Does climate change impact supercell tornadogenesis?

By Alexis HoffmanSURE Fellow

Washington University in St Louis Research at NC State University Summer 2010

Main questions:

• How might something so large as climate change impact something as small as a tornado?

• Will future warming affect supercells?– If so, can we discern an affect on

tornadogenesis?

Why does this matter?

– Supercells produce: • Average of 1,500 tornadoes/year (NOAA)

– $1.1 billion in damages/year– Over 80 deaths/year

• Damaging straight-line winds• Severe hail/rain events

What parameters matter in severe weather?

• Vertical wind shear

• Thermodynamic instability– Warming with increased moisture content – Known with more certainty in climate models

Hypothesis

– �Based strictly on the thermodynamics, future storm severity will increase.• Stronger updraft (vertical velocity)• Faster rotation (vertical vorticity)

Supercells

• Favorable environment:– Warm moist air lies

below cold dry air– Large wind shear– Moderate capping

inversion

• Mesocyclone (deep rotating updraft)

Image Credit: http://www.nssl.noaa.gov/primer/tornado/images/tor_formation_lg.jpg

Tornadoes

-Tornadogenesis: -S t r e t c h i n g *-Tilting-Baroclinic Generation

Image Credit: http://www.stormnetwork.net/56/a-tornado-sweeping-through-costa-ballena-greenhouses-and-trees/

Methods: Climate• A1B Scenario • CCSM3 Monthly Data• Averaged March and April:

– Temperature and Relative Humidity• Averaged over area:

– Oklahoma• Averaged over times:

– “20C” = 1990-1999– “A1B” = 2090-2099

Idealized Supercell SoundingsFuture

CAPE: 3431 J/kg

T(981mb) = +3.4K

CurrentCAPE: 2444 J/kg

Weather Research & Forecast Model

• High-resolution (2km) mesoscale model– Input:

• current and future idealized supercell sounding

• Current sounding from Weisman & Klemp 1982

• Quarter-circle shear hodograph– Ideal for mesocyclone formation

• Ran 8 microphysics schemes

• NCL for graphics and statistics

Maximum Updraft(300m)Images from Kessler-type parameterization, not averages

20C: 70 Minutes A1B: 70 Minutes

MAX = 2.5 m/s MAX = 3.8 m/s

Maximum Updraft

% CHANGE:∆(300) = 25%

∆(750) = 24%

∆(1500) = 18%t = 40-120 mins

p < .01

Maximum Vertical Vorticity (750m)Images from Kessler-type parameterization, not averages

20C: 80 Minutes A1B: 80 Minutes

MAX = 2.9x10-5 s-1 MAX = 3.3x10-5 s-1

Maximum Vertical Vorticity

% CHANGE∆(750) = 41%

∆(1500) = 22%

t = 60:120p < .01

Rotational UpdraftVertical Vorticity * Updraft

~SPC Severe Weather Parameter

% CHANGE• ∆(300) = 112%• ∆(750) = 89%• ∆(1500) = 55%

t = 60:120 mins

p < .01

Maximum Downdraft Speed (4000m)

% CHANGE• ∆(4000) = -31%• ∆(300) = -10%

t = 60:120 mins• Stronger downdraft speed

= stronger RFD

p < .01

Vorticity Evolution

As low vorticity increases, pressure decreases

Pressure drop retards updraft, maximum vorticity to shifts to region between updraft and downdraft

Future simulation is consistent with tornadogenesis signature

Changing the thermodynamics led to:

Stronger rotational updrafts, for a given wind shear Increased stretching

Low-Level/Mid-Level Vorticity SignatureVortex splitChanges are statistically significant

Conclusions

Further Research:

Isolate climate effects with a case study

If (average) wind shear decreases, as it should, will it affect vertical vorticity enough to dampen the thermodynamic effects?

Will hail production significantly change?Will it rain much more?

Given a number of supercells, will the percentage that have tornadoes increase? If a tornado happens in the future, is it going to be stronger?

Approach: Use ensemble of climate modelsUse a higher resolutionUse additional IPCC scenariosUse a past outbreak

Acknowlegements

• GCEP Program• Gary Lackmann• Kevin Hill• Bryce Tyner• Joseph Zambon• Briana Gordon• NC State University (MEAS)

CitationsBrandes, E. A., 1978: Mesocyclone evolution and tornadogenesis: Some observations.

Mon. Wea. Rev., 106:995Š1011. Brooks, H. E., C. A. Doswell III, and J. Cooper, 1994: On the environments of tornadic

and nontornadic mesocyclones. Wea. Forecasting, 9, 606Š618. Davies-Jones, R. P. 1984. Streamwise vorticity: The origin of updraft rotation in

supercell storms. J. Atmos. Sci. 41:2991Š3006. -----, and R. J. Trapp, and H. B. Bluestein, 2001: Tornadoes and tornadic storms. Severe

Convective Storms, Meteor. Monogr., No. 50, Amer. Meteor. Soc., 167Š221. Del Genio, A. D., M.-S. Yao, and J. Jonas (2007), Will moist convection be stronger in a

warmer climate?, Geophys. Res. Lett., 34, L16703, doi:10.1029/2007GL030525. Doswell, C. A., 1987: The distinction between large-scale and mesoscale contribution to

severe convection: A case study example. Wea. Forecasting, 2:3Š16. Held, I. M., and B. J. Soden, 2006: Robust responses of the hydrological cycle to global

warming. J. Climate, 19, 5686Š 5699. Intergovernmental Panel on Climate Change (2007) Climate change 2007: the physical

science basis. Cambridge University Press, Cambridge Klemp, J. B., R. Rotunno, and P. S. Ray, 1981: Observed and numerically simulated

structure of a mature supercell thunderstorm. J. Atmos. Sci., 38:1558Š1580. ------, and ------, 1983: A study of the tornadic region within a supercell thunderstorm. J. Atmos. Sci., 40:359Š377 Lemon, L. R. and C. A. Doswell, 1979: Severe thunderstorm evolution and mesocyclone

structure as related to tornadogenesis. Mon. Wea. Rev., 107:1184Š1197. Locatelli , J. D., M. Stoelinga, and P. V. Hobbs, 2002: A new look at the Super Outbreak

of tornadoes on 3Š4 April 1974. Mon. Wea. Rev., 130:1633Š1651. Markowski, P. M., E. N. Rasmussen, and J. M. Straka, 1998a: The occurrence of

tornadoes in supercells interacting with boundaries during VORTEX-95. Wea. Forecasting,13, 852Š859.

Citations (continued)

Mead, C. M., 1997: The discrimination between tornadic and nontornadic supercell environments: A forecasting challenge in the southern United States. Wea. Forecasting, 12:379Š387.

Moller, A. R., C. A. Doswell III, M. P. Foster, and G. R. Woodall, 1994: The operational recognition of supercell thunderstorm environments and storm structures. Wea. Forecasting,9, 327Š347.

Rasmussen, E. N., and D. O. Blanchard, 1998: A baseline climatology of sounding- derived supercell and tornado forecast parameters. Wea. Forecasting, 13, 1148Š1164.

Rotunno, R. and J. B. Klemp, 1985: On the rotation and propagation of simulated supercell thunderstorms. J. Atmos. Sci., 42:271Š292.

Trapp, R. J., 2000: A clarification of vortex breakdown and tornadogenesis. Mon. Wea. Rev., 128:888Š895.

-------, and B. A. Halvorson, and N. S. Diffenbaugh (2007), Telescoping, multimodel approaches to evaluate extreme convective weather under future climates, J. Geophys. Res., 112, D20109, doi:10.1029/2006JD008345.

-------, and N. S. Diffenbaugh, H. E. Brooks, M. E. Baldwin, E. D., Robinson, and J. S. Pal, 2007: Changes in severe thunderstorm environment frequency during the 21st century caused by anthropogenically enhanced global radiative forcing. Proc. Natl. Acad. Sci. USA, 104, 19 719Š19 723.

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Weisman, M. L. and J. B. Klemp, 1982: The dependence of numerically simulated convective storms on vertical wind shear and buoyancy. Mon. Wea. Rev, 110:504Š520.

APPENDIX

Climate and Severe Storms

• Background– Trapp et al (2007)

• Increase in CAPE attributable to increase in low-level atmospheric water vapor

– Trapp et al. (2009)• Storm forcings will continue to increase in the 21st century,

despite a decrease in vertical wind shear

– Del Genio et al (2009)• Thunderstorms wont change dramatically, but “the most severe

storms may occur more often”

Thunderstorm

A1B Scenario

Image Credit: IPCC Fourth Assessment

Precipitation

• p < .05. • Large variability between ensemble members • ∆(300) = 25%• ∆(750) = 23%

– Middle hr

PrecipitationImages from Kessler-type parameterization, not averages

20C: 40 Minutes A1B: 40 Minutes

Precipitation (4km, t=80mins)

Same Picture: 20C Same time: A1B

Graupel/Hail• “Insignificant” except at 9000m• Variability is very high• Raising freezing level• Increased updraft

Downdraft

Vorticity– Proves a stronger couplet

Idealized Supercell Sounding

Image Credit: Weisman & Klemp 1982

-WRF MODEL

-High-resolution mesoscale model using idealized supercell

-2km scale

-Quarter circle shear hodograph

-Coriolis term is OFF

Idealized Supercell: Current

Statistics– Student’s T-Test for significance between current and

future

– Analyzed final hour• Beginning Hour (Minutes: 10-70)• Middle Hour (Minutes: 40-100)• Last Hour (Minutes: 60-120)

– Significant: p < .01 • some information with p < .05

High-Altitude Precipitation in RFD

Precipitation is transported downward and anticyclonically around the main updraft

Evaporative cooling

This is in 20C at t=90min @ 4km

Limitations:

Only considered A1B

Realistically, capping inversions may change

Increased boundary layer moisture can impede low-level mesocyclogenesis

Sources of errorFaulty future soundingCould not locate maxima/minima with indicesResolution = 2km Conversions of sounding data

US Standard Atmospheric Supplement 1968

Idealized Supercell Hodograph: Future