31{4)[CD], 418-426 Tap chl CAC KHOA HOC VE T R A I OAT 12-2009
DAC DIEM PHAN BO Ml/A TREN LANH THO VIET NAM
PHAM XUAN THANH, NGUYEN X U A N ANH, LE VIET HUY, LE NHU QUAN, BERNARD FONTAINE, NATHALIE PHILIPPON
I. MO DAU
Viet Nam ed khi hau nhiet ddi dm, chju anh hudng eua gid mua, mdt nam cd hai mua : mua mua triing vdi mda gid mua mua he Bae ban edu vd mua khd triing vdi mda gid mua mda ddng Bdc ban edu, xen ke giua hai mda la thdi ky ehuyln tilp. Tuy nhien, do lanh thd Viet Nam nam keo dai theo phuong kinh tuyln, lai ed dia hinh phdc tap, nen dae dil'm phan bd mua tren lanh thd Viet Nam rdt khae nhau.
KJiu vue miln Nam - noi duge coi Id dil'n hinh eho chl dd mua cua khu vuc, mua mua keo dai td thdng 5 de'n thang 10. Lugng mua d day cd lien quan mat thie't vdi boat ddng eiia gid mua Tay Nam chilm ty le 80-90 % tdng lugng mua hang nam. Tren khu vuc miln Bdc, mua khdng phu thudc hodn todn vdo boat ddng eua gid mua Tay Nam, song lugng mua d day vdn duge cung cdp tu lugng dm ciia gid mda mda he dem lai thdng qua boat ddng cua ranh gid mda, dai hdi tu nhiet ddi (HTND), xoay thuan nhiet ddi (XTND)... Mua mua d day cung duge bat ddu vao thdng 5 va kit thuc vao thang 9, 10. Rilng khu vue miln Tmng, do tde dung ehdn gid eila ddy Trudng Son, ludng gid Tay Nam khdng gay mua eho khu vuc nay, lugng mua tdp tmng trong khoang thdi gian td thang 8 de'n thang 12 vdi cue dai mda mua roi vao thang 9, 10. Lugng mua d miln Trung ehu ylu lien quan dlh su boat ddng eua cae dang nhilu ddng khi quyln (front lanh, HTND, XTND...) nhdt la khi cd su ke't hgp cae dang nhilu ddng nay.
Vdn dl phan bd mua tren lanh thd Viet Nam da duge nhilu tac gia dl cap dlh vdi nhung muc tieu va each tie'p can khdc nhau [3, 4, 8]. Tuy nhien, vdn cdn ton tai nhung quan dilm khae nhau vl phan viing cho khu vuc Thanh Hoa va Ninh Thuan - Binh Thuan (xem phdn III). Trong nghien cdu nay, chung tdi mudn lam ro them dae dilm phdn bd eua tmdng mua bang cac cdng eu tho'ng ke mang tinh khach
quan nham xdy diJng cac ehi so mua mua (biln ddi eua lugng mua theo thdi gian nhilu nam) cho tirng khu vuc. Cac chi sd nay se dung lam ylu td du bao mua mda cho nhung nghien edu tie'p theo. Dl lam duge diiu dd, trudc he't, chung tdi dp dtjng phuang phap phan nhdm (clustering) vd ham tuong quan cho chudi sd lieu mua thang cua 44 tram tren lanh thd td nam 1960-2000 de xde dinh nhung khu vue cd tinh dong nhdt vl dae tmng mua. Sau dd, tren mdi khu vue, ehung tdi xde dinh thdi gian keo dai cua mda mua. Cudi eung, ehi sd mua mua duge xae dinh eho tung khu vue trong khoang thdi gian eua mda mua.
II. CO S6 SO UEU VA PHUONG P H A P /. Co sd so'lieu
Sd lieu mua thdng tu nam 1960 de'n 2000 cua 44 tram do mua duge su dung d l xde dinh cac khu virc ed tinh dong nhdt vl dae dil'm mua va xay dung ehi sd mua eho mdi vdng. Day la nhung tram cd cha't lugng td't va dai dien cho 7 vdng khi hau cila Viet Nam, trong dd : khu vue Tay Bdc gom 7 tram : Dien Bien, Hod Binh, Lai Chau, Mai C:hau, Mde Chau, Mudng Te, San La, Yen Chau ; khu vuc Ddng Bdc gdm 7 tram : Bdc Quang, Yen Bdi, Van Chdn, Cao Bdng, Ha Giang, Hdn Gai, Lang San ; khu vuc dong bdng Bdc Bd gdm 7 tram : VTnh Yen, Bdc Giang, Ha Ndi, Ninh Binh, Phil Liln, San Tay ; khu vuc Bac Tmng Bd gom 8 tram : Bdi Thugng, Ddng Hdi, Ha Tinh, Hul, Tuong Duong, Thanh Hod, Tinh Gia, Vinh ; khu virc Nam Tmng Bd gdm 6 tram : Da Nang, Nha Trang, Phan Thie't, Quang Ngai, Quy Nhan, Tuy Hod ; khu virc Tdy Nguyen gdm 4 tram : Bao Lde, Ban Me Thuat, Da Lat, Playeu ; khu vue Nam Bd gom 5 tram : Cd Mau, Cdn Tha, Phu Qudc, Rach Gia, Vdng Tau.
Dl xac dinh ngay bdt ddu mua mua cho khu vuc miln Nam chung tdi sd dung hai nguon sd lieu: 1) sd lieu mua ngay cua 6 tram : Bao Lde, Tay Ninh, Tan Son Nha't, Cdn Tho, Rach Gia, Cd Mau ; 2) sd lieu gid tai lOOOhPa tu ngudn sd lieu tai phan tich cua NCEP/DOE-II [51, tmng binh tren 4 nut ludi (105
°E -107.5 °E, 10 °N - 12.5 °N). Ngdy bdt ddu mda mua cho khu vue miln Bae xac dinh bang dung sd lieu mua ngay td 1979 din 2004 cua 9 tram : Lai Chau, Mde Chau, Ha Giang, Yen Bdi, Lang San, Hdn Gai, Ha Ndi, Nam Dinh, Thanh Hod. Ty le mdt sd lieu eua cae Uam ndy rdt thdp, duy nhdt tram Hdn Gai CO ty le 3,2 %, cac tram cdn lai dlu thdp hon 1 %.
2. Phuong phap
Trong nghien cdu nay, ehdng tdi su dimg phuang phap phan nhdm, bat nhan Id thuat toan K-mean, thilt lap phan bd theo khdng gian eua ede tram. Thuat toan K-mean ehia eae tram cua ma tran X (N dilm * P quan sat) thdnh K nhdm dua tren cue tieu boa binb phuong khoang edeh (sai sd) giua ede dil'm vd trgng tam (gia tri trung binh) eua nhdm, tren tdt ea cae nhdm. Ban ddu, ma tran X duge chia ngdu nhien thanh K nhdm vd tinh trgng tam cua mdi nhdm. Sau do, khoang each eua mdi diem vdi tam cua tirng nhdm,
cung nhu tam cua nhdm ed khoang each gdn nhdt cua timg dil'm duge xac dinh. Cudi cdng dilm se duge di ehuyln tdi nhdm cd tam gdn nhdt. Ne'u nhdm nao bi rut hit phdn tu, dil'm ed khoang edeh xa nhdt tdi cac tam se duge ehuyln din nhdm nay dl thanh lap nhdm mdi. Qua trinh duge lap lai cho din khi tdng binh phuong tdt ca cac khoang edeh dat cilc tilu.
Ma tran X Id trudng mua thang, sdp xlp thdnh 44 hang (tuang dng vdi 44 Uam) vd 492 edt (41nam*12 thing). Thuat todn K-mean duge lap lai 500 Idn vdi tap trgng tdm ban ddu khae nhau cho mdi Idn lap.
Diiu nay dam bao loai bd kha nang may rui vd eho phep dat duge phan nhdm td't nhdt. Trong qua trinh thuc hien, ehdng tdi da dp dung thuat toan K-hiean vdi K = 4, 5, 6, 7, 8. Kit qua cho thdy K = 6 la phd hgp nha't vdi cae vdng khi hau Viet Nam.
Ngoai ra, dl khang dinh lai kit qua eua phuong phap phan nhdm, ehdng tdi su dung hdm tuong quan de xae dinh mdi quan he gida eae khu vuc nhd duge tao ra khi phan nhdm.
III. P H A N B 6 THEO KHONG GIAN CUA TRUCSNG MUA
Trong khi hau, phan bd theo khdng gian eua trudng mua cQng nhu eua eae tmdng ylu td khi
Wgng khae (bdc xa, nhiet dd, dd dm, gid...), thudng duge xae dinh theo cde vdng khi hau. Theo tae gia Nguyin Dde Ngu, Nguyin Trgng Hieu (2004) lanh thd Viet Nam duge chia thanh 7 vdng khi hau : Tay Bac (Bl), Ddng Bde (B2), Dong Bang Bae Bd (B3), Bdc Tmng Bd (B4), Nam Tmng Bd (Nl),Tay Nguyen (N2) va Nam Bd (N3). Theo each phan chia nay, khu vtrc Thanh Hod thude vdng B4 keo dai td Thanh Hoa de'n Hulvd khu vue Ninh Thuan - Binh Thuan thude vung Nl keo ddi td Binh Thuan din Da Ndng.
Tuy nhien, theo tae gia Pham Nggc Toan, Phan Tdt Dde (1993), khu vuc Thanh Hod - Nghe An - Ha Tinh ed tinh chdt ehuyln tilp vdi khi hau dong bang Bae Bd vd duge xlp vao miln khi hau phia Bde, tach biet vdi miln khi hau Ddng Trudng Son (tu Quang Binh de'n Khanh Hod). Khu vue Phan Thilt - Binh Thuan duge xlp vdo miln khi hau phia Nam. Ngoai ra khi nghien edu diln biln lugng mua tren khu vuc ven biln Tmng Bd, tac gia Nguyin Vilt Ldnh (2006) cung cd quan dil'm rang khu vt^c Thanh Hod - Quynh Luu ed cue dai mua mua vdo thdng 9, gidng mdt so' tram d Bdc Bd vd khae so vdi khu vue edn lai cua Bde Tmng Bd (cue dai vdo thdng 10). Cdn khu vue Ninh Thuan - Binh Thuan mang nhdng dae tmng eua mua gid mda Tay Nam (tuang tir nhu khu vuc Nam Bd, khdc ban so vdi khu vue con lai eua Nam Trung Bd) mac dd lUtDtig mua khdng Idn.
Nhu vay, cdn tdn tai nhdng quan dil'm khae nhau vl phan vdng cho khu vue Thanh Hod vd Ninh Thuan - Binh Thuan.
1. Phan bd khdng gian theo phuong phap phan nhdm
Ke't qua dp dung phuang phdp phan nhdm eho chudi sd lieu mua thdng eua 44 tram td nam 1960 de'n 2000 duge trinh bdy trong ede hinh 1, 2 va 3.
Trong dd, binh 1 bil'u diln vi tri ft-am eua cae nhdm, hinh 2 bilu diln bie'n trinh nam eua tdng nhdm va hinh 3 Id biln ddi nhilu nam. Cae tram thudc nhdm 1 (•) tap trung ehu ylu tren khu vue miln Bdc (hr vy tuyln 19 °N), va mdt sd tram thude khu virc Nam Bd (Da Lat, Budn Me Thuat, Phan Thilt, Vung Tdu va Cdn Tho). Diiu nay eho thdy, phdn ldn lanh thd eua Viet Nam ed mua mua trdng vdi mua gid mua Tay Nam, bdt ddu td thang 5 vd cue dai vdo thdng 8 vd ke't thuc vdo thdng 10 (hinh la). Cac Uam trong nhdm nay ed lugng mua trung binh nam biln ddi td 1.500 din 1.700 mm (tmng binh 1.650 mm/nam).
Tren hinh 3a khdng thdy cd xu thi nhilu nam cua nhdm 1, tuy nhien ton tai mdt dao ddng tua hai nam.
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Hinh 1. Vi tri tram cua eae nhdm : nhdm I (•), nhdm 2{>-), nhdm 3 (•), nhdm 4 (A), nhdm 5 ( ),
nhdm 6 ( • )
Nhdm 2 ( • ) va nhdm 3 (•, duy nhdt tram Bdc Quang) bil'u diln nhung tram thude khu vue vung nui cao phia bde, nai ghi duge lugng mua Idn nhdt eua Viet Nam vdi lugng mua trung binh nam khoang 2.340mm (nhdm 2) va 4.850 mm (nhdm 3) trong dd thang 7 lugng mua ldn ban 500 (nhdm 2) va 900 mm (nhdm 3). Cae nhdm 4 (A) vd Idp 5 ( ) gdm eae tram ven biln miln Trung td khoang vy dd 12 °N de'n 19 °N cd bie'n trinh hang nam vdi ciJc dai lugng mua rai vao thang 10, dat khoang 600 mm (hinh 2d,e). Tong lugng mua trung binh nam cho nhdm 4 khoang 2.340 mm va nhdm 5 khoang 2.060 mm. Tuy nhien, chung ta ed thi thdy su khae nhau gida xu thi bie'n ddi nhilu nam gida hai nhdm : nhdm 4 khdng ed xu the' rd rang trong khi nhdm 5 c6 xu the tang (binh 3d,e). Nhdm 6 ( • ) bie'u diln cae tram d miln Tay Nam Bd va Tay Nguyen ed lugng mua tmng binh nam khoang 2.680 mm va cue dai vao thdng 8 (> 400 mm).
Tdm lai, phan bd theo khdng gian cua trudng mua tren lanh thd Viet Nam tdn tai hai ranh gidi khu vue : 1) ranh gidi tai vy dd 19 °N phan ehia khu virc miln Tmng vdi phdn lanh thd phia Bde ; 2) ranh gidi tai vy do 12 °N phan chia khu vue miln Trung vdi phdn lanh thd phia Nam.
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Hinh 2. Biin trinh nam cila lugng mua tuong dng vdi eae nhdm : nhdm I (a), nhdm 2 (b), nhdm 3 (c), nhdm 4 (d), nhdm 5 (e), nhdm 6 (f)
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Hinh 3. Bie'n doi nhilu nam eua lugng mua (theo don vi dd lech ehudn) tuong dng vdi cac nhdm nhdm 1 (a), nhdm 2 (b), nhdm 3 (e), nhdm 4 (d), nhdm 5 (e), nhdm 6 (f)
2. Phan bo'khong gian theo he so'tuong quan Dua uen kit qua cila phuang phdp phan nhdm d tren, chung tdiehia khu vtrc Bde Trung Bd (B4) thanh 2khu vue nhdOdy vydg 19°Nlam ranh gidi):
khu virc phia Bdc (B-B4) gom cac tram : Bdi Thugng, Thanh Hod, Tinh Gia vd Tuong Duong ; khu vuc phia Nam (N-B4) gdm Vinh, Ha Tinh, Dong Hdi va Hul. Tuong tu nhu vay, vdi ranh gidi khoang 12 °N, khu vue Nam Tmng Bd (Nl) eung duge chia thanh : B-NI gom Da Nang, Quang Ngai, Tuy Hod, Nha Trang; N-Nl gom Phan Thilt. Khu virc Tay Nguyen (N2) duge ehia thdnh : B-N2 gom Playeu vd Budn Me Thuat; N-N2 gdm Dd Lat vd Bao Lde. Nhu vay trong sd7 vung khi hau Viet Nam (theo Nguyin Dde Ngu, Nguyin Trgng Hieu, 2004), 3 vdng duge phan chia (B4, Nl vd N2) va 4 vung gid nguyen (Bl, B2, B3 vaN3). Bdng 1 trinh bdy kit qua tuong quan giua cae khu vue mdi duge phan chia vdi nhau va vdi ede vung khi hau khae. Kit qua eho thdy : 1) khu vue B- B4 tuong quan vdi B3 (R = 0,76), vdi B1(R = 0,66) td't hon vdi khu vi;e N-B4 (R = 0,55) ; 2) khu vuc N-Nl tuong quan vdi N3 (R = 0,60) td't ban vdi B- Nl (R = 0,53); 3) khu vuc N-N2 tuong quan vdi N3 (R = 0,49) td't ban vdi B-N2 (R = 0,30). Cd nghia la cac tram khu vuc Thanh Hod ed dae dilm mua gdn
gidng khu viie Bdc Bg hon la vdi khu vue Ha Tinh - Dong Hdi. Tuong tu nhu vay, khu vue Ninh Thuan - Binh Thuan va Lam Dong cd dae diem mua gan gidng khu vuc Nam Bg ban la vdi khu vire Nam Trung Bg va Tay Nguyen. Do dd, ehung toi se ghep Thanh Hod vao khu vue mien Bde va Ninh Thuan - Binh Thuan vdo khu vire miln Nam.
Tdt ea kit qua tren khang dinh mdt diiu, ve phan bd khdng gian cua trudng mua, lanh thd Viet Nam ed thi chia thdnh 3 khu vuc : khu vue miln Bde, khu vue miln Tmng va khu vue miln Nam vdi ranh gidi giua eae miln d vdo khoang vydd 19 °N va 12 °N.
Tie'p theo, chung tdi se xac dinh thdi gian keo dai mua mua va biln ddi nhilu nam cua lugng mua tren tdng khu vire.
IV. P H A N B 6 T H E O THOl GIAN CUA TRUONG MUA
1. Bie'n trinh nam
Nhu da ndi d tren, Viet Nam ed e h l d g khi hau nhiet ddi dm, chiu anh hudng cua gid mua. Tren lanh thd Viet Nam, mot nam ed 2 mua : mua mUa trung vdi mua gid mua mda he Bae ban cdu va mua khd trung vdi mda gid mua mda dong Bde ban cau.
Bl B2 B3 B-B4 N-B4 B-Nl N-Ni B-N2 N.N2 N3
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N-N2 -0,06 0,03 -0,02 -0,07 0,0 0,33*
• 1 0,43 0,45 1 0,30*
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N3 0,19 0,13 0,04 0,15 0,31*
0,51 0,60 0,25 0,49*
1 xen ke gida hai mda la thdi ky ehuyln tilp. Rieng
khu vire miln Tmng, do tac dung chdn gid cila ddy Tmdng Son, ludng gid Tay Nam khdng gay mua eho khu vuc nay. Mda mua d day ehi thuc stJ bdt ddu vdo thang 8-9, khi cd sir boat ddng cua cac nhilu ddng khi quyln (dai bdi tu nhiet ddi, xoay thuan nhiet ddi, front lanh...) tren khu vuc nay, va kit thuc vdo thdng 11-12.
a) Bien trinh nam cCia tugng mUa tren khu vue mien Nam
• Thdi diem bdt ddu miia mualbdt ddu gio miia miia he
Theo quan dilm cua nhilu nhd khoa hgc tren thi gidi, trong khu vuc chiu anh hudng eua gid mda, thdi dilm bat ddu gid mda mua he cung la thdi dil'm bat ddu eua mda mua, ngudi ta xac dinh thdi dil'm nay theo nhilu edeh : 1) thdng qua su thay ddi eua hudng gio thinh hanh ; 2) dita vdo lugng mua, khi nd vugt nguong nao dd ; 3) td hgp gida trudng gid va mua, hode giua gid vd hoat ddng dd'i luu [7].
0 khu vue Nam Bd - noi duge coi Id dil'n hinh cho chidd gid mua khu viJc, chung tdi xac dinh thdi dilm bat ddu mua mua theo each thd 3, ed nghia la sd dung mdt tieu ehudn td hgp giua trudng gid vd hirdng mua. Phuang phap xac dinh va kit qua eu thi da trinh bdy chi tie't trong [7]. d day, ehi trinh bdy vdn tdt ke't qua dat duge, dl minh hoa cho biln trinh nam cua lugng mua tren khu vuc nay. Hinh 4a bil'u diin gia trj tmng binh trugt 5 ngay eua lugng mua tmng binh ngay eila 26 nam eua 6 tram khu vuc Nam Bd (dudng liln net) va van tde gid vy hudng mire lOOOmb, tmng binh 26 ndm cua 4 nut ludi (105°E- 107.5°E, 10°N-12.5°N). Sd' lieu gid Idy td nguon sd lieu tai phan tich ciia Trung tam Qude gia Du bdo Mdi Trudng My/Phdng Nang Lugng (NCEP/DOE-
II). Do thi eho thdy sir ehuyln ddi td mua khd sang mua mua duge ddnh ddu bdi: I) sir tang ddt ngdt vl lugng mua td 3 mm/ngay trong thdng 4 sang 7 mm/
ngdy trong thang 5 ; 2) sir thay ddi hudng gid td ddng-nam (gid tri am) sang tay-nam (gid tri duong).
Tren CO .sd dd, ngdy bdt ddu gid mua mdi nam duge xae djnh khi thoa man diiu kien : 1) tong lugng mua ngdy (sau khi tmng binh frugt 5 ngdy) phai vugt qua 5 mm/ngay va duy tri lien tue 5 ngdy ; 2) td'c dd gid vy hudng phai ldn ban 0,5 m/s.
Dua theo tieu chud'n nay, ngay bdt ddu gid mda trung binh 26 nam la ngay 12 thang 5, vdi do lech ehudn Id 11,5 ngay. Sdm nhdt la ngdy 19 thdng 4 (nam 1979), mu6n nhdt la ngay 9 thang 6 (nam 1993).
• Bie'n trinh ndm cua luong mUa
Tren CO sd sd' lieu mua trung binh thang eua 7 tram thudc khu virc miln Nam td nam 1960 de'n nam 2000, ehung tdi tinh biln trinh ndm cua lugng mua eho khu viJe nay {hinh 4b). Biln trinh nam eho thdy cd mdt cue tilu vao thdng 2 khoang 20 mm vd mdt cue dai vdo thang 8 khoang 350 mm. Mda mua keo ddi tu thang 5 din thang 10 vdi tdng lugng mua khoang 1.700 mm chilm 83 % tdng lugng mua ea nam. Hai thang 4 vd II ed thi xem nhu la ehuyln tilp giua mua khd va mua mua vdi lugng mua mdi thdng khoang 100-150 mm/thang. Lugng mua Uong mda mua phu thudc vao : 1) cudng dd cua ludng gid Tay Nam, lien quan din td'c dd ciia ddng gid kinh hudng tren khu vuc gida An D6 Duong "nguon bom"
vd boat ddng cua cae hinh thi thdi tilt tren khu vue phia bdc vd tren Biln Ddng (ranh gid mda, HTND, XTND), "ngudn hut" ; 2) boat ddng cila cae nhilu ddng khi quyln HTND, XTND, trIn ehinh khu vue nay.
Hinh 4a. Gid tri tmng binh trugt 5 ngdy cua luting mua trung binh ngdy 26 nam tren khu virc Nam Bg (dudng liln net) vd td'c dd gid trung binh tai muc
lOOOhPa (dudng ddt net)
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J F M A M J J A S O N DThang
Hinh 4b. Biln trinh nam cua lugng mua tren khu vue mien Nam
b) Bien trinh nam cCia lugng mua tren khu vac mien Trung
Nhu ehdng ta da bilt, do anh hudng eua dia hinh, mda mua d khu virc mien Tmng khong trdng vdi mda gid mua mua he. Thdi diem bdt ddu mua mua d day khdng trdng vdi thdi dilm ehuyln mda, thdi dil'm ed su thay ddi ca vl diiu kien nhiet lire (bde xa Mat Trdi) va diiu kien ddng lire (gid mua) tren quy md todn edu. Vi vay, se khong cd ea sd chde ehdn dl xay dung ehi tieu cho xde dinh ngdy bdt ddu mda mua tren khu vue miln Tmng. Do dd, chung tdi khdng dl cap de'n thdi dilm bat ddu mda mua tren khu vue nay.
Bie'n trinh nam lugng mua tren khu vtie miln Trung (hinh 5) duge xay dung dua tren co sd sd lieu mua thdng cua 9 tram (4 tram thuge bdc miln Trung : Vinh, Ha Tinh, Dong Hdi, Hul va 5 tram thudc nam miln Tmng: Da Ndng, Quang Ngai, Quy Nhon, Tuy Hod, Nha Trang). Biln trinh mua tren
khu vuc miln Trung the hien hai ciic dai : mOt cue dai ehinh vao thang 10 vd mot cue dai phii vao thdng 5. Mdt cue tilu chinh vao thdng 3 va mOt cue tilu phu vao thang 7.
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200
Hinh 5. Biln trinh nam eua lugng mua tren khu vue mien Tmng
Lugng mua Uong cue dai phu thudng rdt nhd ehi khoang 100 mm/thang. ed cue dai nay la do cac tram khu vijc bae miln Trung ndm d sudn ddn gid Dong Bdc cda ddy Trudng Son. Front lanh (d thdi dilm cudi mda khd) thudng rdt dm (sau khi di qua biln Dong Tmng Hoa vd phia bde Biln Ddng Viet Nam) tdi khu viie ndy thudng gay nen nhdng tran mua nhd de'n mua vda va ed thi gay lu eho khu vue nay (IQ tilu man). Nguge lai, circ dai chinh rdt ldn (tdi 600 mm). Mda mua keo ddi td thdng 8 (thdng cd lugng mua trung binh Idn hon 100 mm/thang) din thang 12 vdi tong lugng mua khoang 1.700 mm chilm 78
% tdng lugng mua nam. Trong dd ede thdng ehinh gida mua mua (thdng 9, 10, 11) khu vtic ndy thudng chiu tae dgng rat manh eua dai HTND, bao, dp thdp nhiet ddi. Dae biet khi ed su kit hgp gida eae hinh the thdi tilt nay vdi boat ddng cda front lanh td phia bde, mua tren khu vuc nay se td to din rat to.
e) Bien trinh nam cCia lugng mua tren khu vue mien BSe
• Thdi diem bat ddu miia mua
Tren khu vuc Bac Bd, hoan luu gid mda mua he chiu sir tae dgng rat Idn cua dia hinh vd trudng khi dp dia phuong. Do anh hudng eua dp thdp Bdc Bd, ludng gid Tay Nam bi ehuyln hudng thdnh ddng- nam [8]. Khi nghien edu khu vtrc Bdc Bg, ehung toi tha'y khdng ed sir thay ddi rd ret vl td'e dd gid trude va sau thdi dilm bdt ddu mua mua [6J. Day Id dilm khae biet so vdi khu vue Nam Bd. Tuy nhien, lugng mua ehil ylu tren khu vue nay vdn ed ngudn gde td lugng dm eung cdp bdi hoan luu gid mda he vd thdi dilm bat ddu mua mua vdn gdn liln vdi thdi dilm
bdt ddu gid mua tren khu vuc nay ndi rieng vd khu v\xc chau A gid mua ndi ehung.
Thdi dilm bat ddu mda mua vd cac nhan td c6 lien quan da trinh bdy ty my, ehi tie't trong |5|. Dudi day, chung tdi chi trinh bdy tdm tdt kit qua nay.
Hinh 6a bil'u diln gia tri tmng binh tmgt 5 ngdy cua lugng mua tmng binh ngdy 26 nam eua 9 tram thudc khu vue Bde Bd (chi sd mua khu vtrc Bde Bd). Sa bd, ed thi thdy mua mua bat ddu vao khoang ddu thdng 5 (dae tmng bdi su tang ddt ngdt cua luong mua tmng binb td 3 mm/ngay len 6 mm/ngdy) va kit thuc vdo cud'i thdng 10 (ddnh ddu bdi su giam ddt ngdt eua lugng mua hJr 5 mm/ngay xud'ng 2 mm/ngdy). Tren co sd gia tri tmng binb h-ugt 5 ngdy cua lugng mua tmng binb 9 tram (chi sd mua eua tdng nam), thdi dilm bdt ddu mua mua eila mdi nam duge xde dinh theo diiu kien : tong lugng mua ngay phai vugt qua gia tn 6 mm/ngay va phai keo dai lien tuc trong vdng 5 ngay. Diiu kien nay thoa man : 1) dae dilm mua tren khu vuc thudng gdn liln vdi mdt hinh the thdi tie't nhdt dinh vd thudng keo ddi theo tung dgt 3-5 ngay ; 2) dam bao vl dd ldn dl loai trd nhung dgt mua trong mua khd do anh hudng ciia khdng khi lanh (thudng cd lugng mua nhd) eung eo thi keo ddi 3-5 ngdy.
Kit qua tinh todn ngay bdt ddu mua mua cho hifng nam hi 1979 din 2004 eho thdy, ngay bdt ddu mua mua tmng binb cho 26 nam la ngay 13 thang 5, sdm nha't la ngdy 23 thdng 4 (nam 1979) va mudn nha't la ngay 2 thang 6 (nam 1982).
• Bie'n trinh ndm cila lugng mua
Bieh trinh mua khu vuc miln Bae duge xde dinh dua uen sd lieu thdng cua 25 tram td vy tuye'n 19°N trd ra {hinh 6b). Tuong tu khu vuc miln Nam, biln trinh nay cung thi hien dae dilm mua eua khu vue nhiet ddi gid mda vdi mdt ctic dai vao thdng 7, 8 vd mdt cue tilu vao thdng 12, 1. Mda mua eung bat ddu thdng 5, kit thuc vdo thang 10 vdi tdng lugng mua khoang 1.600 mm chilm 84 % tdng lugng mua nam. Tuy nhien, lugng mua gida cac thdng mua mua khdng ddng dlu nhu khu vuc miln Nam ma chenh lech nhau kha nhilu. Lugng mua cae thang ddu mua mua (thang 5, 6) phu thudc vao boat ddng eua ranh gid mua. Lugng mua dat cue dai vdo thang 7,8 trung vdi khoang thdi gian ma dai HTND va XTND boat ddng manh. Cac thang cud'i mda, khi boat ddng eua dai HTND va XTND lui xud'ng khu vue vy dd thdp, lugng mua d ddy giam ban. Mdt sd noi, nhu khu vue Tay Bdc, mua mua ke't thuc vao cud'i thdng 9.
9 10 11 12
Hinh 6a. Gia tri trung binh trugt 5 ngdy cua lugng mua trung binb ngdy 26 nam tren khu vuc Bae Bg
(mm/ngdy)
S 300
1 200
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Hinh 6b. Bie'n trinh nam cua lugng mua tren khu vuc miln Bdc 2. Bien ddi nhieu nam cua lupng mua
Vdn dl CO ban eua dir bao thdng ke la xay dung ylu to du bao va tuyln chgn nhan to du bdo sao cho ehung ed quan he td't nhdt vdi ylu to du bao. Ylu td dir bao phai la ehi sd dai dien tdt nhdt eho khu vue duge du bao. Nhdm hudng tdi du bdo thdng ke tdng lugng mua mua eho Viet Nam, ehung tdi xay dung chl sd mua mua, cho tdng khu vue da xae dinh a tren. Chl sd mua mua chinh la biln ddi nhilu nam eiia tdng lugng mua mua Idy trang binh cac tram tren m6t khu vuc dong nha't vl dae tmng mua. Hinh 7 gidi thieu chi so mua mua cua Viet Nam vd tuong dng Id hdm mat dd nang lugng phd eila chung.
Tren khu vuc miln Nam, tdng lugng mua mua dao ddng quanh gia trj tmng binb 1.697 mm vdi dd lech ehudn 148mm va he sd biln ddng 8,7%. Dao ddng cua chudi sd lieu ed chu ky 2-3 ndm, tuang dng vdi ehu ky ed mat dd nang lugng phd dat cue dai (hinh 7b). Trong dd, hai nam 1963 va 1974,
3.3 3 2.6 O 2 to Q.
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13.3 10 8 6.7 5.7 5 4.4 4 3.6 3.3 3.1 2.8 2.7 2.5 2.4 2.2 2.1
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I960 1970 1980 1990 2000 13.3 10 8 6.7 5 7 5 4.4 4 3.6 3.3 3.1 2.9 2.7 2.5 2.4 2.2 2.1
Nam
Hinh 7. Bie'n ddi nhilu ndm eila tdng lugng mua mda (ehi sd mua mda) theo don vj dd lech ehudn, tren khu vue miln Nam (a), khu vue miln Tmng (e) va khu vue miln Bde (e). Hdm mat dd nang lutmg phd
eho chudi sd lieu mua khu vuc miln Nam (b), khu vuc miln Tmng (d) vd khu vuc miln Bdc (f) lugng mua dat di thudng mua thdp cdp 3, ed trj sd
nhd hon -1,96 Idn dd lech chudn (theo phan cdp cua tde gia Nguyin Trgng Hieu, 2005). Nguge lai, nam 2000, lugng mua dat xdp xi di thudng eao cdp 3 (-1-2,61 S). Dj thudng nay gay nen tran Id lieh sd tren ddng bang sdng Cuu Long : dgt lu ldn nhdt kl td nam 1926 [2]. Lugng mua trong khoang thdi gian 40 nam ed xu hudng tang len. Xu hudng ndy ed lien quan deh xu hudng tang eua boat ddng xoay thuan nhiet ddi tren khu vuc ven biln miln Nam [9].
Khu vuc miln Tmng, lugng mua tmng binh mua mua td thdng 8-12 dat 1.682mm, dd lech chudn 296 mm, he sd bie'n ddng 17,6 %. Tuong ta khu vuc miln Nam, bieh ddi theo thdi gian cua tdng lugng mua mda cdng c6 ehu ky 2-3 nam. Nam 1979, lugng mua dat di thudng thdp ca'p 2(-l,75S); nam 1981, dat di thudng
cao cdp 1 (-F2,19S), nam 1999 dat dj thudng cao cdp 2 (-H2,43S). Nam 1999 ghi ky lue vl eudng dd mua cua Viet Nam : 977,6 mm/ngdy tai Hul [10]. Gidng khu vue miln Nam, d day eung ghi duge xu hudng tang cua lugng mua trong chudi 1960-2000.
Tren khu vuc Bdc Bd, lugng mua tmng binh mda mua Id 1.589mm vdi dd lech chud'n 196mm, dd bie'n ddng 12,3%. Dao ddng eua chudi eung cd chuky 2-3 nam. Nam 1991, lugng mua dat di thudng thdp cdp 1 (-1,61S) ; nam 1978, 1994 dat dj thudng cao cdp 1 vd nam 1973 dat dj thudng cao cdp 2
(-F2,56S). Ngugc vdi khu vuc miln Nam va miln Tmng, khu vue miln Bde ghi duge mdt xu hudng giam eua lugng mua trong vdng 40 nam. Xu hudng nay eung cd thi do xu hudng giam eua boat ddng xoay thuan nhiet ddi tren vdng ven biln miln Bde [9].
KET LUAN
DiJa tren chudi sd lieu mua thdng eila 44 tram td nam 1960 din 2000 vd sd lieu mua ngay eua 15 tram td 1979 din 2004, sd dung phuong phap thdng ke, chung tdi rut ra mdt sd kit luan sau :
1. VI khdng gian, lanh thd Viet Nam cd thi ehia thdnh 3 khu vtJc ed tinh ddng nhdt vl dae h-ung mua : khu vuc miln Bdc td vy tuyln I9°N trd ra, khu v\ic miln Trung td vy tuyln I9°N din 12°N va khu vue miln Nam td vy tuye'n I2°N trd vdo.
2. Mda mua tren khu virc miln Nam keo ddi td thang 5 din thang 10, vdi ngay bdt ddu trung binh Id 12 thang 5. Mda mua tren khu vue mien Bde eung keo ddi td thang 5 din thdng 10, vdi ngay bdt ddu trung binh la 13 thang 5. Mda mua tren khu vire miln Trung td thdng 8 din thang 12.
3. Khu vue miln Nam ed chl do mua dn dinh nhdt vdi dd biln ddng 8,7 %, sau dd de'n khu vue miln Bdc (dd bie'n dgng 12,3 %), vd cud'i cdng la khu vuc miln Tmng (dd biln dgng 17,6 %). Mua tren khu vue miln Nam va miln Trung td 1960 de'n 2000 ed xu hudng tang, cdn d khu vue mien Bae cd xu hudng giam.
TAI LIEU DAN
[1] NGUYfiN TRONG HIEU, PHAM THI THANH HUONG, 2005 : Mdt sd di thudng khi hau tieu bilu d Viet Nam thdi ky 1961-2000. Tap chi Khi tugng Thuy van, 5, 7-70.
[2] LE BAC HUYNH, 2000 : Nhan xet bude ddu vl lu lut nam 2000 d ddng bang sdng Cdu Long.
Tap chi Khi tugng Thuy van, 12, 7-9.
[3] NGUYEN VIET LANH, 2006 : Diln biln lugng mua tren khu vire ven biln Trung Bd va mdi quan he giua diln biln mua vdi cac nhilu dgng trong co chl gid mua. Tap chi Khi tugng Thuy van, 10, 9-18.
[4] NGUYfiN DUC NGU, NGUYEN TRONG HIEU, 2004 : Khi hau va tai nguyen khi hau Viet Nam.
Nha xudt ban Ndng nghiep, 295 trang.
[5] PHAM XUAN THANH, B. FONTAINE, N. PHI- LIPPON, 2008 : Xac dinh thdi diem bat ddu mda mua tren khu vue Bdc Bg vd ddnh gia kha ndng du bdo. Te CKHvTDt, T. 30, 1, 13-21.
[6] PHAM XUAN THANH, 2008 : Etude clima- tique de la mousson vietnamienne et applications d la prevision saisonniere. Memoire de these. Centre
de Reeherches de Climatologie, Universite de Bour- gogne, France, 207p.
[7] PHAM XUAN THANH, B. FONTAINE, N.
PHILIPPON, 2009 : Onset of the Summer Monsoon over the Southern Vietnam and its Predictability.
Theoretical and Applied Climatology. DOI : 10.1007/.S00704-009-0115-Z
181 PHAM NGOC TOAN, PHAN TAT DAC, 1993 : Khi hau Viet Nam. Nha xudt ban Khoa hgc va Ky thuat, 312 tr.
|9| NGUYfiN VAN TUYEN, 2007 : Xu hudng boat dgng cua xoay thuan nhiet ddi va bao d khu virc Tay Bde Thai Binh Duong vd Biln Ddng theo cac each phan loai khae nhau. Tap ehi Khi tugng Thuy van, 7.4-70.
1101 TRAN THANH XUAN, TRAN THLJC, 2000 : VI hai tran lu cudi nam 1999 d miln Tmng. Tap chi Khi tugng Thuy van, 1, 4-77.
SUMMARY
Spatial and temporal study of rainfall ctiaracteristics over Vietnam
In this paper, spatial and temporal study of rain- fall characteristics over Vietnam is analyzed using the monthly rainfall data at 44 stations from 1960 to 2000 and daily rainfall data from 1979 to 2004. Clus- tered method is used to analyse the spatial distribution of rainfall. Results showed that Vietnam can be divi- ded into three regions with consistent of rainfall characteristics in each region. That includes: North Vietnam from Thanh Hoa (about 19 °N) ; Central Vietnam from Nghe An to Khanh Hoa (approximately 12 °N) and South Vietnam, including south of Tay Nguyen, Nam Bo and several provinces in Nam Trung Bo (Ninh Thuan - Binh Thuan). Rainy season in the Southern region extends from May to October with the onset is on May 12. The Northern rainy season also starts on May and end on October, the onset is on May 13. The Central region, the rainy season really starts on August and finish on December.
In the period from 1960 to 2000, the Southern of Vietnam has the most stable rainfall regime with fluctuations of 8.7%, the Northern (12.3%), and the Central (17.6%). In the area of Southern and Central, rainfall tends to increase, but the area of North tends to reduce.
Ngdy nhdn hdi : 07-11-2009
Vien Vat ly Dia cdu
(Vien Khoa hgc vd Cong nghe Viet Nam) Trung tdm Nghien eCtu Khi hdu Dijon
(Cong hod Phdp)
