A Shape Metric Methodology for Studying the Evolving Geometries of Synoptic-Scale Precipitation Patterns in Tropical Cyclones

A tropical cyclone (TC) is a cyclonic weather system with compact, centrally organized precipitation. As a TC transitions from a symmetric warm-core cyclone to an extratropical system, or as the TC dissipates, the weather system loses its characteristic central, symmetric qualities. In this article, we demonstrate a shape metric methodology that can be used to assess the overall evolution of and the spatiotemporal positions of significant changes to synoptic-scale precipitation structure. We first illustrate this methodology using three-hourly North American Regional Reanalysis (NARR) accumulated precipitation in Hurricane Katrina (2005) and then extend the analysis to all 2004 to 2015 U.S. landfalling TCs. To quantify the shape of the precipitation pattern, we construct a binary image by limiting the search radius to a distance of 600 km from the TC center and applying a minimum threshold based on the 90th percentile of precipitation observed within the search radius. Using the fundamental geographic concept of compactness, we formulate a suite of shape metrics that encompass the characteristic geometries of TCs moving into the midlatitudes: asymmetry, fragmentation, and dispersiveness. As we demonstrate with Hurricane Katrina, a moving Mann–Whitney U test reveals significant shape changes during the TC life cycle. These evolutionary periods correspond to structural changes observed by National Hurricane Center forecasters. Extending the analysis to all 2004 to 2015 storms, we observe increasing (decreasing) compactness in the eastern and central (western) Gulf of Mexico. Dispersiveness increases prior to landfall in most cases; however, asymmetry and fragmentation increase more commonly in western (vs. eastern) Gulf landfalls.