MS Seminar - Thomas McKenzie
Title: A CLIMATOLOGY OF TROPICAL CYCLONE SIZE IN THE WESTERN NORTH PACIFIC USING AN ALTERNATIVE METRIC. Abstract: The size of a tropical cyclone (TC) is a critical structure parameter that inﬂuences the greatest extent of societal impacts, and can be estimated by several diﬀerent metrics. 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 TC size parameter that objectively uses the sea level 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 calculates the area enclosed by an isobar around a tropical cyclone, and then computes the area ratio of adjacent isobars. These calculations are then compared with an analytically-derived area ratio using Holland (1980) radial pressure formulation. The outermost closed isobar (OCI) is generally determined to be the most outward isobar whose calculated 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) TC data set (1979 – 2014), and the results 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 roughly 30,000 six hour JTWC Best-Track records. 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 calculations using the alternative metric are 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 N. When the small TCs at formation were compared to the large TCs at formation, it was found that there was a statistically signiﬁcant diﬀerence in the geographic distribution of these two groups. The size and position of the monsoon trough generally inﬂuences the size of a forming TC in the WNP. The study includes an examination of several case studies representative of the analysis presented above.