For the AMS 7th Symposium on the Urban Environment

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 (Wilczak, et al 2001), this method does not work well over complex surfaces such as those found in urban areas where mean streamlines may not be horizontal and the degree of tilt may differ for different wind directions (Klipp 2004), 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. A substitute for the surface stress for use inside the roughness sublayer has been developed. It is coordinate system independent, therefore sonic anemometer tilt correction is not necessary. Rotation of the coordinate system into streamwise coordinates is also optional