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Determination of averaging period parameter and its effects analysis for eddy covariance measurements

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Science china Earth sciences 2005年第Z1期48卷 33-41页

作者： sun xiaomin, zhu zhilin, xu jinping, yuan guofu , zhou yanlian & zhang renhua institute of geographic sciences and natural resources research, chinese academy of sciences, beijing 100101, china Graduate School of the chinese academy of sciences, beijing 100039, china institute of geographic sciences and natural resources research Chinese Academy of Sciences Beijing 100101 China Graduate School of the chinese academy of sciences Beijing 100039 China institute of geographic sciences and natural resources research Chinese Academy of Sciences Beijing 100101 China Graduate School of the chinese academy of sciences Beijing 100039 China

It is more and more popular to estimate the exchange of water vapor, heat and CO2fluxes between the land surface and the atmosphere using the eddy covariance technique. To get believable fluxes, it is necessary to correct the observations based on the different surface conditions and to determine relevant techinical parameters. The raw 10 Hz eddy covariance data observed in the Yucheng and Changbai Mountains stations were recalculated by various averaging periods (from 1 to 720 min) respectively, and the recalculated results were compared with the results calculated by the averaging period of 30 mins. Meanwhile, the distinctions of fluxes calculated by different averaging periods were analyzed. The continuous 15 days observations over wheat fields in the Yucheng station were mainly analyzed. The results are shown that: (i) In the Yucheng station, compared with the observations by 30 min, when the averaging period changes from 10 to 60 min, the variations of the eddy-covariance estimates of fluxes were less than 2%; when the averaging period changes less than 10 min, the estimate of fluxes reduced obviously with the reduction of the averaging period (the max relative error was -12%); and when the averaging period exceeds 120 min, the eddy covariance estimates of fluxes will be increased and become unsteady (the max relative error is over 10%); (ii) the eddy covariance estimates of fluxes over wheat field in the Yucheng station suggusted that it is much better to take 10 min as an averaging period in studying diurnal change of fluxes, and take 30min for a long-term flux observation; and (iii) normalized ratio was put forward to determine the range of averaging period of eddy covariance measurements. By comparing the observations over farmlands and those over forests, it is indicated that the increase of eddy covariance estimates over tall forest was more than that over short vegetation when the averaging period increased.

关键词： eddy covariance, various surface conditions, averaging period, parameter, chinaFLUX.

Study of a model for correcting the effects of horizontal advection on surface fluxes measurement based on remote sensing

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Science china Earth sciences 2006年第S2期49卷 273-280页

作者： TIAN Jing, zhang renhua, sun xiaomin, zhu zhilin & zhou yanlian Key Laboratory of Ecosystem Network Observation and Modeling, institute of geographic sciences and natural resources research, chinese academy of sciences, beijing 100101, china Graduate University of the chinese academy of sciences, beijing 100049, china

As well known, the methods of remote sensing and Bowen Ratio for retrieving surface flux are based on energy balance closure; however, in most cases, surface energy observed in experiment is lack of closure. There are two main causes for this: one is from the errors of the observation devices and the differences of their observational scale; the other lies in the effect of horizontal advection on the surface flux measurement. Therefore, it is very important to estimate the effects of horizontal advection quantitatively. Based on the local advection theory and the surface experiment, a model has been proposed for correcting the effect of horizontal advection on surface flux measurement, in which the relationship between the fetch of the measurement and pixel size for remote sensed data was considered. By means of numerical simulations, the sensitivities of the main parameters in the model and the scaling problems of horizontal advection were analyzed. At last, by using the observational data acquired in agricultural field with relatively homogeneous surface, the model was validated.

关键词： fetch horizontal advection energy balance pixel size chinaFLUX.

Surface roughness length dynamic over several different surfaces and its effects on modeling fluxes

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Science china Earth sciences 2006年第S2期49卷 262-272页

作者： zhou yanlian, sun xiaomin, zhu zhilin, zhang renhua, TIAN Jing, LIU Yunfen, GUAN Dexin & yuan guofu Key Laboratory of Ecosystem Network Observation and Modeling, institute of geographic sciences and natural resources research, chinese academy of sciences, beijing 100101, china Graduate University of the chinese academy of sciences, beijing 100049, china institute of Applied Ecology, chinese academy of sciences, Shenyang 110016, china

Roughness length and zero-plane displacement over three typical surfaces were calculated iteratively by least-square method, which are Yucheng Experimental Station for agriculture surfaces, Qianyanzhou Experimental Station for complex and undulant surfaces, and Changbai Mountains Experimental Station for forest surfaces. On the basis of roughness length dynamic, the effects of roughness length dynamic on fluxes were analyzed with SEBS model. The results indicate that, aerodynamic roughness length changes with vegetation conditions (such as vegetation height, LAI), wind speed, friction velocity and some other factors. In Yucheng and Changbai Mountains Experimental Station, aerodynamic roughness length over the fetch of flux tower changes with vegetation height and LAI obviously, that is, with the increase of LAI, roughness length increases to the peak value firstly, and then decreases. In Qianyanzhou Experimental Station, LAI changes slightly, so the relationship between roughness length and LAI is not obvious. The aerodynamic roughness length of Yucheng and Changbai Mountains Experimental Station changes slightly with wind direction, while aerodynamic roughness length of Qianyanzhou Experimental Station changes obviously with wind direction. The reason for that is the terrain in Yucheng and Changbai Mountains Experimental Station is relatively flat, while in Qianyanzhou Experimental Station the terrain is very undulant and heterogeneous. With the increase of wind speed, aerodynamic roughness length of Yucheng Experimental Station changes slightly, while it decreases obviously in Qianyanzhou Experimental Station and Changbai Mountains Experimental Station. Roughness length dynamic takes great effects on fluxes calculation, and the effects are analyzed by SEBS model. By comparing 1 day averaged roughness length in Yucheng Experimental Station and 5 day averaged roughness length of Qianyanzhou and Changbai Mountains Experimental Station with roughness length parameter

关键词： chinaFLUX surface roughness length dynamic spatial heterogeneity Yucheng Experimental Station Qianya