Mechanism Governing the Size of Tropical Cyclone-Like Vortices

The size of tropical cyclones (TCs) is defined as, for instance, the radius of wind speed over 15 m/s (R15), which is as important as the strength of TCs. Factors of changing the size of TCs have been investigated by many studies. However, the mechanism governing the size of TCs has not yet been understood well. The purpose of this study is to understand its mechanism by using simplified TC-like vortices. In order to do so, various numerical experiments were performed using a primitive equation model on an f-plane. In these experiments, various sizes and strengths of external forcing mimicking cumulus heating were given at various distances from the vortex center to examine these effects of the forcing on R15.

When forcing size and strength are constant, the vortex size depends on the distance of forcing from the center. Forcing at outer regions makes the vortex larger than at inner regions. Similar results are obtained with using other forcing sizes and strengths.

These results indicate that we can understand the mechanism governing the vortex size as follows: Whether or not the vortex size increases depends on whether or not the secondary circulation (SC) induced by forcing exceeds initial R15. When the extent of the SC is beyond initial R15, the vortex grows in size. The farther the SC outside the vortex extends, the more the size increases.

In detail, when the SC exceeds initial R15, parcels having large absolute angular momentum (AAM) come into the vortex from the outside, making R15 larger. Thus, it is important whether the SC exceeds initial R15 or not. The extent of the SC can be evaluated by the Rossby radius of deformation. When forcing is given at small (large) Rossby radius regions, i.e. at inner (outer) regions, the SC is confined in narrow (extendes over wide) ranges. Thus, it is difficult (easy) to exceed initial R15, and the size does not (does) increase. When AAM is a Lagrangian conserved quantity, tangential winds at R15 increase by the inflow of parcels with large AAM being far from the vortex. In fact, we can confirm from backward trajectory analyses that parcels come from the edge of the SC. Thus, the farther the SC expands, the larger the vortex size becomes.