The effective height of a two-wavelength scintillometer system

A scaling factor, S, is derived to account for the difference in path-weighted measurement heights of a combined system consisting of a large-aperture scintillometer (LAS) and a millimetre-wave scintillometer (MWS), operating at wavelengths of 0.88μm and about 3mm respectively, and designed to determine the area-averaged latent heat flux. This work extends an earlier derivation of Zef , the effective height for the LAS yielding the area-averaged sensible heat flux. The LAS and MWS have different path-weighting functions, therefore, in general, it is expected that the path-weighted beam heights are different, for paths other than with the beam parallel to the land surface. Their difference will depend on the detail of the experimental set-up. The objective is to introduce a scaling factor that transforms the MWS refractive-index structure parameter measurement to the same pathweighted height of the LAS measurement. By applying S as a prelude to the calculation of the temperature and humidity structure parameters, and the sensible and latent heat fluxes, these quantities are made representative of the same measurement height, thus simplifying the application of two-wavelength scintillometry. The equations presented here enable the analysis of LAS-MWS paths such that they can be selected to optimize S towards its ideal value of unity. For this purpose we provide a new analytical approximation of the LAS path-weighting function. The importance of accounting for S is demonstrated by example applications over varying topography as well as for slanted beams.