The value of the aerodynamic roughness length over a heterogeneous surface that relates the horizontally averaged wind velocity profile to the spatially averaged or aggregated shear stress or momentum flux. The effective roughness length is defined only for heights above the numerical blending height and is dependent on the magnitude, distribution, and length scale of surface roughness variations. The effective roughness length is typically larger than linearly or geometrically averaged roughness lengths. See flux aggregation.