Modeled downward transport of a passive tracer over western North America during an Asian dust event in April 1998

An intense Gobi Desert dust storm in April 1998 loaded the midtroposphere w ith dust that was transported across the Pacific to western North America. The Mesoscale Compressible Community (MC2) model was used to investigate me chanisms causing downward transport of the midtropospheric dust and to expl ain the high concentrations of particulate matter of less than 10-mum diame ter measured in the coastal urban areas of Washington and southern British Columbia. The MC2 was initialized with a thin, horizontally homogeneous lay er of passive tracer centered at 650 hPa for a simulation from 0000 UTC 26 April to 0000 UTC 30 April 1998. Model results were in qualitative agreemen t with observed spatial and temporal patterns of particulate matter, indica ting that it captured the important meteorological processes responsible fo r the horizontal and vertical transport over the last few days of the dust event. A second simulation was performed without topography to isolate the effects of topography on downward transport. Results show that the dust was advected well east of the North American coa st in southwesterly midtropospheric flow, with negligible dust concentratio n reaching the surface initially. Vertically propagating mountain waves for med during this stage, and differences between downward and upward velociti es in these waves could account for a rapid descent of dust to terrain heig ht, where the dust was entrained into the turbulent planetary boundary laye r. A deepening outflow (easterly) layer near the surface transported the tr acer westward and created a zonal-shear layer that further controlled the t racer advection. Later, the shear layer lifted, leading to a downward hydra ulic acceleration along the western slopes, as waves generated in the easte rly flow amplified below the shear layer that was just above mountain-crest height. Examination of 10 yr of National Centers for Environmental Predict ion-National Center for Atmospheric Research reanalyses suggests that such events are rare.