[A-AS08] General Meteorology

[AAS08-P15] Verification of over-the-horizon detection phenomena in the sea near Taiwan by using AIS and shore-based navigation radar

*SHIH-CHIAO TSAI¹, JOU PING HOU¹, MONG LING CHIAO¹, Chih Pin Lee¹, Po-Ruey Lei², Yi Chun Wang¹, xin shang lan¹ (1.National Defense University, 2.ROC Naval Academy)

Keywords:evaporation duct, elevated duct, over-the-horizon radar, WRF model, radio wave penetration angle, minimum trapping frequency

Atmospheric duct effect is the main reason for over-the-horizon (OTH) detection of radar. Whether occurring as evaporation duct over the ocean or as elevated duct at high altitude, it can enable electromagnetic waves to propagate longer with smaller path losses, achieving the OTH detection. The atmospheric duct effect is significantly influenced by parameters in the atmospheric and ocean boundary layer. However, mesoscale numerical weather prediction models have limitations in forecasting atmospheric duct, mainly due to the need for improvement in the model's physical mechanisms and parameter settings, as well as a lack of observational data over the ocean, causing difficulties in analysis and verification. In this study, unmanned aerial vehicles (UAVs) were used for high-resolution atmospheric refractivity profile observations in the sea near Taiwan. The observed data was then compared with simulation results from the Weather Research and Forecasting (WRF) model to fine-tune the optimal parameter settings for the model under specific weather conditions. Additionally, the atmospheric refractivity profiles obtained from UAV measurements were used to simulate the raytracing and propagation loss of radar. Furthermore, shore-based navigation radar and Automatic Identification System (AIS) ships position were used to verify the OTH radar parameters (radio wave penetration angle and minimum trapping frequency) under specific weather conditions. The results indicate that when there is a significant increase in abnormal AIS transmission signals (ship positions at distances greater than 60 kilometers), it is usually associated with higher and stronger atmospheric duct phenomena. The OTH radar phenomena from shore-based navigation radar also show a significant correlation with the occurrence of evaporation duct and elevated duct. The model comparison results reveal that different parameter settings are required under specific weather conditions to achieve optimal atmospheric duct simulation results.