Isentropic Potential Vorticity Analysis of the Outflow Layerof a Typhoon

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A tropical cyclone may be considered to be a heat engine. Its fuel is the latent heat gathered by surface air as it is swept toward the central region of a low-level inflow circ- ulation. The heat is released by condensation during ascent of the moist surface air in the eye-wall. In any heat engine, a heat sink is required for energy to be converted to the kinetic energy of motion. In a tropical storm, such a sink is found in an ele- vated (upper troposphere-lower statosphere) layer of outflow from the storm's central region. The temperature, pressure, humidity, and three-dimensional velocity of the air change drastically before reaching this sink region. However, two other properties are better conserved, that can serve as tracers. These are its equivalent potential temperature (a measure of its entropy) and its potential vorticity (a combined measure of its thermal stability and rotational motion). Calculation of natural three-dimensional fields of these properties requires, however, measurements of unprecedented detail and extent through the life- cycle of a (preferably intense) tropical cyclone. Four observational experiments in the summer of 1990 should provide the data needed to analyze, in particular, the vertical structure of the elevated outflow layer. These analyses should further our understanding of tropical cyclone outflow and its interaction with its tropospheric and stratospheric environment. An ideal case is now available for analysis, since the instru- mental networks were deployed in a region (the western Pacific) and during a period (10-20 September 1990) that allowed them to capture Super-typhoon Flo. The analyses could serve to discriminate between a few cur- rent theories of tropical cyclone development and provide a basis for advancing beyond current statistical-empirical methods for forecasting intensity changes. Also, it is estimated that 5- 10% of the global mass exchange between troposphere and strato- sphere occurs in these storms. Thus, the analyses could help refine our present estimates of atmospheric chemical budgets. Merrill has established a creditable past record in tropical cyclone analyses. He is being supported by ONR and AFGL to par- ticipate in the data editing, reduction and statistical cyclone- track analyses of the summer 1990 measurements. The additional research supported under this award provides an opportunity for complementary use of these unprecedented data sets.