Analysises Of Flux Data At Xilinhot National Climate Observatory

Xilinhot National Climate Observatory is as one representative of the grassland ecosystem of semi-arid region in the north China. Using the Eddy Covariance(EC) System to investigate the effect of the human activity, the carbon/water cycle and the interaction among the soil, the grassland and the atmosphere is significant. Based on the turbulence data observed by the EC and Grader data in 2009 at the Observatory, this paper analyzes(1)several methods about the Eddy Covariance data quality control&assessment.(2) the local characteristics of the EC data and the situation about the local measurement environment.(3) the condition of the local land surface energy balance. The main results are as followings.1. The data quality control technique is more important:the result of the Ogive function calculation shows that the average time-30min be the average time is reasonable. Using the value of automatic gain to be the threshold can identify the most singular value caused by the measurement frozen up in the winter. In the complex terrain, the plane fit method could not correct the affect of the terrain effectively and in the area where the wind direction changes dramatically the deviation of the two correct methods (plane fit &double rotation) increases. The result of the frequency response correction shows that the flux has been underestimated, after the correction the sensitive heat flux, the latent heat and the CO2 flux is increased by 5.29%,9.32% and 9.32% respectively. In the local environment the flux correction results are similar between the Webb method and the Liu method. This difference is less than 0.5%.2. Through the steady state test and integral turbulence characteristics test show than over 80% data pass both tests. The good data number in daytime is more than in night time and in summer is more than in winter. The pre-processing works can eliminate worst data. After the tests, using the good data to analyze the energy balance, the closure of the local energy balance has increased nearly 3%.3. The variance of the velocity components is in conformity with power 1/3 and the fitting coefficients reflets the characteristics of the local topography and underlying surface. Because of the influence of the drought and the different geographical position, the sensitive flux is more than latent heat flux and be the primary form of net radiation flux conversion, the maximum average daily CO2 flux is less than-0.1mg/m2s. The calculation of the FSAM model shows that in the stable condition the source area can be the circle area with the 250m radius. In the unstable condition the radius of the circle source area is 150m. In the source area, the stability and the homogenicity of the local underlying surface is good. In the 4 directions, the growths of the grass are similar and have the same annual rhythms. The observation takes on natural state and 96% fluxes are come from the grass underlying surface. However, the complex terrain make the local vertical velocity is change systematically and the development of the turbulence is restrained in the area where the terrain has a dramatic change. The local extremely cold winter would affect the measure result, such as coming out the abnormal data or the data absence.4. The total energy balance closure (EBC) is 79%. The closure of the energy balance has its own characteristics:the closure in the daytime, growing season and the instability condition is higher than that in the night, non-growing season and the stability condition. The upper 0-5cm soil layer has a significant effect on the energy balance closure and make the EBC increase by 10%. After the examination using the friction velocity threshold, turbulent energy threshold and the steady state & integral turbulence characteristics tests, the EBC can increase by 10% respectively. The final EBC after all the examination and the data processing/correction is 92%.