Climatology of Tropical Cyclone Size in the Western North Pacific Using an Alternative Metric

The size of a tropical cyclone (TC) is a critical structure parameter that is associated with the greatest extent of societal impacts, and it can be estimated by several different metrics depending on the variable used. In this study, a revised method of quantifying the size of a TC is introduced. This method expands upon the work of Merrill (1984) to present an alternative tropical cyclone size parameter that uses the surface pressure field and the area enclosed by it. This new approach is made possible by higher resolution and more accurate gridded meteorological data. The revised method measures the relative area of each closed isobar around a tropical cyclone to compute the area ratio of adjacent isobars. These calculations are then compared with an analytical area ratio derived from Holland (1980). The outermost closed isobar (OCI) is generally determined to be the most outward isobar whose area ratio does not significantly depart from the analytical ratio derived from Holland (1980). The algorithm is applied to a 36-year Western North Pacific (WNP) tropical cyclone data set (1979 – 2014), and the results of this are analyzed statistically and physically. This derived climatology utilized three reanalysis data sets: NASA's Modern-Era Retrospective Reanalysis data set (MERRA), ECMWF's ERA-Interim reanalysis (ERA-I), and NCEP's Climate Forecast System Reanalysis database (CFSR). On average, the algorithm was able to successfully determine an OCI for 75 – 80% of the 6-h best-track storm fixes. The primary reason for the inability of the algorithm to determine an OCI was poor representation of the TC in the gridded reanalysis, especially at and soon after formation. The statistical analysis reveals that TC size measurements using the revised metric is generally in agreement with existing climatologies. These results include a maximum mean TC size in October, a positive relationship between size and age of a TC, interannual variability of size, and an apparent maximum size near 25 degrees North. When the small-sized TCs at formation were compared to the large-sized TCs at formation, it was found that there was a highly statistically significant difference in the geographic distribution of these two groups. The size and position of the monsoon trough, in articular, generally determines where and how a TC forms in the WNP. The expected growth of a TC throughout later in its life cycle, especially during extratropical transition, was well-represented by this analysis. The study concludes with an examination of several case studies representative of the analysis presented above.