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Is there a demographic bias in the Kansas tornado record? Dr. John Heinrichs Samuel Lane Department...
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Transcript of Is there a demographic bias in the Kansas tornado record? Dr. John Heinrichs Samuel Lane Department...
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
40
60
80
100
120
140
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
Year
To
tal
nu
mb
er
of
torn
ad
oe
s
rep
ort
ed
• 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
0
10
20
30
40
50
60
70
80
90
100
Nu
mb
er
rep
ort
ed
Year
Cla
ss
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
0
2000
4000
6000
8000
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
r
-0.05
0
0.05
0.1
0.15
0.2
To
rnad
oes
/yea
r
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
0
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
r
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
To
rnad
oes
/yea
r
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
0
2000
4000
6000
8000
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
r
0
0.05
0.1
0.15
0.2
0.25
To
rnad
oes
/yea
r
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
0
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
r
0
0.1
0.2
0.3
0.4
0.5
0.6
To
rnad
oes
/yea
r
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
0
10
20
30
40
50
60
70
80
Decades 1950-1990
Torn
ado
Eve
nts
1950
1960
1970
1980
1990
Climate Zone 9 Population Change
315896
300050
281610
294805
283585
260000
270000
280000
290000
300000
310000
320000
Decades 1950-1990
Pop
ulat
ion
1950
1960
1970
1980
1990
Climate Zone 7 Population Change
8641593730 97834
107274121835
0
20000
40000
60000
80000
100000
120000
140000
Decades 1950-1990
Po
pu
lati
on
1950
1960
1970
1980
1990
Climate Zone 8 Tornado Events Per Decade
77
5647 53
141
0
20
40
60
80
100
120
140
160
Decades 1950-1990
To
rnad
o E
ven
ts 1950
1960
1970
1980
1990
Climate Zone 8 Population Change
397380
524521 521875 545721 576592
0
100000
200000
300000
400000
500000
600000
700000
Decades 1950-1990
Po
pu
lati
on
1950
1960
1970
1980
1990
Climate Zone 4 Tornado Events Per Decade
23
16
2624
64
0
10
20
30
40
50
60
70
Decades 1950-1990
To
rnad
o E
ven
ts
1950
1960
1970
1980
1990
Climate Zone 4 Population Change
3573034295
3260231052
28056
0
5000
10000
15000
20000
25000
30000
35000
40000
Decades 1950-1990
Po
pu
lati
on 1950
1960
1970
1980
1990
Climate Zone 7 Tornado Events Per Decade
56 51 56
29
124
0
20
40
60
80
100
120
140
Decades 1950-1990
To
rnad
o E
ven
ts 1950
1960
1970
1980
1990
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
0
10
20
30
40
50
60
70
80
90
100
Decades 1950-1990
To
rna
do
Ev
en
ts
1950
1960
1970
1980
1990
Climate Zone 6 Tornado Events Per Decade
61
53
38
54
68
0
10
20
30
40
50
60
70
80
Decades 1950-1990
To
rna
do
e E
ve
nts
`
1950
1960
1970
1980
1990
Climate Zone 6 Population Change
354227
480496
586027
659732
763971
0
100000
200000
300000
400000
500000
600000
700000
800000
900000
Decades 1950-1990
Po
pu
lati
on
1950
1960
1970
1980
1990
Climate Zone 1 Tornado Events Per Decade
30
19
25 27
88
0
10
20
30
40
50
60
70
80
90
100
Decades 1950-1990
To
rnad
o E
ven
t
1950
1960
1970
1980
1990
Climate Zone 1 Population Change
50961
4769344819
42730
38385
0
10000
20000
30000
40000
50000
60000
Decades 1950-1960
Po
pu
lati
on 1950
1960
1970
1980
1990
Climate Zone 2 Tornado Events
48
30
4448
93
0
10
20
30
40
50
60
70
80
90
100
Decades 1950-1990
To
rnad
o E
ven
ts
1950
1960
1970
1980
1990
Climate Zone 2 Population Change
115259
102176
9008484055
73398
0
20000
40000
60000
80000
100000
120000
140000
Decades 1950-1990
Po
pu
lati
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
rnad
o E
ven
ts
1950
1960
1970
1980
1990
Climate Zone 3 Population Change
354523
381647
395936 395900 395439
330000
340000
350000
360000
370000
380000
390000
400000
Decades 1950-1990
Po
pu
lati
on
1950
1960
1970
1980
1990
Graphical representation by decade
• Left graphs show tornado totals by decade, right show population by decade
0
20
40
60
80
100
120
140
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
Year
Nu
mb
er
of
torn
ad
oe
s
rep
ort
s/e
ve
nts
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.