Rdsearch On The Impacts Of Poyang Lake On Typical Weather Process And The Characteristics Of Near Surface Boundary Layer

Poyang Lake is one of the largest freshwater lakes in China, due to its complexity of terrain and the inhomogeneity of water-land surface, significantly influences the structures of temperature and humidity of local boundry layer and the physical process, which can eventually influences the development of the weather system passing over the Lake. Therefore, it is of great scientific signifcane to research and analyze the characteristics of boundary layer over Poyang Lake and the lake effects on transitting weather system. In this paper, the methods of data analysis and numerical simulation have been employed to analyze the characteristics of the Poyang Lake-effect on precipitation, the surface layer over Poyang Lake under typical weather condition, and the Poyang Lake effect on the transitting weather system. The mian contents and results are as follows:(1) Characteristics of Poyang Lake-effect on precipitation.Through the data analysis we found there exists Lake-effect precipitation over Poyang lake. During such events, winds from the north to northwest advect cold and dry air over Poyang Lake, which rapidly warms and moistens the overlying air mass, finally resulting precipitations. Two synoptic weather patterns are favorable for lake-effect precipitation events:a trough-type pattern (～88%of events) from November to June, and a tropical-cyclone-type pattern (～12%of events) from August to September.(2) Characteristics of Poyang Lake-effect on the transiting weather systems. Here, this paper analyaed two typical examples of transitting weather systems that was strenghened or reduced accordingly, it is indicated that such functions of strenghenning or reducing were caused by the functions of cool source or heat source displayed by Poyang lake seansonally. There exists key thransitting point between cool season and hot season upon the end of June and the early of July, before the end of June Poyang lake plays the role as heat source and after the early July palys the role as cool source, this transit is coincided with the time that is the terminal of rainfall period by the end of June and start of hot and dry period.One example analyzed the phenomenon that the convective clouds generated in Mufu Mountains were remarkably weakened or dissipated after passing over Poyang Lake. It is explained that during hot summer the transiting convective clouds were weakened by the function as cool source produced by Poyang lake. The simulating result show that this weakening function by Poyang lake on the transiting convective clouds were caused by the anti-cyclonical circulations, which is initiated by the cool source of Poyang lake, that will cause atmosphere submerging and diverging.Other example analyzed the influence on a heavy precipitation process occurred in north central Jiangxi during19-20June2010. The simulating result shows that it is identical with the real situation when the Poyang lake geographical data added to the experiment of numerical simulation. On views of the vertical transportation and vapor transportation, Poyang Lake could positively feedback the local anomalous increase of precipitation. This kind of positive reaction majorly generated by the heat source function of Poyang lake.(3) Characteristics of surface layer during a heavy rainfall process. By analyzing a local heavy rainfall process over Poyang Lake region,it is found that Poyang lake provided enough water vapor for strong strengthened the unstable development of nearing surface layer. During the heavy rainfall, the sensible and latent heat fluxes, the horizontal and vertical turbulent momentum fluxes increased rapidly; In addition, the turbulent kinetic energy increased greatly. Before the heavy rain occurrence, the mesoscale momentum flux was larger than the turbulent momentum flux, however, during the heavy rainfall, the turbulent flux increased remarkably.(4) Characteristics of near surface layer during a strong wind process. Through the analysis of the characteristics of near surface layer during a strong wind process,it is found that with the passage of cold air front, the wind speed of near surface layer is obeyed under the rule of power exponent, and the wind speed increases with the height of the surface layer, more earlier and stronge wind more produced quickly. And there exists a systematic descending movement. Though the comparison between gusty momentum flux and turbulent momentum flux, it is indicated that the porcess of strong wind from cool air could strenghen the turbulence of near surface, and could strenghen the mutual function between the surface and up atmosphere.