Abstract:

The atmospheric duct in the troposphere comprises strong vertical refractivity gradient structures. It has great scientific significance and great values for propagations of radio waves and functions of surveillance equipments. Using the GPS sounding datasets obtained four times daily at Dongsha sounding station, Nansha sounding station, R/V“Kexue1”, and R/V“Shiyan3” during the South China Sea Monsoon Experiment (SCSMEX) in 1998, the characteristics of the lower atmospheric ducts were analyzed, excluding evaporation duct pre- and post-monsoon onset over the South China Sea (SCS). The results showed that the summer monsoon had an important influence on the lower atmospheric duct over the SCS. The occurrence probability of the atmospheric duct during the pre-monsoon period was larger than that in the active monsoon period and the amplitudes of the duct occurring probabilities pre- and post-monsoon onset were different spatially; they were about 12% over the northern South China Sea (SCS) and 10% over the southern SCS. Moreover, summer monsoon onset elevated the heights of the lower atmospheric duct layers over the northern SCS and reduced the heights of the lower atmospheric duct layers over the southern SCS, whose amplitudes of variation at given altitudes were all within 300 m. In addition, except that the duct strength in the northern SCS became a bit stronger during the active summer monsoon period, the strength in other parts of the SCS decreased, especial those in the southern SCS that were about 40m in thickness and 1M units in duct strength. The duct characteristics in the SCS were determined by the sharp decrease of water vapor with altitude. The summer monsoon brought much warm water vapor into the lower troposphere over the SCS where different atmospheric boundary layers formed, which resulted indifferent atmospheric duct characteristics from the northern to southern SCS.

Key words: South China Sea Monsoon Experiment, monsoon onset, atmospheric duct, statistical analysis