The 6-yr best-track data of tropical cyclones (TCs) in the western North Pacific are used to study the
statistical features of TC size and radial wind profile. A TC size is defined as the azimuthal mean radius of
34-kt surface wind.
On average, the TCs in the western North Pacific have a size of 203 km, and the size is larger for
stronger TCs. Further analyses show that larger TCs tend to move faster than smaller ones, with a 23-24
km difference in size corresponding to a difference of about 10 km h-1 in moving speed. The TCs that
recurve from westward to eastward moving have a mean size of 218 km, significantly larger than that of
those without a turning point (179 km). Regional TC distributions demonstrate that the TCs affecting
the Korean Peninsula and southwestern Japan have the largest mean size (250-280 km). There are also
some large TCs affecting southern Philippines, while TCs over the South China Sea are generally small in
size. Comparison of intensity and size of all TCs during their lifespan demonstrates that a TC tends to
reach its maximum size 6 h after it reaches its maximum intensity, and the decrease rate of size during the
weakening stage of a TC is much smaller than the increase rate of size during its developing stage. Thus,
linear regression relations between TC size and intensity are established for its developing and weakening
stages respectively, which can be used as a forecast tool for TC size.
Features of TC radial wind profile are studied by analyzing a parametric wind model based on the radius
data of 34-, 50-, and 64-kt surface winds. The results show that the shape parameter d most frequently takes
the values of 0.3, 0.4, and 0.5. It generally increases (decreases) as the TC develops (weakens), implying a
sharper (flatter) radial wind profile. Changes in d leads the tendency of intensity. The two parameters for
the asymmetric model, namely p and q, are mostly 0.85-1.05 and 0-0.2, respectively, embodying the fact
that the asymmetric component is generally much smaller than the symmetric component. The asymmetry
in 34-kt surface wind is much stronger than that in 50- and 64-kt surface winds, with the maximum radius
often in the northeast quadrant.