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

Abstract

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毅31忆50义N,116毅15忆07E义). 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毅-213. 7毅 and 326郾 25毅-33. 75毅 and non鄄prevailing wind directions remained 33. 75毅-146. 25毅 and 213. 75毅-326. 25毅, 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