TORNADO RESEARCH
COLLABORATIVE
PROPOSAL: Challenges in Understanding Tornadogenesis and Related
Phenomena
Collaborators: Jerry M.
Straka, Erik N. Rasmussen
On the role of descending rain
curtains in tornadogenesis: Amanda Kis, Jerry M. Straka,
and Katharine M. Kanak
The archetypal tornado vortex signature (TVS) descends from
aloft toward the surface, preceding tornadogenesis by up to tens of
minutes. However, descending TVSs account for only about half of all
verified tornadoes also observed by Doppler radar. Non-descending TVSs
comprise the other half, and form either simultaneously below cloud
base or from the ground upwards. They are observed nearly concurrently
with tornadogenesis, greatly reducing warning lead-time. Non-descending
TVSs form in environments with vertical vorticity and radial inflow
either constant beneath cloud base, or maximized at the surface.
We demonstrate a mechanism in which the rear flanking downdraft (RFD)
and hook echo advect sufficient vertical vorticity at low levels to
form a tornado strength vortex. These experiments are an extension of a
previous study on a purely barotropic mechanism of tornadogenesis.
Using Beltrami flow as the initial condition we simulate a mature
mesocyclone, with a cyclonically rotating updraft maximized at
midlevels surrounded by an anticylonically rotating downdraft. We
perturb the flow from Beltrami by releasing hydrometeors above the
updraft, which then spread out to the edges of the updraft in the
divergent flow and descend. The hydrometeors fall through the downdraft
as a rain curtain, representing the hook echo and transporting angular
momentum to the surface. Near the surface, the hydrometeors converge
inwards, producing and maximizing both vertical vorticity and radial
inflow at the lowest levels as well as tangential wind. As there is no
buoyant force sustaining the storm updraft, modeled storms collapse
after tornadogenesis due to the downward-directed non-hydrostatic
pressure gradient force associated with the low-level vortex. Thus,
this model design can represent tornadogenesis but not tornado
maintenance.
In a series of three-dimensional experiments, a circular plane of
hydrometeors is released above the updraft. The maximum mixing ratio of
the hydrometeors is varied between 1 g/kg to 9 g/kg in order to analyze
the effect of varying precipitation amounts on low-level convergence of
vertical vorticity and tornadogenesis. The results have implications
for tornadogenesis in low precipitation (LP), classic (CL), and high
precipitation (HP) supercells.
Many tornado simulations documented in the existing literature have
been performed using axisymmetric models. We increase realism in a
series of three-dimensional experiments with specified asymmetric rain
curtains. In these experiments, the maximum mixing ratio of the
hydrometeors is set to 5 g/kg, and the plane of hydrometeors released
above the updraft is successively reduced by one-eighth slices. In
axisymmetric simulations, horizontal vorticity cannot be tilted into
the vertical. However, the asymmetric design of some of these
experiments allows horizontal vorticity to be tilted vertically. The
evolution of vertical vorticity in these experiments is analyzed in
order to gain insight into the dynamics of tornadogenesis.
Nocturnal tornadoes and
low-level static stability: Amanda
Kis, Jerry M. Straka, and Katharine M. Kanak
Nocturnal tornadoes, while comprising only about a quarter of
verified tornadoes, produce a disproportionate percentage of tornado
fatalities. Despite their significance, there is little pre-existing
literature on this subject. It seems possible that the dynamics of
tornadogenesis would vary significantly over the diurnal cycle. For
example, the depth and nature of storm inflow may change as the daytime
boundary layer transitions into the nighttime stable layer.
A climatology of significant (F2 – F5) nocturnal (occurring between 3
UTC – 13 UTC) tornadoes in the contiguous United States from 2004 –
2006 reveals that the majority of cases occurred in squall
line-embedded supercells or broken lines of supercells. Over half of
the cases occurred during the cool season, mostly in Gulf Coast states.
The location of the maximum number of cases shifted to the southern and
central Plains during spring. Only one in 69 tornadoes in the
climatology occurred during summer.
Several modeling studies have shown that increasing low-level vertical
static stability impedes tornadogenesis. However, Rapid Update Cycle-2
(RUC-2) model proximity soundings were gathered for cases in the
climatology, and suggest that over half of the tornadoes in the
climatology formed within a stable boundary layer. We will simulate in
a series of numerical modeling experiments mature supercells in
environments of varying low-level vertical static stability, and
examine the formation of tornado strength vortices.
Tornadic Supercells in
Landfalling Hurricanes: Owen Shieh, Jerry M. Straka,
Katharine M. Kanak
Very high resolution numerical simulations of landfalling
hurricanes are being conducted in order to attempt to identify
supercell storms and examine their formation dynamics.
PARTICIPATION
IN VORTEX2
Development of Unmanned Aerial
System for Research in a Severe Storm Environment and Deployment within
the Verification of the Origins of Rotation in Tornadoes Experiment 2
Collaborators:
Erik M. Rasmussen, Brian
Argrow, Jerry
M. Straka, Adam
Houston, Eric Frew and Katharine
M. Kanak
This work is supported by NSF ATM 0446509 and ATM
0733539, NSF ATM-0823794