Study About Law And Correction On Saturation Of Self-developed Large Aperture Scintillometer

Large Aperture Scintillometer(LAS)is an instrument,based on the theory of light transmission combined with Moning-Obukhov similarity theory(MOST),used to measure surface water within the region average of heat flux.A Self-developed LAS,made by microclimate and instrumentation laboratory of China agricultural university,has a high consistent with Eddy Covariance System(EC),and is widely used in domestic for flux measurement.When considering the quality control of LAS data,Chinese scholars mainly rely on the saturation line,proposed by Ochs;however,Foreign researchers do their own theoretical derivation or experimental methods to study the limits of saturation occurs,get different results.To explore for law of saturation occurs about self-developed LAS,this study select two sets of self-developed LAS,with aperture of 0.15 m and of 0.075 m(referred as LAS and LAS1respectively),to study saturation occurs in Shangzhuang(SZ)and Guyuan(GY)with different underlying surface conditions and diverse meteorological conditions,based on criterion of BLS900.The study includes the determination of saturation line and saturation data correction,and results are as follows:(1)Self-developed LAS has the same Cn2 trend with BLS900 in different underlying surface conditions and diverse meteorological conditions.When calibrating with BLS900,R2 value and the linear rate of LAS1 is 0.796 and 0.757 respectively;besides,R2 value of self-developed LAS is 0.829,linear rate is 0.573.Compared with LAS1,self-developed LAS has stronger ability of saturation resistance.Moreover,consistency between LAS and LAS1 is great,R2 value of both LAS are above 0.7,especially R2 can reached into0.851 in sunny time in GY.Consequently,these instruments meet the requirements of this study.(2)The result of experimental data processing,by using ratio method and fitting method,is that the “weak saturation” line of SZ is 0.0922 and of GY is 0.0967,which fulfills with the first-order scattering theory and the inequality of average aperture effect.According to saturated trend of Guyuan experiment,the “saturated” line is 0.3592 by using fitting method.When using “weak saturation” line of SZ and GY for quality control,we realized that saturation rate,from the result of quality control according to“weak saturation” line of SZ,is the largest,which is 29.3% and 15.84% respectively.Therefore,“weak saturation” line of SZ is more applicable for this study.After quality control of experimental data,we realize that the phenomenon of saturation ofself-developed LAS mainly is “weak saturation”,which can be corrected in the future.(3)Consider that influence of scale 10 in turbulence,the method of calculating coefficients of saturation correction is using research method of Kohsiek.Then,the correction coefficients of SZ and GY are 1.0057 and 1.0049 in LAS respectively,and1.0275 and 1.0194 in LAS1 correspondingly.Compared with other method,we found that the correction rate of SZ and GY data is the largest after processed by using the“weak saturation” line of SZ,which are 12.5% and 10.4% respectively.Additionally,R2 changed slightly after correcting LAS and LAS1,of which the linear trend floating around 0.03,which fulfilled theoretical situation.When comparing with BLS900,corrected R2 of LAS1 remain as its original,and its linear relationship changed around0.02,which is satisfied result of correction.(4)According to SZ "weak saturation" line combining with the calculation formula of flux,in the normal circumstances(T = 288 k,P = 105pa),we can deduct the relationship between the theoretical installation height and distance of LAS in different area of flux for avoiding saturation.For instance,in the sensible heat flux is about 150w/m2,optical path length of 2000 m,the effective installation height should be 10 m at least in order to make sure the value of scintillometer meter is located in the unsaturated zone.