Atmospheric Ducts and Their Electromagnetic Propagation Characteristics in the Northwestern South China Sea

Abstract

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.

The non-uniformity characteristics of the evaporation duct in the South China Sea based on CLDAS data

“…The SST data from the CFSR dataset is used in this article. This dataset has been validated by numerous scholars and has been widely used to analyze the spatiotemporal distribution characteristics of evaporation ducts in various sea areas (Shi et al, 2015b;Huang et al, 2022;Yang et al, 2023). Due to the different temporal and spatial resolutions between the CLDAS and the CFSR, the bilinear interpolation and nearest neighbor interpolation to obtain the temporal and spatial resolutions of SST to 1 h and 0.0625°respectively in this article.…”

“…and time of day scale (Shi et al, 2015b;Zhang et al, 2016;Huang et al, 2022;Yang et al, 2023). The daily variation of the evaporation duct height (EDH) is primarily influenced by thermal differences between land and sea.…”

“…However, due to the unique climate environment and surface types of the ocean, there is a severe lack of meteorological observation stations, making it difficult to achieve regional and high temporal resolution meteorological detection. As a result, the most widely used approach currently is to predict based on the models developed from the Monin-Obukhov similarity theory (MOST) by measuring maritime meteorological parameters and sea surface temperature (Zhang et al, 2020;Zhao et al, 2023) or using reanalysis data (von Engeln and João, 2004;Shi et al, 2015b;Sirkova, 2015;Cheng et al, 2021;Huang et al, 2022;Yang et al, 2023). The commonly used diagnostic models for EDH include the Paulus-Jeske (P-J) model (Guo et al, 2018), Musson-Gauthier-Bruth (MGB) (Zhao et al, 2021), Babin-Young-Carton (BYC) model (Zhao et al, 2019), and Naval Postgraduate School (NPS) model (Zhao et al, 2021).…”

Revisiting Enhanced AIS Detection Range under Anomalous Propagation Conditions

“…In the tropospheric duct, the electromagnetic (EM) energy of certain frequencies is trapped and propagated over a long distance as in a waveguide. Recently, considerable efforts have been made in studying the propagation characteristics of tropospheric ducts and investigating the associated over-the-horizon maritime propagation for specific regions or seas [10,11,[15][16][17]. The evaporation duct attracted the attention of the authors of those papers because it is nearly permanently present above large bodies of water at lower and even moderate latitudes [18].…”