Test of Scintillometer Saturation Correction Methods Using Field Experimental Data

Abstract

Saturation of large aperture scintillometer (LAS) signals can result in sensible heat flux measurements that are biased low. A field study with LASs of different aperture sizes and path lengths was performed to investigate the onset of, and corrections for, signal saturation. Saturation already occurs at C 2 n ≈ 0.074D 5/3 λ 1/3 L −8/3 , where C 2 n is the structure parameter of the refractive index, D is the aperture size, λ is the wavelength, L is the transect length, which is smaller than theoretically derived saturation limits. At a transect length of 1 km, a height of 2.5 m, and aperture ≈0.15 m the correction factor exceeds 5% already at C 2 n = 2 × 10 −12 m −2/3 , which will affect many practical applications of scintillometry. The Clifford correction method, which only depends on C 2 n and the transect geometry, provides good saturation corrections over the range of conditions observed in our study. The saturation correction proposed by Ochs and Hill results in correction factors that are too small in large saturation regimes. An inner length scale dependence of the saturation correction factor was not observed. Thus for practical applications the Clifford correction method should be applied.

On the Discrepancy in Simultaneous Observations of the Structure Parameter of Temperature Using Scintillometers and Unmanned Aircraft

“…While it is better to prevent saturation (by reducing the path length or increasing the observation level), one can also correct the C 2 n signal. Based on an extensive comparison of different saturation correction methods, Kleissl et al (2010) recommended the Clifford correction method (Clifford et al 1974). For both the WURLAS and BLS instruments, the saturation correction factor m = C 2 n cor /C 2 n uncor was calculated for nine values of C 2 n uncor .…”

Developments in Scintillometry

“…This experiment, conducted in collaboration with the Center of Ecology and Hydrology, focused on the testing of the prototype of a radiowave scintillometer. A second experiment (Kleissl et al 2008) was performed in collaboration with New Mexico Tech in Sevilleta National Wildlife Refuge, a park in New Mexico. This latter experiment focused on the issue of saturation of the scintillometer signal: Under which conditions does the simple relationship between scintillometer signal and the structure parameters of the refractive index no longer hold, and can we model those deviations?…”

Infrared and millimetre-wave scintillometry in the suburban environment – Part 1: Structure parameters

“…However, the complexity of the energy balance (Oke, 1987) means this is usually not attempted in urban areas. In particular the significant storage heat flux (Offerle et al, 2005) and contribution from anthropogenic activities (Klysik, 1996;Allen et al, 2011) are both very difficult to measure.…”

“…This technique requires that a value of the temperature-humidity correlation coefficient, r T q , be assumed. Often r T q is taken to be ±1, indicating perfect correlation, as in Green et al (2001) and Meijninger et al (2002a), but other values have also been used: Kohsiek and Herben (1983) used r T q = 0.87; Evans (2009) used r T q = 0.8; and Meijninger et al (2006) used measured r T q from a nearby EC station with values between −0.5 and 0.9. Previous studies measuring r T q with fast-response sensors suggest daytime values tend to be smaller than 1, typically around 0.8.…”

“…Previous studies measuring r T q with fast-response sensors suggest daytime values tend to be smaller than 1, typically around 0.8. For example: 0.75 at a flat, homogeneous site (Kohsiek, 1982); 0.76 over sandy soil with patchy vegetation (Andreas et al, 1998); 0.70-0.95 for unstable conditions over heterogeneous farmland (Meijninger et al, 2002a). Nocturnal values of r T q down to −1 are rarely seen (Andreas et al, 1998;Meijninger et al, 2006).…”