Contrasting Various Metrics for Measuring Tropical Cyclone Activity

Popular metrics used for measuring the tropical cyclone (TC) activity, including NTC (number of tropical cyclones), TCD (tropical cyclone days), ACE (accumulated cyclone energy), PDI (power dissipation index), along with two newly proposed indices: RACE (revised accumulated cyclone energy) and RPDI (revised power dissipation index), are compared using the JTWC (Joint Typhoon Warning Center) best-track data of TC over the western North Pacific basin. Our study shows that, while the above metrics have demonstrated various degrees of discrepancies, but in practical terms, they are all able to produce meaningful temporal and spatial changes in response to climate variability. Compared with the conventional ACE and PDI, RACE and RPDI seem to provide a more precise estimate of the total TC activity, especially in projecting the upswing trend of TC activity over the past few decades, simply because of a better approach in estimating TC wind energy. However, we would argue that there is still no need to find a ¡§universal¡¨ or ¡§best¡¨ metric for TC activity because different metrics are designed to stratify different aspects of TC activity, and whether the selected metric is appropriate or not should be determined solely by the purpose of study. Except for magnitude difference, the analysis results seem insensitive to the choice of the best-track datasets