Statistical Characteristics Analysis Of Atmospheric Ducts Over China Seas And Numerical Modeling Study On Their Evolution Mechanism

The research of atmospheric ducts, a world focus of national defense, has a significant influence on radar detections and radio communications. China has paid more attention to the issue of atmospheric ducts in recent years. However, the research of ducts is restricted by lack of observations over vast oceans. There is a gap to practical guiding for the low spatial-temporal resolution of statistics. For mechanisms of ducts, currently, local researchers mainly focus on the theories of radio propagation and the weather situation diagnosis, but rarely on the thermodynamic and dynamic mechanisms of ducts. Fortunately, high-resolution numerical models offer an alternative way of gaining insight into ducts. In this thesis, based on the reanalysis data and numerical simulation results, the distribution features and occurrence frequency of atmospheric ducts over China Seas are statistically analyzed, and the mechanisms of ducts evolution during some typical synoptic processes are also studied by numerical models.First of all, to get an overall understanding, NCEP-FNL reanalysis data is used in statistic analysis of atmospheric ducts and its climate genesis over China Seas; second, meso-scale atmospheric model MM5 with a very high vertical resolution provides an annual variation of simulated ducts. According to the simulation results, the main weather types which are favorable for ducts over a regional sea at east are studied in details using empirical orthogonal function (EOF) decomposition; meanwhile, MM5 results are used to diagnose evaporation duct directly, and the relationship between the seasonal distributions and marine conditions, including ocean oceanic circulations and atmospheric conditions near the sea surface; third, the association of ducts with sea fog over Yellow Sea is analyzed in a case study; fourth, WRF model with its 3DVAR module is chosen for data assimilation tests and development mechanism research of ducts in a sea-land breeze circulation; at last, the advantages of various numerical models are discussed, and some issues of atmospheric ducts are prospected.Referring to methods in meteorology, some ideas for duct research are introduced: (1) EOF for ducts. Based on simulation results, EOF is an effective approach to study synoptic features of atmospheric ducts over a regional sea. (2) Meso-scale model can be used to simulate evaporation duct by refining model vertical levels reasonably, but not depend on evaporation duct model.The results are summarized as follows:(1) there exists a close relationship between the variance of appearance probability and types of the ducts and eastern Asia monsoon activities; coastal zones along the Bohai Sea, the Yellow Sea and the East China Sea have a distinct high occurrence probability during summer. (2)Three top weather situations are favorable for duct occurrence of the regional sea at east: southern to a cold anticyclone, north-western to a tropical low, and between a northern low and a southern high, respectively. In the first situation (accounts for 41%) duct height is relatively high (1300～1600m), and duct strength is much intense (17M) in the second (accounts for 7%). The similarity of these two types is that air divergence and subsidence results in inversion, and strong water vapor advection causes large vertical humidity gradient in lower atmosphere. However, in the third situation (accounts for 4%) advection plays an important role. (3) There obviously exists a belt-region with high evaporation duct occurrence(EDO) along the Kuroshio, and EDO is also relatively higher in some seasons near the regions of Taiwan warm current, the Yellow sea warm current and the Tsushima Warm Current. (4)Ducts occur at the top of sea fog with a height of 100-200m,where relative humidity gradient can be up to 50%/100m; due to the long wave radiation cooling in sea fog, more stable the atmospheric layer is, more intense the ducts are; ducts have a much more rapid response to humidity gradient than sea fogs do. (5) Daily variations of duct occurrences is related to the sea-land breeze in east and southeast coast; sea wind leads to a thermal internal boundary layer where ducts appear in when dry air raising over land and subsidence over sea. The air near sea surface is warming caused by compression and has a huge humidity gradient with upper air. Vertical mixing intense when land air flow over sea and the humidity gradient reduces, then the ducts disappear. This process can be accelerated by lee waves.