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Section II
The Dynamics of Tropical Cyclones
Prof. Kerry A. Emanuel
Director
Center for Meteorology and
Physical Oceanography
Massachusetts Institute of Technology

[[image: Drawing of arrows and clouds against a blue sky]]

[[bold]]Introduction[[/bold]]

In spite of decades of intense scientific research, the physics of tropical cyclones remains poorly understood. This state of affairs exists mostly because these storms begin and end in parts of the earth's atmosphere that are very poorly observed, and because the reconnaissance aircraft that are used to observe hurricanes can only sample the lowest half of the circulation. The prospects for better measurements are now increasing with the growing concern for the influence of these catastrophic events on society. In early 1991, the United Nations declared that tropical cyclones deserve special attention as part of the International Decade of Disaster Reduction, owing to their disproportionate role in loss of life and damage. Several months later, a devastating storm swept through Bangladesh, killing at least 150,000. Several facets of tropical cyclones are relatively well understood, and serve to delineate the questions that will need to be answered before more accurate forecasts of cyclone movement and intensity can be made.

[[bold]]Hurricane Intensity[[/bold]]

Tropical cyclones derive their energy from the thermodynamic disequilibrium that generally exists between the tropical oceans and atmosphere. The amount of this disequilibrium is reflected by the undersaturation of near-surface air; the relative humidity is usually near 80%. The amount of latent heat transfer from the ocean necessary to saturate this air represents an upper limit to the amount of heating

[[bold]]I-7[[/bold]]