Validation of fluxes of an extra large aperture scintillometer at Cabauw using Sky Arrow aircraft flux measurements

An extra-large aperture scintillometer (XLAS) has been operated at the Cabauw tower for several years over a path of 9.8 kilometers, at an average height of 43 meters (Kohsiek et al., 2002). This yields a long term record of the area-averaged sensible heat flux. During the RECAB summer campaign on July 27 2002, the Sky Arrow flux aircraft was flown close to the path of the scintillometer at approximately 70 meters (and two more levels) at four times of the day (Vilà-Guerau de Arellano et al., 2004).

First the aircraft data are used to validate the fluxes obtained from the XLAS. Since the aircraft measures the local flux at flying altitude, whereas the scintillometer estimates the surface flux, the flux divergence below the plane needs to be taken into account. This flux divergence is estimated from the temperature profile along the Cabauw mast. The fluxes of the scintillometer and the aircraft match very well, within their respective accuracies.

Secondly, the variability of the aircraft-derived flux along the path is investigated. Despite the rather homogeneous land use, some -statistically significant- differences could be found when the flight-lag was subdivided into five parts. Some of this variability was constant between different times of the day, and should thus be linked to properties of the underlying surface, whereas other variations appeared to be random.

Finally, the spatial variability of the structure parameter of the refractive index along the flight lag is investigated using the aircraft data, in order to study how well the scintillometer represents the path-average structure parameter.

Kohsiek W., Meijninger W., Moene A., Heusinkveld B., Hartogensis O., Hillen W. and DeBruin H. (2002). An extra large aperture scintillometer for long range applications. Boundary-Layer Meteorol., 105, 119-127.

Vilà-Guerau de Arellano, J., Gioli, B., Miglietta, F., Jonker, H.J.J., Klein Baltink, H., Hutjes, R.W.A., Holtslag, A.A.M., Entrainment process of carbon dioxide in the atmospheric boundary layer. Journal of Geophysical Research 109, D18110.