Observed Diurnal Variations of Over-the-Horizon Propagation Loss in Evaporation Duct Over the South China Sea

An evaporation duct is formed over the sea surface owing to the evaporation of seawater, and the increase in the evaporation duct height (EDH) will lead to a sharp decrease in the profiles of atmospheric modified refractivity near the sea surface. Evaporation duct reduces the path loss (PL) of electromagnetic (EM) wave propagation, which has a significant effect on the performance of radar and communication systems. The trapping ability of the evaporation duct fluctuates greatly within a single day; however, the effect of the diurnal variation of the evaporation duct with measured PLs has seldom been investigated. In this letter, the diurnal variations of measured PLs of three over-the-horizon links are recorded by the developed EM propagation systems for two days in the northern part of the South China Sea. Combined with the data of meteorological observation and reanalysis results, the diurnal variation of the evaporation duct is investigated through numerical simulations. The results demonstrate that the measured PLs exhibit strong diurnal variation with a fluctuation of about 50 dB. Besides, the EDH shows a good correlation with the variation of PLs: in one day, during 06:00-08:00, the measured PLs are high, and the EDH is low; during 17:00-19:00, the measured PLs are low, and the EDH is high.