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In the past years there has been a renewed interest in the use of scintillometers for the measurement of sensible and latent heat flux This interest is partly invoked by the need to infer the energy fluxes over areas that match in size with satellite pixels or the grid of a numerical model In this respect it is of importance to exploit the path limits of a scintillometer One of the problems encountered in extending the path length is saturation of scintillation To avoid saturation one may use large apertures or choose a longer wavelength such as microwave or radiowave or install the scintillometer at a higher level above the ground Here our experiences with an extra large aperture scintillometer XLAS operating over a path of 9 8 km are reported The instrument has transmitter and receiver apertures of 31 cm and uses a 0 94 m 100 mW light emitting diode as light source The height of the beam over the surface is 40 m The transmitter was installed in the TV tower near the town of IJsselstein and the receiver in the KNMI meteorological tower near Cabauw both at 40 m height The path crosses mainly pasture land and some low built-up area mainly situated near the transmitter The scintillometer was first installed in August 2000 dismantled in October of that year and re-installed in October 2001 with the aim to get a full year of data From publications in the literature it can be concluded that in our set-up saturation gets significant at a sensible heat flux of 100 Wm-2 or more Corrections may be made for higher heat fluxes We compare the scintillometer fluxes with eddy correlation fluxes obtained near the receiver end of the optical path In doing so one faces the issue of comparing a local measurement with an area-averaged one To gain insight in the degree of homogeneity of the area we will analyse thermal pictures of the surface beneath the scintillometer path In this way we hope to get better to grips with the performance of the scintillometer as a device for measuring area-averaged heat fluxes

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Long range scintillometry

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with scintillometrie as keyword