Optical large aperture scintillometeyr (LAS) has been shown, in recent years, to provide reliable 'area averaged' turbulent sensible heat flux over of several hundreds of meters to several kilometers over both homogeneous (De Bruin et al 1995) and heterogeneous sites (Lagouarde et al 2002). Sensible heat flux (H) is derived from the refractive index structure parameter (Cn~2) which is directly related to the variance of the log amplitude fluctuations (σ~2) of the EM wave received by the LAS (Wang et al 1978). In an ideal (non-absorbing) atmosphere σ~2 is directly related to refraction of the emitted EM wave by turbulent eddies. Kagawa et al (1996) demonstrated that absorption (water vapor) can enhance the measured variance, which in turn can result in the overestimation of H. In practice σ~2 is bandpass filtered (0.03-400Hz) to eliminate absorption, instrument drift and high frequency noise. Following Nieveen et al (1998), for pastureland, this has been reduced (0.1-400 Hz). In order to control the quality of our scintillometery measurements and the adequacy of the filter used, spectral analyze has been performed.
展开▼
