Large intensity changes in tropical cyclones: A case study of supertyphoon flo during TCM-90

A unique dataset, recorded during the rapid intensification and rapid decay of Typhoon Flo, is analyzed to isolate associated environmental conditions and key physical processes. This case occurred during the Tropical Cyclone Motion (TCM-90) field experiment with enhanced observations, especially in the upper troposphere beyond about 300 km. These data have been analyzed with a four-dimensional data assimilation technique and a multiquadric interpolation technique. While both the ocean thermal structure and vertical wind shear are favorable, they do not explain the rapid intensification or the rapid decay. A preconditioning phase is defined in which several interrelated factors combine to create favorable conditions: (i) a cyclonic wind burst occurs at 200 mb, (ii) vertical wind shear between 300 and 150 mb decreases 35%, (iii) the warm core is displaced upward, and (iv) 200-mb outflow becomes larger in the 400-1200-km radial band, while a layer of inflow develops below this outflow. These conditions appear to be forced by eddy flux convergence (EFC) of angular momentum, which appears to act in a catalyst function as proposed by Pfeffer and colleagues, because the EFC then decreases to small values during the rapid intensification stage. Similarly, the outer secondary circulation decreases during this stage, so that the rapid intensification appears to be an internal (within 300 km) adjustment process that is perhaps triggered during the preconditioning phase. Rapid decay occurred over open ocean when the environmental factors of ocean thermal structure, and vertical wind shear, positive 200-mb EFC, and vigorous outflow into the midlatitudes appear favorable. However, the EFC extending down to 500 mb and inducing a second shallower secondary circulation is hypothesized to account for the rapid decay by leading to a less efficient energy conversion.