Friday, 9 August 2013: 2:30 PM
Multnomah (DoubleTree by Hilton Portland)
Many severe weather outbreaks over the central United States initially develop in the southwestern portion of the Great Plains, particularly in western Oklahoma and Texas, often along the dryline. Commonly in the spring season, a lee trough associated with the Rockies sharpens the air mass and moisture gradients between dry air from the Desert Southwest and Mexico and warm moist air from the Gulf of Mexico. The deeper boundary layer to the west advects over and caps the shallower moist air, giving rise to convective available potential energy (CAPE). However, predicting convective initiation (CI) has proven challenging, creating a need for an improved fine-scale understanding of processes that describe along-dryline variability. Some previous studies have noted the existence of misocyclones, vortices of 200 m to 2 km width, propagating along the boundary, which may locally enhance the vertical motion of air parcels. Many theories exist on how misocyclones form in the dryline environment. For example, some studies identify that pre-existing horizontal vorticity in the form of horizontal convective rolls (HCRs) is tilted vertically and stretched to generate misocyclones.
Case studies from the spring of 2012, including that of the 30 April dryline event, are used to test current working theories. Dual-Doppler Ka band (TTUKa) radar data are objectively analyzed with 40 m grid spacing and ~15 s temporal resolution for unparalleled detail in the analysis. Supposing HCRs generate misocyclones, one expects vertical wind shear to be a necessary condition for misocyclone formation and that vortex genesis will occur in a spatially periodic manner proportional to boundary layer depth. As data show no clear periodicity, an alternative theory to misocyclone genesis is consideredthat misocyclones form from dynamic fluid-flow instability. To test this notion, second derivative 2-D wind fields are calculated to determine Rayleigh and Fjortoft instability criteria. Lastly, an attempt to collocate misocyclones with satellite imagery of clouds is offered to test if areas of the dryline near misocyclones have enhanced vertical velocity and such vortices are therefore a possible CI source.
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