Determining the validity of the boundary layer budget method for estimating greenhouse gas fluxes over complex terrain using idealized simulations
The difficulty with the Lagrangian approach is the ability to track an airmass. Changes in scalar concentration in the airmass are due not only to surface fluxes but also to horizontal advection in and out of the sampled airmass and from entrainment or detrainment at the top. Because observations of horizontal and vertical advection are typically unavailable or highly inaccurate, these terms are not usually calculated and the change in airmass concentration is assumed to be caused by surface fluxes only. This assumption works well for flat, homogeneous terrain, but in mountainous terrain horizontal and vertical advection is likely not negligible due to the presence of thermally-driven wind systems.
Here we assess the violation of the BLB assumptions when applying this method to passive trace gas fluxes such as CO2 and methane in mountainous terrain. We use idealized simulations in which trace gas surface fluxes are prescribed. We then calculate the surface fluxes using the BLB method and compare these to the prescribed surface fluxes. We show that the BLB method has the potential to work over mountainous terrain as long as a suitable sampling strategy is used.