Use of variance techniques to measure dry air-surface exchange rates

Abstract

The variances of fluctuations of scalar quantities can be measured and interpreted to yield indirect estimates of their vertical fluxes in the atmospheric surface layer. Strong correlations among scalar fluctuations indicate a similarity of transfer mechanisms, which is utilized in some of the variance techniques. The ratios of the standard deviations of two scalar quantities, for example, can be used to estimate the flux of one if the flux of the other is measur+-d, without knowledge of atmospheric stability. This is akin to a modified Bowen ratio approach. Other methods such as the normalized standard-deviation technique and the correlation-coefficient technique can be utilized effectively if atmospheric stability is evaluated and certain semi-empirical functions are known. In these cases, iterative calculations involving measured variances of fluctuations of temperature and vertical wind velocity can be used in place of direct flux measurements. For a chemical sensor whose output is contaminated by non-atmospheric noise, covariances with fluctuations of scalar quantities measured with a very good signal-to-noise ratio can be used to extract the needed standard deviation. Field measurements have shown that many of these approaches are successful for gases such as ozone and sulfur dioxide, as well as for temperature and water vapor, and could be extended to other trace substances. In humid areas, it appears that water vapor fluctuations often have a higher degree of correlation to fluctuations of other trace gases than do temperature fluctuations; this makes water vapor a more reliable companion or "reference" scalar. These techniques provide some reliable research approaches but, for routine or operational measurement, they are limited by the need for fast-response sensors. Also, all variance approaches require some independent means to estimate the direction of the flux.

Sensible heat flux estimation by flux variance and half‐order time derivative methods

“…[Wesley, 1988], the value of C•(1.3) found in this study is large. However, as mentioned before, the measurements in the pine forest were performed near the canopy-atmosphere interface.…”

On the Anomalous Behaviour of Scalar Flux–Variance Similarity Functions Within the Canopy Sub-layer of a Dense Alpine Forest

“…McNaughton and Brunet (2002) even argued that interactions between such inactive and active eddy motions responsible for momentum transfer are also possible within the ASL and proposed a simplified mechanism for its onset. Likewise, a large number of studies (Wesley 1988;Weaver 1990;Padro 1993;Andreas et al 1998) have shown how source-sink in-homogeneity at the ground induces departure between measured FV similarity functions and predictions from MOST.…”

Variance methods to estimate regional heat fluxes with aircraft measurements in the convective boundary layer

“…The variance methods are based on 22 flux-variance relationships derived on the basis of similarity arguments and established through the 23 determination of the constant parameters in the derived relationship. Such relations have been established 24 and verified extensively through experiments in the surface layer and it now appears possible to derive 25 surface fluxes with sufficient accuracy (e.g., Wesely, 1988;Katul et al, 1996). In contrast, for the CBL, the 26 relevant flux-variance relationships are still not fully understood and far from established.…”