A new methodology for classification of tropical cyclones: the importance of rainfall

Abstract

A new hazard index is presented to estimate and rank Tropical Cyclone or Hurricane severity according to storm damage and death toll at landfall. The index is derived and tested for hurricanes making landfall on the continental United States, being the most comprehensive data available. The index uses the three characteristic meteorological aspects of hurricanes that lead to fatalities and damage; wind, rainfall and storm surge. The contributions of each of these three aspects are parameterised by sub-indexes that include the key physical parameters describing the impact of each process and which are combined to determine the total hazard. Rainfall is identified as an important and frequently dominant hazard in terms of damage and death toll, but is not included in any current hazard scales or indexes. The new rainfall sub-index adopts rainfall intensity, storm rainfall area, and the forward speed of the system to estimate the rainfall hazard. The new hazard index, applied to recent U.S. hurricanes (2003–2012) at landfall, has better skill than existing scales in terms of ranking the severity of the events in terms of both damage and death toll. Further, the land characteristics of rainfall impacted areas are assessed to demonstrate flood-related vulnerability. Maps of hydrological soil type, land use and elevation are combined to generate NRCS curve-numbers as the primary hydrological indicator and the population at risk (exposure) as the social indicator. Including the land characteristics provides some improvement in the model-data correlation but significant variance remains, partially attributed to uncertainty in the social data.