Abstract
This Chapter examines theoretical and operational aspects of coordinate systems. A distinction is made between the vector basis, a local property of a coordinate system, and the overall coordinate frame consisting of the vector basis and coordinate lines, a global property of the flow that is determined by the flow field in three dimensions. Point measurements can only define the vector basis. Because in field campaigns many components that enter into the mass balance in complex flows are severely under-sampled, a properly chosen coordinate frame for point measurements should optimize our estimates of the surface-air exchange and should maximize information for diagnostics purposes.
The strengths and weaknesses of three operational coordinate systems for point measurements (instrument, natural wind, and planar fit) are examined in detail. That error in scalar fluxes due to coordinate tilt is usually small for small tilt angles does not negate the need for coordinate rotation because the tilt error can introduce a systematic bias to the time integrated flux. On the other hand, it is also important that over-rotation be avoided in post-field data analysis. Tilt errors caused by contamination from the streamwise and cross-wind fluxes should be treated differently.
Appendix B outlines a method for rotation into the planar fit coordinate. The scheme relies on the straightforward vector operation and avoids the need for rotation angles.
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Lee, X., Finnigan, J., Paw U, K.T. (2004). Coordinate Systems and Flux Bias Error. In: Lee, X., Massman, W., Law, B. (eds) Handbook of Micrometeorology. Atmospheric and Oceanographic Sciences Library, vol 29. Springer, Dordrecht. https://doi.org/10.1007/1-4020-2265-4_3
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DOI: https://doi.org/10.1007/1-4020-2265-4_3
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