Severe Weather: Tornadoes Harold E. Brooks NOAA/National Severe Storms Laboratory Norman, Oklahoma

Definition Tornado-violently rotating column of air in contact with the ground associated with a thunderstorm or developing thunderstorm –Waterspout-tornado over water –Intensity measured by damage with Fujita scale (F0-F5)

US Averages Mean annual damage~$500M –Highly variable –Expect a $1B tornado once per decade Expected death toll~40 per year

Regression slope=14/year

2008 Max, min 10 th, 90 th %iles 25 th, 75 th %iles Median

Permanent Homes Mobile Homes

Changes on individual days Preliminary reports are what comes in within a few days Final reports –Bad data –Late reports come in (+) –Multiple reports of same tornado (-)

T=86 W=129 H=411

T=57 W=125 H=593

m=-0.1 m=-0.4

Fujita scale Rates tornado damage from 0 to 5 Tornadoes in US rated by maximum damage point

Wurman and Alexander (2004) Relationship between tornadic winds and damage (more than just a function of F-rating—damage is a complex function of wind duration, acceleration, direction, etc.)

F2+ Tornadoes ( )

nontornadic tornadic

June 24, 2003 near Manchester, SD (Courtesy of Tim Samaras) In situ observations in tornadoes

“Ingredients” for severe thunderstorms-the supercell Thunderstorms –Low-level warm, moist air –Mid-level (~2-10 km) relatively cold, dry air –Something to lift the warm, moist air Organization –Winds that increase and change direction with height over lowest few km –From equator at surface, west aloft

Tornadogenesis Deep, weak rotation starts –Environmental wind shear Rotation near surface starts –Probably due to evaporation of rain → temperature gradients → rotation Rotation intensifies (ice skater effect?)

Environmental conditions Significant severe thunderstorms (10% biggest events, including F2+ tornadoes) –More consistent in observations, more easily distinguished

Environment-Event Relationships Ingredients based –Define events in terms of environmental conditions Storm “strength”-CAPE or Wmax Organization-0-6 km wind shear Initiation?

Datasets Storm Prediction Center severe weather reports (1991-9) Environmental conditions from “reanalysis” –1.9x1.9 lat/lon spacing, every 6 hours –3.75 million soundings Focus on significant severe thunderstorms Smooth using Gaussian kernel

Significant Severe Weather Locations ( )

Shear Wmax CAPE

Wmax CAPE

Hail Wind Tornado Conditional Probability of Events Given Any Significant Event

Interannual Variability of Environments Departures from long-term average Look at patterns of more and less than normal

Hail (3 in) Wind (75 kt) Tornado (F3) Conditional Probability of Really Big Events

Climate model simulations Three main groups (so far) –GISS (parameterized updraft) –Oklahoma/Melbourne –Purdue Look at favorable conditions –Concentrate on changes in model world

(Del Genio et al., 2007) Changes in Environments in Doubled CO2 (GISS)

(Marsh et al., 2009)

Convective Initiation Biggest frequency change in environments in summer Initiation less probable in summer for given environment

Trapp et al. (2008) Regional Analyses

S06 (m/s) CAPE N DSEV,P (days)

Summarizing Trapp Statistically significant increases in CAPE, decreases in shear Severe weather days increase in all regions, not significant in Southern Great Plains

What do we know? Reports aren’t very reliable Wind vs. hail/tornado Intensity is function of shear Future –Favorable severe environments may go up –Frequency increase for wind more likely –Intensity distribution may shift slightly –Initiation is still a question