An Objective Measure of TC Size Using the Evolution of the Area Of Closed Isobars—Algorithm, Resulting Climatology, and Physical Insights

The size of a tropical cyclone (TC) is a critical structure parameter that influences the greatest extent of the TC’s societal impacts, which can be estimated by several different metrics. This parameter is important to Department of Defense (DoD) weather operations as forecasters rely on accurate and timely TC size estimates to mitigate military operational risk. In this study, a revised method of quantifying the size of a TC is introduced that exploits the wealth of gridded data available today. This method expands upon the definition of the radius of outermost closed isobar (e.g. Merrill 1984) to present an alternative TC size parameter, which objectively uses the sea level pressure field and the area enclosed by it. The revised method calculates the area enclosed by an isobar around a TC, and then quantifies how this area changes as pressure increases (e.g. the ratio of areas of two consecutive isobars). These calculations are then compared with an analytically-derived area ratio (derived from Schloemer 1955 and Holland 1980). The TC’s outermost closed isobar is generally determined to be the most outward isobar whose calculated area ratio does not significantly depart from the analytical ratio. This approach has the benefit of excluding baggy, but closed, isobars that may more represent the environment of the TC rather than the TC itself.

The algorithm is tested on a 36-year Western North Pacific tropical cyclone data set (1979–2014) using sea level pressure grids from three reanalysis databases: NASA’s Modern-Era Retrospective Reanalysis, ECMWF’s ERA-Interim reanalysis, and NCEP’s Climate Forecast System Reanalysis. On average, the algorithm was able to successfully calculate an outermost closed isobar for 73–80% of the roughly 30,000 six hour Joint Typhoon Warning Center Best-Track records. The primary reason for the inability of the algorithm to determine an outermost closed isobar was due to poor representation of the tropical cyclone in the gridded reanalysis, especially at and soon after formation, and thus is a deficiency of the dataset and not the algorithm. This research may provide DoD weather analysis and forecasting systems with enhanced opportunity to mitigate TC impacts to military operations including TC analysis and forecasting performed at the Joint Typhoon Warning Center and within the 557th Weather Wing. The talk will conclude with a discussion of the climatology of TC size found, including comparisons to prior climatologies and physical causes of the spatiotemporal patterns found.