What Causes Haboobs to Propagate Faster or Slower?

Haboobs, or dust-carrying cold pools, contribute a significant percentage of emitted dust in certain regions. For example, haboobs are responsible for more than 70% of the lofted dust in the Sahara, and for up to 30% of lofted dust in the southern Arabian Peninsula. Haboobs also pose critical aviation and public health hazards. Given these important impacts, our understanding of the physical processes governing the propagation of haboobs is imperative. These include the role of sloped topography, aerodynamic surface roughness, and surface processes. Laboratory and observational work have also established that the mass of suspended particles is critical for the negative buoyancy of certain density currents, such as pyroclastic flows or oceanic turbidity currents. The impact of dust mass on haboob buoyancy, and consequently on haboob propagation, has yet to have been explored. Such an impact would be analogous to the reduction of buoyancy in updrafts due to condensate loading and will hereafter be referred to as the “aerosol buoyancy effect” (ABE). The goal of this research is therefore to explore the impacts of these aforementioned factors on the propagation of haboobs.

A set of high-resolution simulations using the Regional Atmospheric Modeling System (RAMS) will be presented. These idealized haboob simulations constitute a factor-separation experiment where the following factors are explored. The topography is varied across sloped planes with grades of 0%, 1%, 3%, and 5% being representative of topography in Arizona, a region in which haboobs are frequently observed. The direction of slope (e.g. north-facing vs south-facing) is also varied as this impacts the solar irradiance received by the land surface, and hence the surface fluxes. Simulations are run testing various aerodynamic roughness lengths which also alter the surface fluxes of heat and momentum that act to dissipate the haboob. Finally, tests are run with the ABE being either enabled or disabled. Preliminary results suggest the following: as the grade of a slope is increased, haboobs propagate further in the downslope direction than the upslope direction. North facing slopes in the Northern Hemisphere (which receive less solar irradiance) and surfaces with lower roughness lead to reduced dissipation of haboobs via surface fluxes and thus faster propagation compared to south facing slopes and surfaces with greater roughness. Finally, haboobs in simulations with the ABE enabled are found to propagate further downslope compared to haboobs in simulations with the ABE disabled. The relative importance of these factors and interactions between the factors will be explored through the factor separation methodology. Implications of these results on forecasting haboobs and on improving parameterizations in global models will also be discussed.