Thursday, 6 November 2014: 4:30 PM
Madison Ballroom (Madison Concourse Hotel)
Two high resolution (250 m grid spacing) real-data simulations of the 3 May 1999 Oklahoma City, Oklahoma tornadic supercell and associated tornadoes using single- and triple-moment microphysics are analyzed to characterize the microphysical, thermodynamic, and dynamic impacts on vertical accelerations in and near the tornado-like vortices (TLVs). Trajectory analyses indicate that the behavior of the TLVs is systematically different between the single- and triple-moment experiments, with the TLV in the triple-moment experiment being substantially more intense and longer-lived than in the single-moment case. The triple-moment scheme in this case produces less rain and hail mass in the low levels, and drop size distributions of rain biased toward larger drops, relative to the single-moment scheme, all of which contribute to less latent cooling and warmer outflow. It is found that the intensity and longevity of the TLV are tied to weaker negative thermal buoyancy in air flowing into the TLV in the triple-moment case, in agreement with previous observational and modeling studies. Additionally, the contribution to buoyancy from pressure perturbations, which is often neglected in many applications, is found to be of prime importance in the low levels of the TLV, where strong negative pressure perturbations lead to substantial positive buoyancy. This contribution more than compensates for the slight negative thermal buoyancy in the triple-moment case. Finally, more parcels entering the TLV in the triple-moment experiment have a history of dynamically induced descent, whereas buoyantly driven descent is more prevalent in the single-moment experiment.
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