Understanding and Diagnosing Tropical Cyclone Structure Variations

In this study, we will report on progress toward understanding and diagnosing tropical cyclone structure changes using a combination of satellite-based, and other observational datasets. In particular we will report on progress toward the development of an objective satellite-based tropical cyclone (TC) size parameter, relationships between TC size variability and environmental Total Precipitable Water (TPW) and a method to produce statistical-dynamical forecasts of operationally important wind radii. Methods and results discussed are applicable other basins and future satellites. TC size will be defined as the areal extent of the cyclonic circulation associated with a TC. It is recognized that TC size is determined by both environmental changes and changes related to the strength and/or intensity of the TC, where TC strength is defined here as the shape of the outer symmetric TC vortex well removed from the radius of maximum winds.

Variations of TC size and strength are difficult to measure in the data sparse tropical oceanic regions. While, TC size metrics do exist, such as the radius of outer closed isobar (ROCI), the radius of gale force winds (R34), they are subjectively determined by a number of agencies using non-uniform and temporally changing methods. These metrics 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. Recent model analyses are used to estimate the TC size, while, best track data information and infrared (IR) satellite imagery are then used to refine TC size estimates. This refined TC size metric will permit improved TC size climatologies, as well as, allow for the examination of trends and variations over a long period of time and many regions. Our current understanding of how TCs grow is also relatively limited and largely based on idealized modeling studies. Using the TC size metric developed here, analyses from numerical analyses, and satellite-based TPW record, we examine observationally the purported relationship between atmospheric moisture and TC size, as well as other environmental factors related to TC size and strength variations. The goal is to determine the likely mechanisms in which the TC size and strength increase, especially while those storms intensify, dramatically increasing the destructive potential. This will increase our overall understanding, allow for detailed model diagnoses and create necessary information needed to statistically estimate tropical cyclone size and strength changes. Finally, it is desirable to produce forecast guidance for significant wind radii along with statistical-dynamic intensity forecasts to operational centers. This can be done by using a combination of numerical analyses and forecasts, and initial information derived from IR satellite imagery. Details of this approach and example forecasts will be discussed.

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.