Numerical Study On The Formation Mechanism Of Atmospheric Duct During Typhoon And Sea-land Breeze Processes

With the wide application of electromagnetic technology in various fields, the atmospheric duct is gradually becoming a hot research topic in many countries. The emergence of atmospheric duct strongly depends on weather conditions. In this thesis, three duct cases are selected and investigated, which are during one typhoon process and two sea-breeze processes, respectively. First, the relationship between the evolution of atmospheric duct and the weather system change are carefully analyzed by using observational data as more as possible. Second, the processes of the three duct cases are simulated by using a new meso-scale atmospheric model WRF (Weather Research and Forecasting), and the formation mechanisms are studied in detail based on numerical simulation output with high spatial-temporal resolution.Observational analysis shows that the occurrence of atmospheric duct is closely related to the weather situation. During the typhoon process, the atmospheric duct process occurred in a subsidence area west to the typhoon. The high humidity in the near surface layer is caused by the outside low-level flow in the typhoon early stage bringing plenty of moisture from sea. 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 meanwhile, the sink flow prevents the upward diffusion of water vapor in near surface layer, which results in humidity decreasing with height. The formation of atmospheric duct in sea breeze processes are related to the sea breeze circulation which transports plenty of water vapor to the near surface layer, while the sink flow of the sea breeze circulation prevents the vertical mixing of water vapor.Using WRF model and its3DVAR (3-Dimensional Variational) assimilation module, the three duct cases are simulated in detail. The results of comparison with in observations indicate that the WRF modeling with high-quality initial conditions provided by cycling-3DVAR well reproduces the evolution of the atmospheric duct. Based on numerical simulation output with high spatial-temporal resolution, the formation mechanism of atmospheric duct is studied in detail, and the results are as follow.1) During the typhoon process, the existence of both humidity gradient and inversion cause the duct. Humidity gradient is a key factor to the formation of this duct, and the inversion enhances its strength. The outside low-level flow in the typhoon early stage brings plenty of moisture from the sea. The dry mass is transported from the north by the outside high-level flow in the typhoon late stage; sink down causes an intense 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, the inversion formation hinders the upward transport of water vapor, so that the humidity gradient develops further.2) The type, height and other parameters of the duct which occurres during the sea-land breeze process are influenced by weather background. Before sea breeze occurres, if there is a trough associated with strong sink flow behind controlling the region, the cold air into the the bottom of the warm air and then the inversion forms at a high level, and the clear-sky weather caused by the sink down flow is favorable for the sea-breeze occurring. The humidity in the near sea surface layer becomes larger because the sea wind brings plenty of water vapor, and the dry air mass is transported from high-level flow in the late stage, so an intense gradient of humidity which is prerequisite for the formation of duct appears. The inversion formation hinders the upward transport of water vapor, so that the humidity gradient develops further. The duct forms in the inversion layer at last which is at a high level. When it is controlled by a stable high-pressure before the sea-land breeze occurs and the sink down which due to high pressure is not strong enough to directly cause the inversion, the height and type of this duct which appears near sea surface layer are controlled by the sea-breeze circulation, and they are lower than the duct which above mentioned.