DO4Models box model results

Within the framework of the Dust Observations for Models (DO4Models) project, the performance of three commonly used dust emissions schemes is investigated in this paper using a box model environment. We constrain the model with field data (surface and dust particle properties as well as meteorological parameters) obtained from a dry 5 lake bed with a crusted surface in Botswana during a three month period in 2011. Our box model results suggest that all schemes fail to reproduce the observed horizontal dust flux. They overestimate the magnitude of the flux by several orders of magnitude. The discrepancy is much smaller for the vertical dust emission flux, albeit still overestimated by up to an order of magnitude. The key parameter for this mismatch is the 10 surface crusting which limits the availability of erosive material even at higher wind speeds. In contrast, direct dust entrainment was inferred to be important for several dust events, which explains the smaller gap between modelled and measured vertical dust fluxes. We conclude that both features, crusted surfaces and direct entrainment, need to be incorporated in dust emission schemes in order to represent the entire 15 spectra of source processes. We also conclude that soil moisture exerts a key control on the shear velocity and hence the emission threshold of dust in the model. In the field, the state of the crust is the controlling mechanism for dust emission. Although the crust is related to the soil moisture content to some extent, we are not able to deduce a robust correlation between state of crust and soil moisture.