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Global Guide to Tropical Cyclone Forecasting: CHAPTER 2: TROPICAL CYCLONE STRUCTURE

APPENDIX: COMPUTATION OF AVERAGE WINDS AROUND TROPICAL DISTURBANCES AND CYCLONES

This method uses variable weights around the circle to represent the shear directly over the disturbance more accurately, while still preserving the conservatism of area averaging. Fig. A2.1 shows a common situation in which the tropical cyclone is situated several hundred kilometres equatorward of a jet. The following page indicates the vector components (u,v) of the total wind at eight points around the circle and shows two different ways of computing their average. Note that uniform weighting gives a resultant wind speed of 19 kt. The suggested variable weighting gives a reduced speed of 12 kt because the zonal winds north and south of the center (and the meridional winds east and west) receive less weight.

Figure A2.1 : An example of upper-level flow around a tropical cyclone to illustrate the variable-weighting method of computing vector average winds.

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