Tuesday, 6 August 2013
Holladay-Halsey (DoubleTree by Hilton Portland)
Numerical simulations were performed of the 15-16 April 2003 severe storms outbreak over West Texas using the Regional Atmospheric Modeling System (RAMS) in order to assess the role(s) that lofted mineral dust from the U.S. desert southwest might have played in the development and evolution of the convection. This required the creation and implementation of various parameterizations that allowed dust to potentially serve as cloud condensation nuclei (CCN), giant CCN (GCCN), and ice nuclei (IN) in the model. A control simulation was performed (CTL) wherein dust effects were excluded. In a second simulation, only dust radiative effects were considered (RAD). In a third simulation, dust indirect microphysical and direct radiative impacts were simulated (DST). Dust serving as CCN reduced warm rain formation. Thus, cloud droplets were transported aloft, enhancing freezing, aggregation, as well as the production of graupel and hail. Graupel and hail were relatively small in size due to reduced single particle riming efficiencies. Dust serving as GCCN and IN played secondary roles. The DST simulation yielded the weakest precipitation accumulation, as much of the total hydrometeor mass was in the form of small ice particles that were transported into the anvil regions of the storms. The DST convective line was associated with relatively weak cold pools and multiple long-lived supercells, while the RAD and CTL simulations produced widespread cold pools, which hindered supercell formation. However, the absorption of solar radiation within the dust plume in the RAD simulation reduced the amount of radiation that reached the surface prior to convection, resulting in increased atmospheric stability compared to the CTL case. The CTL simulation initially yielded more widespread convection, while the RAD simulation produced a long-lived supercell. The results suggest that dust indirect microphysical and direct radiative impacts may at times greatly influence the development of severe storms. In this study, dust actually increased the potential for supercells.
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