Angular Momentum Transport and Tropical Cyclone Size Changes

This paper is the second part of a comprehensive study on tropical cyclone (TC) size. In part one, the climatology of TC size and strength over the western north Pacific (WNP) and the North Atlantic have been established based on the QuikSCAT satellite data during the period 1999 – 2009, which represents the largest dataset ever assembled for size studies. In this part, the mechanisms that are likely responsible for TC size changes are explored through analyses of angular momentum (AM) transport associated with the TC system.

Based on the data from part one and the Climate Forecast System Reanalysis data, TCs are classified into 4 distinct categories: i.e. intensity increase and size increase (I↑S↑), intensity increase and size decrease (I↑S↓), intensity decrease and size increase (I↓S↑), and intensity decrease and size decrease (I↓S↓), with all changes in intensity and size occurring within 24 h. It is found that the AM transports from these 4 categories are consistent with the hypothesis and in both ocean basins. Changes in AM export at the upper and import at the lower level appear to be an important dynamic factor that affects TC intensity and size respectively. These mechanisms also provide an explanation as to why the correlation between intensity and size is weak, which has never been discussed before.

To provide a preliminary picture of the evolution of the synoptic flow, the patterns associated with different sizes (small, medium and large) of WNP TC are presented. The AM transports are then linked to these synoptic flow patterns associated with the different changes of TC size. The extent and magnitude of the low-level wind surges as well as the radial winds are related to the changes in TC size, which agrees well with the AM transport concept.