FLEXPART model sensitivity study for the footprint of a Swiss tall tower site

Flux towers are essential tools for collecting measurements of trace gas concentrations/fluxes to investigate source regions of greenhouse gases (GHGs) and other pollutants. Most flux towers provide observations at heights of several meters to tens of meters and therefore only sample potential source regions in their immediate vicinity, i.e., these towers have small so-called footprints. Here we are interested in estimating the footprint of one of the few European tall towers located close to Beromünster, Switzerland. The tower was initially set up as a CarboCount CH site — a dense GHG observation network run for four years (2012 - 2015) — and is continued since by the University of Bern. Measurements are taken at an altitude of 212m above ground. This relatively high observation height results in a larger tower footprint and therefore the tower observations are predestined for a source analysis on a much larger scale than typical for flux towers.

We will present preliminary results of a sensitivity study performed with the Lagrangian atmospheric transport model FLEXPART using different meteorological input data of various spatial resolution, different model internal time step settings, as well as two
different convection schemes used in the convective atmospheric boundary layer — a Gaussian Hanna-type turbulence model and a more realistic skewed turbulence scheme in which a larger area is occupied by downdrafts than by updrafts.

The range of simulated footprints will be used in combination with emission inventories of CO₂ and CH₄ to simulate observations at the tower. By comparing the simulated with the actual observations at the tower we aim to evaluate the quality of the simulated footprints for the respective input data and model setting.