Attenuating the Absorption Contribution on $${C_{n^{2}}}$$ Estimates with a Large-Aperture Scintillometer
Tóm tắt
Large-aperture scintillometers (LAS) are often used to characterize atmospheric turbulence by measuring the structure parameter of the refractive index
$${C_{n^{2}}}$$
. However, absorption phenomena can lead to an overestimation of
$${C_{n^{2}}}$$
. By applying an accurate numerical filtering technique called the Gabor transform to the signal output of a LAS, we improved our knowledge of the accuracy of the measured
$${C_{n^{2}}}$$
by determining and attenuating the contribution of absorption. Two studies are presented on a 12-day dataset using either fixed band pass or adaptive filtering. The first consists of evaluating the best-fit filter for which the resulting
$${C_{n^{2}}}$$
is independent of meteorological conditions, especially crosswind, and the second consists in accurately reconstructing the signal to remove absorption, without losing information on
$${C_{n^{2}}}$$
. A reference
$${C_{n^{2}}}$$
(hereafter ‘reconstructed
$${C_{n^{2}}}$$
’) is created by accurately removing absorption from the scintillation spectrum, and is used to evaluate each filter. By comparing the ‘reconstructed
$${C_{n^{2}}}$$
’ with a raw
$${C_{n^{2}}}$$
measured with a scintillometer, in the presence of absorption, we found that the average relative contribution of absorption to the measurement of
$${C_{n^{2}}}$$
is approximately 9%. However, the absorption phenomenon is highly variable; occasionally, in the worst cases, we estimate that the absorption phenomenon could represent 81% of the value of
$${C_{n^{2}}}$$
. Some explanations for this high variability are proposed with respect to theoretical considerations. Amongst the fixed band-pass filtering used, we conclude on the preferential use of a band-pass filter [0.2–400 Hz] for
$${C_{n^{2}}}$$
, as its performance is only slightly affected by the crosswind, and that the mean absorption contribution is reduced to 5.6%, with a maximum value of 60%. Using an adaptive filter on the 12-day dataset really improves the filtering accuracy for both points discussed—the independence of meteorological conditions and the quality of signal reconstruction.
Tài liệu tham khảo
Avila R, Vernin J, Masciadri E (1997) Whole atmospheric-turbulence profiling with generalized scidar. Appl Opt 36: 7898–7905
Beziat P, Ceshia E, Dedieu G (2009) Carbon balance of three crop succession over two cropland sites in South-West France. Agric For Meteorol 149: 1628–1645
Clifford SF (1971) Temporal-frequency spectra for a spherical wave propagating through atmospheric turbulence. J Opt Soc Am 61: 1285–1292
de Bruin HAR, Vanden Hurk BJJM, Kohsiek W (1995) The scintillation method tested over a dry vineyard area. Boundary-Layer Meteorol 76: 25–40
Foken T (2008) Micrometeorology. Springer, Germany, 308 pp
Green AE, Green SR, Astill MS, Caspari HW (2000) Estimating latent heat flux from a vineyard using scintillometry. Terr Atmos Ocean Sci 11: 525–542
Hartogensis OK, Watts CJ, Rodriguez J-C, de Bruin HAR (2003) Derivation of the effective height for scintillometers: La Poza experiment in Northwest Mexico. J Hydrol 4: 915–928
Hill RJ, Clifford SF, Lawrence RS (1980) Refractive index and absorption fluctuations in the infrared caused by temperature, humidity and pressure fluctuations. J Opt Soc Am 70: 1192–1205
Ingensand H (2002) Concepts and solutions to oversome the refraction problem in terrestrial precision measurement. In: FIG XXII international congress, Washington, DC, April 19–26 2002, 12 pp
Irvine M, Lagouarde J-P, Bonnefond J-M, Grimmond S, Oke TR (2002) Spectral analyses of optical scintillations: refraction and absorption components in an urban zone (Marseille, France). In: Proceedings of 4th AMS symposium on urban environment, May 2002, Norfolk, VA, pp 217–221
Kaimal J, Wyngaard J, Izumi Y, Cote O (1972) Spectral characteristics of surface layer turbulence Q J R Meteorol Soc 98:563–589.
Kleissl J, Gomez J, Hong S-H, Hendrickx JMH, Rahn T, Defoor WL (2008) Large aperture scintillometer intercomparison study. Boundary-Layer Meteorol. 128: 133–150
Kleissl J, Watts CJ, Rodriguez JC, Naif S, Vivoni ER (2009) Scintillometer intercomparison study-continued. Boundary-Layer Meteorol 130: 437–443
Kolmogorov A (1941) Dissipation of energy in a locally isotropic turbulence. C R Acad Sci URSS 32: 16
Lagouarde JP, Bonnefond JM, Kerr YH, McAneney KJ, Irvine M (2002) Integrated sensible heat flux measurements of a two-surface composite landscape using scintillometry. Boundary-Layer Meteorol. 105: 5–35
Lee RW, Harp JC (1969) Weak scattering in random media, with application to remote probing. Proc IEEE 57: 375–406
Lüdi A, Beyric F, Mâtzler C (2005) Determination of the turbulent temperature–humidity correlation from scintillometric measurements. Boundary-Layer Meteorol. 117: 525–550
McAneney KJ, Green AE, Astill MS (1995) Large-aperture scintillometry: the homogeneous case. Agric For Meteorol 76: 149–162
Meijninger WML, Green AE, Hartogensis OK, Kohsiek W, Hoedjes JCB, Zuurbier RM, de Bruin HAR (2002) Determination of area-averaged water vapour fluxes with large aperture and radiowave scintillometers over a heterogeneous surface-flevoland field experiment. Boundary-Layer Meteorol 105: 63–83
Moene AF (2003) Effects of water vapour on the structure parameter of the refractive index for near-infrared radiation. Boundary-Layer Meteorol 107: 635–653
Moene AF, Meijninger WML, Hartogensis OK, Heusinkveld BG, de Bruin HAR (2005) The effect of finite accuracy in the manufacturing of Large Aperture Scintillometers. Internal Report 2005/1, Meteorology and Air Quality Group, Wageningen University, Wageningen, The Netherlands, 19 pp
Nieveen JP, Green AE, Kohsiek W (1998) Using a large-aperture scintillometer to measure absorption and refraction index fluctuations. Boundary-Layer Meteorol 87: 101–116
Ochs G, Wilson J (1993) A second generation large-aperture scintillometer. NOAA Tech. Memo. ERL WPL-232, 31 pp
Qian S, Chen D (1993) Discrete Gabor transform. IEEE Trans Signal Process 41: 2429–2438
Solignac PA (2009) Conception, Réalisation et Mise en oeuvre d’un scintillomètre: Influence de la vapeur d’eau dans la bande 940 nm. PhD Thesis, Université de Toulouse, 175 pp
Solignac PA, Selves JL, Béteille JP, Gastellu-Etchegorry JP (2007) Scintillometer data processing enhancement by Gabor transform and expansion. In: Proceeding of IEEE IMTC, May 2007, Warsaw, Poland, 4 pp
Solignac PA, Brut A, Selves JL, Béteille JP, Gatsellu-Etchegorry JP, Keravec P, Béziat P, Ceshia E (2009) Uncertainty analysis of the computation methods to derive the sensible heat flux from scintillometer. Atmos Meas Technol 2: 741–753
Strohbehn J (1968) Line-of-sight wave propagation through the turbulent atmosphere. Proc IEEE 56: 1301–1318
Tatarskii VI (1961) Wave propagation in a turbulent medium. McGraw Hill, New York, 232 pp
Vernin J, Chadid M, Aristidi E, Agabi A, Trinquet H, Vander Swaelmen M (2009) First single star scidar measurements at Dome C, Antartica. Astron Astrophys 500: 1271–1276
Wang T, Ochs G, Clifford S (1978) A saturation-resistant optical scintillometer to measure \({C_{n^{2}}}\) . J Opt Soc Am 68: 334–338
Ward HC, Evans JG, Grimmond CSB (2011) Effects of non-uniform crosswind fields on scintillometry measurements. Boundary-Layer Meteorol 141: 143–163