A coordinate system independent surface stress

A small amount of instrument tilt can degrade the accuracy of eddy correlation flux measurements, yet in the field, it is difficult to mount sonic anemometers perfectly vertically. Although instrument tilt can be corrected with the planar fit tilt correction method, this method does not work well over complex surfaces such as those found in urban areas, nor is it meant to be applied to data taken from within the roughness sublayer. In addition, the presence of the vertical surfaces of building walls complicates the definition of surface normal, which may no longer be parallel to the gravity vector.

In this presentation, a substitute for u'w' for use in roughness sublayers is derived from coordinate system independent information about the turbulent kinetic energy elipsoid obtained from sonic anemometer data.