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Different approaches to modeling supercell mineral dust ingestion pathways
Robert B. Seigel, Colorado State University, Fort Collins, CO; and S. C. van den Heever
Lofted dust via outflow from mesoscale convective cold pools is a common occurrence that alters both the microstructure of clouds and the radiative budget. On a global scale, dust directly impacts the radiative budget through the absorption and scattering of both shortwave and longwave radiation. Additionally, dust can indirectly affect the climate through microphysical processes by acting as effective aerosols that alter cloud properties and cloud radiative feedback effects (known as the aerosol indirect effect). On the mesoscale, lofted dust modifies precipitation processes within convection by acting as IN, CCN, and GCCN. In extreme cases, strong outflow lofts high concentrations of dust such that visibility is dramatically reduced and air quality is degraded, which becomes a danger to the public, air travel, and military forces.
Before one can understand the microphysical and radiative impacts of mineral dust on mesoscale convection, dust ingestion pathways must first be understood to know which region of the storm will be most affected. Using the Regional Atmospheric Modeling System (RAMS) with an interactive dust model and a maximum grid spacing of 300 meters, an idealized supercell is simulated under three different mineral dust representations. Approach one (referred to as the Background simulation) utilizes a specified horizontally homogeneous vertical profile of dust with no surface emission. In contrast, the second approach (referred to as the Supercell Lofting simulation) contains no background dust, but uses the interactive dust model with surface emission. Additionally, a third approach (referred to as the Boundary Lofting simulation) imposes a low-level boundary near the supercell to further understand how boundary lofted dust becomes entrained into supercells.
Quantitative results are dramatically different between the three approaches, whereby the Background simulation demonstrates the most significant dust ingestion. In the Background simulation, the vast majority of mineral dust ingested into the supercell occurs ahead of the rear-flank downdraft (RFD) cold pool within the boundary layer. Dust ingestion within the Supercell Lofting simulation occurs at the interface between the RFD cold pool and ambient air via turbulent mixing. While, the addition of a low-level boundary (Boundary Lofting simulation) enhances mineral dust ingestion by imposing additional lofted dust from the boundary that becomes ingested ahead of the RFD cold pool. Results from all three approaches will be presented.
Session 7, Theoretical and modeling studies of mesoscale processes I
Tuesday, 2 August 2011, 4:00 PM-6:00 PM, Marquis Salon 456
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