The spatial contribution of translation speed to tropical cyclone wind structure

The contribution of translation speed to the tropical cyclone wind structure is an important component of the storm's asymmetries. A literature review shows most parametric wind schemes still follow the Jelesnianski (1966) formulation, which superimposes half the translation speed at Rmax then decreases sharply to a weight of 0.1 at 100 km and 0.04 at 300 km. The best 2D empirical fit using zunzun.com for this relationship, formulated as weight (w) as a function of radius (in km), is:

w = (a + br^c)^(-1/d)

where a=6.03, b=0.0107, c=2.8, and d=2.8. Note this relationship is constant for all intensities (Vmax) and translation speeds.

Some schemes will adjust Vmax at Rmax using the Schwerdt (1979) relationship which weights the contribution of translation speed to Vmax as a high ratio at slow speeds, but decreases to a moderate ratio at fast speeds. For example, the contribution of a 5-knot translation speed is 4.1 knots to Vmax (weight of 0.83), of a 15-knot translation speed is 8.3 knots (weight of 0.55), and a 25-knot translation speed is 11.4 knots (weight of 0.46). However, the Schwerdt relationship contains no radial or intensity dependence. Both the Schwerdt and Jelesnianski relationships appear to be formulated qualitatively based on experience and apparent reasonableness of results, not on a data analysis.

Using H*WINDS, we have taken a first step in developing simple empirical weight equations based on wind asymmetries, intensity, and translation speed. These results will be presented at the conference.