The propagation of electromagnetic waves beyond the line of sight can be caused by atmospheric ducts, which are significant concerns in the fields of radar and communication. This paper utilizes data from seven automatic weather stations and five radio-sounding stations to statistically analyze the characteristics of the atmospheric ducts in the northwest region of the South China Sea (SCS). After verifying the practicality of numerical analysis data from NCEP CFSv2 and ECMWF in studying atmospheric ducts using measured data, we analyzed the spatial–temporal distribution characteristics of the height of the regional evaporation duct and the bottom height of the elevated duct. The study found that the NCEP CFSv2 data accurately capture the evaporation duct height and duct occurrence rate in the study area, and the elevated duct bottom height calculated from ERA5 and the measured data have good consistency. The occurrence rate and height of the evaporation duct in coastal stations in the northwest of the SCS vary significantly by month, demonstrating clear monthly distribution patterns; conversely, changes in the Xisha station are minimal, indicating good temporal uniformity. For lower atmospheric ducts, the difference in occurrence rates between 00:00 and 12:00 (UTC) is negligible. The occurrence probability of elevated ducts in the Beibu Gulf area is relatively high, mainly concentrated from January to April, and the Xisha area is dominated by surface ducts without foundation layers, mainly concentrated from June to August. Monsoons play a critical role in the generation and evolution of atmospheric ducts in the northwest of the SCS, with the height of the evaporation duct increasing and the bottom height of the elevated duct decreasing after the onset of the summer monsoon. In the end, we simulated electromagnetic propagation loss under different frequencies and radiation elevation angles in various duct environments within a typical atmospheric duct structure.

Based on air temperature, relative humidity, air pressure, and wind speed data from 4 buoy stations in the northern South China Sea, the homogeneity of the evaporation duct height (EDH) obtained from sea surface temperature (SST) from buoys and SST from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 Reanalysis (ERA-5) data are compared using the Naval-Postgraduate-School (NPS) model. The findings are given as follows: (1) the SST of the ERA-5 has highly accurate (the difference between the buoys and ERA-5 is mainly falling in –0.5°C~1.5°C) and shows strong correlations (≥0.95) with the observed data. (2) The probability of the EDH difference at –2m~2m obtained from the ERA-5 is lower than the actual observation, which underestimates the uniformity of the EDH. (3) The SST difference of buoys and ERA-5 at 0°C to 1.0°C has a more significant impact on the assessment of non-uniformity of EDH than the SST difference at –0.5°C~0°C and 1.0°C~1.5°C (take up above 60%). In conclusion, although the uniformity of the EDH may be underestimated based on the ERA-5 data, it still demonstrates the uniformity characteristic of the EDH quite well.

The propagation of electromagnetic waves beyond the line of sight can be caused by atmospheric ducts, which are a significant concerns in the fields of radar and communication. This paper utilizes data from seven automatic stations and five radio-sounding stations to statistically analyze the characteristics of the atmospheric ducts in the northwest region of the South China Sea (SCS). After verifying the practicality of numerical analysis data from NCEP CFSv2 and ERA5 in studying atmospheric ducts using measured data, we analyzed the space-time distribution characteristics of the height of the regional evaporation duct and the bottom height of the elevated duct. Using the parabolic equation model, we simulated electromagnetic propagation loss under different frequencies and radiation elevation angles in both uniform and non-uniform duct environments within a typical atmospheric duct structure. The study found that the NCEP CFSv2 data accurately captures the evaporation duct height and duct occurrence rate in the study area, and the elevated duct bottom height obtained from the inversion of ERA5 and the measured data has a good consistency. The occurrence rate and height of evaporation duct in coastal stations in the northwest of the SCS vary significantly by month, demonstrating clear monthly distribution patterns. Conversely, changes in the Xisha station are minimal, indicating good temporal uniformity. For lower atmospheric ducts, the difference in occurrence rates between 00:00 and 12:00 (UTC) is negligible. The occurrence probability of elevated ducts in the Beibu Gulf area is relatively high, mainly concentrated from January to April, and the Xisha area is dominated by surface ducts without foundation layer, mainly concentrated from June to August. The monsoon plays a critical role in the generation and evolution of atmospheric ducts in the northwest of the SCS, with the height of the evaporation duct increasing and the bottom height of the elevated duct decreasing after the onset of the summer monsoon. Electromagnetic propagation simulations demonstrate that higher frequency and lower elevation angles of radiation sources in the trapping layer of the evaporation duct make it easier to be trapped. As the evaporation duct height decreases, the amplitude of the "sinusoidal fluctuation" of the propagation loss also decreases. Frequency changes of the radiation source in the surface duct environment have minimal impact on electromagnetic propagation loss, but the elevation angle of the radiation source is a critical factor. The frequency of the "sinusoidal fluctuation" in propagation loss is higher in a hybrid duct environment compared to a uniform surface duct. Additionally, the propagation loss increases faster with distance at the height of the evaporation duct, resulting in greater electromagnetic propagation loss.

The uniformity of evaporation duct height (EDH) plays a crucial role in the accurate prediction of electromagnetic propagation. This paper examines the non-uniformity of EDH in the South China Sea using data from 5 buoy stations and 14 automatic weather stations. The factors contributing to the non-uniformity of EDH are analyzed using the Spearman rank correlation method, and the impact of sea surface temperature (SST) error on the non-uniformity of EDH is also examined. The findings are as follows: (1) the Xisha Islands (XS) show the most non-uniformity in EDH, while the distributions of EDH in the Hainan Island (HN) and Nansha Islands (NS) are fairly uniform. (2) The primary factor related to the non-uniformity of EDH in the HN region is relative humidity when ASTD≥0, and wind speed when ASTD<0. However, the main factor related to the non-uniformity of EDH in the XS and NS regions is atmospheric temperature when ASTD≥0, and wind speed when ASTD<0. (3) The SST errors mainly affect the percentage distribution of EDH difference when its absolute value is less than 2m. When the absolute value of EDH difference is more than 2m, the impact of SST errors is relatively minor.

Evaporation ducts, which are created due to evaporation from the ocean, significantly affect electromagnetic propagation in coastal and marine areas. Based on air temperature, relative humidity, air pressure, sea surface temperature, and wind speed data from 4 buoy stations in the northern South China Sea, the evaporation duct height (EDH) characteristics of Typhoon Kompasu (202118) are analyzed using the Naval-Postgraduate-School (NPS) model. The findings are given as follows: 1) The increased mass, momentum, and heat exchanges between the air and sea during a typhoon significantly affect the horizontal homogeneity of temperature and humidity from the ocean surface, especially in the areas where the center of the typhoon passes through. 2) The low-value center of EDH forming at the typhoon center, and typhoon Kompasu increases the non-uniformity of the EDH between the area centered around the typhoon and other regions, with a maximum EDH difference of over 10m. 3) The main impact factors of EDH between buoy stations at different locations of Typhoon Kompasu are different.