Similarity theory for local and spatially averaged momentum fluxes

Tower and aircraft data from a number of field studies are analyzed to: (1) estimate the roughness lengths for local and area-averaged momentum fluxes over various surface types; (2) examine the applicability of the ψ-stability formulations of Paulson and Dyer for momentum fluxes; (3) investigate the possibility that tower flux measurements over tall vegetation are often inadvertently made in the roughness sublayer where Monin–Obukhov (M–O) similarity theory is not valid. With tower and aircraft data, it is shown that the term ψ(zom/L) evaluated at the roughness height becomes significant with relatively large roughness length and small measurement height. The usual omission of this term in M–O theory is not justified for flux calculations over rough surfaces such as forests. In stable conditions (z/L<0.5), the Dyer ψ-stability formulation predicts the tower-based momentum fluxes reasonably well over both forest and short vegetation sites. In weakly unstable conditions (−1