Numerical Simulation of Atmospheric Duct in Typhoon Subsidence Area

The atmospheric duct is a kind of anomalous refraction phenomena in the troposphere atmosphere. It can change the normal propagation characteristic of the electromagnetic wave, and has a significant influence on radar detections and radio communications. The emergence of duct strongly depends on the weather conditions and it often occurs in the subsidence area west to a typhoon.With the rapid development and extensive application of atmospheric numerical models, high-resolution numerical modeling has become an important tool to get insight of duct.Using the WRF model, the atmospheric duct process occurred on 31 August 2002 over Nanjing region in a subsidence area west to Typhoon Rusa is studied in details. The WRF numerical simulation reproduces well the evolution of the duct, which starts to form in the evening of 31 August, reaches the strongest level the next early morning, weakened and disappeared rapidly after sunrise. Based on numerical simulation output with high spatial-temporal resolution, results show that humidity gradient is a key factor to the formation of this duct, and the humidity invertion enhances its strength. The outside low-level flow in the typhoon early stage brings plenty of moisture from the sea to Nanjing region in the near surface layer. The typhoon moves northeastward and dry air mass is transported from the north by the outside high-level flow in the typhoon late stage, and sink down due to high pressure, so an intense gradient of humidity which is prerequisite for the formation of duct appears in the near surface layer. The subsidence itself is not strong enough to directly cause the inversion, but the clear-sky weather caused by it is favorable for long-wave radiation cooling during night time, which is the primary cause for the inversion formation. The inversion formation hinders the upward transport of water vapor, so that the humidity gradient develops further. Besides, the simulation results also reflect the marine atmospheric duct.These results also show that the high-resolution atmospheric meso-scale numerical simulation can be used as an effective means of studying and forecasting duct.