Near-storm environments of outbreak and isolated tornadoes
Alexandra Anderson-Frey, Yvette Richardson, Andrew Dean, Richard Thompson, Bryan Smith
Between 2003 and 2015, there were 5343 outbreak tornadoes and 9389 isolated tornadoes reported in the continental United States. Here, the near-storm environmental parameter-space distributions of these two categories are compared via kernel density estimation, and the seasonal, diurnal, and geographical features of near-storm environments of these two sets of events are compared via self-organizing maps (SOMs). Outbreak tornadoes in a given geographical region tend to be characterized by greater 0–1-km storm-relative helicity and 0–6-km vector shear magnitude than isolated tornadoes in the same geographical region and also have considerably higher tornado warning-based probability of detection (POD) than isolated tornadoes. A SOM of isolated tornadoes highlights that isolated tornadoes with higher POD also tend to feature higher values of the significant tornado parameter (STP), regardless of the specific shape of the area of STP. For a SOM of outbreak tornadoes, when two outbreak environments with similarly high magnitudes but different patterns of STP are compared, the difference is primarily geographical, with one environment dominated by Great Plains and Midwest outbreaks and another dominated by outbreaks in the southeastern United States. Two specific tornado outbreaks are featured, and the events are placed into their climatological context with more nuance than typical single proximity sounding-based approaches would allow.
A Significant Tornado in a Heterogeneous Environment During VORTEX-SE
Timothy Coleman, Anthony Lyza, Kevin Knupp, Kevin Laws, Wes Wyatt
On 1 March 2016, an EF2 tornado occurred near Birmingham, AL, and was examined as part of VORTEX-SE. The boundary-layer environment near the tornadic supercell was heterogeneous in space and unsteady in time, with what typically would be considered an excellent proximity sounding. In this case, however, the proximity sounding severely underestimated the CAPE. SPC mesoanalyses substantially underestimated the CAPE and wind shear as well. Tornadogenesis occurred near a weak, frontogenetical thermal boundary, where evaporation from antecedent light showers had also increased dewpoint values. A local maximum in surface dewpoint (and instability), and a local maximum in helicity both existed near the region of frontogenesis. As a QLCS moved into the region of higher CAPE air,
Observations of Near-Surface Vertical Wind Profiles and Vertical Momentum Fluxes from VORTEX-SE 2017: Comparisons to Monin–Obukhov Similarity Theory
Paul Markowski, Nathan Lis, David Turner, Temple Lee, Michael Buban
Observations of near-surface vertical wind profiles and vertical momentum fluxes obtained from a Doppler lidar and instrumented towers deployed during VORTEX-SE in the spring of 2017 are analyzed. In particular, departures from the predictions of Monin–Obukhov similarity theory (MOST) are documented on thunderstorm days, both in the warm air masses ahead of storms and within the cool outflow of storms, where MOST assumptions (e.g., horizontal homogeneity and a steady state) are least credible. In these regions, it is found that the nondimensional vertical wind shear near the surface commonly exceeds predictions by MOST. The departures from MOST have implications for the specification of the lower boundary condition in numerical simulations of convective storms. Documenting departures from MOST is a necessary first-step toward improving the lower boundary condition and parameterization of near-surface turbulence (“wall models”) in storm simulations.