Is there a demographic bias in the Kansas tornado record?

Dr. John HeinrichsSamuel Lane

Department of GeosciencesFort Hays State University

Overarching research goal• Identify the climatological factors that influence tornado

occurrence in Kansas• Tornadoes cause many deaths and injuries each year, predictions

can assist disaster managers• Simulations of global climate change suggest increases of

extreme weather

Argonia, KS May 29, 2004 (http://www.mesoscale.ws/pic2004/040529-33.jpg)

Background

• Previous research indicated a clear relationship between the El Niño/Southern Oscillation and tornado frequency (in particular, tornado activity increases in the western portion of the state after La Nina events)

• Higher frequency of tornadoes in counties containing major metropolitan areas was noted

Background (continued)

• During that research, observed that Kansas tornado report occurrence shows dramatic positive trend with time

• Raised question about whether the observed increase was due to more observers (i.e., tornadoes were just as common in the past, we just missed them)

Kansas tornado record

0

20

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1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Year

To

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• Average number of tornadoes per year: 56• Standard deviation: 30• Note “lull” in tornadoes in 1970s, steep increase afterwards

Previous research on the topic

• Divided opinions:– Aguirre et al., 1994: population and income not related to

detectability of weak (F0 and F1) tornadoes across the continental US – excellent paper but needs to be confirmed and updated (also county level may be too fine)

– Anderson et al., 2005: spatial variability of tornado reports may be modeled by a measure of human population density

Data

• Tornado data obtained from the Severe Storms database at the National Climatic Data Center (NOAA/NESDIS) for 1950-2004

• 3087 total tornado reports• All data assigned counties

(post 1993, data set has more precise locations)

• Population data for 1950, 1960, 1970, 1980, 1990, 2000 from US Census Bureau

19501955196019651970 1975 1980 1985 1990 1995 2000

F0F1

F2F3

F4F5

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90

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Nu

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Year

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Reported tornadoes by class since 1950

F0

F1

F2

F3

F4

F5

• Dramatic increase in F0 tornadoes since 1975 (note that in the 70s, more F1s and F2s then F0s, though!); F4s and F5s have been stable over time

• Perhaps the weakest tornadoes were missed previously?

Methodology

• Spatial unit of analysis: climate division• Calculate tornado trends (all tornadoes and F0s only),

population trends for each division over 1950-2004, 1975-2004

• Test null hypothesis that the tornado and population trends are correlated across the divisions

Results: All tornadoes 1950-2006

• Divisions 3,7,8 have same trend sign, rest are opposite

• Division 6 (largest increase in population) had a decrease in tornadoes!

Comparison of trends for population, all tornadoes, 1950-2004

-2000

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12000

1 (NW) 2 (NC) 3 (NE) 4 (CW) 5 (CC) 6 (CE) 7 (SW) 8 (SC) 9 (SE)

Climate division

Peo

ple

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r

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0.05

0.1

0.15

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To

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Population trend 1950-2000 Tornado trend (all) 1950-2004

Statistical results for all tornadoes, 1950-2004

• Pearson’s correlation coefficient (assumes normality) over all divisions: -0.41

• Spearman’s rank correlation (nonparametric): -0.23

• Critical value for 7 degrees of freedom: 0.521

• Can reject null hypothesis at 95% level of significance

Results for all tornadoes 1975-2004

• Larger tornado trends, all positive

• 5 divisions with same trend sign

• Division 6: greatest population growth but lowest tornado trend

Comparison of trends for population, all tornadoes, 1975-2004

-2000

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2000

4000

6000

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10000

12000

14000

1 (NW) 2 (NC) 3 (NE) 4 (CW) 5 (CC) 6 (CE) 7 (SW) 8 (SC) 9 (SE)

Climate division

Peo

ple

/yea

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0

0.1

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0.8

To

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Population trend 1970-2000 Tornado trend (all) 1975-2004

All tornadoes 1975-2004

• Pearson’s correlation: -0.19

• Spearman’s rank correlation: -0.12

• Null hypothesis can be rejected

Results for F0 tornadoes 1950-2004

• Every division has positive tornado trend

• Signs of tornado and population trends opposite for 5 divisions

Comparison of trends for population, F0 tornadoes, 1950-2004

-2000

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2000

4000

6000

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10000

12000

1 (NW) 2 (NC) 3 (NE) 4 (CW) 5 (CC) 6 (CE) 7 (SW) 8 (SC) 9 (SE)

Climate division

Peo

ple

/yea

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0

0.05

0.1

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0.25

To

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Population trend 1950-2000 Tornado trend (F0) 1950-2004

F0 tornadoes 1950-2004

• Pearson’s correlation: -0.12

• Spearman’s rank correlation: 0.02

• Null hypothesis can be rejected

Results for F0 tornadoes 1975-2004

• Tornado trends greater then full time period• 4 divisions with opposite sign

Comparison of trends for population, F0 tornadoes, 1975-2004

-2000

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2000

4000

6000

8000

10000

12000

14000

1 (NW) 2 (NC) 3 (NE) 4 (CW) 5 (CC) 6 (CE) 7 (SW) 8 (SC) 9 (SE)

Climate division

Peo

ple

/yea

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0

0.1

0.2

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To

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Population trend 1970-2000 Tornado trend (F0) 1975-2004

F0 tornadoes 1975-2004

• Pearson’s correlation: -0.23

• Spearman’s rank correlation: -0.02

• Null hypothesis can be rejected

Climate Zone 9 Tornado Events Per Decade

5447

40

71

51

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70

80

Decades 1950-1990

Torn

ado

Eve

nts

1950

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Climate Zone 9 Population Change

315896

300050

281610

294805

283585

260000

270000

280000

290000

300000

310000

320000

Decades 1950-1990

Pop

ulat

ion

1950

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Climate Zone 7 Population Change

8641593730 97834

107274121835

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60000

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100000

120000

140000

Decades 1950-1990

Po

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1950

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1970

1980

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Climate Zone 8 Tornado Events Per Decade

77

5647 53

141

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100

120

140

160

Decades 1950-1990

To

rnad

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1990

Climate Zone 8 Population Change

397380

524521 521875 545721 576592

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100000

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300000

400000

500000

600000

700000

Decades 1950-1990

Po

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1950

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Climate Zone 4 Tornado Events Per Decade

23

16

2624

64

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60

70

Decades 1950-1990

To

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1950

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Climate Zone 4 Population Change

3573034295

3260231052

28056

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5000

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15000

20000

25000

30000

35000

40000

Decades 1950-1990

Po

pu

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on 1950

1960

1970

1980

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Climate Zone 7 Tornado Events Per Decade

56 51 56

29

124

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40

60

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100

120

140

Decades 1950-1990

To

rnad

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ts 1950

1960

1970

1980

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Climate Zone 5 Population Change

194908

214003

198284

202967

196327

185000

190000

195000

200000

205000

210000

215000

220000

Decades 1950-1990

Po

pu

latio

n 1950

1960

1970

1980

1990

Climate Zone 5 Tornado Events Per Decade

65

36

49

32

94

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Decades 1950-1990

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Climate Zone 6 Tornado Events Per Decade

61

53

38

54

68

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Decades 1950-1990

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`

1950

1960

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1980

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Climate Zone 6 Population Change

354227

480496

586027

659732

763971

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100000

200000

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400000

500000

600000

700000

800000

900000

Decades 1950-1990

Po

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1950

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1990

Climate Zone 1 Tornado Events Per Decade

30

19

25 27

88

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Decades 1950-1990

To

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1950

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Climate Zone 1 Population Change

50961

4769344819

42730

38385

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30000

40000

50000

60000

Decades 1950-1960

Po

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on 1950

1960

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Climate Zone 2 Tornado Events

48

30

4448

93

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Decades 1950-1990

To

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1950

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Climate Zone 2 Population Change

115259

102176

9008484055

73398

0

20000

40000

60000

80000

100000

120000

140000

Decades 1950-1990

Po

pu

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on 1950

1960

1970

1980

1990

Climate Zone 3 Tornado Events Per Decade

38

46

22

26

62

0

10

20

30

40

50

60

70

Decades 1950-1990

To

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1950

1960

1970

1980

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Climate Zone 3 Population Change

354523

381647

395936 395900 395439

330000

340000

350000

360000

370000

380000

390000

400000

Decades 1950-1990

Po

pu

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1950

1960

1970

1980

1990

Graphical representation by decade

• Left graphs show tornado totals by decade, right show population by decade

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1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

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Yearly Total Tornado days

Another interesting result

• If multiple tornado reports on the same day in the same county are combined (as done for the ENSO study), trend is largely eliminated

• Difference between reported tornadoes and tornado events/days is greatest after 1990

Conclusions

• Can reject hypothesis that increases in tornado report frequency are related to population change

• Results consistent with Aguirre et al. (1994)• In fact, correlations surprisingly suggest very

weak inverse relationship• Divisions containing KC/Topeka, Wichita (rapid

growth areas) have very different trend patterns• Much of apparent growth of weak tornadoes is in

multiple events

Discussion

• Is the observed increase in tornadoes a real one? Not necessarily!– Changes in reporting process?– More attentiveness/awareness?– More spotters/chasers?

• Could it be that the observed increase is because we are better at seeing what goes on in large mesocyclones with tornado swarms?

• Future work needed to test these hypotheses

References

• Aguirre, B. E., Rogelio Saenz, John Edmiston, Nan Yang, Elsa Agramonte, Dietra L. Stuart (1994). "Population and the Detection of Weak Tornadoes." International Journal of Mass Emergencies and Disasters 12 (3): 261-277.

• Anderson, C.J., Wikle, C.K., Zhou, Q., and J.A. Royle (2005). "Population Influences on Tornado Reports in the United States" In Review.