Severe Typhoon Koinu passed south of Hong Kong on 8 and 9 October 2023,
triggering the issurance of the Increasing Gale or Storm Signal No. 9,
the second highest tropical cyclone warning signal in Hong Kong. Koinu
was a difficult case for TC warning service due to its compact size and
rather erratic movement over coastal waters of Guangdong. To monitor
Koinu's movement and wind structure, the Hong Kong Observatory utilized
various observational platforms, including meteorological aircraft,
ocean radar, and synthetic aperture radar on polar orbiting satellites.
The paper presents major observations derived from these measurements.
The aircraft probe and drosonde data suggested boundary layer inflow,
warm core structure, eyewall updraft and downdraft, and high turbulence
in the eyewall of the typhoon. The weather radar observations indicated
occurrence of a waterspout in the vicinity of the typhoon. Additionally,
the study evaluates the forecasting performance of the AI-based
Pangu-Weather model, and the results highlight its better performance
than the global numerical weather prediction models in forecasting
tropical cyclones in the region. The documentation of these observations
aims to provide valuable references for weather forecasters and
stimulate further research on forecasting this type of tropical
cyclones.