Characteristics Analysis And Change Rule Of Atmospheric Duct In The South China Sea

Atmospheric duct is a special stratification in the troposphere, which could affact electromagnetic wave propagation, and damage radar detector system and communication system. Our country has a vast sea area and abundant marine resources, especially the South China sea, which play a important role in the development of national defense and the economy, which is also the high appearance probability area of atmospheric duct. This demands that we study the atmospheric structure to find the rule of atmospheric duct appearance and the vaild way of detection and predication. According above-named causes, in this paper, we investigate the characteristics and change rules of evaporation duct over the South China Sea (SCS). Statistical analysis, diagnostic analysis and evaporation duct model(NPS,Naval Postgraduate School) will be used to releave the temporal and spatial distribution features of atmospheric duct in the SCS and find the rule of atmospheric duct appearanc. The mainly results are as follows:(1) During the South China Sea Summer Monsoon Experiment (SCSMEX) in1998, the occurrence probability of the evaporation duct during the period of pre-monsoon is higher than that during the active period, and which could be increases with the decrease of the latitude, it is higher over the southern SCS than over the northern SCS. There is a diurnal feature of evaporation duct over the SCS, which tends to show low-high-low in general. The daily oscillation of the duct strength is intensive, while the duct height changes little. Compared with the variance of the duct height and strength after the monsoon, the variance before the monsoon is smaller. The duct strength over the southern SCS is stronger than that over the northern SCS, but the duct height over the southern SCS is lower than that over the northern SCS. It could be caused by the changes of the near surface atmospheric boundary layer due to the monsoon onset. Around onset of the summer monsoon, the vapor pressure mutation is obvious over the SCS, that causes the vapor which in the boundary layer over near sea surface sharply reduce, and led to the structrue of upper-layer dry and lower-layer wet which is beneficial to the occurrence of evaporation duct.(2) In the atmospheric environment over the sea, the atmospheric duct is other type ducts except evaporation duct, which including surface duct and elevated duct. The big probability of the occurrence of the strong atmospheric ducts over the South China Sea (SCS) was in the low height The probability of the occurrence of the lower strong atmospheric ducts over the SCS increased from the north to the south of SCS, whereas the height of the atmospheric duct layer decreased gradually. The probability of the occurrence of the lower strong atmospheric ducts in the nighttime (12GMT) was a bit higher than the other time of the diurnal evolution and the height of duct layers displayed a significant difference on the space. The height of the duct layer was lower in the late of nighttime (18GMT) and more stable.The diurnal feature is more obvious over the southern SCS than which over the northern SCS.(3) Using the NPS evaporation duct model to calculate the height of evaporation duct. The analysis shows:under stable condition of atmospheric, the air-sea tempertature difference is bigger than0℃, evaporation duct height depends remarkably on air-sea tempertature, but the change of evaporation duct height composites at different relative humidity is very little. In the statue of low wind speed, low relative humidity, big air-sea tempertature difference, the height of evaporation duct easy to extend to the higher height. But In the statue of big wind speed, high relative humidity, small air-sea tempertature difference, the height of evaporation duct difficult to extend to the higher height.