with NOAA's Hurricane Research Division in their annual field program of research. Graduate students from the military services will be trained.
