杨树人工林生态系统通量贡献区分析

采取通量源区模型FSAM(Flux Source Area Model)利用2009年北京大兴区杨树人工林生态系统碳水通量涡度相关观测资料,分析了不同大气条件下生态系统的通量贡献区分布特征。研究结果表明:(1)该站通量贡献区随大气稳定条件增强而增大。除326.25°—3.75°方向生长季不稳定条件下外,生长季的通量贡献区范围普遍大于非生长季的贡献区范围;(2)通量贡献区与观测高度、冠层高度、地表粗糙度、风向以及大气稳定度有关,当风速风频较大,大气不稳定时,湍流扩散作用强烈,贡献区范围较小;(3)该观测场在2009年以不稳定大气条件为主,通量信息主要来源于距离观测塔50—400 m范围,且69.3%的信息来源于通量塔偏北风与偏南风方向,其中42.56%的信息来自于偏南风方向;(4)随着大气稳定程度加强,通量来源最少区从塔偏西方转为偏东方,在大气稳定度条件和风向的共同作用下,生长季时主要通量贡献区在塔偏南方向,而非生长季时主要通量贡献区在塔偏北方向。

Footprint analysis of turbulent flux over a poplar plantation in Northern China

Footprint source area analysis is an important approach to study qualitatively and quantitatively the relationship between the flux information and the surface condition.Fluxes measured with eddy covariance system over a heterogeneous surface are usually of uncertainty,hard to control and difficult to interpret.As a result,the credibility of energy and mass exchanges between the atmosphere and the land surface could be questionable.Therefore,it is critically important to identify the fluxes measurement by improved understanding the footprint source area.We calculated the footprint source area from continuous flux measurement by an eddy covariance system from Jan 1,2009 to Dec 31,2009 using FSAM(Footprint Source Area Model) model over a poplar plantation in the southern part of Beijing(DanXing;39°31′50″N,116°15′07E″).Our objectives are to analyze the spatiotemporal distribution of footprint source area and to interpret the representativeness of the flux measurement data.Changing with the seasons,the prevailing wind directions were 146.25°—213.7° and 326.25°—33.75° and non-prevailing wind directions remained 33.75°—146.25° and 213.75°—326.25°,respectively.On the spatial variation,stable stratification inhibits turbulence diffusion and contributes to the downwind flux information.Our results indicated that the more stable the atmosphere stability conditions were,the larger the source areas were.During the study period,the source areas in the dormant season were larger than those in the growing season,except for the wind direction between 326.25° to 33.75°.According to our result,the source area became smaller while the wind frequents and strength became higher.Based on computing the one year measurement data,the upwind ranges were about 50—400 m mainly characterized with unstable stratification around the measurement site.Moreover,our results indicated that 69.3% of the flux information came from northwest and southwest,and approximately,42.56% of which came from the southwest.According to the combined action of atmosphere conditions and wind directions,the footprint that measured in the growing season was from the southern part of the tower.In addition,with the stronger of the atmosphere conditions,the smallest region of source area changed from west to east.On the time scale,unstable stratification dominated the measurement site during almost all wind directions,only when the winds blew from the east did it switch to stable stratification.In particular,during the growing season when the winds came from the north,the source regions were mostly affected by atmospheric conditions.Generally speaking,the source area is related to the measurement height,canopy height,roughness length,wind direction and atmospheric conditions and the source areas enlarge with the stability of atmospheric stratification.