A dust storm over the elevated Black Rock Desert in northwestern Nevada

This study describes synoptic and mesoscale analyses of the dynamical processes that caused a dust storm in late February 2002 over the Black Rock Desert (elevation = 1190 m) in northwestern Nevada. The analyses included multi-scale observations and numerical simulations. The study showed that fast (~ 6-9 hours) adjustment processes occurred within the exit region of a curved jet streak exhibiting significant mesoscale thermal wind imbalance, and as a consequence velocity divergence spread across the exit region to the right side due to the unbalanced acceleration within the exit region. The ascending branch of this direct circulation shifted in time and the associated adiabatic cooling strengthened the baroclinic zone thereby enhancing the cross-stream frontal structure aloft. As the cooling intensified, the integrated mass flux convergence on the right side of the jet resulted in a low-level isallobaric wind oriented orthogonal to the exit region of the jet and directed towards the Black Rock Desert. In time, increasing ageostrophic flow strengthened the near-surface cold front creating ideal conditions for the low-level vertical shear and buoyancy resulting in the organization of turbulence kinetic energy over the Black Rock Desert. The timing of the vertical transport was ideally suited to the state of the desert crust, and the high momentum air scoured out the weakened surface crust producing reduced visibilities in several Sierra Nevada leeside metropolitan areas. Both analyses and numerical simulations clearly supported the evidence of geostrophic imbalance in the synoptic scale circulation which was the main driving force behind the high momentum air transported from the middle troposphere to the elevated surface accompanying the transformation from a balanced thermally indirect, transverse ageostrophic circulation about the curved jet streak's exit region to an unbalanced thermally direct circulation.