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Tropical cyclone (TC) intensity observations considerably improve forecast models. They

are particularly used to continuously measure TC intensity for landfalling cyclones to improve their

forecast. For example, TC Irving, which operated in the Gulf of Tonkin, South China Sea, on 23–24 July

1989, was observed by a conventional weather radar installed at the Phu Lien Observatory in North

Vietnam. The maximum wind speed was calculated by the hyperbolic-logarithmic approximation

(HLS-approximation) of spiral cloud-rain bands (SCRBs) of recorded TC radar images. The data

spanned about 15 h. Ground-based estimates of the cyclone intensity were obtained from pressure

measurements at two coastal weather stations. A comparison of these estimates with the HLS wind

resulting from the HLS approximation of SCRBs showed satisfactory synchronization. In particular,

radar and meteorological data indicated cyclone intensification near landfall and rapid cyclone

intensification after landfall. Both intensifications were accompanied by polygonal eye shapes. This

study demonstrates the feasibility of using the HLS-approximation technique for retrieving TC

intensity variation from conventional weather radar data.