31(3), 279-286 Tap chi CAC KHOA HOC VE TRAI DAT 9-2009
NGUYEN NHAN VA QUY LUAT CUA THOl TIET MUA L 6 N , "MUA L 6 N TRAI MUA"
VUNG BAC TRUNG BO (GIAI DOAN 1987-2006)
I. M 6 DAU
0 Viet Nam, trong khoang hai chuc nam trd lai day, do nhidu nguyen nhan khae nhau, sy "gia tang"
cde hien tugng thien tai do mua Idn, thdi didm xudt hien khdc thudng ciia cac dgt mua Idn khdng vao mha mua (ggi la "mua ldn trii miia") vd nhirng thiet hai nghiem trgng do ehiing giy ra da trd thanh nhirng
"vdn dd ndng", thu hiit sy chii y cila todn xa hpi.
Ben canh dd, viee nghien cufu, thdng ke, phan loai cic hinh thd thdi tidt (HTTT) mua ldn ludn la vdn dd rdt duge quan tam khdng ehi eua cdng tie dy bio mi cdn cila cic nhd khoa hgc do dnh ehdt dae biet, khd ndm bat cila ban than hien tugng. Mua ldn va "mua Idn trdi mua" thudng di kem Iii lut, Iii quet, lu dng, sat Id ddt... nen nghien ciru nguyen nhan vd kha nang xudt hien cic dang tai bidn mdi tmdng nay la rdt cdn thidt.
Mua Idn trong miia mua va cud'i miia mua cd the sinh lu lut anh hudng xdu ddn ddi sdng, kinh td xa hpi cila ngudi dan ; Uiy nhidn khdng thie'u nhiing nam ban ban, cd kha nang mdt miia thi "mua ldn trai miia" - mua sdm ddu miia lai hd trg cho san xudt, ddi sdng ciia ngudi din.
Nghien curu nguyen nhan, quy luat boat ddng cila thdi tidt mua ldn, lam rd them tinh ehdt hai mat cua hien tugng "mua Idn trai miia" Id rdt edn thie't, cd y nghia khoa hgc va thyc tidn rd ret.
Tren eg sd phan loai cae HTTT gay mua ldn, kdt hgp vdi cic .so lieu do mua mat ddt thdi ky 1987- 2006, bai bio Uinh bdy cac kdt qua thd'ng ke, phan tich nguyen nhan didn bidn eila thdi de't mua Idn vung Bde Tmng Bd vdi mong mudn nhtmg quy luat boat dpng eua thdi ddt mua ldn, "mua ldn trdi miia"
riit ra tu nghien ciru nay se cd ieh cho cdng tic dy bao cQng nhu phdng ngira, giam nhe thiet hai eho cic tinh Bac Trung Bd.
II
, .,•, NGUYEN KHANH VAN, D 6 LE THUY DIA BAN, SO UEU VA PHUONG PHAP
NGHIEN CLTU
Dd nghien ciru vd mua ldn vd "mua ldn trii miia"
viing Bac Tmng Bd, chiing tdi lya chgn giai doan tu 1987 - 2006 vdi ly do Uong khoang mpt hai thap ky gdn day, do mdt sd nguydn nhan khic nhau, mua Idn chang nhirng xdy ra thudng xuyen ban, gay IQ lut nghiem trgng ban ma cdn xudt hien "bdt thudng ban" gay khd khan eho cdng tac dy bio mua ldn va phdng tranh thien tai do mua ldn.
Khu vyc Bdc Tmng Bp duge lya chgn de nghien curu mua ldn la mpt viing ndi tidng nhieu thien tai, didu kien khi hau khdc ngbiet va cho tdi nay vdn cdn la mdt trong nhirng khu vyc cd dieu kien kinh td - xa hdi va kha nang phdng chd'ng thien tai thdp ban cic khu vyc khic.
Bac Tmng Bd la mdt viing dia ly Ur nhien kha rOng ldn bao gdm sau tinh : Thanh Hoa, Nghe An, Ha Tinh, Quang Binh, Quang Tri va Thua Thien - Hud. Day la khu vyc ed dia the kha dae biet, phdn phia tay la dai Bdc Trudng San, dia binb dde tu tay sang ddng vdi cic mire dd khie nhau, nai thi thoai thoai vdi mdt sd luu vyc sdng rOng va thdp ddn ra bid'n ; trong khi noi khic, canh vimg niii phia tay la cic ddng bang hep ven bid'n ; va tai mpt sd khu vyc tu cic day niii phia tay lai xudt hien nhiing dai niii chay ngang ra ngay sit bidn, tao nen cic "bdy dia hinh" giy mua.
Phan tich dae did'm hinh thii dia hinh viing Bdc Tmng Bp cho thdy : tuf bdc vao nam dia thd toan viing gidng nhau d ehd phia ddng ddu la dudng bd bidn, phia tay la ddy Bdc Tmdng Son. Cdn sy khic nhau thd hien d dia thd ciia nhirng dai ddng bang hep, rpng khae nhau, nhung thung liing sPng cd dd dd'c, dp md vd phia bid'n khae nhau. Ndu coi cic dae didm hinh thii dia hinh nay nhu la mpt trong
cic tic nhan gay nen sy phan hda chd dp mua va d mdt chimg myc nhdt djnh anh hudng de'n tinh chdt cac dgt mua (mua binh thudng, mua ldn...) chung tdi phan chia Bdc Tmng Bd thdnh hai khu vyc :
1) Khu vyc Bdc Deo Ngang (BDN), gdm ba tinh Thanh Hda, Nghe An, Ha Tinh: ddy Tmdng San each xa bidn, dia hinh chuydn tidp ddn tir viing nui sang viing ddi va ddng bdng, dai ddng bang phit tridn rdng, ding kdnhdt la ddng bang Thanh Hda, Nghe An.
2) Khu vyc Nam Deo Ngang (NDN), bao gdm cic tinh Quang Binh, Quang Trj, Thira Thien - Hud:
chidu ngang hep, ddy Trudng San chay song song vdi dudng bd bidn, dia hinh chuyen hda ddt ngpt, ddng bang hep chay ven bidn.
Vd sd lieu mua va cic HTTT, tren ca sd cic sd lieu thu thap trong 20 nam (1987-2006) bao gdm sd lieu mua quan trie tai cic tram trong khu vuc nghien ciru va sd lieu tii phin tich ciia Ca quan khi tugng Nhat Ban (Japanese 25 years Re-Analysis JRA25) [7], ehiing tdi da phan rich va thd'ng ke cic HTTT vd td hgp cua chiing giy ra cic dgt mua ldn va rdt ldn trong khu vyc.
Trong nghidn ciru nay, chung tdi phan ra hai loai:
mua Idn va mua rdt Idn. Trdn eg sd cae kdt qua nghien curu vd mua Idn da duge tidn hdnh [1,3,4], deu chi dd xic dinh dgt "mua Idn" va "mua rdt ldn" trong khu vyc quan tam duge dd xudt trong bdng 1.
Bdng 1. Tieu chi xac djnh ddt "mua I6n" va "mua ra't I6n"
Loai mua y ^ . ' ^ ^ ^ S ' ^ " . Dien mua
• mua/ngay mua keo dai • Mua ldn
Mua rat ldn > 100 mm
> 50 mm ^ > 50 % sd 2 ngay ^^^^ j^Q^g khu vuc Theo so dd phan viing khi hau Viet Nam cua Nguyen Dire Ngu va Nguydn Trgng Hieu [2], Bdc Tmng Bd la mdt trong 7 viing khi hau Viet Nam.
Khie vdi nhieu viing cd chd dp mua miia he, chil ydu Id do gid miia Tiy Nam nhu vung Tay Bdc, Ddng Bac, Tay Nguydn, Ddng bang Nam Bd, viing Bac Tmng Bp (vd Nam Tmng Bd) Id nhirng viing cd chd dp mua lech sang cudi thu ddu ddng, chil ydu do gid mua Ddng Bdc. Trdn thuc td mua mua d Bde Tmng Bd bao gdm hai thdi ky, thdi ky mua "tidu man" (tu thing V ddn thing VI) va thdi ky mua ehinh vu (tu thang VIII ddn thing XI, XII). Chinh vi thd
"mua trii mda" d Bde Tmng Bd duge xem la nhirng trin mua ldn, dien rdng hoac khdng rPng, xdy ra
vao cic thing khdng phai la miia mua chinh (cac thing tuf III ddn VII).
III. KET QUA NGHIEN CLTU VA TOAO LUAN /. Cac HTTT va td hop cac HTTT gay mua Idn cho khu vuc Bac Deo Ngang
a) Phan bo thdi gian cCia eae dgt mUa Mac dii mua ldn va rdt Idn chil ydu xua't hien tu thing VIII ddn XI, nhung trong mdt sd nam xudt hien
"IQ ddu man", mua ldn cd thd vao thdng V, thang VI (1989, 1989, 1994, 1999 va 2001) {bang 2). Trong 20 nam, da xd'y ra 67 dgt tai khu vyc BDN, tmng binh 3,4 dgt/nam, nam it nhdt la 1 dgt (1987, 1998 va 2002), nam nhidu nhdt la 6 dgt (1989, 1994, 2005).
Trong 67 dgt nay, cd 26 dgt mua rdt Idn (bang 2, cic sd trong ngoac), chidm ~38,8 %. Trong sd 26 dgt mua rdt Idn nay, nhidu nha't cd 3 dgt xdy ra trong nam 1990, cdn lai la 1 hoac 2 dgt/nam. Trong khu vyc nay, mua tap tmng vao ba thing VIII, IX va X. Trong 20 nam quan trde : trong thing VIII cd 13 dgt mua ldn, 3 dgt mua rdt Idn ; thing IX : 18 dgt mua Idn, 11 dgt mua rdt Idn va thing X : 16 dgt mua ldn, 9 dgt mua rdt Idn. Chi trong ba thing nay, sd dgt mua da chidm 70 % (dd'i vdi mua Idn) va 88 % (ddi vdi mua rdt Idn) trong tdng sd cic dgt mua (bang 2).
b) Cae HTTTva to hap cCia chung gay mua ldn Nguydn nhan gay ra cic dgt mua Idn trong khu vyc BDN cd nhidu, dd la bao, ip thdp nhiet ddi (AT ND), dai hpi hi nhiet ddi (ITCZ, HTND), ichdng khi lanh (KKL), ranh thdp (RT), xoiy thdp (XT), gid miia tay nam (SW), gid ddng nam (SE). Tuy nhidn, ndu chl dan thudn mPt dang thdi tidt khd cd thd gay ra mdt dgt mua ldn trdn dien rdng, ngay ea khi cd bao va ATND. Nhung dgt mua Idn va rdt ldn thudng duge sinh ra bdi td hgp cila 2 hoac 3 loai HTTT hoac xdy ra ddng thdi, hoac gdi ddp nhau {bang 3)
Tir bang 3, ta thdy cd 7 loai HTFT gay mua Idn, trong dd, dai HTND la mdt HTTT dien hinh gay ra 20 dgt mua ldn (-29,8 %), 7 dgt mua rdt ldn (-27 %).
Tidp ddn la HTTT td hgp sd 4 {XT, RT, gio SE, NT KH, dai thapD) gay ra tdi 14 dgt mua ldn, (-20,9%), 2 dgt mua rdt ldn (-7,7%). Cac HTTT khic nhu bao va td hgp dai HTND va.bao(A TND) cung chidm mpt sd lugng ding kd (tuang iing la 12 dgt vdi - 18 % va 13 dgt chidm -20 % dd'i vdi mua ldn va 6 dgt vdi - 23 % va 7 dgt chidm -27 % dd'i vdi mua rdt ldn. KKL cung giy ra mua cho khu vyc ndy, khoang 10,4-11,9 % dd'i vdi cahai loai mua (bang 3).
Bdng 2. Phan bo theo thdi gian (n§m, thang) cac ddt mua ldn va rat ldn (cac so trong ngoac) d Bac Deo Ngang (thdi ky 1987-2006)
1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Tdng sd
IV V
1
1
1 2
1 6
VI
1
1
1 3
VII
1(1) 1
1 1 1
1 2(1) 8(2)
Thing VIII 1(1) 1 1(1) 1(1) 1 1 1 1
1 2
1 1 13(3)
IX 1(1) 1(1) 1(1) 1(1) 1 2 2(2) 1
1(1) 1(1) 2(1) 3(1) 1(1) 18(11)
X 1(1) 2(1) 2(1) 2(1) 1 1(1) 1 1 1 2(2) 1(1)
1(1) 16(9)
XI XII
1 1
1(1) 3(1)
Tdng sd 1(1) 2(2) 6(2) 5(3) 3(2) 3(1) 3(2) 6 4
5(2) 2 1 3(2) 2(1) 5(1) 1(1) 3(1) 2(1) 6(2) 4(2) 67(26) Bdng 3. Phan bd
Sd
rj^ Hinh the' thdi tie't 1 Bao hoac ATND 2 Hoi tu nhiet ddi 3 KhOng khi lanh
cac HTTT va to hdp cac HTTT gay mUa Idn va d Bac Deo Ngang (ttidi ky 1987-2006) IV V VI
1 1 1
Thing VII VIII 2(2) 3(1) 2 4
IX 4(2) 8(5)
rat Idn (so trong ngoac)
X XI
1(1) 12(6) 5(2) 20(7) 5(2) 2(1) 7(3)
Tdn sudt (%) 17,9(23,1) 29,8(26,9) 10,4(11,6) XT hoac RT vd gid SE,
HTKH, dai thdp, ddng xidt gid tay, viing thdp phia tay, gid NE-E
1 1 3(2) 1 14(2) 20,9(7,7)
Bao (hoae ATND) vd KKL
6 HTND va Bao (ATND) 7 HTND va KKL
Tdng so 6 3
3 8(2)
4(2) 13(3)
5(4) 18(11)
1(1) 1(1) 16(9)
13(7) 19,5(26,9) 1(1) 1,5(3,8) 3(1) 67(26) 100(100) Thdi gian keo ddi cila ede HTTT gay mua dao
ddng tuong ddi nhidu : tir 2 ddn 7 ngdy (ddi vdi mua ldn), tu 2 ddn 3 ngay (dd'i vdi mua rdt ldn), trong dd cd sy phan biet giiia cic loai HTTT va td hgp cic HTTT. HTND giy mua ldn trung binh khoang
3 ngay, cd dgt keo ddi tdi 7 ngay, KKL tmng binh khoang 3 ngay, dai nhdt la 5 ngay, HTND va KKL trung binh khoang 3 ngay, HTND va Bao : trung binh khoang 2,7 ngdy, cic HTTT cdn lai khoang 2,4 ngay {bdng 4).
Bdng 4. Thdi gian mua Idn va ra't Idn (ngay) cua cac loai HTTT va to hdp cua chung ci Bac Deo Ngang (thdi ky 1987-2006)
Sd TT 1 2 3 4 5 6 7
Hinh the thdi tidt Bao hoac ATND
HTND KKL
XT hoac RT va gid DOng Nam (SE), HTKH, Bao (hoac ATND) va KKL
HTND va Bao (hoac ATND) HTND va KKL
dai thdp
Thdi doan keo dt Tdng sd
(ngay/dgt) 29/12 60/20 21/7 33/14 35/13 3/1
Trung binh 2,4 3,0 3,0 2,4 2,7 3
li (ngdy) Dai nha't
3 7 5 4 5 3
Ngdn nhdt
2 2 2 2 2 3 Dd'i vdi mua rdt ldn, hdu bdt cic HTTT ed thdi
doan keo dai trung binh la 2 ngay. Duy nhdt cd dai HTND eho mua rdt ldn trong 3 ngay {bang 5).
2. Cac HTTT va td hpp cac HTTT gay mua Idn cho khu vuc Nam Deo Ngang
a) Phan bo thdi gian cua cae dot mUa
Nhu trdn da phan rich, do vi tri dia ly va didu kien hinh thii dia hinh BDN va NDN cd nhtmg khdc biet nha't dinh nen thdi gian xudt hien ciia mua Idn tai khu vyc NDN chil ydu tir thang VIII ddn thing XI.
Khic vdi BDN, tai khu vyc nay, mua ldn cd thd xudt hien rdt sdm (tuy ban hiru) vao thing III, IV (nam 1999), thing V (1989, 2001), thing VI (1989, 1992, 2000 va 2004). Trong 20 nam, da xdy ra 111 dgt mua ldn d NDN {bdng 6), tmng binh 5,6 dgt/nam, nam cd it nhdt la 2 dgt (2006), nam nhidu nhdt la 10 dgt mua ldn (1998). Trong 111 dgt nay, ed 27 dgt mua rdt Ion (bang 6, cic sd trong ngoac), chidm -24 %.
Trong dd, nam nhidu dgt mua rdt ldn - 4 dgt la cic nam 1995, 1996, 1999 va 2002, edn lai la 1, 2, 3 dgt/nam. Trong khu vuc nay, mua tip tmng vao ba thing IX, X va XI. Trong 20 nam, vao thing IX da
ghi nhan duge 16 dgt mua ldn, 5 dgt mua rdt ldn, vao thing X : 38 dgt mua ldn, 13 dgt mua rdt ldn va thing XI : 26 dgt mua ldn, 6 dgt mua rdt ldn.
Chi trong ba thing nay, sd dgt mua da chidm 72 % (dd'i vdi mua ldn) vd 89 % (dd'i vdi mua rdt ldn) trong tdng sd cac dgt mua (bang 6).
b) Cac HTTT va to hgp eae HTTT gay mi/a ldn
Tugng ty nhu BDN, nguyen nhan cila cae dgt mua ldn khu vyc NDN eung do boat ddng eila bao, ATND, dai HTND (ITCZ), KKL, RT, XT, gid mua Tay Nam (SW), gid dong nam (SE)... va mua ldn, mua ra't Idn thudng do td hgp cila 2 hoac 3 loai HTTT hoac xd'y ra ddng thdi, hoac gdi tidp nhau (bang 7).
Tir bang 7, ta thdy ed 7 loai HTTT gay mua ldn, trong dd, KKL la mpt HTTT didn hinh gay ra tdi 41 dgt mua ldn (-37 %), 6 dgt mua rdt ldn (-22 %). Tidp ddn la HTTT to hgp so 4 {XT, RT, gid SE, HTKH, dai thapD) gay ra tdi 20 dgt mua ldn (-18 %), 7 dgt mua rd't ldn (-26 %). Cic HTTT khie nhu bao va dai HTND eung chie'm mpt sd lugng ding kd tuang img la 13 dgt vdi -11,7 % va 17 dgt chidm -15,3 % dd'i vdi mua ldn; dd'i vdi mua rdt ldn mdi loai binh thd tren cd 5 dgt -18,5 % (bang 7).
Bdng 5. Thdi gian mua rat Idn (ngay) cua cac loai HTTT va to hdp cua chung d Bac Deo Ngang (thdi ky 1987-2006)
Sd TT
1 2 3 4 5 6 7
Hinh thd thdi tidt Bao hoic ATND
HTND KKL
XT hoac RT va gid Ddng Nam (SE), HTKH, Bao (hoac ATND) va KKL
HTND va bao (hoae ATND) HTND va KKL
dai thdp
Thdi doan keo dai (ngdy) Tdng sd Trung
(ngay/dgt) binh 12/6 2,0 18/7 2,6 6/3 2,0 4/2 2,0 14/7 . 2,0
2/1 2,0 Ddi nha't
2 3 2 2 2 2
Ngdn nhdt
2 2 2 2 2 2
Bdng 6. Phan bo theo thdi gian (nam, thang) cSc ddt mua Idn va rat Idn (so trong ngoac) d Nam Deo Ngang (thdi ky 1987-2006)
Nam 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998
1999 1 2000
2001 2002 2003 2004 2005 2006
Tdng .sd 1
III IV
Thdng
V VI VII VIII IX X XI XII
1
1 1(1)
1(1) 2(1) 2(1) 2(1) 1
1
1(1) 1(1) 1
1 2 2 3 2
3(1) 2(1) 1 2(2) 3(1) 2(1) 2
3(2) 3(2) 1(1) 3(2) 3
1 2(1) 3(1) 4(2) 1(1) 2 2(1)
1 2 1(1)
2(1) 9(1) 16(5) 38(13) 26(6) 2 2 12(1)
Tdng sd 6 3 8 6 4 6(1) 4(1) 5 6(4) 9(4) 5(2) 10(3) 7(4) 7(2) 5(2) 7(4) 4 3 4 2 111(27) Bdng 7. Phan bo cac HTTT va to hdp HTTT gay mua Idn va rat Idn (so trong ngoac)
d Nam Deo Ngang (thdi ky 1987-2006) Hinh the thdi tiet
11 Ill IV V VI v n 1 Bao hoae ATND 1 2 1 2 HTND
3 KKL
Thing „.:.
VIII IX X XI XII •—'=""
1 2(1) 3(1) 3(3) 13(5) 3(1) 9(3) 5(1) 17(5)
13(3) 19(2) 9(1) 41(6)
Tdn sudt (%) 11,7(18,5) 15,3(18,5) 37,0(22,2) XT hoae RT vd gid
SE, HTKH, dai thdp, ddng xie't gid tay, viing thdp phia tay, gid NE-E
1 1 1 1 11(7) 1 20(7) 18,0(26,0)
Bao (hoac ATND) va
KKL 1(1) 3(1) 3 7(2) 6,3(7,4)
(hoac ATND) 7 HTND vd KKL
Tdng so
1 1(1) 4 2(1) 2 10(2) 9,0(7,4) 3 3 2,7 1 1 2 4 2(1) 9(1) 16(5) 38(13) 26(6) 12(1) 111(27) 100(100)
Thdi gian keo dai cua cac HTTT gay mua dao ddng tuang ddi nhidu : tir 2 deh 8 ngdy (ddi vdi mua Idn), tu 2 ddn 7 ngdy (ddi vdi mua rdt ldn), Uong dd c6 sy phan biet giOa cic loai HTTT va td hgp cic HT
TT. Dai HTA/D giy mua tmng binh khoang 3,6 ngay, ed dgt keo dai tdi 8 ngdy, KKL khoang 3 ngay, dai nha't 6 ngay ; HTND va Bao : Uung binb 2 ngay, cac HTTT cdn lai khoang 2,5 ddn 3 ngay {bdng 8, 9).
Sd
Bdng 8. Thdi gian mua Idn va rat Idn (ngay) cda cac loai HTTT va to hdp cua chung d Nam Deo Ngang (thdi ky 1987-2006)
Hinh thd thdi tie't Bao hoac ATND
HTND KKL
XT hoaC RT va gid SE, HTKH, dai thdp, ddng xidt gid tay, vimg thdp phia tay, gid NE-E
Bao (hoac ATND) va KKL HTND vd bao (hoac ATND) HTND va KKL
Thdi doan keo dai (ngay) Tdng sd
(ngay/dgt) 36/13 61/17 124/41 ' 59/20
22/7 22/10
9/3
Trung binh
2,7 3,6 3,0 3,0 3,1 2,2 3,0
Dai nha't
5 8 6 7 4 3 5
Ngdn nhdt 2 2 2 2 2 2 2
Sd
Bdng 9. Thdi gian mua rat Idn (ngay) cua cac loai HTTT va to hdp cua chung d Nam Deo Ngang (thdi k y l 987-2006)
Hinh thd thdi tidt Bao hoac ATND
HTND KKL
XT hoac RT va gid SE, HTKH, dai thdp, ddng xidt gid tay, viing tha'p phia tay, gid NE-E
Bao (hoae ATND) va KKL HTND va bao (hoae ATND) HTND va KKL
Thdi doan kdo ddi (ngdy) Tdng sd
(ngay/dgt) 10/5 17/5 18/6 ' 22/7
8/2 4/2
Trung binh
2,0 3,4 3,0 3,1 4,0 2,0
Dai nha't 3 6 3 7 4 2
Ngdn nha't 2 2 2 2 4 2
3. Cac HTTT gay "mua Idn trai mUa" d Bdc Trung Bp
Nhu da ndu d tren, "mua trii miia" g Bdc Tmng Bd la nhirng uan mua ldn, dien rdng hoae khdng rdng, xdy ra vao cic thing khdng phai la mua mua chinh , dd la cic thing tir III ddn VII. Dudi day la kdt qua nghien ciJru, thd'ng ke phan bd ciia cic dgt mua nay.
a) "Ml/a Idn trai mua" khu vac Bae Deo Ngang
Bdng 10 Uinh bdy sy phan bd cic HTTT va td hgp cic HTTT giy "mua Idn trii miia" d BDN. Trong 20 nam cd tdng cdng 16 dgt "mua Idn trii miia", xudt hien thudng xuyen hen ca la trong cic thing V va VII (mdi thing - 37,5 % tdng sd dgt) va nguyen nhan chinh la do boat ddng cua bdo {ATND) hoac do sy hien dien cua loai HTTT td hgp .so 4, nhu : XT, RT, hgi tu tren cao, HTKHU
Bdng 10. Phan bo c^c HTTT va to hdp cac HTTT gay "mua Idn va rat Idn trai mua"
CI Bac Deo Ngang (thdi ky 1987-2006)
s
TT 1 2 3 4
Hinh thd thdi ddt Bao hoac ATND
HTND KKL
XT hoac RT va gid SE, HTKH, dai thdp, hdi tu trdn cao
Tdng sd
V 1 1 4 6
Thina VI
1 1 1 3
VII 4 2 1 7
Tdng sd 6 3 1 6 16
Tdn sudt (%) 37,5 18,7 6,3 37,5 100
b) "Mua rat ldn trai mua" khu vac Bae Deo Ngang Trong sd 16 dgt "mua Idn trii miia" d khu vyc BDN da thd'ng ke tren bang 10, chi ghi nhan ed mdt dgt "mua rdt ldn trdi miia", xd'y ra vdo cae ngay 22-24/7/1989, do boat ddng cila bao dd bp vao Thanh Hda.
c) "Maa Idn trai mua" khu vUc Nam deo Ngang Theo phan loai thd'ng ke, d khu vyc NDN trong 20 nam cd ta't ca 10 dgt "mua ldn trii miia" {bang 11). Trong dd cd niia sd trudng hgp mua ldn (50 % tdng sd dgt) ed nguyen nhan la do bdo hoae ATND,
40 % tdng sd trudng hgp "mua Idn trii mtia" ed nguyen nhan do boat ddng eua HTTT td hgp XT hoac RTva gio SE, HTKHO
d) "Maa rat ldn trai mua" khu vac Nam Deo Ngang
Trong sd 10 dgt "mua Idn trai miia" (bang 11), ehi quan sat thdy ed 1 dgt "mua rdt ldn trai miia" do boat dpng eiia bao (trong dai HTND) dd bd vdo Nam Dinh trong cde ngdy 22-23/7/1996.
Vd tinh chat hai mat ciia hien tugng "mua ldn trai mua" d Bde Tmng Bd ta thdy :
Bdng 11. Phan bo cac HTTT va to hdp HTTT gay "mua Idn trai miia" cl Nam Deo Ngang (thdi ky 1987-2006)
Thdng rp,' .. Tdn sudt
^ Tong so ,„ , VI VII ^ (%)
s
TT 1 2 4
Hinh the thdi ddt Bao hoac ATND
HTND
XT hoae RT vd gid SE, HTKH, dai thdp.
ddng xidt gid tay, viing thdp phia tay.
gid NE-E
Tdng sd
III IV V
1 2 2
1 1 1 1 1
1 1 2 4 2
5 1 4 10
50 10 40 100
Cic dgt "mua ldn trii miia" vao cic thing III-VII - thdi did'm khd ban cila todn viing ed thd gay nen lu
"ddu man" d mdt sd nai, tuy nhien d noi khdc, cic dgt mua nay da bd .sung mdt lugng nude ding kd eho san xudt va ddi sdng cila ngudi dan Bdc Trung Bp.
KET LUAN
Trdn co sd cic sd lieu mua quan trdc tai cac tram trong khu vyc nghien eiru va sd lieu tai phan tich cua Co quan khi tugng Nhat ban (Japanese Re- Analyzed JRA25), vdi cic ddu ehi phin loai mua ldn va mua rdt Idn, chiing tdi da phan tich va thdng ke ede HTTT giy ra cac dgt mua ldn, "mua ldn trii miia" thdi ky 1987-2006, trdn hai khu BDN va NDN cua Bac Tmng Bp, ke't qua nhu sau :
1. Tmng binh hang nam cd 3,4 dgt mua ldn vd rdt ldn (trong dd cd 1,6 dgt mua rdt Idn) trdn dien rdng gay lu lutd ba tinh BDN. Dd'i vdi ba tinh NDN, trung binh ed 5,6 dgt mua Idn va rdt ldn (trong dd cd 2,7 dgt mua rdt Idn). Cic dgt mua cd the xd'y ra tir thing III ddn thing XII, nhung chu ydu tap tmng vao thing VIII ddn X dd'i vdi BDN va tir thing IX ddn XI ddi vdi NDN.
2. Ndu nhu d BDN cic H-TTF: dai HTND (29,8 %), XT hoac RT va gid SE, HTKH, Dai thdp... (20,9 %),
HTND va bao hoac ATND (19,5 %), Bao hoac ATND (17,9 %) la 4 HTTT ed tdn sd xudt bien Idn hon ca, thi d NDN, ngoai 3 hinh thd gidng nhu d phia bdc' vdn chie'm tdn sd ding kd la dai HTND (15,3 %), XT hoac RT vd gid SE, HTKH, Dai thdp... (18,0%), Bao hoac ATND (11,7 %), edn cd hinh the KKL va hinh thd nay cd tdn sd cao nhdt (-37,0 %).
3. Td hgp cila 2 HTTT xdy ra ddng thdi hoac gdi tidp nhau cd nhidu kha nang gay mua Idn vd rdt Idn tren dien rOng, thdi gian mua keo ddi, gay ngap lut nghiem trgng. Thdi gian keo dai cua cic HTTT gay mua dao dpng Uong khoang thdi gian 2-8 ngay, chii ydu til 2 din 4 ngay, sd dgt keo dai ban 5 ngay khOng nhidu, thudng do KKL dgt chinh vd tang cudng cung nhu dai HTND.
4. Cic HTTT xudt hien vao giua miia (Vlll-X eho BDN, IX-XI eho NDN) thudng gay ra mua Idn vd rdt ldn. Nhirng dgt mua nhu vay thudng do KKL ke't hgp vdi boat dpng eila ddi gid DOng manh, hay KKL ke't hgp vdi Bao, ATND hoae dai HTND.
5. Vd "mua ldn trdi mua" :
a) 6 BDN cd 16 dgt (tir thing V ddn thing Vll), Uong dd cd 1 dgt "mua rdt Idn trdi miia" ; thdi didm
xudt hien : thing VII (7 dgt) va thing IV (6 dgt).
Ddi vdi NDN, cd 10 dgt, xudt hien tir thdng III ddn thdng VII, trong dd cd 1 dgt "mua rdt ldn".
b) Nguydn nhan chinh cua cic dgt "mua ldn trii miia" d BDN chii ydu la do 3 loai HTTT : i) Bao hoac ATND (37,5 %), ii) HTTP tO hgp XT, RT va gid SE, HTKH... (37,5 %) va iii) HTND (18,7 %).
Cdn g phia NDN nguyen nhan chu ydu ehi la hai loai HTTT : i) bao hoac ATND (50 %), ii) HTTP td hgp RT, XT va gid SE, HTKH... ( 40 %).
6. Vd tinh chdt hai mat cila "mua Idn trii miia" : cde dgt mua nay xudt hidn tir thing III ddn thing VII la thai didm khd ban ciia Bdc Trung Bp, bdn canh hieu irng gay ngap lut va mdt sd dang thidn tai nhu tmgt Id da't, lu quet, lu dng, d mpt mirc dp nhdt dinh,
"mua ldn trii miia" cung bd sung mdt lugng nude ding kd cho ddi sdng, san xudt ciia ngudi dan viing Bdc Tmng Bd.
Cdng trinh hodn thanh vdi sy tai tia cila "Chugng trinh Phdng Uanh va giam nhe thidn tai", Vien Khoa hgc va Cdng nghe Viet Nam, nam 2008-2009.
TAI LIEU DAN
[1] VtJ DINH HAI, 2000: Cic dang hinh the thdi ddt giy mua Idn d midn Tmng. Tuydn tap vd dy bio KTTV Idn thil V, 1996-2000.
[2] NGUYfiN DLTC NGCT, NGUYEN TRONG HIEU, 1985. Phan viing Khi hau Viet Nam. Tdng cue KTTV, Ha Ndi.
[3 ] NGUYEN KHANH VAN, BUI MINH TANG, 2003 : Dae didm hinh thd thdi ddt gay mua lu, lut ldn va dae biet ldn d cic tinh Quang Binh, Quang Tri, Thua Thien-Hud (giai doan 1997-2001). Tap chi Cac Khoa hgc vd Trii Ddt, sd dae bidt ky nidm 10 nam thdnh lap Vien Dia ly, T. 25, 4, 339-345.
[4] NGUYfiN KHAMH VAN, BUI MINH TANG, 2004 : Dae diem hinh thd thdi de't gay mua lu, lut ldn d ede tinh Thanh Hda, Nghe An, Ha Tinh (thdi ky 1997- 2001). Tap ehi Cac Khoa hgc vd Trii Ddt, T. 26,1, 50-59.
[5] Tap chi "Dae did'm khi tugng thily van" cic nam tu 1987 ddn 2006. Tmng tam Dy bio KTTV Trung ugng.
[6] Sd lieu quan udc mua (tai Tmng tam Dy bio KTTV Tmng uong vd Tmng tim Tu lieu) giai doan 1987-2006.
[7] Sd lieu tai phan tich cila Co quan khi tugng Nhat ban JRA25 (Japanese 25 years Rean Alysis) giaidoan 1987-2006.
SUMMARY
Causes and occurring frequency of heavy rain,
"unusual heavy rain" in North Central of Vietnam (period 1987-2006)
Study on heavy rain and "unusual heavy rain" in North Central of Vietnam shows that:
Statistically, In the North of Ngang pass (NNP) there are 3.4 heavy rain and very heavy rain spells annually (of which, there are 1.6 very heavy rain spells). In the South of Ngang pass (SNP) there are 5.6 heavy rains and very heavy rain spells, of which, there are 2.7 very heavy rains spells. Rain spells occur from March to December but tend to concentrate in the period from August to October for NNP and from September to November for SNP.
Heavy rains are caused by many synoptic weather situations (SWS). However, some main situations causing heavy rain in different areas can be summa- rized as follows :
- SWS causing heavy rains In NNP are : inter- tropical convergent rain (29.8%), tropical cyclone or low trough and SE wind, mehdianward convergence...
(20.9%), Inter-troplcal convergent rain and hurricane or tropical low pressure (19.5%), hurricane and tropical low pressure (17.9%).
- In SNP : apart from the 3 SWS causing heavy rains as in the NNP: Inter-troplcal convergent rain (15.3%), tropical cyclone or low trough and SE wind, mehdianward convergence... (18.0%), hurricane and tropical low pressure (11.7%), there is a remarkable cause from cold air with very high frequency (37.0%).
The combination of the above two SWS at the same time or In continuity can potentially cause heavy rain in large area and for a long time. Time of heavy rain spell can be vaned from 2-8 days, mainly : 2-4 days, the 5 days spell is limited, normally combined of main cold air and additional cold air, or inter-tro- plcal convergent rain.
In NNP "unusual heavy rain" can occur 16 spells/20 years in period from May to July, of which, there Is one "unusual very heavy rain". In SNP 10 spells/20 years in period from March to July and of which, there is one "unusual very heavy rain", too.
Ngdy nhan bdi : 02-7-2009 Vien Dia ly
(Vien Khoa hgc vd Cong nghe Viet Nam) Trung tdm Du: bdo Khi tugng Thuy vdn TW
(TT KTTV Qudc gia)
