Examination of Global Satellite-Based Tropical Cyclone Size Variations

In this study we define tropical cyclone (TC) size as the areal extent of the cyclonic circulation associated with a TC. TC size is both influenced by the shape of the TC vortex, and by the environment in which the TC is contained, but these two factors can be quite different. For instance, it is relatively common for a small TC vortex (~200 – 300 km) to be embedded in a large region (e.g., within a monsoon or easterly wave) of cyclonic flow. It is also important to note that the outer region of the TC where size changes occur are characterized by relatively low Rossby Numbers and inertial stability (i.e., Coriolis accelerations are important and the mass field adjusts to changes in the winds). For a variety of reasons including operational procedures and forecast priorities, the reliability and availability of TC size information is poor, and relatively few studies of TC size variability have been compiled. This shortcoming has limited our understanding of the temporal and spatial evolution of TC size. As one may imagine, variations of TC size are difficult to measure in the data sparse tropical oceanic regions. In addition, the current TC size metrics such as the radius of outer closed isobar (ROCI) and the radius of gale force winds (R34), are subjectively determined by a number of agencies using temporally changing, non-uniform methods, that are dependent upon TC intensity and/or translation speed, and may not be representative of TC size as defined here. As a result, the most basic inter-basin climatologies of TC size are unreliable and difficult to interpret. The unfortunate outcome is that the few studies addressing TC size variation and forecasting are dependent on the existing, yet inhomogeneous TC metrics and are difficult to reconcile in a unified manner. Nonetheless, TC size variations are known to be important for characterizing the overall TC wind field and the minimum central pressure, and thus are important for a variety of public warning, weather and climate applications. To overcome many of the shortcomings associated with records and measures of TC size, this study develops an objective TC size metric that can be applied globally given the historical geostationary satellite and best track intensity and location information. This presentation will concentrate on the development of a unified global tropical cyclone size metric, the creation of a global TC size dataset and the use of the resulting dataset to determine inter-basin climatologies that begin to investigate the causes of TC size changes.

Disclaimer: The views, opinions, and findings contained in this report are those of the authors and should not be construed as an official National Oceanic and Atmospheric Administration or U.S. Government position, policy, or decision.