| This thesis examines the conversion of Atlantic tropical cyclones (TC) into extratropical cyclones (extratropical transition; ET) and presents arguments for the climatology, lifecycle definition, and the physical processes behind ET. Extratropical transition is the conversion of a symmetric, vertically stacked, warm-core tropical cyclone with a maximum intensity in the lower troposphere into an asymmetric, cold-core and tilted extratropical cyclone with a maximum intensity in the upper troposphere. This transition usually occurs with movement into the middle latitudes, and is partially a result of the increased shear, baroclinicity and synoptic-scale disturbances at those latitudes.; After an introduction to the topic in Chapter 1, a comprehensive climatology of extratropically transitioning tropical cyclones in the Atlantic basin is presented in Chapter 2. Storm tracks and intensities over a period from 1899 through 1996 are examined, with a focus on the more reliable post-1950 era database. Extratropically transitioning tropical cyclones represent 50% of landfalling tropical cyclones on the east coasts of the United States and Canada, and the west coast of Europe, combined.; Atlantic transition occurs from 24°N through 55°N, with a much higher frequency between the latitudes of 35°N to 45°N. Transition occurs at lower latitudes at the beginning and end of the season, and at higher latitudes during the season peak (August--September).; The structural evolution of the 61 tropical cyclones from 1979--1993 in Chapter 2 were further examined using 1.125° ECMWF reanalyses in Chapter 3. A reliable indicator for the start of extratropical transition was the mean 850--600hPa thickness difference between the semicircles right and left of storm motion.; The fourth chapter examines in detail a case study of extratropical transition through an application of the diagnostics developed in the first two chapters as well as conventional measures of tropical and extratropical cyclone evolution.; The end result of extratropical transition is not necessarily a cyclone that has a frontal, cold-core structure. As viewed within the generalized cyclone phase space defined here and in other related research, extratropical transition represents only one potential post-tropical evolution of a tropical cyclone, and not necessarily the final transition for the cyclone. (Abstract shortened by UMI.)... |
