Rapid intensification of tropical cyclones (TCs) is not predicted well, which could mean the public would not have time to prepare if such an event occurs close to the coast. Four nonlinearly interactive processes (ocean thermal structure changes, atmospheric boundary layer and convective transports, and external factors such as static stability and eddy angular momentum transports) may be involved in TC intensity changes.
The objective of this study is to contrast the rapid intensification of Supertyphoon Flo with the simultaneous slow intensification of Typhoon Ed during the Tropical Cyclone Motion (TCM-90) field experiment. No correlation of intensity change with the passage of Flo over a warm-core ocean eddy was evident. A composite of six thermal structure comparisons of Flo and Ed indicates Flo had greater convective instability in the mid-tropospheric and higher temperatures in the upper troposphere. The 850-200 mb vertical shears were well below the usual inhibiting threshold value for TC intensification during the pre-conditioning period, and then decreased further during the rapid intensification stage of Flo. A more significant decrease in vertical shear occurs between 300 and 150 mb during the pre-conditioning period, which is consistent with a cyclonic wind burst at 200 mb, and the upward shift in the warm core. The origin of this cyclonic wind burst is traced to an angular momentum flux convergence event initiated at outer radii. In contrast to the forcing assumed in some numerical simulations, this flux convergence is short-lived, is only significant early in the pre-conditioning period, and in particular is not significant during the rapid intensification stage.