Possible causes of the variation in fractal dimension of the perimeter during the tropical cyclone Dan motion

We calculated the fractal dimensions D _b of the perimeter of tropical cyclone (TC) Dan based on the satellite GMS-5 infrared sensor images from 1800 UTC, 1 October 1999 to 1200 UTC, 9 October 1999. The fractal dimensions D _b were used to characterize objectively the temporal change of TC complex structure. Our results show that the change of fractal dimension during TC Dan motion can be divided into three stages. The statistically significant difference does not exist either between D _m1 and D _L or between D _m3 and D _L, but it exists between D _m2 and D _L, where D _mi denotes the mean value of D _b in i-th stage (i=1, 2 and 3); D _L denotes Lovejoy’s fractal dimension calculated based on satellite and radar data within the size range (1?1.2×10⁶ km²), which is used as a “normal value” of the fractal dimension of the cumulus cloud perimeter for the global tropical region. TC Dan turns to the north from the west abruptly at the end of the second stage. The emergence of the second stage with high fractal dimensions may be viewed as a possible premonition for the track turning. Our results also show that there are two kinds of processes resulting in the translation from the first stage to the second stage. One is the interaction of TC circulation and an adjacent small scale convective cloud cluster, causing to the complexity increase of a local segment of the perimeter. The other includes the fragmentation of a strong convective area within the TC inner region, the self-organization of the small strong convective cloud clusters, the emergence, development, and merger of the small scale non-convective holes, and the formation of a gap of the perimeter, causing to the complexity increase of the whole TC perimeter.