Footprint analysis of turbulent flux over a poplar plantation in Northern China
杨树人工林生态系统通量贡献区分析

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.