A new scaling term for use in roughness sublayers

Sonic anemometer data from the inertial sublayer, usually several roughness heights above a canopy, are easier to interpret than data from within the roughness sublayer. Some of the problems of using data from within the roughness sublayer include the fact that the data are representative of only a local area, flows are typically not fully adjusted to the ever changing surface properties, and the presence of obstacles makes tilt correction extremely problematic. However, knowledge of flow and fluxes within the roughness sublayer will improve our understanding of urban and forest transport.

A substitute for u* is proposed as a tool for use in scaling schemes within the roughness sublayer. Traditional u* calculations can be distorted by the influence of nearby vertical surfaces such as building walls or flow around the sides of trees, as well as being sensitive to sonic anemometer tilt. This substitute utilizes invariants of the Reynolds stress tensor which are independent of the choice of coordinate system, making it more reliable for use in the roughness sublayer. It has the added advantage of approximating standard u* values when applied to data from the inertial sublayer. The empirical formula is based on data from lab flows as well as atmospheric field data from CASES99 and JU2003.