An approximate footprint model for flux measurements in the convective boundary layer

An explicit footprint model for flux measurements of passive scalars in the lower part of the convective boundary layer (CBL) is introduced. A simple footprint model is derived analytically in an idealized CBL. The simple model can simulate the overall characteristics of the flux footprint. Then a method is proposed to adjust the analytical solutions to those from a Lagrangian stochastic model that considers more realistic atmospheric conditions in the vertical direction. The adjusted footprint model is a function of Monin-Obukhov length (L), roughness length, receptor height, and CBL depth (h). Comparison between the results from the adjusted footprint model and stochastic model suggests that the adjusted footprint model can well simulate the streamwise extent of the footprint within the dimensionless upwind distance X < 1, which accounts for a majority of the footprint. The model applies to stabilities of -L/h between 0.01 and 0.1 and roughness lengths between 10^-5 and 2 × 10^-3h in the lower part of the mixed layer (from 0.1h to 0.6h).