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Tornadogenesis: Our Current Understanding and Unanswered Questions for VORTEX2The Second Verification of the Origins of
Rotation in Tornadoes Experiment
Photo by Herb Stein
Yvette Richardson Penn State University
AMS MeetingKansas City, MO12 February 2009
VORTEX2
• Second highly coordinated field phase of an ongoing, broad investigation of tornadogenesis, tornado structure, and the relationship between tornadoes, their parent thunderstorms, and the larger-scale environment
• Dates: 10 May – 13 June 2009; several weeks in 2010
• Steering Committee: – Howie Bluestein (University of Oklahoma) – Don Burgess (Cooperative Institute for Mesoscale Meteorological Studies)– David Dowell (National Center for Atmospheric Research)– Paul Markowski (Penn State University)– Erik Rasmussen (Cooperative Institute for Mesoscale Meteorological Studies) – Yvette Richardson (Penn State University)– Lou Wicker (National Severe Storms Laboratory)– Josh Wurman (Center for Severe Weather Research)
VORTEX2
• Four foci – tornadogenesis– near-ground wind field in tornadoes– relationship between tornadoes, their parent
thunderstorms, and the larger-scale environment
– storm-scale NWP, supercell predictability
Pre-VORTEX1 Methods
• Visual observations
• Fixed site instrumentation
• Numerical simulations
• Limited mobile assets
Pre-VORTEX1 Understanding
• Virtually all violent and most strong tornadoes are associated with supercells
• Supercells are defined by rotational signatures (mesocyclones) generally identifiable on radar
Doug Raflik Steve Hodanish
ObjectivesBasic Structure of a Supercell
reflectivity
after Lemon and Doswell (1979)Eric Nguyen
What we knew before VORTEX1
• Supercells acquire net cyclonic rotation aloft by tilting environmental streamwise (horizontal) vorticity in the updraft
Importance of ambient horizontal vorticity
from COMET
pre-existing vertical vorticity at the surface
initially negligible vertical vorticity at the surface
zeroth order assumption: no baroclinity or viscous effects; vortex lines are material lines
What we knew before VORTEX1 The importance of a downdraft
Roberto Giudici
Courtesy of Dave Blanchard
NSSL archive photo
Courtesy of Dave Blanchard
Horizontal vorticity also can be generated baroclinically by buoyancy gradients associated with a storm’s cool outflow, and the tilting of this vorticity can be important in the development of low-level rotation in supercells.
What we knew before VORTEX1Importance of storm-generated horizontal vorticity
shaded and contoured
Vertical cross-section across gust front
• I (outer region): flow here conserves its angular momentum, therefore it spins faster as it approaches the axis
• II (core): extends from the axis to the radius of max wind
• III (corner): flow becomes primarily vertical from horizontal
• IV (boundary layer): radial inflow contracts the vortex, yielding intense winds
• V (parent updraft): updraft aloft
Laboratory vortex structure depends strongly on swirl ratio
What we knew
before VORTEX1
Basic tornado structure
Davies-Jones 1986
What did VORTEX1 add?
• Mobility!– mobile soundings– mobile mesonets – mobile radars
(mainly airborne)
Summary of VORTEX1 Findings
• Modification of attitudes toward low-level mesocyclones
– very large fraction of supercells contain circulations at low levels, and probably even at the surface
• The thermodynamic properties of the downdrafts may exert some control on tornado formation, intensity, and longevity
– warmer downdrafts associated with tornadoes
• Increased awareness of the sensitivity of supercells to the near-storm environment
– storm-boundary interactions – mesoscale variability away from obvious mesoscale
boundaries
v’
v’
Shabbott & Markowski (2006)
Summary of VORTEX1 Findings
• Striking kinematic similarities between tornadic and nontornadic supercells on the mesocyclone scale
nontornadic tornadic
Smaller field campaigns after VORTEX1
• mobile dual-Doppler intercepts
• tornado-scale radar observations
• mobile mesonet intercepts
Marquis et al. (2008)
tornado
gust fronts
Bluestein et al. (2003) Wurman (2002) Lee et al. (2004)
What are the biggest unanswered questions in the study of tornadogenesis?
nontornadic nontornadic
tornadic nontornadic
Why do storms with seemingly similar structure differ in their tornado production?
What are the biggest unanswered questions in the study of tornadogenesis?
Wakimoto et al. (2004)
tornadic
tornadic
nontornadic
Why are the most intense mesocyclones not necessarily the ones most likely to be associated with tornadogenesis?
A
C
Wakimoto and Fujita (1981)
DOW1 image from 2 June 1995
Tornadoes and low-level mesocyclones are virtually always found to have anticyclonic vorticity on the opposite side of the hook echo/RFD—such vorticity couplets suggest that the tilting of vorticity by a downdraft is important.
purely barotropic process
purely baroclinic process
neg
ativ
e b
uo
yan
cyno baroclinic vorticity generation; vortex lines are frozen in the fluid; vortex lines passing through low-level vorticity maximum form U’s (not observed).
observed case: vortex lines passing through low-level vorticity maximum form arches
Straka, J. M., E. N. Rasmussen, R. P. Davies-Jones, and P. M. Markowski, 2007: An observational and idealized numerical examination of low-level counter-rotating vortices toward the rear flank of supercells. E. J. Severe Storms Met., 2(8), 1-22.
Markowski, P. M., J. M. Straka, E. N. Rasmussen, R. P. Davies-Jones, Y. Richardson, and J. Trapp, 2008: Vortex lines within low-level mesocyclones obtained from pseudo-dual-Doppler radar observations. Mon. Wea. Rev., 136, in press.
The fact that baroclinic vorticity generation seems to be important does not necessarily imply that TG becomes increasingly likely as the baroclinity (and cold pool strength) increase!
What role, if any, dosecondary gust fronts play?
Marquis et al. 2008
RFD
Wurman et al. 2007
What are the biggest unanswered questions in the study of tornadogenesis?
Lewellen et al. (1997,2000,2004)
What are the biggest unanswered questions in the study of tornadoes?
• How valid is our understanding of the corner flow region and low-level structure of tornadic flow, which is based on laboratory studies, numerical simulations, and very limited observations?
Wurman and Alexander (2004)
What are the biggest unanswered questions in the study of tornadoes?
What is the relationship between observed winds and structural damage?
1 May 2008
Storm-environment interactions(on days when storm motion or road network is not conducive to tornadogenesis mission)
2 June 1995
cloud shadow
VORTEX2
• Four foci – tornadogenesis– near-ground wind field in tornadoes– relationship between tornadoes, their parent
thunderstorms, and the larger-scale environment
– storm-scale NWP, supercell predictability
What is needed?• Integrated wind and thermodynamic data with
high temporal resolution covering spatial scales from tornado to whole storm
• Thermodynamic observations above the ground– Previously limited to a paucity of direct
measurements made by soundings and suspect indirect observations from retrievals based on Doppler wind syntheses
• Dual-polarization measurements to characterize hydrometeors, particularly in the rear-flank region
• Observations in the corner flow region
VORTEX2 Participating Instruments
• Radars:– 2 X-band DOW mobile radars– 2 C-band SMART-Radar mobile radars– Rapid-scan X-band DOW mobile radar– NOXP, X-band, dual-pol mobile radar– W-band UMASS mobile radar– XPOL, X-band, dual-pol mobile radar
• Mobile mesonets• Mobile soundings• Stick Net• In situ tornado probes• UAV systems• Photography units• Disdrometers
• DOW6• DOW7• DOW5 (“Rapid-DOW”)• UMass W-band• UMass XPOL• Smart-Radar 1• Smart-Radar 2• NOXP
Remote Sensing
In situ measurements
Tornado in-situ probes
Stick Net
Mobile Mesonets
Rawinsondes
Disdrometers
VORTEX2 Unmanned Aerial Systems
Must identify an aircraft suitable for supercell operations.
Must identify takeoff/landing sites
Must have pilots and spotters for each aircraft who have passed FAA medical exam requirements.
Must have aircraft in sight.
Certificate Of Airworthiness is uncertain.
In 2009, will only be used in a subset of V2 domain.
Nested deployment strategy
If possible, take advantage of fixed observing systems in OK
Situational Awareness Display
Will include mobile mesonet data, radar data, roads, and terrain
Available over the mobile digital network
Vortex-2 Operations
Center (VOC)
•Colocated in the Hazardous Weather Testbed (HWT) in Norman National Weather Center•Provides forecasting (both day 1 & 2) support for field operations•Provides backup support to field operations to teams out of contact •Interfaces between Spring Fcst Experiment, SPC, and VORTEX-2•Relays VORTEX-2 reports to NWS field offices •Helps Vortex-2 logistics coordinator in Boulder •Safety net for field teams having difficulties (vehicle issues, health problems, etc.)
Summary
• Decades of severe storms research has resulted in conceptual models of tornado structure and genesis.
• Many of these conceptual models cannot be fully evaluated with existing datasets.
• VORTEX2 will provide the opportunity to collect a comprehensive dataset of wind and thermodynamic measurement with high spatial and temporal resolution spanning several scales of motion.