PLACEMENT HEIGHT OF EDDY-CORRELATION SENSORS ABOVE A SHORT TURFGRASS SURFACE
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abstract
Variation in measured sensible heat flux density Fh with sensor height above short turfgrass during mainly unstable conditions was investigated using the eddy correlation (EC) technique. Our data showed that EC-measured Fh values at 0.25 and 0.38 m above the turfgrass were 15% and 10% lower, respectively, than that at the 1.00 m height. There was no statistical difference in the EC-measured Fh at 0.50, 1.00 and 1.25 m. An analysis of `footprints' shows that at least 90% of the measured Fh at a height of 0.5 m was from our experimental site, decreasing to less than 70% at the 1.5 m height. Sensor placement at the 0.5 m height would result in little reduction, as did occur at heights less than 0.50 m, in the covariance between vertical wind speed fluctuation and air temperature fluctuation, owing to small-sized eddies being contained between the separation distance of the sonic anemometer transducers. We speculate that measurements closer than 0.5 m to the surface differed from those at l.00 m as a result of small-sized eddies near the surface being contained between the sonic separation distance and therefore not completely detected by the sonic anemometer. 1995.
