Since the launch of the QuikScat in 1999, valuable ocean-surface wind data have been available for the study of tropical cyclone structure. In this paper, we will present results from detailed analyses of these data around tropical cyclones over the western North Pacific (WNP) for the period 1999-2001. Because of possible contamination near the center, the focus is on the outer-wind structure. Temporal evolutions of the size (radius of 15 m/s winds) and outer-wind strengths will be presented.

The size variations are found to be consistent with those found in previous studies. An interesting finding is that the average size during this period over the WNP generally tends to be smaller than those from other years. This result is explained in terms of the environmental flow patterns (typical of La Niņa conditions) in which the tropical cyclones were embedded. Such an observation further emphasizes the control of the environment on the size evolution of a tropical cyclone.

The evolution of the outer-wind strength as well as the entire outer-wind structure for different tropical cyclones can be classified into several types, each of which can be explained in terms of the changes in the environmental flow patterns. In some cases, internal dynamics apparently also has an important contribution.