Observed Over-the-Horizon Propagation Characteristics and Evaporation Duct Inversion during the Entire Process of Tropical Cyclone Mulan (202207)

An evaporation duct, which often occurs in the air-sea boundary layer, has a considerable impact on the remote sensing detection capability of electromagnetic (EM) systems. Tropical cyclones (TCs) substantially increase the transfer of water vapor and heat in the air-sea boundary and inevitably affect EM propagation environments; however, observations of duct environments and their effects on EM propagation during a TC process have seldom been reported. In this article, X-band EM propagation systems that can operate continuously in the TC environment were specially developed, and observations of multiple over-the-horizon propagation links during TC Mulan (202207) were presented. The path loss (PL) of five links and the surface variations both in Mulan's area of high wind speed (WS) and eye were recorded. The average PL of a 53-km link was approximately 155 dB in the high WS area, while a 30 dB increase in PL was detected in Mulan's eye. The inversion and simulation results show that the evaporation duct height (EDH) was high in the high WS area, which led to low PL in EM propagation; conversely, EDH was low inside Mulan's eye, resulting in high PL.