Internal boundary layer devleopment over a salt pan: Measured and modelled dust emissions

Abstract:

A key component of a dust emission scheme is the threshold for sediment transport, which is a function of soil size distribution, soil moisture, air and soil particle density, and surface roughness. For a particular region or landform that is not vegetated the variable that controls the transport threshold the most is soil moisture. This is because it is assumed that the other components vary little (air and soil particle densities) or are kept constant (soil size distribution and surface roughness). This puts the emphasis very heavily on soil moisture and wind stress as the key drivers of dust emission for specific landforms and dust emission schemes. Dust emission measurements were undertaken in 2011 on Sua Pan in Botswana, a large, flat, unvegetated salt pan, as part of the Dust Observation for Models (DO4 Models) campaign. The observations consisted of 11 climate stations placed within a 144 km². Out of the measured and calculated erodibility parameters responsible for predicting transport threshold within current schemes, surface soil moisture and aerodynamic roughness length varied the most over the duration of the project and spatially across the pan. In 2011, the pan was drying from extensive flooding and rainfall in the previous wet season. Within the dry winter season months of June through September the dominantly eastern winds dried the pan developing a directional increase in aerodynamic roughness length over time. This cumulated in a region of maximum roughness length with the highest potential for dust emissions. In some cases, the aerodynamic roughness length of the bare soil increased by three orders of magnitude within the three month period. This increase in roughness almost doubles the modelled threshold shear velocity required for this surface to be emissive. The temporal and spatial variability of the calculated transport threshold is explored with observed data and compared with the modelled transport threshold for this region for the campaign duration. These results suggest that for dry saline lakes and other evaporite and unvegetated emissive surfaces the surface roughness changes dramatically from season to season and can have a direct effect on the soil surface emissivity. In conclusion, more research is needed to resolve the variation in spatial and temporal dust production within these landform.