測(cè)量儀器本身是否會(huì)對(duì)測(cè)量結(jié)果造成偏差？
                                                 ——在渦度協(xié)方差系統(tǒng)中，如何確保測(cè)量準(zhǔn)確

        三維超聲風(fēng)速儀是渦度協(xié)方差測(cè)量系統(tǒng)中的核心測(cè)量組件。有研究表明，在對(duì)風(fēng)速進(jìn)行測(cè)量時(shí)，哪怕超聲風(fēng)速儀傳感器的體積很小，也會(huì)對(duì)風(fēng)速測(cè)量結(jié)果產(chǎn)生偏差^{【1，2，3，4，5，6】}。另外，如果采用合體式設(shè)計(jì)思路，即把三維超聲風(fēng)速儀和氣體分析儀合二為一。由于氣體分析儀位于三維超聲風(fēng)速儀采樣空間內(nèi)部或與其非常接近^{【7，8】}，風(fēng)速的測(cè)量誤差就會(huì)很大（圖1）。

圖1 若物體距離三維超聲風(fēng)速儀太近，如氣體分析儀，就會(huì)導(dǎo)致其風(fēng)速測(cè)量不可靠。

 
        理論上，渦度協(xié)方差系統(tǒng)測(cè)量同一渦旋的風(fēng)速和其對(duì)應(yīng)的氣體密度。但在實(shí)際測(cè)量時(shí)，卻不能這樣。合體式設(shè)計(jì)思路，由于其測(cè)量組件本身就會(huì)對(duì)渦旋造成擾動(dòng)，這種擾動(dòng)所導(dǎo)致的測(cè)量誤差很難被量化，且不可進(jìn)行后續(xù)訂正【6，7，8，9】。
 
那怎么辦呢？研究表明，一個(gè)簡(jiǎn)單的解決方案就是采用分體式思路：三維超聲風(fēng)速儀和氣體分析儀以一定間距（10-20cm）分開測(cè)量。這種分體式測(cè)量，只需對(duì)原始數(shù)據(jù)做一個(gè)簡(jiǎn)單的數(shù)據(jù)訂正就可以得到準(zhǔn)確結(jié)果【10，11，12】。
 
LI-COR的渦度協(xié)方差測(cè)量系統(tǒng)以嚴(yán)謹(jǐn)?shù)目蒲谐晒麨橐罁?jù)，采用分體式設(shè)計(jì)思路（圖2），確保了渦度通量數(shù)據(jù)的準(zhǔn)確、可靠。

圖2 LI-COR分體式渦度協(xié)方差測(cè)量系統(tǒng)設(shè)計(jì)思路

 
參考文獻(xiàn)
[1] Wyngaard, J. C., 1981. The effects ofprobe-induced flow distortion on atmospheric turbulence measurements. Journalof Applied Meteorology, 20: 784-794.
[2] Wyngaard, J. C., 1988. Flow-distortioneffects on scalar flux measurements in the surface layer: Implications forsensor design. In Hicks, B. B. (Eds) Topics in Micrometeorology. A Festschriftfor Arch Dyer. Springer, Dordrecht.
[3] Frank, J. M., W. J. Massman, and B. E.Ewers, 2013. Underestimates of sensible heat flux due to vertical velocitymeasurement errors in non-orthogonal sonic anemometers. Agricultural and ForestMeteorology, 171-172: 72-81.
[4] Horst, T. W., S. R. Semmer, and G.Maclean, 2015. Correction of a non-orthogonal, three-component sonic anemometerfor flow distortion by transducer shadowing. Boundary-Layer Meteorology, 155(3): 371-395.
[5] Frank, J. M., W. J. Massman, E.Swiatek, H. A. Zimmerman, and B. E. Ewers, 2016. All sonic anemometers need tocorrect for transducer and structural shadowing in their velocity measurements.Journal of Atmospheric and Oceanic Technology, 33(1): 149-167.
[6] Huq, S., F. De Roo, T. Foken, M.Mauder, 2017. Evaluation of probe-induced flow distortion of Campbell CSAT3sonic anemometers by numerical simulation. Boundary-Layer Meteorology, 165(1):9-28.
[7] Horst, T. W., R. Vogt, and S. P.Oncley, 2016. Measurements of flow distortion within the IRGASON integratedsonic anemometer and CO2/H2O gas analyzer. Boundary-Layer Meteorology, 160(1):1-15.
[8] Dyer, A. J., 1981. Flow distortion bysupporting structures. Boundary-Layer Meteorology, 20(2): 243-251.
[9] Grare, L., L. Lenain, and W. K.Melville, 2016. The influence of wind direction on Campbell Scientific CSAT3and Gill R3-50 sonic anemometer measurements. Journal of Atmospheric andOceanic Technology, 33(11): 2477-2497.
[10] Moore, C. J., 1986. Frequency responsecorrections for eddy covariance systems. Boundary-Layer Meteorology, 37: 17-35.
[11] Horst, T. W., and D. H. Lenschow,2009. Attenuation of scalar fluxes measured with spatially-displaced sensors.Boundary-Layer Meteorology, 130(2): 275-300.
[12] Mauder, M., and T. Foken, 2011.Documentation and Instruction Manual of the Eddy-Covariance Software PackageTK3.

 

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