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R E S E A R CH AR T I C L E

Differences in tornado activities and key tornadic

environments between China and the United States

Ruilin Zhou | Zhiyong Meng | Lanqiang Bai

Department of At mospheric and Oceanic

Sciences, S cho ol o f Physics, Peking

University, B eijing, China

S cho ol o f Atmospheric S ciences, S un Yat-

sen University, Z huhai, China

Correspondence

Zhiyo ng Meng, Depa rtment of

Atmospheric a nd Oceanic S ciences,

Schoo l of Physics , Peking University,

B eijing 1 00871, China.

Email: [email protected]

Funding information

National Nat ural S cience F ounda tion of

China, Grant /Award Numbers: 4187 5051,

419 05043, 42030 604

Abstract

The y early tornado oc currence i n C hin a is approx imately 5 1 0% o f that in the

United States , an d t he peak mon th o f torn a do occurren c e in C hi na i s Jul y ,

w h i l e t h a t i n t h e U n i te d St a t e s i s M a y . H o w e v e r , t h e t w o c o u n t r i e s ha v e n e a r l y

th e s ame land a rea an d s im ilar l at i tud in a l lo c at i o n . T h is st u dy a t te m pt s t o di s-

clos e t he dif ferenc e s in t he key t ornadic e n v ironm e nts th at a re m o st poss ib l y

responsi bl e for th e l arge di screpanc y o f t o r n a do o c c u r r e n c e s o b s e r v e d b e t w e e n

th e t w o c o untri e s. The r egi o n wi th t he hi ghest torn a do dens ity i n C hin a ( JS) i s

comp ared w it h th ree s im il arl y s iz e d r egio ns in t he U n it e d S tat e s, i ncl udi ng

t w o to r n ad o - p r o n e r e g i o n s (U C a n d U SE 1) and o ne regio n similar to J S

( U S E 2 ) . T h e r e s u l t s sh o w th a t JS h a s a mu c h l o w e r t o r n ad o fr e q u e n c y t h a n

U C a n d U S E 1 , m a inl y du e t o JS 's mu c h l o we r l i k e l i h o o d o f m u l t i p l e t o r n a d o e s

o c c u r r i n g i n a sh o r t p e r i o d d u r i n g i t s t o r n a d o s e a s o n. C o m pa r e d w i t h U C a n d

U SE 1 , J S fe a t u r e s r e l a ti ve ly lo w m e a n v a l u e s o f t h e s i g n i f i c a n t t o r n a d o p a r a m -

e t e r ( S T P ) d u r i n g th e i r r e s p e c t i ve t o r n a d o se a s o n s. T h e S T P , i n c o r po r a t i n g s e v -

e r a l to r n a d o - r e l a t e d v a r i a b l e s , w a s f o u n d t o s h o w a m o n o t o n i c r e l a ti o n s h i p

w i th t h e to t a l to r n ad o co u n t a t a se a s o n al sc a l e a n d p e r f o r m w e l l to e x p l ai n

m o n t h l y v a r i a t i o n s in t o r n a d o f r e q u e n c i e s . C o m pa r e d w i th t h e ta r g e t r e gi o n s

in the U ni ted S tat e s , JS has a si mi lar magni tude a nd de creasi n g trend o f the 0

6-k m v er tical wind s hear from sprin g to sum mer, but a later i ncrease in i nsta-

bil ity. Res ultin g from a b alanc e betwee n t h e de c r e a s i n g v e r ti c a l s h e a r a n d t h e

inc r e a sin g t h e r m o d y n a m ic in st ab il it y, JS f e a tu r e s a d e l a y e d mo n th l y p e a k o f

ST P a n d t h u s a d e l a y e d to r n a do s e a s o n , w h i c h m a k e s JS ha v i n g a wo r s e k i n e -

m a t i c e n v i r o n m e n t f o r t o r n a d o g e n e s is wi th fa r l o w e r l o w - l e ve l v e r ti c a l w i n d

s h e a r d u r i n g i t s t o r n a d o se a s o n th a n t h a t o f th e U n i t e d S t a t e s .

KEYWORD S

C h i n a , d i f f e re n c e , e n v i r on m e n t a l f e a t u re s , sign ific an t t o rn ado par amet e r, to rn a do , th e

U ni t e d S t a t e s

1 | INTRODUCTION

T o r na d o o ccu r r e n ce i n the U ni te d St a te s i s si gn ifi ca nt ly

h i g h e r t h an t h a t i n o t h e r r e g i o n s i n t h e w o r l d ( B r o o k s

et al., 2 003; Farne y and Dix on, 201 5). B ased on the

t o r n a d o d a ta b a s e fr o m NO A A ' s S t o r m P r e d i c t i o n C e n tr e ,

t h e a v e r ag e an n u a l n u m b e r o f U . S . t o r n a d o e s i n t h e l as t

t h r e e d e c a d e s w a s a p p r o x i m a t e l y 1, 2 0 0 an d t o r n a d o e s

w e r e m o s t a c t i v e i n t h e U n i t e d S ta t e s in M a y . D e s pi t e

Chi na's s im il ar latitudinal l ocati o n a nd l a nd area, the

Re c e iv ed : 6 O c to b e r 2 02 0 Re v is e d: 3 J un e 2 02 1 A cc e pt ed : 7 J u ne 20 2 1

D O I : 10 .1 0 02 / jo c .72 4 8

Int J Climatol. 2 0 21 ;1 1 8 . w ile y on lin el ibr a r y .c o m/ jo ur n a l/ j o c © 2 02 1 Ro ya l Me te or o lo gi ca l So c ie ty 1

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a nn ua l to r n ad o co u n t in Ch ina i s le s s th an o ne - te n th t ha t

i n the U n i te d St a t e s, a n d th e p e a k n u m b e r o f t o r n a d o e s

occ u rs 2 m onth s l ater in C hin a th an in t he U nit ed St ates

( Fa n a n d Y u , 2 0 1 5 ; C h e n . , 2 0 1 8 ) . T h e f a c t o r s th a tet al

caus e t orna do ac ti vities to be so di ffe rent betwe e n t he

t w o co u n tr i e s r e m a i n u n c l e a r . T h i s s t u dy a i m s t o d i s c l o s e

t h e d iff e r e n c e s in t h e ke y t o r n a d ic e n v i r o n m e nt s t h a t a r e

m o st p o ssi bl y r esp o n sib le for the large torn ad o occur-

r e n c e di sc r e pa n c y b e t we e n C h i n a a n d th e U n it e d S t a te s .

T o r n a do e s t e n d t o f o r m in e n v i r o nm e n ts wi t h s u b -

s t a n t i a l l o w -l e v e l sh e a r , in ad d it ion to m o is tu re, ins ta b il -

i ty an d v e r t i c a l m o ti o n , wh i c h a r e th e t h r e e i n g r e d i e nts

o f d e e p m o i s t co n v e c ti o n (B r o o k s . , 2 0 0 3 ; Gr a m set al

et al et al. , 2 0 1 2 ; A n d e r s o n - F r e y ., 2 0 1 8 ) . M a n y p r o x im it y

s o u n d i n g s tu d i e s ha v e i n di c a t e d th a t fa v o u r ab l e t o r n a d i c

e n v i r o n m e n t s t e n d to h a v e l o w v a l u e s o f th e l i f t e d c o n -

d e ns at i o n le v e l ( L CL ) , wh ich i s co r r e l a te d wi t h hig h e nv i -

r o nm e nt a l hu mi di t y a t lo w l e v e ls (G r a ms . , 2 0 1 2 ;et al

Thom pson ., 2012 ). I nstability, wh ich i s o f ten me a-et al

s u r e d by c o n v e c t i v e av a i l a b l e p o t e n t i a l e n e r g y ( C A P E )

a n d r e l e a s e d by l a r g e -s c a l e v e r ti c a l m o t i o n , i s e s s e n ti a l

f o r sp e e d i ng u p th e s ur f a c e a i r p a r c e l v e r t i c al l y du r i n g

d e e p c o n v e c t i o n ( E m a n u e l , 1994 ). Com pared with other

c o n v e c t i v e m o d e s , s u p e r c e l l t h u n d e r s to r m s a r e c h a r a c t e r -

ized by deep an d p ers istent r otatin g u pdrafts, w hic h seem

t o h a v e a st r o n g e r a b il it y t o p r o d u c e t o r n a d o e s (Du d a

and G allus , 201 0). In a dditio n t o t h e t h e r m o d y n a m i c

p a ramet e r o f m oist ure a nd in st abil it y, t he ki nem at ic

p a r a m e t e r o f th e 0 6 -km ve c to r s h e a r m a g n i t u d e (S HR 6 ) ,

w h ic h i s im p o r ta n t f o r th e d e v e lo p m e n t o f t h e m id l e v e l

r o t a t i o n o f t h e me s o cy c l o n e i n a su p e r c e l l t h r o u gh t h e

t il ti ng o f i ts a ss o cia t e d ho r iz o nt a l vo r t i cit y b y a s to r m -

s c a le u p d r a f t , i s o f t e n u s e d to a s s e s s th e p o te n t i a l o f su p -

erce lls (R as mu s s en a n d B la n c ha r d, 199 8; Th ompson

et al. , 2 0 0 3 ) . T h e c o m b i n a t i o n o f th e C A P E a n d S H R 6

parameters has b een sh own t o be e ffec tive in d is crimin at-

i n g be t w e e n s e v e r e a n d no n s e v e r e t h u n d e r s t o r m s

( Brook s . , 2003; Taszar ek ., 2017 ), and t heet al et al

C A P E S H R6 p a r a m e t e r s p ac e i s o f t e n in ve sti g a te d to

o f f e r v a l u a b l e in s i g h t i n t o t o r n a d i c e n v i r o n m e nt s

( e .g ., Brook s, 2 009; Grünwald an d Brook s, 20 11;

A nd e r s o n - F r e y . , 2 0 1 6 ) .et al

L o w -l e v e l e n v i r o n m e n t a l s t o r m - r e l at i v e h e l ic it y

( SR H ) al s o p r o v i d e s a g o o d r e f e r e n c e f o r t h e d e v e l o pm e n t

o f l o w - l e v e l r o t at i n g u p d r a f ts a n d th u s t h e p o te n ti a l f o r

t o r n a d o g e n e s i s ( K e r r a n d D a r k o w , 1 9 9 6 ; Da v i e s -

Jones, 2015; Coffer . , 2 019) . S RH is calculated by inte-et al

g r a ti ng the mu l ti p li ca ti o n o f t he st o r m- r e la t iv e v e lo ci t y

v e c t o r a n d t h e s tr e a m w is e v o r ti c i ty fr o m t h e g r o u n d to

som e s e lec ted heigh t (Davies -Jones ., 199 0). I n a net al

e n v i r o n m e n t w i t h a l a r g e l o w - l e v e l S R H v al u e , wh e r e

l o w e r - t r o p o s ph e r i c h o r i z o n ta l s t r e a m w i s e v o r t i c i t y is

m o r e do m in a n t t h a n l o w e r - t r o p o s p h e r i c h o r i z o n ta l

cro sswis e v ortic ity, t he horizon tal vorte x tube s a re more

i n c l i n e d t o b e t i l t e d a n d st r e t c he d i n t o s tr o n g n e a r -

s ur fa c e ve r t i c a l v o r t i c i t y ( No w o t a r s ki a n d J e n s e n , 2 0 1 3 ;

Markow ski an d R ic hardson, 2 014) .

A l tho u gh t o r n a dog e n e si s i s u l tim ate ly d e p e n de nt o n

smaller-scale p roce sses, i t has long been know n t o be

closely r elated to large-to -mes os cale environmen ts. Such

l a rg e -t o - me so s c al e e n v i r on me n t s a r e o f t en d e p ic t ed b y t h e

f o ur e n v ir o nment a l p ar a m et er s de s cri b e d a b o v e , n a m e l y,

C A P E, SH R 6, L C L a n d S R H (e . g. , Ra s m u s se n an d

Blanc hard, 199 8; Rasm ussen, 2003; P otvi n . , 2 010;et al

A nderson -Frey ., 201 6). T hese en vironmental parame-et al

ters are a cc oun ted f o r in t he STP ( Thompson ., 2003 ),et al

w h ich w a s dev e l op ed t o d i s cr imi n at e si gni f i ca nt t o r n a di c

en vironm ents from non-tornadic environm ents and h as

b e e n w i d e l y u s e d f o r o p er a ti o na l t or n ado for e c asts

( T hompson . , 20 12). T he non di mension al energy-et al

h e lic i ty i nd e x (E HI ) , wh i ch c o mbin e s C A P E w it h l o w -

l e v el S RH , is a lso co mm o nly e m p loy e d a s a m e an s o f id e n -

t i fy i n g t o r na do p o t e n tia l ( H art a n d K o r o tk y , 19 91;

R a smusse n, 20 03). To date, it r em ai ns unclear h ow th es e

t o rn a do- f av ou r a bl e e n v ir o nme n ta l pa r amete r s d iffe r

b e tween C hina and t he United States.

Tornado f requenc y h as been dem o nstrated to be cor-

r e l a te d wi t h mo nt hl y t o se a so n al e nv ir o n me n ta l fe a tu r e s.

A n e m p i r i c a l in d e x b as e d o n mo n thl y a v e r a g e d co n v e c -

t i v e p r e c i p i ta t i o n a n d S R H h a s b e e n d e m o n s t r a te d a s

havi ng some s kil l i n a ccountin g f o r mon thl y torn ado

c o u n ts i n t h e U n i te d St a t e s ( T ip p e t t . , 2 0 1 2 ; T i pp e t tet al

et al., 201 4). T he obse rved sh ift in t he seas onali ty o f tor-

n a d o r e p o r t s o v e r t h e G r e a t P l a i n s h a s b e e n a c c o m p a n i e d

b y c o r r e s p o n d i n g t r e n d s i n a n e n v i r o n m e nt a l i n d e x t h a t

is a f unction o f C APE a n d SR H ( Lu ., 20 15). In a ddi-et al

t i o n , t h e s u m o f t h e d a i l y m a x i m u m o f t h e S T P s h o w s

co n si st e nt c ha ng e s i n it s sp a t ia l di st r ib u ti o n wi th th e to r -

n a d o f r e q u e n c y in th e U n i t e d S t a t e s a t m o n t h l y a n d s e a -

s o na l s ca le s , a nd th e sp a ti a l d e p e nd e nc e o f to r n ad o

f r e qu e n c y o n th e S T P wa s f o u n d to v a r y se a s o n a l ly

( Ge n s i n i a n d Br o o k s , 2 0 1 8 ; G e n s i n i a n d B r a v o d e

Guenn i, 2019 ). Th ese r e sults im ply t hat u nderstandin g

t he e nv ir o n me n ta l d if fe r e nc e s in to r n a d o -p r o n e m o nth s

m a y be a k e y pe r s p e c t i v e f o r u n d e r s ta n d i n g t h e di f f e r -

e n c e s in fr e q u e n c y o f t o r n a d o e s a n d t h e mo n t h l y v a r i a -

t ion s in t orna d o es be tw een C hin a a n d t he U ni te d S ta t e s.

In th e p resen t study, torna d o r e cords f r om 2007 to

2 0 1 6 w e r e c o l l e c t e d t o r e v e a l ke y e n v i r o n me n t a l fa c t o r s

t h a t m a y h av e m a de t o r n a d o e s i n Ch i n a co n s i d e r a b l y le s s

f r e qu e n t a n d o c c u r r i n g 2 mo n t h s l a te r t h a n tho s e in t h e

Un ited States . T o ac hieve the se g oals ef fectively a nd eff i-

ciently, s pec ial a tten tion was p rim a rily paid to high-

i nci dence torn ado regions a nd to th e p rim e m o nths of

t o r n a d o o c c u r r e n c e . T h e r e s t o f th e pa p e r i s o r g a n i z e d a s

f o l lo w s. S e c ti o n 2 int r o d u c e s t h e d a ta a n d m e th o d s. T h e

2 Z HO U .ET A L



res ults a re presented i n Sec tion 3, followed by a d isc u s-

s i o n i n Se c ti o n 4 o n t yp i c a l sy n o pt i c si t u a t i o n s , t h e p o t e n -

t i a l i m p a c t o f to p o g r a p h y a n d so m e s tu d y l i m i t a ti o n s .

S e c t i o n 5 g i v e s a b r i e f s u m m ar y .

2 | DATA AND METHODS

2.1 Tornado databases|

The t ornado inf orm ation f or cont ig uous Ch ina f rom 200 7

to 20 16 w a s o b tain e d fr o m v a r iou s s o u rc es with cros s-

ch eckin g, inc luding the Y earboo k o f M eteorological

Di sasters i n C hi na ( C hin a Meteor ol ogical A dm ini strati on ,

2 0 0 5 2 01 7) , t h e t o r na d o d a t a se t c o ver i ng c o n t i gu o us

C h i na d ur i ng t h e p e r i od 1 94 8 2 0 1 2 c o m p il e d b y C h e n

et al. ( 2018 ), alm o st all t he published paper s o n t orna do es

i n C hi n a , th e d a m a g e s u r v ey s co n d u c te d b y l o c al m e te o r o-

l o gic a l ag e n c ie s a n d t h e r e p o r ts a r c h i v ed th ro u g h we b

s e arc h e s. Qu a li ty c o n t r ol o f t h e t o r n ad o r e c or ds w a s pe r -

f o r me d b y r e v i e wi ng t h e n a ti o na l a n d r e g io n a l r a dar

r e f le c ti vi ty m o s a i c i m a g e s ( a v ai l a bl e a t h t t p: / /d ata .c m a .cn/

e n ) , a nd t o r n a do r e c or d s th at w e r e no t a s s o c ia te d w i th a

c o n ve c tive s t or m we r e r e m o v ed .

W e d ef ine d t h e s t u dy pe r io d a s 2 0 0 7 2 01 6 f o r t w o

m ai n r e a s ons . F i r st , t h e to r n ado d a t a c o l le c te d in t h e s e

y e a rs h a ve mu c h l e s s u n c er t a inty tha n t h o se co l l e cte d

be fo re 2 007. B ecau s e the C hina New Ge ner ation W eath er

R ad ar (C I NR A D ) n e two r k b e g an t o ta k e s h a p e a f te r 20 0 6 ,

w e w e r e a b l e t o p e r fo r m q ua li ty c o n t r ol o n t o r n a d o

r e cor d s wi th t he av a il ab il ity o f r a d a r r e fle c tiv it y m o saic

i ma ge s du r in g t he ch o sen st u d y p e r io d . W it h th e wi de u se

o f s m a rt p h o n es si n c e 2 0 0 7, f ar fe w er t o rna d o r ep o r ts

sh o ul d h av e b e e n mi sse d. S e cond , t hi s st u dy a i ms t o d i s-

c u s s t h e e n v ir onm en tal pa r a m et e rs b e h ind t h e la r g e d i f fe r -

enc es i n t he total n umber o f torn ado es b e tween China

a n d t h e U n i te d S t a te s r a t h er t h a n t h e c l i m a to lo g ica l v a r i -

abi lity a ss oci at ed w it h th e t o rnado count s. The high -

i n cid e nce a r e a s a n d p rim e m o n t hs o f t o r n adoe s r e v e a le d

f r o m t h e se 1 0 - y e a r d a ta a r e co n s i st e nt w i th t h os e de t e r-

m ined i n p re v iou s st u die s w ith l o ng e r stu d y pe r i o ds i n

both China (Chen ., 2018 ) an d the U nite d S tate set al

( e .g . , A s hl e y , 2 0 0 7 ; T i p pe tt . , 2 0 1 2) . C on s e qu en tly ,et al

u s i ng th e 1 0-y e a r da t a fr o m 2 0 0 7 2 01 6 s h o ul d b e r e a s on-

a b l e f o r th i s st u d y.

A to t a l o f 9 8 3 t o r n a d o r e p or ts w e r e c a ta lo g u ed i n

C h i na fr o m 2 0 0 7 t o 2 0 1 6 . T h i s t o rna do d a t a s e t h a s bee n

m ad e a va i l ab le a t th e P e k in g U n i ve rs i t y O p e n R e s ea r ch

Data Platf orm at https://do i.or g/ 10.18170/DV N/QKQHTG

( Z ho u . , 20 20 ). T h e t o r nad o i nf or m at ion o v e r th e sa m eet al

p e rio d i n the co n t i gu o us U n i t e d St a t es w as d e r i v e d fr om

N OAA' s St o r m P redi cti o n C en tr e ( SP C ) sev ere w e a th er

d a tab as e ( a v a i la ble a t ht tps: / /ww w .s pc .n o a a. g ov/ w cm /

# d ata ) , wh ich h a s b e e n u s e d w i d el y i n t h e li t e r at ur e . D u r -

ing t he 10 studied ye ars , a total o f 11,79 5 t ornadoes w e re

c a tal ogu e d in t h e U n it e d S ta t e s, wh ich is a p p r o xim a te ly

12 time s t he n u mber recorded in Chi na.

2.2 Selection of target regions|

T o a c h i e v e o u r g o al s , w e f o c u s e d o n r e g i o n s w i t h h i g h

fre quency o f t ornadoes. F i g ure 1a,b sh o ws th at m o st tor-

n a d o e s o c c u r in e a s te r n Ch in a, w it h t h e hi g h e st to r n a d o

fre quency o ccurri ng in Ji angsu P rov i nce (the r ed box i n

F i g u r e 1 a ) , w h e r e th e t e r r a i n i s g e n e r al l y fl at . W e t h u s

s e lected a r egion o f i nterest con fin e d b y t he area within

31 3 5 N an d 1 18 122 E ( 4 × 4 ) and labe lle d t hi s r egion

a s J S . In t h e U n i t e d S t a t e s , t h e h i g h e s t to r n a do f r e q u e n c y

i s l o cated i n t he region to the e as t o f t he Roc ky M oun-

t a ins . W e c h o se th e 4 × 4 a r e a co nf in e d by 3 6 4 0 N

a n d 9 7 1 0 1 W a s t h e h i g h -i n c i d e n c e r e g i o n a n d la b e l l e d

i t U C ; th is r e g i o n is lo c at e d i n t h e c e n t r a l an d so u t h e r n

G r e a t P l a ins o f th e U n it e d S t a te s ( F ig u r e 1 c , e ).

I n a d di ti o n to t h e ce n t r a l a n d so u th e r n Gr e at P l a i n s ,

M i s s i s s i p p i a n d Al ab a m a , w h i c h a r e l o c a t e d i n th e s o u th -

e a s t e r n U n i t e d St a t e s , h a v e t h e s e c o n d - h i gh e s t t o r n a d o

f r e qu e n c i e s in t h e U n i te d S ta t e s a n d ha v e b e e n c o n f i r m e d

t o b e t h e tw o s ta t e s th a t h o l d t h e hi g h e s t to r n a do fa t a l i ty

rates o th er than Arkans as ( Ash ley, 200 7; Colem a n a n d

Dixon , 2014 ). Thus, w e ch o se th e 4 × 4 a r e a wi th in 31

35 N a nd 86 90 W ( Figure 1c ,f) a s a n o th er high-

i nci d e nce r e g io n in t he U nit e d S t at e s a nd l a be l le d t hi s

r e g i o n a s U SE 1 . W e f u r the r c h o s e t h e 4 × 4 a r e a wi th in

32 3 6 N a nd 78 82 W a s U SE 2 (Figure 1c, d) t o create a

t a r g e t r e g io n in t h e U n it e d S t at e s t h a t r o u g h ly m a t c h e s

J S i n C h i n a i n t e r m s o f a si m i l a r to r n a do co u n t , l a ti tu d e ,

t e r r a in e l e v a ti o n a n d l o ca ti o n r e l at iv e t o a c o a stl in e .

2.3 Environmental parameters|

T h e l o n g -t er m s p a ti al me a ns o f r e l e v an t e n v ir o nm en ta l

p a ram e te r s c a n b e u s e d t o r e pr e se n t t h e o v e ra l l e n v i r on -

m e n ta l f e a tu r es o f th e fo u r ta rg e t r e g io n s i n a sta t i st i ca l

s e n se . T h e e n vir o nm e n ta l p a r a m et e r s w e r e c a l cu l a te d

u s ing t h e N at i o n al C e n tr es fo r E nv i ro n m ent al P r e d i ct i o n

F i n al Op e r a ti on a l G l o b a l A n a ly s i s d a ta se t ( N C EP F N L ,

a v a il ab l e o n l i ne a t ht t p s : // rd a . uca r. ed u / da tas e ts / d s0 83 . 2/ )

with a 6 -hr inter val a nd 1 × 1 ho ri z o nt al r e s o l uti on . I n

t h e ca lcu l ations o f at m osp h er ic p a ra m e te rs, o n ly NC E P

F N L d a ta o ve r t h e g r i d p o i n ts l o c at ed o n l a n d o r n e a r t h e

c o ast we r e u s e d (F i gu r e 1 b ,d ) f o r t w o m a i n r e a s o ns . F i r st,

t h er e ar e a l m o st n o to r n a do r e p o r ts o ve r t h e o c e an i n t h e

nort heast c orner o f J S o r in t he south e ast corn e r o f U SE 2.

W a te r s po u ts o v e r o c e a ns ha v e l i t tle c h a n c e o f b e i n g

ZH OU .ET A L 3



d e te c ted an d a r e di ffic u lt t o ve r i fy du e to i nsu ffi cie n t r a d a r

c o v er a ge a n d i m p r a ctic al d a m ag e s u r ve ys . A d d i t io n a ll y,

t h e bo u n da r y l a y e r co n d it i ons o v e r t h e o c e a n a r e s u b st a n-

t i a ll y d if f e re nt f r o m tho s e o v er l a n d , w h i c h w o u ld a ff e ct

t h e d e r i ve d c o n v e ct i ve p ar a me t e rs e xa m i ne d i n t h i s s t u d y.

S e v e r a l to r n a d o - r e l a t e d a t m o s ph e r i c pa r a me t e r s w e r e

c o m p o s i t e d o ve r t h e s t u d i e d s p a t i a l an d t e m p o r a l r a n g e s

f o r t h e f o u r t a r g e t r e g i o n s d u r i n g t h e t o r n ad o - p r o n e

m o n th s: 0 1 -k m S R H (S RH 1 ) , m i xe d - la ye r C A P E

( M LC A PE ), S HR 6, mi xe d - la y e r L C L ( M LL C L ), S T P a nd

E H I . M L C A P E w a s ca l c u l a t e d u s i n g a pa r c e l li f t e d f r o m a

m ix e d l ay e r i n t h e lo w e s t 1 0 0 h P a . I n c al c u l at i n g S R H 1 ,

t h e st o r m m o t io n wa s e st im a te d u si n g t h e m e t h o d o f

Bunke r s . (2 000) . T he STP w as c a lculated followinget al

t h e p r o c e s s o f T h o m p s o n . (2 00 3 ) u s i n g a m i x e d- l ay e ret al

m e th od ( a lso w it hi n th e l o wes t 10 0 hPa).

STP=M L CA P E

100 0 J kg 1

×

2 0 0 0 m M L L C L

1500 m

×S R H1

100 m2 s 2

×

S HR 6

20 m s 1

FIGURE 1 ( a ) S p a ti a l d i st ri b u ti o n o f to r n a d o o c cu rr e n ce s i n co n t i g uo u s C h i n a f ro m 20 0 7 to 2 0 1 6 d i s p l a y e d o n 1 × 1 l at i tu de

l o n g i tu d e g ri d s . Th e r e g i o n i n th e r e d b o x i s e n l a rg e d i n ( b ) . Th e r e d do ts i n ( b ) d e n o te t he l oc a t io n s w h e re t he NC E P F N L da t a w e r e us e d to

c a l cu l a t e e n v i ro n m e n t a l f e a t ur e s. ( c) A s i n ( a ) b u t f o r t h e t o r na do e s th a t o cc ur re d i n th e c on t ig uo u s U n i t e d S t a te s d u ri n g t h e s a m e pe ri o d .

( d ) ( f ) S a m e a s i n ( b ) b ut c or re s p o n d i n g t o th e s e l ec t e d t a r g e t re g io n s i n th e U n i t e d St a t e s, na me l y , U S E 2 , U C , an d U S E 1 (i n d i c a t e d b y t he

b l u e b o x es a n d b l a c k a rr ow s ) . N C E P F N L d a t a ov e r t h e b l ue gri d po i nt s i n (d ) ( f ) w e re u s e d t o ca l c ul a t e e n v i r on m e n t a l f e a t u re s . T he co l o ur

b a r i s e mp lo y ed fo r ( a ) ( f )

4 Z HO U .ET A L



E HI w as calc ulated accordin g t o th e modif ied versi o n

outlined in R a smus sen (2 003) .

EH I=M LC A PE S R H 1×

1600 00

T h e s e at m o s p h e r i c p a r a m e t e r s w e r e f i r st ca l c u l a t e d

for e very 6-h r data time s e ries for th e grid poin ts located

o n l a n d o r n e a r t h e c o a s t i n e a c h o f t h e fo u r ta r g e t

r e g i o n s . T he v a l ue s o f t h e s e e n v i r o n m e n t al p a r a m e t e r s

o n g r i d s w i t h i n 1, 0 0 0 k m o f a tr o p i c a l cy c l o n e

( T C) c e n t r e d u r i n g t h e p e r i o d a f f e c t e d by T Cs w e r e

rem o ved t o e l im inate th e i nfl u ences o f T Cs on the e nvi-

ronm ental c h a r a c te r is tic s. T h e s e a tmo s ph er i c p a rameters

w e r e t h e n a v e r a g e d i n e a c h o f t h e f o u r r e g i o n s th r o u g h -

o u t th e i r r e s p e c t i ve t o r n a do -p r o n e m o n t h s t o o b t ai n

m e an va l u e s.

T h e t r o p i c a l cy c l o n e b e s t- t r ac k d a t a we r e o b t a i n e d

f r o m t h e HU R D A T 2 d a t ab a se ( L an d se a a n d

Franklin, 2 013, a v a ila b le o n l ine a t https:// ww w.nh c.

n o a a . g o v / d a ta / h u r d a t/) an d t h e d at a b a s e co mp i l e d b y

R e g i o n a l S p e c i a l i ze d M e te o r o l o g i c a l C e n t r e T o k yo

( R S M C , av a i l a b l e o n l i n e a t ht tp : / / w w w . j m a . g o . jp / j m a/

j m a - e n g / j m a - c e n t e r / r s m c -h p- p u b - e g /b e st tr a c k . h tm l) .

2.4 Tornado event and its tornado-|generating efficiency

Torna d o o u tb r ea ks of ten o cc u r i n t he U ni ted S t a tes bu t

a r e r a r e in C h i n a . A t o r n a d o o u t b r e a k e v e n t i s g e n e r a l l y

desc ribe d a s t he occurre nc e o f 1 0 o r m ore torn a do e s asso-

c i a te d w i t h t h e sa m e sy n o p t i c - s c a l e st o r m s ys te m

( Galway, 1 977; Glic kman, 20 00). I n o ur attem p ts to seek

t h e ca u s e s o f t h e d i f f e r e n t t o r n a d o n u m b e r s o b se r v e d

be tw een th e fou r ta r g e t r e gi o ns, i t is i mp ort an t to t ak e

i nto a cco u n t th e fr e q u e ncy wi th w hic h m u lt ip l e t o r na -

does occur in a s hort p eriod a nd m e as ure t he effec ts o f

t h is ty p e o f o c c u r r e n c e o n t h e to t a l t o r n a d o c o u n t s.

I n t h is w o r k , t o r n a do e s t h a t o c c u r a t cl o se ti m e s a n d

l o c a t i o n s a r e c o n si de r e d o n e t o r n a d o e v e n t . T h e t o r n a d o

event d ataset is b u ilt a s follows.

1. For e ach torn a do , a c o rresponding torna d o e vent is

d e t e r m i ne d t o i n c lu d e t h i s t o r n a d o a n d t h e t o r n a do e s

t ha t o cc ur in i ts p r o xi mi ty wi th in 6 h r a nd wi th in a

1 × 1 r egi o n r elat ive to th is t ornad o . T he in it ial se t

o f t h e n u m b e r (N t ) o f t o r n a d o e s t h a t o c c u r i n a t o r -

n a d o e v e n t is t h e n o b t a i n e d . T h i s da t a s e t i s la b e l l e d

t h e o l d t o r n a d o e ve n t d a ta s e t, a n d i t c o n ta i n s m a n y‘ ’

d u p li c a te t o r n a d o r e p o r ts .

2 . T h e t o r n a d o e v e n t wi t h t h e l a r g e s t N t in t h e o l d‘ ’

d a ta s e t i s se l e c t e d a n d m o v e d fr o m t h e o l d d a ta s e t t o‘ ’

t h e n e w da t a se t . T o r n ad o e v e n t s w it h th e s am e N t‘ ’

v a l u e s a r e p r o c e s s e d in ch r o n o l o g i c a l o r d e r . T h e t o r -

nado reports t hat o c cur in thi s torn ado e vent are iden-

t i f i e d a n d r e m o v e d f r o m t h e r e m a i n i n g t o r n a d o e v e n t s

in t h e o ld da t a se t . F o r t h e o ld d at a se t , t h e N t v a l u e s‘ ’ ‘ ’

f o r a l l t o r n a d o e v e n t s a r e t h e n u p d a te d , a n d t h e to r -

n a d o e v e n t s w i t h N t 0 ar e r e m o v e d .=

3 . T h e se c o nd st e p i s r e p e at e d u n ti l t h e o l d da t a s e t i s‘ ’

e m p t y .

4. Thes e t ornado even ts in the n ew da taset a re the n‘ ’

used for t he fol low ing a nalys is .

To a sses s th e pot ent ia l for mu l ti pl e torn a does to occ u r

i n a sh o r t p e r i o d i n a g i v e n r e g i o n , th e t o r n a do -

g e n e r at i n g e f f ic i e n c y o f t o r n a do e v e n t s is d e f i n e d a s t h e

n u m be r o f t o t al t o r n a d o r e p o r t s (N t o r ) d i v i d e d b y t h e

num ber of total tor nado events (Nte), wh ich r ef lects t he

a v e r a g e n u m b e r o f t o r n a d o e s th a t o c c u r p e r to r n a d o

event. It sh ould b e noted t hat th is d efi ni ti o n o f t he

t o r n a d o -g e n e r a ti n g e f f i c i e n c y o f to r n a do e v e n t s mo s t l y

con si ders th e to rna d o num ber a nd does not i ncl u de i nf o r-

m a ti on suc h as t he d urat ions , p at h le ngth s or int e ns it ie s

o f to r n a do e s . I n t h i s s tu d y , a n e f f i c i e n t to r n a d o e ve nt

d o e s n o t in di c a t e a l o n g -l i v e d , lo n g -t r a c k e d o r se v e r e t o r -

n a d o e v e n t .

3 | RESULTS

3.1 Tornado activity|

Th e c haract eris ti cs of t o rnad o a c ti v it ies i n JS a nd UC , th e

regi ons wi th t he h ighes t torn a do freque nci e s in C hin a

a n d t h e U n i t e d S t a t e s , r e s p e c ti v e l y , a r e e x a m in e d fi r s t . A s

s ho wn i n F i gu r e 2a , JS ha s a si mi la r t e m po r a l d i str i bu -

t i o n o f t o r n a d o e s t o th at o f a l l o f Ch i n a , w h i c h i s po s s i b l y

d u e to t h e m u c h h i gh e r f r e q u e n cy o f to r n ad o e s i n J S t h a n

i n oth e r r e g ion s, i ncl udi ng in the southern p art o f C hi na.

T h e sa m e i s al so tr u e fo r U C r e la t iv e to th e co n ti g u o u s

U n i t e d S t a t e s ( F i gu r e 2a ) . C o ns id e r in g t h a t t h e i r h ig h e s t

t o r n a d o d e n s i ti e s a n d mo n th l y va r i a ti o n s i n t o r n a d o

num ber are s imilar to th ose o f t he wh ole country, U C

a n d J S c an b e u s e d a s r e p r e s e n t a t i v e a r e a s o f t h e U n i t e d

S ta t e s a n d C h i n a , r e s p e c t i v e l y . F i g ur e 2 a s h o w s th a t th e r e

a r e a l s o so m e t o r n a d o e s t h a t o c c u r i n A pr i l a n d M ay i n

C h i n a , m o s t o f w h i c h a r e l o c a t e d i n t h e s o u t h e r n p a r t o f

China (Chen ., 2 018) . S outhern C hina is not t he fo cuset al

o f th i s s t u d y d u e t o i t s m o u n t a i n o u s te r r ai n a n d i ts r e l a -

t iv e l y l o w t o r na d o de nsi ty (F i gu r e S 1 ) .

T o a n a l y s e th e m o s t r e p r e s e n t a t i v e e n v i r o n me n t a l

ch aracteristic s o f d iff e rent re gi ons a t t he s am e time sc ale,

w e de f i n e d t h e t o r n a d o se a s o n o f e a c h r e g i o n a s t h e t h r e e

c o n se c u t i v e m o n t h s fr o m th e m o n t h b e f o r e to t h e m o n t h

ZH OU .ET A L 5



a f t e r t h e m o n t h o f p e a k to r n a d o o c c u r r e n ce . J S h a s a

p e a k t o r n a d o o c c u r r e n c e i n J u l y (F i g u r e 2 a ) wi t h a t o r -

n a d o s e a s o n f r o m Ju n e t o A u g u s t . In th e U C a r e a o f t h e

U n i t e d S t a t e s , t h e t o r n a d o s e a s o n l as t s f r o m A pr i l to

J u n e , wi t h t h e m o s t a c ti v e m o n th in M a y ( F i g u r e 2 a) .

M e a n w h i l e , b o t h U S E 1 a n d U SE 2 ha v e a to r n a do s e a s o n

f r o m M a r c h t o M a y ( F i gu r e 2a ) .

A lt ho u g h s o m e t o r n a d oe s a r e k no w n t o o c c u r i n a s s o -

c i a ti o n w i t h T C s , t h is w o r k f o c u s e s o n l y o n t o r na d o es t h a t

a r e no t a ss o c ia t e d w i t h T Cs . T C t o r n a d oe s a r e i d e nt i fi e d

w ithin 1, 000 km of the c entr e o f t heir parent TCs

( M cCaul, 199 1). A s show n in Figure 2b, T C t ornado es

h a v e l i tt l e e f f e c t o n t h e ti m i ng o f tor n a do s e a so n s o r o n

t h e t o r n ad o n u m be rs in t h e f o u r t a r ge t r e gio n s. T h e r e su lt s

s h o w t h a t T C t o r n a do e s t e n d to p r e v a il i n

t h e so u thea ste r n U n it e d S tate s f r o m Ju l y t o S e p te m b er .

Du rin g t he t o r na d o se a s ons t hr o u gh ou t t he 10 s tu die d

y e a rs , n o T C t o r n a do was o b s e r ved i n U C o r U SE 1 , a n d

o n l y o n e T C t o r n a do w a s id e nt i fi e d i n U S E2 i n M a y 2 0 1 2 .

I n J S du r in g i t s t o r n ado s e a s o ns , T C t o r na doe s a cco un t f o r

o n l y 1 0 % o f t h e t o t a l t o r n ad o e s, i n c l ud i ng fo u r N t > 1 T C

t o r na do e v e n t s , a l tho u gh JS c on s t itu te s a s pa ti a l p ea k o f

TC-ass oc iate d t ornadoes in Ch ina ( Bai . , 2020) . F or aet al

con siste nt com p arison, th e TC tornadoes were r emov ed

f r o m t h e f o l l ow in g d i s c u ss i on, if no t o t h e rw ise s t a t e d.

D uri ng the t ornado s e ason in JS, a total o f 135 non-

T C t o r n a d o e s w e r e r e c o r d e d f r o m 20 07 t o 2 0 1 6 , a c c o u n t -

i ng f or 23 % o f a l l reported non-T C torn adoes i n C hi na

from Ju ne to August in th is period. In c omparison, there

a r e 72 3 n o n -T C t o r n a d o e s r e c o r d e d i n U C d u r ing it s t o r -

nado s e asons from 200 7 t o 20 16, ac countin g for 10.7% of

a ll r e p o r te d no n - T C to r n a do e s in t h e U n i te d St a t e s in t h e

s a m e p e r i o d ; t h i s v a l u e e x c e e d s t h e n u m b e r o f t o r n a d o e s

t h a t w e r e r e c o r de d i n J S d u r i n g i t s to r n ad o s e a s o n s b y

fi ve ti mes .

Th e r esul ts show that a l arge di fferenc e i n t he poten-

t ia l f o r mu l t i p le t o r n a d o e s to o c c u r w it h in a sh o r t p e r i o d

ex is ts b e tween JS and U C i n th e ir respec tive tornado sea-

s o ns. T he num ber of Nt 1 torn a do even ts i n JS is m u ch>

s ma ll e r th an t ha t in U C , w hil e the n u mbe r o f N t 1 t o r -=

n a d o e v e n t s i n J S i s c l o s e t o th a t i n U C (F i gu r e 3 a ) . A

t o ta l o f 100 no n -T C to r n a d o e v e n ts w e r e i de n ti fi e d i n J S

d u r i n g i t s to r n a d o s e a s o n s (J u n e A u g u s t ) t h r o u g h o u t th e

s tu d y p e r i o d , i n c lu d i n g 78 t o r n a d o e v e n t s o f N t 1 a n d=

22 t o r na d o e ve nts o f N t 1 ( Fi g u r e 3 a) . M e a n whi le ,>

1 6 9 t o r n a d o e ve n t s w e r e i d e n t i f i e d i n U C , w i t h 72 t o r -

nado events of N t 1 a nd 97 tornado e ve nts o f N t 1= >

r e c o r de d d ur in g i t s to r n a do s e a s o n s (A pr i l J u n e )

t h r o u g h o u t t h e s tu d y p e r i o d ( Fi g u r e 3 a ) . I n U C , the

m o nthly di strib uti o n o f a ll tornado e vents is si mi lar to

t h a t o f N t 1 t o r n a d o e v e n t s , w i t h th e pe a k m o n t h do m ->

i n a te d b y N t 1 to r n ad o e ve nts ( F i g u r e 4a , c ) . In co n tr a s t ,>

t h e N t 1 t o r n a d o e ve n ts d o m in at e in J S (F ig u r e 4 b , c ).=

T h e di f f e r e n c e in th e n u m be r o f t o r n a d o e v e n t s o f

N t 1 d e t e r m i n e s t h e d i f f e r e n c e i n t h e to t a l t o r n a d o>

c o u n t be t w e e n J S a n d U C . F i g u r e 3 b s h o w s th a t 9 0 % o f

t o r n a d o e s i n U C a r e c h a r a c t e r i ze d b y N t 1 t o r n a d o>

events, while the m ajority ( 57.8%) o f t ornadoes in JS cor-

res pon d t o N t 1 torn ad o e vents. Th e d ifferen ce in t he=

t o t a l t o r n a d o c o u n t b e t w e e n J S a n d U C i s mo r e o b v i o u s

f o r hi g h - N t t o r n a d o e ve n ts . A t o ta l o f 5 1 t o r n a d o e v e n ts

o f N t 4 co n t r i b u t e to ne a r l y t h r e e - q u a r t e r s o f t h e to r ->

n a d o r e p o r t s in U C , w h i l e th e r e i s o n l y o n e ca s e o f

N t 4 r e c o r de d i n JS (no t sh o wn) .>

Si mi lar to U C , out st a nd in g con tri but ions of t o rnad o

e v e n t s o f Nt 1 t o t h e to t a l to r n ad o c o u n t s w e r e a lso>

ob served in USE 1 and U SE2 (Figure 3b). Durin g th e tor-

n a d o s e a s o n s ( M a r c h M a y ) , mo r e th a n 8 0 % o f the t o ta l

t o r n a d o c o u n t s we r e a t t r i b u t e d t o t o r n a d o e v e n t s o f

N t 1 in U S E 1 a n d U S E 2 . F i g u r e 3 a , b sh o w s th at t h e>

m a in dif ferenc e in the t otal tornado count between USE1

a n d J S co m e s f r o m t o r n a d o e v e n t s o f N t 1. E ve n>

t h o u g h U S E 2 h as fa r f e w e r t o r n a d o e v e n t s o f N t = 1 a nd

a sl i g h t l y gr e at e r n u m b e r o f N t 1 to r n ad o e v e n t s t h an>

FIGURE 2 M o n th l y d is tr i b ut i on s o f a l l to rn a d o es ( a ) a n d n o n -T C to r na d o e s (b ) f r o m 20 07 to 2 016 i n th e f o ur ta rg e t r eg i on s ( s ho w n b y

th e l e f t ax i s ) , c o n ti g u o us C h in a ( CH N ; sh o w n b y t h e l e f t ax i s) a n d th e U n i t ed S t a t es ( U S A ; s ho w n b y t h e ri g ht a x i s )

6 Z HO U .ET A L



JS, t he higher li kel iho od of high -N t torn a do events i n

USE2 (1 0 N t 4 cas e s in U SE2) h e lps it to e xc eed JS i n>

t e r m s o f t h e i r t o t a l t o r n a d o n u m b e r s .

I n agree men t w ith th e n um b er s a n d pr o po rtion s of

N t 1 to r n a do e v e n t s ( F i g u r e 3a ) , t h e t o r n a d o ->

g e n e r at i n g e f f i c i e nc y o f t o r n a d o e v e n t s ( N t o r / N te )

d e c r e a s e s mo n o to n i c a l l y f r o m U C ( 4 . 2 8 ) t o U SE 1 ( 3 . 8 9 ) ,

U S E 2 ( 2. 7 1 ) a n d J S ( 1 . 3 5 ) d u r i n g t h e i r co r r e s po n d i n g t o r -

nado season s, in di cating th e d ecreasing likelihood of

m u lt ip le t o rnad oes o c currin g d u ri ng a sin gle e vent in

FIGURE 3 ( a ) N u m be r s o f N t 1 ( re d ) a n d N t 1 ( b lu e ) n o n - T C t o r na do e v e n t s i n t he f o ur t a rg e t r e g i o ns d ur i ng th e i r re s p e ct i v e> =

to r n a d o s e a s on s ( U C, A p ri l to Ju n e ; U SE 1, Ma rc h to M a y ; U S E2 , Ma r ch t o M a y ; J S , J u n e to A ug us t) . ( b ) A s i n ( a ) , b ut fo r th e n u mb e r s o f

t o r n a d o r e p o rt s f ro m N t 1 n o n - T C t or n a d o e v en t s (r e d ) an d Nt 1 n on - T C to rn a d o e v e n t s ( b l ue )> =

FIGURE 4 M o nt h ly di s t r ib u t i o n s o f N t 1 n on - T C to rn a d o e v e n t s ( a ) , N t 1 n o n -T C t o rn a d o e v e nt s (b ) , a l l n o n -TC to r na d o e v e n ts> =

( c ) , a n d t o r n a d o - g e n er a t in g e f f i c i en c y o f t o r n a do e v e n t s ( d ) f r o m 2 00 7 t o 2 0 1 6 i n t h e fo u r ta r g e t re g io n s

ZH OU .ET A L 7



t h e s e r e gi o n s . H o w e v e r , th i s f e at u r e f a i l s t o be

m ai n t a i n e d o u t s i d e o f t h e to r n a do s e a s o n . In t h e t h r e e

t a r ge t r e g i o n s o f t h e U n it e d S t a te s , t h e to r n a do -

generatin g ef fic ien cy of torna d o e v e n t s d e c l i n e s o b v i o u s l y

i n s u m m e r a n d b e c o m e s s i m i l a r to t h a t o f J S i n J u l y a n d

A ug u s t ( F i g u r e 4 d) , wi t h fe w e r o c c u r r e n c e s o f N t 1 to r ->

n a d o e v e n t s i n s u m m e r t h a n i n s p r i n g ( F i g u r e 4a ) . Nt 1=

t o r n a d o e v e n ts be c o me mo r e co m m o n in s um m e r t h a n

N t 1 t o r na d o e ve nts (F ig u r e 4a ,b ).>

3.2 Environmental features related to|total tornado counts

T o a c c u r a t e ly d e s c r ib e t h e to r n a do d e n s i t y wi t h r e s p e c t

t o ar e a, we d iv i de d t h e to t a l t o r n a d o co u n t i n e a c h r e g i o n

b y the a rea o f t he region. A s s hown in Figure 1b ,d, b oth

t h e no r t h e a s t c o r n e r o f J S a n d t h e s o u t h e a s t co r n e r o f

U S E 2 c o v e r so m e o c e a n a r e a s , w he r e a l mo s t no t o r n a -

d o e s w e r e r e c o r de d . L at i t u d i n a l d i f f e r e n ce s a l s o c o n tr i b -

ute t o t he diff erences i n the lan d area. T he actual land

areas o f U C, USE1 , U SE2 a nd JS are 155, 78 5, 165, 819,

13 0,62 0 a n d 116, 195 km , r e s p e c ti v e l y . T h e to t a l t o r n a d o

c o u n t in e a c h o f th e f o u r t ar g e t r e gi o n s i s s c a l e d by i ts

l a n d a r e a a n d co n v e r te d t o a n a m o u n t p e r 4 0 0 4 0 0 k m×

h e r e a f te r , i f n o t o th e r w i s e s ta t e d . F i g u r e 5a , b sh o w s t h a t

t h e ar e a- sc a le d t o t al t o r n a d o c o u n ts d e c r e a s e f r o m U C

( 743) to USE1 (3 98), JS (18 6) and U SE 2 (18 3) durin g their

cor r espondin g t ornado season s.

T o e x a m i n e t h e m ai n e n vi r o n m e n t al fa c t o r s t h a t

a f f e c t t h e a r e a - s c a l e d t o ta l t o r n a d o c o un ts , t h e a v e r a ge

v a l u e s o f se ve r a l to r n a do - r e l at e d p a r a m e t e r s w e r e

FIGURE 5 ( a) T h e ar ea -s ca le d to ta l to rn a do co u n ts ( sh o w n b y th e l e ft ax i s) a n d th e a ve ra ge S RH1 , SH R6, ML L C L a nd ML C AP E val u e s

( s ho w n b y t h e ri g ht a x i s ) i n th e f ou r t a r g e t re g io n s du ri n g th e i r r e s p e ct iv e to rn a do s ea so n s . ( b ) Th e ar e a-s c al e d t ot al t or n ado c o un ts ( s h ow n

by the left axis ) and t he aver age c o mpos ite i ndic es EH I and STP ( s h ow n b y th e r i g h t a x i s) i n t he fo ur t a rg e t r e g i o ns d ur i ng t h e i r re s p e c t i v e

to r n ado s eas on s

8 Z HO U .ET A L



c a l c u l a t e d f o r e a c h o f th e f ou r r e g i o n s t h r o u g h o u t t h e i r

res pec ti v e torna d o sea s o ns u si ng th e m e th od d e s crib e d i n

S e c t io n 2 .3 .

T h e t a r g e t r e g i o n s i n t h e tw o c o u n t r i e s a r e n o ti c e a b l y

dif ferentiated by th eir e n v ironm e ntal features durin g

t h e i r r e s pe c t i v e t o r n a d o s e a s o n s . I n t h e J S r e g i o n o f

C h i n a , the ki n e m a t i c e n v i r o n m e nt is u n f av o u r a b l e fo r

t o r n a d o e s a n d s u p e r c e l l s , w i t h l o w me a n v a l u e s o f S R H 1

a n d S H R 6 , w h il e t h e t h e r m o d yn a m ic c o n d it io n s se e m to

b e c o n du c i v e to th e o c c u r r e n c e s o f s e v e r e s t o r m s , w i t h

r e l a ti v e l y h i g h M LC A P E a n d lo w M L LC L v a l u e s

( Fi g u r e 5a ) . C o m p a r e d w i t h J S , U C i s c h ar a c te r i ze d by a

m o re f a vourab le ki nematic e n v ironm e nt w i th hi gher

S R H 1 an d S HR 6 v a l ue s b u t i s a l s o c h a r a c te r i ze d by a

d r ier e nvi r onmen t wi th a h igher M LL C L val ue

( Fi g u r e 5a ) . U S E 1 a n d U S E 2 h a v e s i m i l a r e n v i r o n m e n t s

w i t h h i g h S HR 6 b u t lo w C A PE v a l u e s ( F i g u r e 5 a ) . T h e

p h e n o m e n o n i n w h i c h m a n y t o r n a d o e s o c c u r in hi gh -

s h e a r bu t l o w - C A P E e n v i r o n m e nt s i n t h e so u t h e a s t e r n

U ni t e d S ta t e s ha s g r a d u a ll y r e ce i v e d m o r e sig ni fi ca nt

atten tion r ec en tly ( e.g ., S h er b u r n a n d P a r ker , 201 4; Lon g

et al., 2018 ; B rown an d N owotarski, 202 0).

R e la t iv e t o t h e a b s e n c e o f c o n si st e n t va r i a ti o n

o b s e r v e d b e t w e e n a p ar t i c u l a r k i n e m a t i c o r t h e r m o d y -

n a m i c p a r a m e t e r a l o n e an d t h e to t a l to r n ad o co u n t , th e

co m p o sit e in d e x ST P sh o ws a co n sis te n t ch a ng e wi th

t h e mo n o t o n i c d e c r e as e i n t he a r e a -s c a l e d to t a l to r n a do

c o u n t fr o m U C t o U S E 1 , J S a n d U SE 2 ( F ig u r e 5 b ). U C

h a s a m u c h h i g h e r S T P va l u e w it h a mu c h h i g h e r to ta l

t o r n a d o c o u n t th an t h e o th e r t a r g e t r e g i o n s, f o l lo w e d b y

U S E 1 w i t h t h e se c o n d- h i g h e s t to t a l to r n a do co u n t . J S

a n d U S E 2 h av e si m i l a r S T P a ve r ag e s , w h i c h is c o n s i s t e n t

w i t h t h e i r co m p ar a b l e t o t a l t o r n a d o co u n t s . A l t h o u g h t h e

E H I i s a l s o a c o m p o s i t e pa r a m e t e r o f b o t h k i n e m a t i c a n d

t he r m o dy na m ic fa ct o r s si mil a r t o t he ST P , it p e r f o r ms

w o rse th an t he ST P i n a s sess ing the t otal tornado count

o f a g i v e n r e g i o n ( F i g u r e 5b ) .

T o v e r i f y t h e e f f e c t o f t h e S T P o n t h e t o t al t o r n a d o

c o u n t , w e c al c u l at e d S p e a r m a n ' s r a n k c o r r e l a t i o n c o e f f i -

cient ( Spearm an's ; K endall an d G ibb o ns, 1 990) , whichρ

ass e sses h ow w ell th e r ela tio n s h ip b etw een tw o s ets o f

data can b e d es crib ed us ing a m o n o t o n i c f u n c t i o n . T h e -p

v a l u e w a s o b t a i n e d fo r t e s t i n g t h e h y p o th e s i s th a t t h e r e is

n o c o r r e la t io n b e t we e n t h e t wo d a ta se ts a g ai n s t th e a lt e r -

n a t iv e h y p o t h e si s o f a no n ze r o c o r r e la t io n . T h e S T P a v e r -

a g e s a n d t h e to t a l to r n a d o c o u n ts we r e o b ta i n e d fo r e a c h

t a r ge t r e g i o n d u r i n g t h e co r r e s p o n d i n g t o r n ad o s e a s o n i n

e a c h y e a r o f t h e 1 0 - y e a r s t u d y p e r i o d, p r o v i di n g a s a m p l e

c o n ta i n i n g 4 0 S T P a v e r a g e s a n d 4 0 c o r r e s p o n d i n g to t a l

t o r n a d o co u n t v a l u e s . A s s h o w n in F i g u r e 6, hi gh t o ta l

t o r n a d o co u n t v al u e s te n d to b e a c c o m pa n i e d b y hi gh

S T P a v e r a g e s . T h e S p e a r m a n ' s b e tw e e n t h e 40 ST Pρ

a v e r a g e s a n d 4 0 t o t al t o r n a d o c o u n t v a l u e s is

a p p r o x i m a t e l y 0. 8 3 , a n d th e c o r r e s p o n d i n g - v a l u e i sp

m u ch low e r t han th e si gni fic an ce level o f . 05. T his fur-

t h e r c o n f i r m s t h a t t h e t o t a l t o r n a d o c o u n t i n a gi v e n

r e g i o n c o r r e l a te s st r o n g l y w it h t h e S T P , i n d ic a ti n g t h a t a

r e g i o n w i th a hi g h m e a n S T P v a l u e m a y ha v e m o r e t o r -

n a d o o c c ur r e n c e s .

3.3 Environmental features related to|monthly variation of tornado activity

A s disc ussed in Sec tion 3.1, the t hree target regi ons o f the

U n i t e d S ta t e s h a v e m an y m o r e t o r n a d o e s a n d r e l a t i v e l y

hi gher torn ado-generating e ffi ci enci es of tornado e ven ts

i n sp r in g th a n i n su m me r , wh il e t he JS r e g io n o f C hin a

h a s t h e m o s t ac t i v e t o r n a d o o c cu r r e n c e i n s u m m e r . T o

e x p l o r e th e m a i n e nv i r o n m e n t a l fa c to r s t h a t c o n t r i b u te

to m o nthly vari a tions i n t ornado ac ti vi ty, t he monthl y

a v e r a g e s o f th e s e t o r n a d o - r e l a t e d p a r a m e t e r s w e r e ca l c u -

l a te d a s d e scr i be d in S e ct i o n 2. 3 .

O n e in te r e s t i n g f e a t ur e o f S H R 6 i s th a t it s m e a n

v a l u e s in t h e th r e e ta r g e t r e g i o n s o f the U n i t e d St a t e s a r e

a l l su b s t a n ti a l l y l a r g e r t h a n i t s m e an v a l u e i n J S

( Fi g u r e 5a ) , s u g g e s t i n g th a t th e U n i te d St a te s m a y ha v e

m o re f a vourab le ki nem a ti c e nviron men ts for th e g en esis

of s u per cells (Markow ski an d R ic hardso n , 2010 ). Our

a n a l y s e s s h o w tha t thi s i s n o t be c a u s e t h e U n i te d S ta t e s

h a s a ge n e r a l l y la r g e r SH R 6 t h a n C h i n a a t a n y ti m e o f

t h e y e a r , a s w e s pe c u la t e d b e f o r e . T h e f o u r t a r g e t r e g i o n s

a c t u a l l y h a v e s i m i l a r S HR 6 val ues in t he s a me m o nths,

w i t h s i m i l a r d e c r e a s i n g t r e n d s fr o m M a r c h t o A u g u s t

FIGURE 6 A s cat te r pl o t b e t we e n t h e ST P ave r age s an d th e

ar e a- sc al e d t ot al t or n ado c o un t s i n t h e fo u r t a r g e t r e g i o n s d ur i n g

t he i r 1 0 s t u d i e d t or n a d o s e a s o ns

ZH OU .ET A L 9



( Fi g ure 7a) . T he mon thl y vari a ti ons in S HR6 in t he f o ur

t a r ge t r e g i o n s p r o b a b l y de pe n d o n c h a n g e s i n t h e ma g n i -

t u de o f th e w in d a t a n a l ti tu d e o f 6 k m a bo v e g r o u nd

l e v e l (A GL ) r a t h e r th an t h e su r f a c e wi n d . T h e w in d

s p e e d s a t a n al t i t u de o f 6 k m A GL a r e u s u a l l y st r o n g e r

t h a n the w i n d sp e e d s a t t h e s u r f a c e , a n d th e a v e r a g e d

s u r f a c e w i n d ma g n i tu d e d o e s no t c h a n g e o bv i o u s l y fr o m

m o nth t o m ont h (F ig u r e 7b) . I n co ntra s t, a s t he m id - t o

h i g h - l e v e l w e s t e r l i e s w e a k e n a n d t h e j e t s t r e a m s m o v e

pole ward fro m s p ring to summ er, th e wind speed at an

a lt i tu d e o f 6 km A G L i n e a c h t a r g e t r e gi o n sh o ws a si m i -

l ar d e cr e a si ng t r e nd a nd s imi l a r m a g n i tu d e o f v a r i at i o n

a s th o s e o f S H R 6 fr o m M a r c h t o A ug u s t. In a dd i ti o n to

t h e me a n v al u e s , t h e bo x - a n d - w h i s k e r pl o t o f S H R 6

sh o ws a r o u g h ly sy m m e t r ic d is tr i b u t io n w it h si m il a r

m e di a n a n d me a n v a l ue s (F i g u r e 8 a) , d e m o n st r a t i ng th at

a hi gh e r mean value i s main ly cont ri bute d b y a larger fre-

q u e n c y o f h i gh e r va l u e s .

T h e e a r l i e r t o r n a d o s e a s o n s o b s e r v e d i n th e thr e e

r e g i o n s o f t h e U n i te d St a t e s c o m pa r e d to t h a t o b s e r v e d i n

JS are p oss ib ly d ue to th e d if ferent m o nthl y variations

i n i n s ta b i l i t y a m o n g th e s e ta r g e t r e g i o n s . T h e m e a n v a l u e

o f M L C A P E s h o w s a r a p i d i n c r e a s e in U C , U SE 1 a n d

U S E 2 b u t ma i n t a i n s a m u c h lo w e r v al u e in J S f r o m

M a r c h t o M a y ( F i g u r e 7a ) . Di f f e r e n t f r o m SH R 6 , th e b o x -

a n d - w h i s k e r p l o t s o f M LC A P E h av e m e a n v a l u e s mu c h

l a r ge r th an th e ir m e d ia n va l u e s, whi ch a r e qu i t e clo s e t o

z e r o i n s p r i n g ( F i g u r e 8b ). A h i g h e r m o n t h ly a ve r a g e o f

M L C A P E i n d i c a te s a h i g h e r f r e q u e n c y o f st r o n g l y un s t a -

b le e nvi r onmen ts i n t hat m onth . I n th e s p ringtim e , warm

a n d h u m i d a i r i s tr a n s p o r te d f r o m th e G u l f o f M e x i c o to

U C , U SE 1 a n d U SE 2 t h r o u g h th e p r e v a i l i n g so u t h e r l y

fl ow s i n the low e r troposp here an d t he s tron g low - level

s o u t h e r l y j e t a t 9 2 5 hP a o v e r t h e so u t h e r n G r e a t P l a i n s

( Fen g . , 20 19; Molina and A llen, 2019 ). In c o ntrast,et al

J S s t a r ts to h a ve a l ar g e M LC A P E m e a n v a l ue o n l y f r o m

J u n e ( F i g u r e 7a ) . T h e d e l a y e d hi g h M L CA P E va l u e s

o b se r v e d i n J S c o m pa r e d w it h t h o s e in th e U . S . lo c a ti o n s

a r e p o s s i b l y d u e t o t h e t r a n s i t i o n o f ba r o c l i n i c z o n e s f r o m

S o u th C hi na i n M a r ch an d Ap r i l to th e m id - la ti t u de J S

r e g i o n i n J u n e a n d J u l y ; th i s tr a n s i t i o n i s a s s o c i at e d w i th

t h e no r t h w a r d p r o pa g a t i o n o f th e s u m m e r m o n s o o n

( T ang . , 2020) . R esulting from th ese season al trans i-et al

t i o n s o f ba r o c l i n ic i t y a n d mo i s t u r e , h i gh M L C A P E va l u e s

a p p e a r i n J S i n s u m m e r ( Fi g u r e 7a ) . C o n s e q u e n t l y , t h e

t h r e e ta r g e t r e g i o n s o f t h e U n it e d St a t e s r e a c h a g o o d

m a tch be t w e e n S H R 6 a n d M LC A P E e a r l i e r th a n J S , a n d

t h u s h av e h i g h e r S H R 6 v a l u e s th a n JS i n th e i r

cor r espondin g t ornado season s.

In a d di ti on to S HR 6, m onth ly a ve r ag es of S R H1 a l so

s h o w a n o b vi o u s d e c r e a s i n g tr e n d f r o m s p r i n g t o s u m m e r

i n t h e t ar g e t r e gi o n s o f th e U n i t e d S ta t e s , a n d t h e a v e r -

a g e s in U C a r e c l e a r l y la r g e r th an t h o s e o f U SE 1 a n d

U S E 2 i n M a y a n d Ju n e (F i gu r e 9 a ) . T h e JS r e g i o n o f

C h i n a , h o w e v e r , h a s a r e l at i v e l y f l a t m o n t h l y v a r i at i o n in

S RH 1 wi th a s mal l p e ak in J u ly ( F igure 9 a). T h e S RH 1

a v e r a g e s o b s e r v e d in J S in s pr i n g a r e m u c h s m al l e r t h a n

t hose i n t he U.S . loc at i ons but b e come sl ight ly larger in

J u ly . S imi la r t o S H R6, th e S R H1 h a s a r o u g h l y s y m m e tr i c

d i s t r i b u t i o n as s h o w n in it s b o x -a n d - wh is k e r pl o t

( Fi g ure 8c) w hic h a lso s hows that th e d ec reasin g t rend of

m o nthly a veraged SR H1 from s pring to sum mer in th e

U.S. l o cations m ain ly r esul ts from m u ch l e ss o ccurrenc es

o f h i g h S R H 1 v al u e s g r e a te r t h a n 1 0 0 m s . SR H1 m e a -

s ur e s th e p o t e n t i a l fo r c y c l o n i c u pd r a f t r o t a t i o n , a n d

e n vi r o n m e n t s w i t h l a r g e S R H1 v a l u e s ma y h av e m o r e

s tr e a m w i s e v o r t e x l i n e s t i l te d u pw a r d a t l o w e r l e v e l s t o

f o r m s t e a d y l o w - l e v e l me s o c y c l o n e s t h a t st r e n g t h e n l o w -

l e v e l u p dr a f ts t o p o s s ib ly t r i gg e r t o r n a d o ge ne s i s t h a n

e n vi r o nm e nt s wi th sm al le r S RH 1 v a lu e s . T he t ar g e t

r e g i o n s i n t h e U n i t e d S ta t e s h a v e a r e l a t i v e l y hi g h e r

t o r n a d o -g e n e r a ti n g e f f i c i e n c y o f t o r n a d o e v e n t s th a n J S

d o e s , p o s s ib l y d u e i n p a r t t o t h e i r hi g h e r SR H 1 va l u e s

o b s e r v e d d u r i n g t h e i r e ar l i e r t o r n a d o s e a s o n s ; t h i s is

FIGURE 7 ( a ) M o nt h ly v a ri a t i on s i n S H R 6 ( so l i d l i n e ; sh o w n

b y t h e l e f t ax i s ) a n d ML CA P E ( d a s h e d l i n e ; s h o wn b y t he ri g h t

a x i s ) i n U C, U S E 1, U SE 2 an d J S . ( b ) M o n th l y v a r i a ti o n s i n wi n d

s p e e d a t t he su rf ace (d a s he d l i ne ) a nd at an a lt i t u d e o f 6 k m A GL

( s ol i d l i n e ) i n U C , U S E 1 , U SE 2 a n d J S

10 Z HO U .ET A L

of 19Differences in tornado activities and key tornadic environments between China and ...


con siste nt w ith th e r e sults of Anders on-Frey . (2 018) ,et al

w h o fo u n d tha t t h e SR H 1 v a l u e s ar e h i g h e r f o r o u t b r e a k

t o r n a d o e s th an fo r i s o l a t e d t o r n a d o e s . S R H1 v a l u e s a r e

se nsi ti v e t o t he ma g ni tu d e a n d o r ie nta t io n o f lo w -l e v e l

w i n d s h e a r , a s w e l l a s to th e e s ti m a t e d s t o r m mo t i o n s .

M o n t h l y v a r i a t i o n s i n SR H1 a r e c l o s e ly r e l a te d to th e

m ag n i t u de o f th e a v e r a g e w i n d s p e e d a t a n a l t i t u d e o f

1 km AG L , wh ic h s h o w s a s i m il a r t r e n d f r o m s p r i n g t o

su m m e r t o tha t o f S RH 1 (F ig u r e 9a ,c ).

T h e M L L C L i s g r e a t l y d e t e r m i n e d b y t h e l o w - l e v e l

m o ist u re. R elat ive to U S E1 a nd US E2 near th e G u l f o f

M e x i c o a n d A tl a n t i c Oc e a n , t h e U C r e g i o n h as l e s s

m o ist u re tra n sp orted from t he G u lf of M e xic o an d i s th u s

general ly c haracte r iz ed by l o w e r r elat ive h umid i ty a t the

s u r f a c e a n d h i g h e r M L L CL th an t h e U SE 1 a n d U SE 2

r e g i o n s (F i g u r e 9 b , d) . T h e in c r e a s e in M L L CL o b s e r v e d

i n UC in sum mer poss ibl y resul ts f rom in creased surface

t e mp e r a t u r e s. I n c o nt r as t wi t h U C , J S ha s a n a p p a r e nt

i ncrea s e in re la ti v e hu mi d i ty a t th e s u r fa ce a nd a

d e c r e a s e in M L L CL fr o m s p r ing t o s u m m e r (F ig u r e 9 b , d ),

b e n e f i t i n g f r o m a l ar g e a m o u n t o f m o i s t a i r t r a n s po r t e d

f r o m t h e So u t h C h i n a Se a a n d In di a n Oce a n, in flu e n ce d

b y t h e su m m e r mo n s o o n . T h e t o r n a d o se a s o n s i n U C a n d

JS c o rrespond to t he mon th s wh en th eir M LLCL mon thly

m e a n v a l u e s a r e r e l a t i v e l y l o w (F i g u r e s 2b a n d 9 b ) . A s

w it h S H R 6 a n d SR H1 , t h e M L L C L h a s a r o u g h l y s y m -

m e tri c d ist rib u ti on a s show n in i ts box -a n d- w his ker p l o t

( Fi g u r e 8d ).

The mon thl y di st rib u ti ons o f t hes e kinem a ti c a nd

therm o dy n am ic p ara m eters d if fer from e ach o th er in a

gi ven r egi o n; thus, t he mon ths i n whi ch t he para meters

b e s t m a t c h sh o u l d be th e m o s t f a v o u r a b l e m o n th s f o r

FIGURE 8 B o x -a nd -w h is ke r s pl o ts o f S HR6 ( a) , ML CA PE (b ) , SR H1 (c ) , M L L CL (d ), E HI ( e ) an d ST P (f ) i n th e f ou r ta rge t re g io n s fr o m

M a rc h to A ug us t dur i n g t he 1 0 st ud i ed ye a r s . Th e s h a d e d bo x e s s p a n t h e 2 5 t h 7 5 t h pe rc e n t i l e s, an d t h e w h i ske r s e x t e n d d ow n w a r d to t h e–

1 0 t h a n d u p w a rd to t h e 9 0 t h p e r ce n t i l e s . Me a n v a l u e s are m a rke d b y d ow n w a rd - p o i n t i n g t ri a n g le s a n d me d i a n v a l u e s are m a rke d b y c i rc l es

ZH OU .ET A L 11



t o r n a d o e s to o c c u r . T h e co n f i g u r a t i o n o f t h e s e p a r a m e -

t e r s wa s e xa m in e d th r o u gh t h e ca lc u l a ti o n s o f EH I a n d

S T P i n th i s st u dy . T h e r e su lt s sh o w th at m o n t h l y a v e r -

aged E HI shows a n i ncon sisten t tre nd with th e m onthly

variation s in t ornado frequen cies in U C a nd USE 2, w hil e

t h e mo n thl y a ve r a g e d S T P r e f l e c ts t h e mo n th l y v ar ia ti o n s

i n th e a r e a - s c a l e d n u m b e r s o f to r n a do e s i n t h e fo u r t ar g e t

r e g i o n s we l l (F i g u r e 1 0 ) . Fo r a g i v e n r e g i o n , t h e p e a k

m o nth o f t he m o nthly a ve raged STP corres po nd s to the

p e a k m o n t h o f t h e a r e a - s c a l e d nu m b e r o f t o r n a d o e s

( Fi g u r e 1 0b) . Co mp a r ed wi th th e mon thl y a v e ra ge d EH I,

w hi ch o nly com bi nes S RH1 a nd CA PE, th e m o nthl y

a v e r a g e d S T P i s a be t te r in d i c a t o r o f t h e ki n e m a ti c an d

t he r m o dy n am ic e n vi r o nm e nt a l c o n f i g u r a ti o n s a s s o c i a t e d

w i t h t o r n a do o c c ur r e n c e s . As w i t h M L C A P E , b u t u n l i k e

S H R6, S RH 1 a nd MLL C L , EH I a n d S T P h ave m ean

v a l u e s m u c h la r g e r t h a n t h e i r m e d i a n v a l u e s , w h i c h a r e

cl o se t o z e r o i n e v e r y m o nth ( Fi g u r e 8 e ,f) . T he mo nth ly

FIGURE 9 M o nt hly va riat ions i n S RH1 ( a), M LLCL ( b) , w in d sp eed at an a lt i tu de of 1 km A GL (c ), and relat iv e hum i dity at the s ur fac e

( d) i n th e f ou r ta rge t r eg i on s

FIGURE 10 ( a ) Mo n th l y v a r i a ti o n s i n th e E H I (s h o wn by th e l e f t ax i s ) a n d th e a r e a -s c a l e d n u mb e r o f n o n -T C to r na d o e s (s h o wn by th e

r ig h t a x is ) i n th e f o ur ta rg e t r eg i on s . ( b ) M o n th l y v a r ia ti o n s in th e S T P ( sh o w n b y t he l ef t a x i s) a nd th e ar e a - sc a l e d n u mb e r o f n o n -T C

t o r n a d o e s ( s h o w n b y t h e r i g h t ax i s ) in t h e fo u r t a r g e t re g i o n s

12 Z HO U .ET A L



a v e r a g e s o f E HI a n d ST P a r e g r e a tl y a f f e c t e d b y e x t r e m e

v a l u e s t h a t r e p r e s e n t f a v o u r a b l e k i n e m a t i c a n d the r m o -

d y n a m i c e n v i r o n m e n ts f o r t o r n a d o g e n e s i s ( F i g u r e 8e , f ) .

4 | DISCUSSION

In the p revious section , th is w ork r evealed th a t t he di ff er-

e nt li k e l iho o d s o f mu l ti pl e to r n a do e s o ccu r r i ng i n a sho r t

p e r i o d b e t w e e n t h e s e t a r ge t r e g i o n s a r e m o s t p o s s i b l y

r e s po n s i b l e fo r t h e i r la r g e d i s c r e p an cy i n to r n a do o c c u r -

renc es. T he n u mb er o f to r n a do e v en ts th a t o c cur at the

sam e tim e is also of interest from th e v iewpoin t of

t h e sy n o p t i c c o n t r o l o f t o r n a d i c e n vi r o n m e nt s . W e e x a m -

i n e d t h e nu m b e r o f to r n ad o d a y s ( Nt d ) i n t h e f o u r ta r g e t

regions a nd th e r a tio o f th e nu m b er o f t o r n a do even ts

( N t e ) t o t h e N td , w h ic h ca n b e u s e d t o imp l y t h e a p p r o x i -

m at e a v e r a ge n u m b e r o f t o r n a d o e ve n ts co n t r o ll e d b y t h e

sam e s y nopti c sit u ati o n (Tab le 1). The r esul ts sh ow th at

t h e r a t io s o f Nt e in JS , U C , U S E 1 a n d U S E 2 t o t h e i r

r e s pe c t i v e N t d a r e 1 . 2 8 , 1 . 4 6 , 1 .6 6 a n d 1. 4 1 , r e s p e c t i v e l y ,

s u g g e s ti n g th a t th e U . S . r e g i o n s h a ve l a r g e r s y n o p ti c co n -

t r o ls o n t h e ir t o r n a d ic e n v i r o n m e n t s th an t h e JS r e g io n ,

w i t h m o r e t o r n a d o e v e n t s o c c u r r i n g pe r t o r n a d o da y .

S y n o p t i c s i t u a t i o n s h a v e a n i m p o r t an t in f l u e n c e o n

the o ccurr e nce of severe storms. T he c o mmon ly us e d

t o r n a d o -r e l a te d p a r a m e t e r s e x a m i n e d in t h i s st u dy d o n o t

m e as u r e fo r c ing fo r a sce n t, w hic h is an i mp o r t a nt fa c to r

i n t h e fo r m a t i o n o f s e v e r e s t o r m s . La r g e - s c a l e u p w ar d

m o ti on forc ing i s o ften ass o ci ated w ith apparen t

sy n o pt i c-s ca le w e a th e r s y st em s. Ri sin g m o ti o n is ty p i ca ll y

com mon ahead o f a trough or n e ar fron tal s yste ms th at

a c c o m p a n y e x t r a tr o p i c a l c y c l o n e s .

T o o b t ai n a g e n e r a l i de a o f t h e s y n o p t i c fe a t u r e s t h a t

are favourab le for t ornadoes , w e e xami ned t he synoptic

si tuation s duri ng al l 22 Nt 1 torn ado e vents in J S a nd>

c o m p a r e d t h e ty p i c a l s y n o p ti c s i tu a t i o n i n J S w i t h th o s e

i n t h e ta r g e t r e g i o n s in t h e U n i t e d S t a te s . F o r e a c h t o r -

n a d o e v e n t , t h e cl o s e s t N C E P F N L d a t a b e f o r e t h e m e a n

t i m e o f o c c u r r e n c e s o f a l l t o r n a d o e s w e r e u s e d . T h e

r e s ul t s s h o w th a t mo s t N t 1 t o r n a d o e v e n t s in JS>

o c c u r r e d in f r o n t o f a so u t h w e s t n o r t h e a s t - o r i e n t e d–

t r o u g h a t 5 0 0 hP a , wi th t h e s u b t r o p i c a l h i g h lo c at e d i n

t h e e a s t o r so u t h e a s t ( F i g u r e S3 a k ) . S o m e t o r n a d o–

e v e n t s w e r e m a i n l y c o n tr o l l e d b y a sh o r t ( F i g u r e S 3 l q )–

o r a d e e p t r o u g h ( F i g u r e S 3 r t ) a l o n e , w i t h t h e s u b t r o pi -–

cal hi g h located farther to th e south. The r em aini ng tw o

t o r n a d o e v e n ts we r e ma i n l y co n tr o l l e d b y t h e s u b tr o p i c a l

h i g h al o n e (F i g u r e S3 u v ) . T h e up p e r - l e v e l j e t s t r e a m s–

m o v e p o l e w a r d a n d be co m e we ak e r fr o m sp r i n g t o su m -

m e r ( Man ney ., 2 014) an d a re thus loc a ted more toe t a l

t h e no r t h o f t h e t a r g e t a r e a i n Ch in a th an t h o se in t h e

U n i t e d S t a te s d u r i n g th e to r n a d o se as o n . O v e r E a s t

C h i n a , t h e r e g i o n wh e r e t o r n a d o e s a r e m o s t f r e q u e n t i n

C h i n a , e x t r a tr o p i c a l cy c l o n e s a r e w e a k e r a n d l e s s fr e -

q ue nt in sum mer th a n i n spring (Lee . , 2 019) . F ewe t a l

N t 1 to r n a do e v e n t s w e r e f o u n d to be a s s o c ia t e d w i t h>

e v i de nt su r f a c e c y c l o n e s a c c o m pa n i e d by th e u p p e r - l e v e l

j e t ( F i g u r e S3 n q ) . R e l a ti v e to J S, e v e n t h o u g h t h e t yp i c a l–

p a t te r n s f a v o ur a b l e f o r mu l ti p l e to r n a do e s i n

U . S . l o c a t io n s we r e a lso m o st l y l o c a t e d i n f r o n t o f a d e e p

t r o u g h o r s o m e t i m e s in fr o n t o f a s h o r t t r o u g h , t h e y w e r e ,

h o w e v e r , a c c o m p a n i e d by m u c h st r o n g e r a n d w i d e r

u p p e r - l e v e l j e t s t r e a m s a n d m o r e i n t e n s e a n d l a r g e r s u r -

f a c e c y c lo n e s (F ig u r e s S 4 , S 5 a n d S 6 ). E x t r at r o p ic al

c y c lo n e s w i t h s t r o n g u p p e r - l e v e l j e t s t r e a m s e n a b le th e

p r o d u c t io n o f e n v ir o n m e nt s t h a t a r e r i c h in v e r t ic a l wi n d

s hear in t he warm s e ctor, t hus e nhan cin g t he pote nti a l o f

t o r n a d o o u t b r e a k s ( M e r c e r . , 2 0 1 2 ; T o c h i m o t o a n de t a l

N i i n o , 2 0 1 6 ) . In a d d i t i o n , e x t r a t r o p i c a l c y c l o n e s o v e r t h e

U n i t e d S t a t e s i n s u m m e r a r e ma i n ly c o n f i n e d t o r e l a -

t iv e l y hi g he r l a ti t ud e s t ha n th o se i n sp r in g (W e r n li a nd

Schw ie rz, 2006 ). A s a r esult, the frequen cy of

e x tr a t r o p i c a l c y c l o n e s d e cr e a s e s i n U C , U S E 1 a n d U S E 2 ,

w h i c h ma y b e p a r t l y r e s p o n s ib l e fo r t h e d e c r e a s e in t h e i r

m e a n v al u e s o f S R H 1 a n d SH R 6 fr o m sp r i n g to s um m e r .

E x am p l e s o f t h e m o s t co m m o n p a t t e r n s o b s e r v e d i n

J S ( F i g u r e S 3 a ) , U C ( F i gu r e S 4 a ) , U SE 1 ( Fi g u r e S 5 a ) a n d

USE2 (Figure S6 a ) w ere s elec ted t o furth er investi gate

their d etailed d if ferenc es in en vironm ental features a t d if -

fe rent lev e ls (F ig ures 11 and S7) . J S w as inf luen ced by a

d eep trou g h t o th e w est a nd a s u btrop i ca l hi g h to t he e a st

i n th e mid d l e t rop o sp here, wi th s y nop ti c- sca l e s ou th wes t-

erl y f low s in t he low e r t roposphere. T he values o f S HR 6

w e re genera l ly sm a ll i n JS. T he strong southwes ter ly

f l o w s e n h a n c e d t h e no r t h e a s t w a r d t r a n s po r t o f mo i s t u r e

fro m the south to JS as w e ll as en hanci ng t he low -l e vel

w i n d s p e e d , w h i c h w as fa v o u r ab l e f o r t h e d e v e l o p m e n t o f

i nst a bi li ty a n d la r g e S R H1 v a lu e s (F i gu r e S 7 i, e ). H o w -

ever, t he re gi on with a g ood configu r ation o f kin ematic

a n d th e r m o d y n a m i c c o n d i t i o n s w i t h a hi gh S T P v a l ue

w a s li mi te d to o nly a sma l l a r e a ne a r J S ( F ig u r e S7 m). I n

con tras t, i n t he cases o f th e US loc ati ons, intens e surfac e

c y c l o n e s w i th s tr o n g u p p e r - l e v e l j e t s p r o du c e d l a r g e a r e a s

w i t h s t r o n g v e r t i c a l w i n d s he a r a n d l o w - l e v e l st o r m r e l a -

t iv e he l ici t y (F ig u r e S 7 ). In a dd i ti o n, t he U C r e g io n w a s

TABLE 1 N u mb e rs o f to rn a d o da y s ( N td ), to r na d o e v e n ts ( N te )

a n d th e r a t i o s o f N t e to N t d in t h e fo u r t a r g e t re g i o n s d u ri n g t he i r

r e spec tive to rn ad o seas o n s

UC USE1 USE2 JS

N td 1 16 6 4 39 78

N te 1 69 1 06 55 10 0

Nte/Nt d 1.4 6 1.6 6 1.4 1 1.28

ZH OU .ET A L 13



con trolled b y a lo n g d r y lin e a n d ac co mpa n ied b y a s outh-

e r l y je t a t 9 2 5 h P a ( F i g u r e S 7 j ) . Dr y a i r f r o m t h e w e s t e r n

h i g h l an ds w a s a dv e c te d o ve r w ar m h u m i d a i r fr o m th e

G u l f o f M e x i c o , p r o d u c i n g a la r g e a r e a w i t h h i g h po t e n -

tial ins tab il i t y a nd convergenc e f or convec ti on ini ti a ti on.

T h e be t t e r m a tc h o f t h e k i n e m a ti c an d t h e r m o d y n a m i c

p a r a m e t e r s i n U C a n d t h e m o r e f a v o u r a b l e k i n e m a t i c

con di t ions in US E1 and U SE 2, hel p ed p r od uce w id er

areas wi th hi g her S TP values t han t hose in JS

( Fi g u r e S 7 ) . T h e s e h i g h -S T P a r e a s w e r e fa v o u r a b l e f o r

t r i g g e r i n g w i de s p r e a d t o r n a d o o ut b r e ak s.

Th e tornado-generating e ff ici e ncy o f t ornado eve nts is

als o greatly inf luenc e d by the con vec tive m o de s o f t he

parent stor ms. Relative to a ll other n on-supercell con ve c-

t i v e m o d e s , s u p e r c e l l sy s t e m s w e r e f o u n d t o be mo r e p r o -

d u c t iv e o f t o r n a d o e s ( D u d a an d G a ll u s , 20 1 0 ) . S m i th

e t a l . ( 2012 ) show e d that th e ratio o f non -s upe r cell torn a -

d o e s t o s u p e r c e ll t o r n a d o e s c o u ld d i f f e r a m o n g d if f e r e n t

r e g i o n s , a n d t h e e s t i m a te d p e r c e n t a g e o f r i gh t- m o v i n g

s up e r c e l l to r n a d o e s w a s gr e at e r t h a n 8 0 % i n U C an d

g r e a t e r t h a n 70 % i n U S E 1 a n d U S E 2 . C o m p a r e d w i th the

t a r g e t r e g io n s in t h e U n it e d S t at e s, JS fe a t u r e s m u c h we a -

ker a mb ient environ men t she ar during its torn a do s e a-

s o n , w h i c h i s un f a v o u r a b l e f o r t h e d e v e l o p m e n t o f

s up e r c e l l s . T h e p r o p o r t i o n o f s u p e r c e l l to r n a do e s i n J S i s

probab ly low e r th an t hose i n UC, U SE1 a nd USE 2. T he

detail ed stati stic a l characteris ti cs of convecti ve m o des

t h a t a r e c o n d u c i v e t o t o r n a d o e s i n C h i n a r e m a i n u n c l e a r

a n d a r e w o r t h e x p l o r i n g i n f u t u r e r e s e a r c h .

I n a d d it io n , t h i s st u d y o n ly f o c u s e s o n t o r n a d o - pr o n e

r e g i o n s a n d o n t h e p r i m e m o n t h s o f t o r n a d o o c c u r r e n c e

i n the t wo studied countries ; t he en vironm en tal

FIGURE 11 E x a mp l es o f

th e s yn o ptic s it u atio n s of ty pica l

h i g h - N t t o rn a d o e v e nt s i n JS ( a ),

U C (b ) , U SE 1 ( c) , a n d U S E 2 ( d ) ,

i n cl u d i ng th e g e op ot e n t i a l

h e ig h ts ( b lu e c o nt o ur s; 1 0 g p m)

a t 5 0 0 h P a , se a l e v e l p r e ss u re

( re d c o n to u rs ; h P a ) a nd th e

w in d sp e ed s g re a t e r th a n 3 5 m

s a t 20 0 h P a ( s h a d i n g , m s ) .

T h e l o ca ti o ns o f to rn a d oe s a re

denoted b y b lack dots a n d the

tar ge t re g io n s ar e i n di ca te d b y

b l a ck b o x e s. Th e ti m e o f N C EP

F N L da ta , d e t e r m i n e d a s t h e

cl o s e st ti m e b e f o re t h e m e a n

ti m e o f o cc u rr e nc e s o f al l

to r na do e s o f th e c o rr es po n di n g

t o r na do e v e n t , h a s b e e n l a b e l l ed

a t th e t op o f t he co rr e sp on d i n g

p a n e l

14 Z HO U .ET A L



d i f fe r e n c e s o b t a i n e d he r e i n ca n n o t be e x tr a p o l a t e d t o a l l

a r e a s in e i t h e r co u n t r y . T o r n a do a c t i v i t i e s a n d t o r n a do -

r e l a te d e n v i r o n m e n t a l p a r am e t e r s h a v e o b v i o u s r e g i o n a l

d i f f e r e n c e s a n d s e as o n a l v a r iat ion s. F o r e xamp l e , th e

t o r n a d o - p r o n e r e g i o n i n th e s o u t h e r n p a r t o f C h i n a ,

w h i c h is l o c a t e d i n G ua n g d o n g P r o v i n c e a n d l ab e l l e d GD

( a 4 × 4 a r e a wi t hin 2 1 25– N a nd 1 11 115– E , a s sho w n

i n F i gu r e S 1 a ) , ha s a sp r i n g t i m e t o r n a d o p e a k

( Fi g u r e S1 b,c ). T h e e v o lu t i o n s o f SH R 6 , SR H 1 , M L C AP E

a n d S T P i n G D ar e q u it e c lo s e t o th o se o b se r ve d i n U S E 1

r a t h e r t h a n in J S (F i g u r e S 2 ). M LC A P E i n c r e as e s m u c h

e a r l i e r i n G D t h a n i n JS . A g o o d m a t c h be t w e e n

M L C A P E an d S H R 6 o c c u r s d u r i n g t h e s t a g e w h e n S R H 6

i s s ti ll q u it e l ar g e . In ad d i ti o n, GD i s c ha r a cte r iz e d by

l o w e r M L LC L v a l ue s t h a n a l l f o u r o t h e r t a r g e t r e g i o n s

( Fi g ure S 2). T hese envi ronme ntal f eatures sh o uld h ave

p r o d u c e d a hi g h f r e q u e n c y o f t o r n a do e s a n d a h i g h

t o r n a d o - g e n e r a t i n g e f f i c i e n c y o f t o r n ad o e v e n t s in GD , as

t h o se o b se r v e d in U S E 1 . H o we v e r , G D e v e n h a s f a r f e we r

t o r n a d o e s th an J S , p o s sib ly b e c a us e o f i ts m o u n ta i n o u s

t o p o gr a p h y . In co n t r a s t , m o s t a r e a s i n t h e J S r e g i o n a r e

plai ns (F igure S1 a ), and t he good mat ch bet wee n S H R6

a n d M L C A P E o c c u r s d u r i n g a s ta g e i n w h i c h th e

S HR 6 v al u e s h a v e a l r e a dy la r g e l y d e c r e a s e d d u e t o t h e

h i g h e r la t it u d e o f JS th a n th at o f G D .

A s o b s e r v e d i n G D , te r r a i n m a y ha v e a n i m p o r t a n t

i mp a ct o n t he d i ffe r e n ce s in t o r na d o a ct iv i ti e s be t we e n

t h e U n i te d S ta t e s a n d C h in a . P r e v io u s st a ti st ic a l a n a l y se s

o v e r th e G r e a t P l a i n s a n d Ar k an s a s h a ve s h o w n t h a t t e r -

r a i n r o ug h ne s s h a s a n e g at i ve e f fe c t o n th e t o r n a d o pr o b -

a b i l i ty a t a f i x e d p o p u l at i o n d e n s i t y ( Ja g g e r ., 20 1 5 ;e t a l

E l s n e r . , 2 0 1 6 ; H u a a n d C h a v a s , 2 0 1 9 ) . Ch i n a h a se t a l

fai r ly m o unt a inou s r egi o ns in t he s outh, from G uang-

d o n g t o Z h e j i an g P r o v inc e s. S im il a r to t h e U n it e d S t at e s ,

m o s t t o r n a d o e s i n Ch i n a a l s o o c c u r i n pl a i n s (F i g ur e S 1 ) .

C o n s e q u e n t l y , fa r f e w e r t o r n a d o e s a r e p o s s i b ly t o o c c u r

i n C h i n a t h a n in t h e U nit e d S t at e s be c au s e t h e r e a r e

m an y few e r p l a ins i n C hi na, a nd the o nly large-extent

plai n a rea i s l ocated farther to t he north a nd is ch aracter-

i ze d b y a n u n f a vo u r a b l e k i n e m a ti c e n v i r o n m e n t f o r t o r -

n a d o o c c u r r e n ce s d u r i n g t h e t o r n a d o s e a s o n .

Fi nall y, it shoul d b e m e ntion e d t hat t here i s stil l som e

room f o r i mprovem e nt i n th is s tudy, but u nfortunate ly, it

i s d i f f i c u l t to i m p r o v e t h i s s t u d y d u e to the l i m i t a ti o n s o f

t o r n a d o d a ta in C h i n a . R e l a t i v e t o th e c u r r e n tl y u s e d

1 0 - y e a r t o r n a d o r e c o r d s , a l o n g e r s t u d y p e r i o d s h o u l d

y i e ld r e s u l ts t h a t a r e cl o s e r t o th e n a t u r e o f t h e r e l a t i o n -

sh ip b e tween th e t ornado num bers a nd en vironm e n tal

p a r a m e t e r s . S i n c e a 1 0 - y e a r pe r i o d i s n o t su f f ic i e n t l y lo n g

t o e li min a te t he e ffe c ts o f s ta t is ti ca l no i se , i t i s d iff icu l t t o

d e f i n e a p r e c i s e 9 0 -d a y p e a k s e a s o n. If a l o n g e r - te r m t o r -

n a d o d at a b a s e w e r e a v a i l a b l e i n C h i n a , de f i n i n g th e

t o r n a d o se a s o n a s a c o n se c u t i v e 9 0 - d a y p e r i o d r a t h e r

t h a n a s t h r e e ca l e nd a r m o n t h s , a s w a s d o n e in t h i s st u d y,

m ig h t m o r e p r e c i s e l y r e p r e s e n t t h e p e a k t o r n a do o c c u r -

r e n ce s . In ad d i t i o n , it i s in e v i t a b l e th at so m e to r n a do e s

a r e no t de t e ct e d an d r e c o r de d, e s p e cia l l y we a k

t o r n a d o e s o c c u r r i n g i n r u r a l ar e a s w i t h l o w p o p u l a t i o n

d e ns it ies . H o wever, b as e d o n v ersi on 4 o f t he Col u mb ia

U n i v e r s i ty G r i d d e d P o p u l a t i o n o f th e W o r l d d a ta s e t

( Ce nt e r f o r I n t e r n a t i o n a l E a r t h Sc ie n c e I n f o r m a t i o n

N e tw o r k CI ES IN Co lu mb ia Un i v e r s i t y , 2 0 1 6 ) , e a s te r n— —

a n d so u t h e r n Ch i n a a r e m u c h mo r e d e n s e l y p o p u l a t e d

t h a n th e c e n t r al a n d so u t h e a st e r n U n i te d St a te s. C o n s e -

q ue ntl y, the mi ssi ng rec o rds in J S a nd GD c o uld t hus be

at leas t c lose to t hos e i n U C , USE 1 and U S E2. T his li mi -

tati on sho ul d not a ff ect t he m ai n fi ndin gs in thi s study.

5 | SUMMARY

T h i s s t u d y wa s a i m e d a t u n d e r s ta n d i n g t h e su b s ta n ti a l

d i f fe r e n c e s in th e t o r n a d o fr e q u e n c y a n d t o r n a do s e a s o n

t im in g b e t we e n C h i n a a n d the U n it e d St a te s by co m p a r -

i ng th e k e y t o r n a d o - a s s o c i a t e d e n v i r o n m e n ta l f e a t u r e s i n

t he hi g h-i nc id e nce r e g i o ns.

The r esu l ts sh ow th at the mu c h h ig her p oten ti a l in

t h e o c cu r r e n c e o f mu l t i pl e to r n a d o e s o v e r a s h o r t p e r i o d

i s pr o ba b l y r e s po n s i b l e fo r th e la r g e r to r n a do n u m b e r s

r e c o r de d in th e U n it e d S t at e s t h a n in C h i n a . D u r ing th e

e a r l i e r to r n ad o s e a s o n , th e t ar g e t r e gi o n s i n t h e U n i t e d

S ta t e s h a v e a p pa r e n tl y h i g h e r t o r n a d o -g e n e r at i n g e f f i -

c i e n c i e s o f t o r n a d o e v e n t s th an th e J S r e g i o n o f C h i n a . I n

t h e ta r g e t r e gi o n s o f t h e U n it e d St a te s , th e d e c r e a s e

i n t o r n a d o - g e n e r a t i n g e f f i c i e n c y f r o m s p r i n g t o s um me r

i s po s si bly d u e in p a r t t o the d e cr e a s e s in t he SR H1 a nd

S H R 6 v a l u e s .

The r esu l ts a ls o s how th at th e S TP h as a bet ter c o rre-

l a ti o n wi th t o r na d o a ct iv i ty t ha n d o ind i v id u a l ki ne m a ti c

o r t h e r m o d y n a m i c p a r a m e t e r s . T h e S T P a v e r a g e s s h o w a

m o notonic d ecreas e w ith t he dec r ease in the a rea-sc aled

t o ta l to r n a d o c o u n t fr o m U C to U S E 1 , J S, a n d U S E 2 d u r -

i ng t he i r r e s p e ct iv e to r n ad o se a so n s. F o r a g i v e n r e g i o n,

t h e mo n t h w i th t h e ma x i m u m m e a n S T P v a l ue i s a l s o

t h e mo n t h w it h p e a k t o r n a d o o c c u r r e nc e s . C o m p a r e d

w it h the t ar g e t r e g i o n s in t h e U n it e d S ta t e s, t h e J S i n

China r eac hes th e p eak STP in a l ater mon th a nd thus

h a s a d e l ay e d t o r n a d o s e a s o n .

C o m p a r e d w it h t h e J S r e g i o n in C h i n a , t h e t a r g e t

r e g i o n s in t h e U n i t e d St a te s h a ve m o r e f a v o u r a b l e k in e -

m a ti c e nviron me nts for sup e rcel ls, wi th m uc h high e r

S H R6 v al u e s. One int e r e s ti ng f ind i ng i s th at t hi s is no t

b e ca u se SH R6 is g ene r al ly la rg er i n the U ni ted St a tes

t h a n th a t i n C h i n a a t a n y ti m e o f t h e y e a r . T h e t ar g e t

r e g i o n s in t h e U n i t e d S t a te s a c t u a l l y ha v e m e a n v al u e s

a n d de cr e a s i n g t r e n d s o f SH R 6 fr o m s p r i n g to su m m e r

ZH OU .ET A L 15



t h a t a r e si m i l a r t o t h o s e in t h e J S r e g i o n o f C h i n a . H o w -

e v e r , th e U ni te d S t a t e s r e g i o n s ha v e m u c h e a r l i e r

i ncrease s i n ins tab i lity than t he JS regi on of Chi na,

r e s ul t i n g in a n e a r l i e r g o o d m a t c h be t w e e n S H R 6 an d

M L C A P E a n d t h u s a h i g h e r S H R 6 v a l u e du r i n g th e i r e a r -

l i e r t o r n ad o se a s o n s. T h e s e e n v i r o n me n t a l fe a t u r e s m a y

have resulted f r om th e w eaker, less f r equent, a nd more

n o r t h e r n u pp e r - l e v e l j e t s t r e a m s a n d e x t r a t r o p i c al

cyc lones in th e torn a do season s o f J S t han t heir counter-

parts in t he Un ited Sta tes .

ACKNOWLEDGEMENTS

T h i s w o r k w a s s up p o r t e d b y t h e N a ti o n a l N a tu r a l S c i e n c e

Foundation of Chin a ( Grant 4187 5051, 4 19050 43 and

42 03060 4). W e t h a n k Y ipen g H u a n g (X ia m en Meteoro-

l o gi c a l B u r e a u , Ch i n a ) f o r h i s c o n s tr u c ti v e c o m m e n ts .

W e a l s o t h a n k t h e a n o n y m o u s revie wers f o r t hei r ins ight -

f u l co m m e n t s in i m p r o v i n g o u r w o r k .

AUTHOR CONTRIBUTIONS

Ruilin Zhou: Da ta cu ra ti on; f orma l a na ly s is; inv e s ti g a-

t i o n ; me t h o d o l o g y; so f t w ar e ; v a l i d at i o n ; v i s u a l i za t i o n ;

w r it in g- ori gi nal draf t; w r it ing- review & e di ti ng. Zhiyong

Meng: C o n c e pt u a l i za t i o n ; f or m a l a n a l y s i s ; fu n d i n g

a cq u isi ti o n; inv e s ti g at io n; m e tho d o l o g y ; p r o je c t a d mi nis -

t r a ti o n ; r e s o u r c e s ; s u p e r v i s i o n ; v i s u a l i za t i o n ; w r i t i n g -

r e v i e w & e d i t i n g . Fo r m a l a n a l y s is ;Lanqiang Bai:

fu n di ng a c qu is i ti o n; in v e st ig a t i o n ; so f t w a r e ; w r i ti n g -

r e v i e w & e d i t i n g .

ORCID

R u i l i n Z h o u https://o rcid.org/00 00-0003 -12 57-9980

Z h i y o n g M e n g https:// orcid.org/0 000-0002 -25 27-1056

L a n q i a n g B a i h t t p s :/ /o r c i d .o r g / 0 0 0 0 - 0 0 0 2 - 0 3 0 4 - 5 6 5 6

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ZH OU .ET A L 17



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SUPPORTING INFORMATION

A dd i t i o n a l s u p p o r t i n g in f o r m at i o n m a y be fo u n d o n l i n e

i n t h e S u pp o r t i n g In fo r m a t i o n s e c ti o n a t th e e n d o f t h i s

a r t i c l e .

How to cite this article: Z ho u , R . , M e ng , Z . , &

B a i, L . (20 2 1 ). Di ffe r e n ce s i n to r n a do a ct iv i ti e s a nd

k e y to r n a di c e n v i r o n m e n ts b e t w e e n C h i n a a n d t h e

U n i t e d S t a t e s . ,I n t e r n a t i o n al J o ur n a l o f C l i m a to l o g y

1 18.– https :/ /doi.org/10 .1002/ joc . 7248

18 Z HO U .ET A L



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