Abstract
Intraseasonal oscillation of the evaporation duct, lasting 30–60 days, has been identified over the South China Sea (SCS) summer monsoon region based on multiple reanalyses and observational data. The boreal summer intraseasonal oscillation (BSISO) causes anomalies at the air–sea boundary and thus plays a dominant role in modulating the variation of the evaporation duct. The height and strength of the duct enhance/suppress during the negative/positive phase of the BSISO over the SCS. This results from the fact that active BSISO convection reduces solar radiation reaching the sea surface by increasing cumulus cloud cover, whereupon precipitation and water vapor transported by the enhanced southwest jet increase humidity over the air–sea boundary. Reduced air–sea temperatures and humidity differences lead to a weaker evaporation duct. Usually, the temporal evolution of the evaporation duct lags 2–4 days behind the BSISO, with the center of evaporation duct anomalies farther south than the BSISO. Simulated electromagnetic fields substantively influence the condition of the evaporation duct, with obvious over-the-horizon and radar blind spot effects in the typical negative phase of the BSISO, which is very different from standard atmospheric conditions.
Highlights
The atmospheric duct is a type of atmospheric junction formed in the troposphere, with a significant impact on the propagation of electromagnetic waves
boreal summer intraseasonal oscillation (BSISO) propagates northeastward across the South China Sea (SCS), intraseasonal variation of meteorological factors are observed along the propagation pathway from the equator to the Northwest Pacific, including outgoing long wave radiation (OLR), sea surface temperature (SST), evaporation, rainfall, and wind [10,11,12,13,14,15,16,17,18,19,20]
A trapping layer can be formed from sea level to the top of the evaporation duct, causing the original electromagnetic blind area to disappear; the duct can change the trajectory of electromagnetic radiation, resulting in the development of a blind area (90 to 110 km) at the maximum calculated height (500 m), which is far from the starting position of the blind zone under standard atmospheric conditions
Summary
The atmospheric duct is a type of atmospheric junction formed in the troposphere, with a significant impact on the propagation of electromagnetic waves. The height of the evaporation duct is less than 40 m [2] It is the kind of atmospheric duct with the highest probability of occurrence and the greatest impact on electronic equipment in the ocean. BSISO propagates northeastward across the SCS, intraseasonal variation of meteorological factors are observed along the propagation pathway from the equator to the Northwest Pacific, including outgoing long wave radiation (OLR), sea surface temperature (SST), evaporation, rainfall, and wind [10,11,12,13,14,15,16,17,18,19,20]. We present general features of electromagnetic propagation in the duct environment
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