On the discrepancy in simultaneous observations of C_T^2 by scintillometers, sonics and unmanned aircraft during LITFASS-2009 and LITFASS-2010

In recent years large aperture scintillometers (LAS) have been increasingly used to determine the path-averaged surface fluxes of sensible heat. The basic parameter derived from LAS is the structure parameter of the refractive index of air (C_n²). It can be linked via the structure parameter of temperature (C_T²) to a path-averaged surface sensible heat flux with Monin Obukhov Similarity Theory (MOST). MOST is only valid above homogeneous conditions, whereas the terrain below the scintillometer path is often heterogeneous. Therefore, there is a need to validate the path-averaged structure parameter from a LAS system along a heterogeneous path with other independent data sources, e.g. airborne measurements or LES.

This study is an elaboration of the study of Beyrich (2012). They compared C_T² obtained with the unmanned meteorological mini aerial vehicle (M²AV) with C_T² obtained with the LAS for five flights on one single day during LITFASS-2009. Processing the data using standard procedures, they found that C_T² obtained from the M²AV data is systematically larger than from the LAS data. Here, we investigate if similar differences can be found for other days, and if these differences can be reduced or explained through a more elaborate processing of both the LAS data and the M²AV data. This elaborate processing includes translation of all signals to a reference height, exact synchronization of averaging times for all systems, correction of the scintillometer signal for saturation, proper treatment of the effect of humidity on the refractive index, the use of alternative methods to derive the structure parameter from the M²AV data, and application of the scintillometer path-weighting on the M²AV data. As a side-effect of this analysis we are able to evaluate the relative importance of these elaborate data processing options for derived C_T².

One important difference between the three measurement systems is the data they provide on which C_T² is based: a time series (sonic) , an instantaneous path average (LAS) and a space-time series (M²AV). To investigate if the derived C_T² depends on the nature of the underlying temperature dataset (time series at a fixed point versus a space-time series) we also show a sensitivity analysis of C_T² from virtual flights in a Large Eddy Simulation model with different mean ground speeds.

We conclude that the difference reported in Beyrich (2012) can be found for other days during LITFASS-2009 and LITFASS-2010 as well. C_T² obtained from the M²AV data is larger than from the data of the LAS and the sonic. A more elaborate data analysis does affect the C_T² values of all systems, but it does not substantially reduce the discrepancy among them. For the LAS-derived values the additional processing steps that have the largest effect (in terms of bias) are the saturation correction and the humidity correction (where it was found that the exact method used does make a significant difference). For the M²AV-derived values the most important step is the application of the scintillometer path-weighting function: due to the surface heterogeneity C_T² varies along the path and as the scintillometer does not weigh all parts equally, the M²AV data should use the same relative weighting.

Consequently, the deviation between C_T² from the M²AV on the one hand and from the LAS and sonic on the other hand cannot be explained so far.

 

References

Beyrich, F, Bange, J, Hartogensis, OK, Raasch, S. and Braam, M, van Dinther, D, Gräf, D, van Kesteren, B, van den Kroonenberg, AC, Maronga, B, Martin, S and Moene, AF, 2012: Towards a validation of scintillometer measurements: The LITFASS-2009 experiment, Boundary Layer Meteorology DOI 10.1007/s10546-012-9715-8