USING A LARGE-APERTURE SCINTILLOMETER TO MEASURE ABSORPTION AND REFRACTIVE INDEX FLUCTUATIONS

    loading  Checking for direct PDF access through Ovid

Abstract

The contributions of refraction and absorption fluctuations to the measured scintillations are observed for a near-infrared absorption region using a NOAA designed large-aperture scintillometer. The logarithmic amplitude spectra are shown to decay with frequency as f−8/3 for both the absorption and scattering mechanisms. For the absorption mechanism this is in line with similar observations made at microwave and infrared frequencies. However, for finite transmitting and receiving apertures, theory predicts a stronger decay of the scattering mechanism due to aperture averaging. The spectral shape is characterised by a region of low frequency absorption, higher frequency refraction and separated by a flattish transition zone. The upper observed corner frequency (fC2) compares well with values calculated using the measured transverse windspeed (v) for a known aperture radius. The lower corner frequency (fC1) position is shown to be sensitive to the ratio of the real and imaginary part of the refractive index structure parameter, (CnR2/CnI2)3/8, and v. The part of the spectrum associated with the absorption scintillations is observed to be much less than that due to refraction through the day until evening, when decreasing CnR2 causes CnR2/CnI2 to decrease and absorption to become significant. If absorption is ignored, this may have consequences for calculating nocturnal surface fluxes. During unstable, daytime conditions the large aperture scintillometer is most sensitive to refractive scintillations despite having an infrared source transmitting in a lossy atmosphere. But also under these conditions, the low frequency absorption part of the spectrum is observable.

Related Topics

    loading  Loading Related Articles