MesoscaleM. D. Eastin Tornadoes QT Movie. MesoscaleM. D. Eastin Tornadoes Significant Events in U.S....

Mesoscale M. D. Eastin

Tornadoes

QT Movie


TornadoesSignificant Events in U.S. History

The Fujita Scale

U.S. Tornado Climatology

Mesoscale Observations

• Scales of Motion• WSR-88D look at the 3 May 1999 Oklahoma City tornado• Damage Patterns

Tornado Structure

• Core Observations• Conceptual Model of Air Flow

Tornadogenesis

• Supercell Tornadoes• Non-supercell Tornadoes

Tornado Forecasting


The Tri-State Tornado:

• Occurred on 18 March 1925

• 695 confirmed fatalities (deadliest in US history)

• Damage suggests F5 intensity• Over 15,000 homes destroyed

• Continuous 219 mile track• Current thought is that it was actually a family of tornadoes spawned by the same storm

Significant Tornado Events

Griffin, IN


The Super Outbreak:

• Occurred on 3 April 1974

• Total of 148 tornadoes in 13 states (Most on one day in U.S. history)

F5=7 F4=24 F3=35

• 315 confirmed fatalities• Over 5,000 people injured• Severely damaged over 900 sq miles


Parker City, IN (21)

F4



48 confirmed fatalities$1.5 billion in damages

3 May 1999 Oklahoma Outbreak


The Fujita-ScaleEstimating Tornado Intensity:

• Developed by Dr. Ted Fujita in 1971• Updated in 2007

• Designed to bridge the gap between the Beaufort and Mach scales

• Note that there are more than 6 F-scale categories in the original scale


The Fujita-ScaleDamage Examples:

• To date, no instrument has proven reliable enough to accurately (and regularly) measure the maximum wind speeds within tornadoes.

• The determination (an estimate) of any tornado’s intensity is always done via post-storm damage surveys of the area

Important Note:

Tornado “intensity” is a function of building construction quality, whether any buildings were even damaged, forward motion of the tornado, as well as many other factors…

F0 F1

F2 F3

F4 F5


The Enhanced Fujita-ScaleA Re-evaluation of Tornado Intensity in the Modern World:

• Based on damage to a single family house using traditional construction practices (i.e. modern quality and design), assuming the house was in compliance with common building codes and was regularly maintained

In general, lower winds speeds are required to produce the same damage

• Officially adopted by the NWS for use beginning February 1, 2007

• More information can be found at: http://www.spc.noaa.gov/faq/tornado/ef-scale.html


Tornado Climatology

From Brooks et al. (2003)

Tornado Alley:


Tornado Climatology


Annual Cycle:


Tornado Climatology


Also see: http://www.nssl.noaa.gov/hazard/tanim/torw8099.html

Seasonality:


Tornado Climatology


Number of Tornado Reports:

• The upwards trends are believed to be artificial

• The trend likely reflects:

• An increase in population density• Improved reporting procedures• Organized networks of “storm spotters”


Tornado ClimatologyDeath:

• Annual total number of deaths has steadily decreased over the last 50 years

• During the period 1950-1999: 4,460 total deaths (average of 89 per year) 40,522 total tornadoes (average of 810 per

year)

Damage:

• Survey of damage from all tornadoes in the period 1950-1995 (after an adjustment for wealth and inflation)

• Total Damage: $19.3 billion• Annual Average: $0.42 billion

• Survey of adjusted damage from only major (F4-F5) tornadoes during the same period

• Account for 2.3% of all tornadoes• Total Damage: $10.2 billion• Annual Average: $0.22 billion


Mesoscale ObservationsMultiple Scales of Rotation:

Mesocyclone: 2000-7000 m in diameter – Most often detected by NWS Doppler radar

Tornado: 100-1000 m in diameter – Rarely observed by NWS Doppler radar (TVS) and never by ASOS (more on this later…)

Suction Vortices: 1-50 m in diameter – Recently observed by high-resolution Doppler radar


Mesoscale ObservationsMultiple Scales of Rotation: Mid-level Mesocyclones

• Persistent rotation observed in supercells by NWS Doppler radar (automated algorithms)• Typical altitudes → 2-7 km AGL Less than 25% of radar-detected mid-level mesocyclones produce tornadoes

NEXRAD-88D RadarOklahoma City

3 May 1999

Mesocyclone Automated Detection Algorithm:

Looks for quasi-symmetric mesocyclonic circulations (large horizontal shears) thatvertically correlate through a >3.5 km depthand are persistent for >10 minutes

Mesocyclonesdetected by the

automated algorithm


Multiple Scales of Rotation: Low-level Mesocyclones

• Altitude = 1-2 km AGL Associated with hook echo Wall cloud

• Often difficult to detect by NWS Doppler radars if more than 50 km from the radar (due to non-zero beam elevation angles and Earth’s curvature)

• If detected, probability of a tornado increases.

Less than 40% of radar-detected low-level mesoscyclones produce a tornado

From Wakimoto et al. (2003)


Storm-relative windsRadar reflectivity

Storm-relative windsRadar reflectivityDoppler radial velocity

Storm-relative windsVertical vorticityVertical motion

Radarbeams

Airborne Doppler

Synthesis(800m AGL)


Multiple Scales of Rotation: Tornado Vortex Signature (TVS):

• Historically, a tornado has shown up on operational NWS radars as a region of enhanced gate-to-gate (adjacent beams) horizontal shear

• When the horizontal shear exceeds some criteria, a TVS is identified

Note: The NEXRAD WSR-88D radar can not resolve a tornado’s circulation.

An identified TVS is highly suggestive that a tornado is present

Not all reported tornadoes are associated with a radar-detected TVS (~60%)

Not all storms with a radar-identified TVS produce a tornado (~80%)

TVS(possible tornado)


Mesocyclone


3 May 1999, Oklahoma City Tornado


Tornado Damage Patterns

From Wakimoto and Atkins (1996)


Photogrammetric Studies:

• Use multiple photographs to diagnose structure and air flow patterns

• Pioneered by Ted Fujita in the 1960s

• Must know many details as a function of time:

• Camera location• Tornado location• Time of each photo• Camera / film specifics

• Assumes visible “features” move with the local wind

(Is this a good assumption?)

Tornado Core Observations


Totable Tornado Observatory (TOTO):

• Developed by Dr. Howard Bluestein (Univ. Oklahoma) and his graduate students in the early 1980s• Designed to record basic surface observations inside a tornado vortex

• Never successfully deployed

• Motivation for a popular movie?


From Bluestein et al. (1983)


From Lee et al. (2004)

Data from anF4 Tornado

Hardened In-Situ Tornado Pressure Recorder (HITPR):

• Developed and deployed by the annual TWISTEX Project since 2003 (http://en.wikipedia.org/wiki/TWISTEX)

• Designed to record basic surface observations inside a tornado vortex• Successfully deployed in multiple tornadoes



Doppler on Wheels (DOWs) :

• Vehicle-mounted Doppler radars can get very close (sometimes too close) and resolve the circulation

• Multiple radars deployed each year (most recently during VORTEX-2)

From Wurman et al. (1997)



Doppler on Wheels (DOWs) :

• Suctions vortices have been observed (and photographed) by storm chasers for decades

• The DOWs have recently provided the first direct quantitative observations of “suction vortices”

(http://www.cswr.org/)

FromWurman(2002)



From Gallus et al. (2005)

Tornado Vortex Chambers:

• Create artificial tornadoes in laboratories• Primary source of quantitative information before the DOW radars (pre-1990s)

• Two important parameters in a vortex chamber

Γ = Circulation of the flow about the central axis Q = Rate of air flow through the chamber top

• The ratio of Γ to Q is called the swirl ratioswirl ratio (S):

Tornadoes form in vortex chambers when the swirl ratio is large

w

v

Q

rS to

2



Tornado Vortex Chambers:

• When the swirl ratio is very small (a), no vortex develops at the surface (notice the descending motion near the axis of rotation)

• As the swirl ratio is increased (b), a vortex develops at the surface (note the inflow and updraft just above the surface much like a tornado). This is called a one-cell vortex (one updraft)

• As the swirl ratio further increases (c), a downdraft develops along the central axis, producing a cloud–free, or “hollow”, center to the tornado (which is often observed by storm chasers)

• At very large swirl ratios (d), the downdraft penetrates to the surface and creates a two-celled vortex (with two updrafts). This results in multiple suction vortices (e) (as observed in nature)



Five Flow Regions and Radial Pressure Profile:

Conceptual Model of Air Flow

Outer Region (I):Inward spiralingair that conserves angular momentum(spins faster as it approaches the tornado axis)

Core Region (II):Inside the maximumwinds, including thefunnel cloud, dust,and debris.(cyclostrophic balance)

r

p

r

v

12

Corner (III):Region where air turns upward frombeing horizontal flow to primarilyvertical flow

Boundary Layer (IV):Flow interacts withground and surfacefriction enhancesthe radial inflow

Rotating Updraft (V):Parent updraft andmesocyclone

Pressure profile:Assumes an idealized vortexstructure in order to relate theflow field to the radial pressuregradient: Rankine Vortex

Burgers-Rott Vortex

2maxmin vp


Supercell TornadogenesisNot well understood!

Two current theories have considerable observational and numerical modeling support Both theories may work in concert Each assumes the following circulations are present in the parent supercell:

• Mid-level mesocyclone generated by tilting and stretching of horizontal vorticity• Low-level mesocyclone generated by tilting and stretching baroclinically-enhanced streamwise vorticity within the vortical updraft• Mature forward and rear-flank downdrafts and their associated gust fronts

Updraft

FFD

RFD

Mid-levelFlow

Inflow

Upper-levelFlow

PrimaryUpdraft

HorizontalVorticityVectors

Inflow alongthe gust front

acquiresstreamwise

vortcity


Negligible Vertical Vorticity at the Surface: Downdraft Required

• Simple tilting of low-level horizontal vorticity by the primary updraft cannot produce vertical vorticity at the surface since the air rises away from the surface during tilting (top scenario)

However, if an adjacent downdraft (i.e. the RFD) is involved in the tilting process, then vertical vorticity can be advected toward the surface (during titling) and subsequently stretched into a tornado (bottom scenario)

Barotropic contribution

Supercell Tornadogenesis


Negligible Vertical Vorticity at the Surface: Downdraft Required

The near-surface horizontal vorticity can be enhanced when the RFD is driven by negative buoyancy

• Recall, horizontal buoyancy gradients produce horizontal vorticity:

The downward advection of any such horizontal vorticity will increase the total available horizontal vorticity to be tilted toward the surface and then stretched into a tornado

Baroclinic contribution

Mechanism produces tornadoes soon after RFD reaches surface


x

B

t

B-


Ample Vertical Vorticity at the Surface: NO Downdraft Required

Strong horizontal shear located along RFD or FFD gust fronts produce large near-surface vertical vorticity

Storm-relative inflow slowly advects any vertical vorticity “pockets” along the gust fronts toward the primary mesocyclonic updraft where stretching and low-level convergence increase the near-surface vertical vorticity, producing a tornado

Mechanism produces tornadoes before RFD reaches the surface or in between RFD “surges”



DOW Radial Velocity Observations

MesocycloneCenter

Gust FrontVortices

Numerical Simulation

Mesocyclone

Gust FrontVortices

Supercell TornadogenesisAmple Vertical Vorticity at the Surface: NO Downdraft Required



Numerical Simulation Movie #1(A Top View)

Numerical Simulation Movie #2(A Surface Observer View)


Non-Supercell TornadogenesisEven Less Understood!!!

• Often occurs along low-level lines of horizontal shear and convergence (e.g. gust fronts and air-mass boundaries)

• Large pre-existing low-level vertical vorticity is stretched by the updrafts of ordinary growing cumulus clouds

• Produces weak, short-lived tornadoes (EF0 - EF2)


Tornado ForecastingContinuous monitoring of ALL available observations:

Use real-time radar data to monitor storm formation and evolution Use surface observations to monitor storm-relative inflow and cold pool characteristics Use nearby soundings (rawinsondes and rapid-update numerical models) to monitor standard forecast parameters (CAPE, SREH, EHI, etc.)

Other useful forecast parameters:

Vertical Shear 0-1 km AGL:

• Large values favor tornadoes• Strong shear implies large horizontal vorticity near the surface that can be tilted into the vertical by updrafts and downdrafts (especially the RFD)

Mixed-layer LCL:

• Small values favor tornadoes• Moist boundary layers limit negative buoyancy in downdrafts and prevent strong cold pools from “under-cutting” the primary updraft (see next slide…)


Tornado Forecasting

Weak Cold Pools

Moderate Cold Pools

Strong Cold Pools

Surface Density Potential Temperature Perturbations(observed by mobile mesonets during VORTEX)


TornadoesSummary:

Significant Events in U.S. History → Why are they significant?

The Fujita Scale → Basic concept and reason for recent changes

U.S. Tornado Climatology → Basic characteristics and trends


• Scales of Motion → Ability / Methods used to observe each scale• Damage Patterns → Basic structure and reasons for such structure

Tornado Structure

• Core Observations → Various methods and laboratory results• Conceptual Model of Air Flow → Basic characteristics of each region

Tornadogenesis

• Supercell Tornadoes → Important physical processes (and when)• Non-supercell Tornadoes → Important physical processes

Tornado Forecasting → Methods and additional useful parameters


ReferencesAgee, E. M., J. T. Snow, and P. R. Clare, 1976: Multiple vortex features in a tornado cyclone and the occurrence of tornado

families. Mon. Wea. Rev., 104, 552-563.

Atkins, N. T., J. M. Arnott, R. W. Przybylinski, R. A. Wolf, and B. D. Ketchum, 2004: Vortex Structure and Evolution within Bow Echoes. Part I: Single-Doppler and Damage Analysis of the 29 June 1998 Derecho. Mon. Wea. Rev., 132,

2224-2242.

Bluestein, H. B., 1980: The University of Oklahoma Severe Storms Intercept Project – 1979. Bull. Amer. Meteor. Soc., 61, 560-567.

Bluestein, H. B., 1983: Surface meteorological observations in severe thunderstorms. Part II: Field experiments with TOTO.J. Climate Applied Meteor., 22, 919-930.

Bluestein, H. B., 1999: A history of severe storms intercept field programs. Wea. Forecasting, 14, 558-577.

Brooks, H. E, C. A. Doswell, and M. P. Kay, 2003: Climatological estimates of local daily tornado probability in the United States. Wea. Forecasting, 18, 626-641.

Burgess, D. W., and L. R. Lemon, 1990: Severe thunderstorm detection by radar. Radar in Meteorology. D. Atlas, Ed., Amer. Meteor. Soc., 619-647.

Davies-Jones, R., 1986: Tornado dynamics. Thunderstorm Morphology and Dynamics, 2nd ed, E. Kessler, Ed., University of Oklahoma Press, 197-236.

Fujita, T.T., 1981: Tornadoes and downbursts in the context of generalized planetary scales. J. Atmos. Sci., 38, 1511-1534.

Gallus, W. A., Jr., C. Cervato, C. Cruz-Neira, G. Faidley, and R. Heer, 2005: Learning storm dynamics with a virtual thunderstorm. Bull. Amer. Meteor. Soc., 86, 162-163.

Klemp, J. B., 1987: Dynamics of tornadic thunderstorms. Ann. Rev. Fluid Mech., 19, 369-402


ReferencesKlemp, J. B., and R. Rotunno, 1983: A study of the tornadic region within a supercell thunderstorm. J. Atmos. Sci.,

40, 359-377.

Lee, B. D., and R. B. Wilhelmson, 1997: The numerical simulation of nonsupercell tornadogenesis. Part II: Evolution of a .family of tornadoes along a weak outflow boundary. J. Atmos. Sci., 54, 2387-2415.

Markowski, P. M., E. N. Rasmussen, and J. M. Straka, the occurrence of tornadoes in supercells interacting with boundaries during VORTEX-95. Wea. Forecasting, 13, 852-859.

Rotunno, R., 1986: Tornadoes and tornadogenesis. Mesoscale Meteorology and Forecasting, P. S. Ray, Ed., Amer. Meteor. Soc., 414-436.

Trapp R. J., and R. Davies-Jones., 1997: Tornadogenesis with and without a dynamic pipe effect. J. Atmos. Sci., 54, 113-133.

Wakimoto, R. M. and N. T. Atkins, 1996: Observations on the origins of rotation: The Newcastle tornado during VORTEX-94. Mon. Wea. Rev., 124, 384-407.

Wakimoto, R. M., and J. W. Wilson, 1989: Non-supercell tornadoes. Mon. Wea. Rev., 117, 1113-1140.

Wakimoto, R. M., C. Liu, and H. Cai, 1998: The Garden City, Kansas storm during VORTEX-95. Part I: Overview of storm’s lifecycle and mesocyclogenesis. Mon. Wea. Rev., 126, 372-392.

Wakimoto, R. M., H. V. Murphey, D. C. Dowell, and H.B. Bluestein, 2003: The Kellerville tornado during VORTEX: Damage survey and Doppler radar analyses. Mon. Wea. Rev., 131, 2197-2221.

Wicker, L. J., and R. B. Wilhelmson, 1995: Simulation and analysis of tornado development and decay within a three-dimensional supercell thunderstorm. J. Atmos. Sci, 52, 2675-2703.

Wurman, J., 2002: The multiple-vortex structure of a tornado. Wea. Forecasting, 17, 473-505.

Wurman, J., J. M. Straka, E. N. Rasmussen, M. Randall, and A. Zahari, 1997: Design and deployment of a portable, pencil beam, pulsed, 3-cm Doppler radar. J. Atmos. Oceanic. Technol., 14, 1502-1512.

MesoscaleM. D. Eastin Tornadoes QT Movie. MesoscaleM. D. Eastin Tornadoes Significant Events in U.S....

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