Effect of Coordinate Rotation on Estimating Surface Heat and Water Vapor Fluxes over Mountainous Terrain

Y. Y. Chen and M. H. Li

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Institute of Hydrological Sciences, National Central University, Taoyuan, Taiwan

Coordinate rotation is often applied to transform measured turbulence data before flux calculations. Different rotation approaches may cause significant variations in estimated fluxes. For example, mountain-valley circulation furthers the requirement of selecting a proper rotation technique. This study applied three coordinate rotation approaches, including double, triple, and planar-fit rotations, for computing heat and water vapor fluxes over a mountainous terrain with the eddy covariance method.

The experiment was conducted in an experimental watershed, Lien-Hua-Chih (LHC), located in the central Taiwan. Vegetation type is a mixture of
natural deciduous forest and shrubs with a canopy height of about 17 m. The observation tower was built to a height of 22 m. The site is a typical
mountainous terrain with significant meso-scale circulation. Figure 1 shows the day time (left) and night time (right) wind direction and speed at the LHC site. Prevailing wind direction is NW in day time and ES in night time.

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Turbulence data above canopy were measured with an eddy covariance system, a 3-D sonic anemometer (Young 81000) and a krypton hygrometer (Campbell KH20). Raw data sampled at 10 Hz was recorded in a data logger, CR23X, and stored by a laptop computer. More than two weeks of data were collected for fluxes comparisons between different coordinate rotation approaches. Figure 2 shows effects of wind directions on determining b1/b2 coefficients of the planar fit rotation due to tilt angle variations associated with the mountainous terrain.