Attenuating the Absorption Contribution on {C_{n2}} Estimates with a Large-Aperture Scintillometer
Abstract
Large-aperture scintillometers (LAS) are often used to characterize atmospheric turbulence by measuring the structure parameter of the refractive index {C_{n2}} . However, absorption phenomena can lead to an overestimation of {C_{n2}} . 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_{n2}} 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_{n2}} is independent of meteorological conditions, especially crosswind, and the second consists in accurately reconstructing the signal to remove absorption, without losing information on {C_{n2}} . A reference {C_{n2}} (hereafter `reconstructed {C_{n2}} ') is created by accurately removing absorption from the scintillation spectrum, and is used to evaluate each filter. By comparing the `reconstructed {C_{n2}} ' with a raw {C_{n2}} measured with a scintillometer, in the presence of absorption, we found that the average relative contribution of absorption to the measurement of {C_{n2}} 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_{n2}} . 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_{n2}} , 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.
- Publication:
-
Boundary-Layer Meteorology
- Pub Date:
- May 2012
- DOI:
- 10.1007/s10546-011-9692-3
- Bibcode:
- 2012BoLMe.143..261S
- Keywords:
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- Absorption;
- Adaptive filtering;
- Atmospheric turbulence;
- Optical propagation;
- Scintillometer;
- Structure parameter of refractive index