Mass conservation above slopes in the Regional Atmospheric Modelling System (RAMS)

This paper examines the mass balance in calculations with the Regional Atmospheric Modelling System (RAMS). An error is pointed out that concerns the calculation of the surface fluxes on slopes. This error affects all the prognostic variables in RAMS when sloping terrain is involved. Here we explain how the error can be corrected. To study the impact of the error, we compared simulations with the uncorrected and corrected model. The model contains CO₂ transport, and online mass balance calculations were performed for this tracer. Without correction, effective surface CO₂-fluxes on mountain slopes were found to be enhanced under certain common conditions to several times the parameterized fluxes. Neglecting this error may cause substantial deviations in both forward and inverse model calculations. After the correction a very good closure of the mass balance is obtained. The correction also modifies the meteorological parameters, although the consequences were limited compared to the CO₂-fluxes.