Synoptic ice motion from AVHRR imagery: Automated measurements versus wind-driven theory

R EMOTE

SENS

E N V I R t W ~9 79 - hi5 (198,9)

Short Communication

Synoptic Ice Motion from AVHRR Imagery: Automated Measurements Versus Wind-Driven Theory Michael ]. Collins' Centre fi)r Research in Exp(~mental Space ~'itmce. Ontario

T h i s m~t,' d,'scrilu's the applicatitm of a rece,ll!l Huci,tah'd theory of auhmmled sea icc mt,li, m d,'tccti, m 1o A VHRR imagemj. It titrllu'r s'ceLs It, point out the m~iquc abili/tl of A VHRR to prot,idc s!t,oplic mcasurenwnls of sca ice r,t'h,cittj which art' useful in t)alidaling regional scab' ice modHs. 'lTw measurements or(' cornl)art'd u,ilh a simph' u,imt driocn tlu'ory of i('c motion a , d /hi' results h'mt support /o the lim'arizh~g assumplio,s of lht' mod,'l. INTRODUCTION Satelhte imagery has It,rig prowded scientists with an invaluable data set [r,,m which sea ice m,.,tJon measurements may be extracted. The tra,ht,.m'.d rneasurernent technique has been t,, visually ~den tify promu,ent ice I'eatttres, ,,r tie-points, in a pair of photographs (DeRyeke, 1973; Hall and Rothr,.,ck, 1!-)81). The advent ol' mapped ,llgit',d satelhte imagery has ,,pened the do,,r to autonlated techniqlles which dramatic',dly reduce the mea stlrernerit time These new meth,,ds 'also prl,duce a regldar grid ,.,1"motk, rl measurements Ul c,,ntrast to the irregular and olten sparse fiehl ,,1 marnval nLea sllremerlts. The pld.,bshed act'omlts of the ,.,omputati,,n',d methods have been limited to synthetic ape~Jre radar (SAR) from Seasat (Fily arid Rothroek. 1987; Addre,~.~ ,'on,-,!,r,~rideri,'e io M I (_iidlin,~. (3erlir,-' Ior BP,,awh Irl

~,'Hp ~p,'ICI-* S('i, II'a+rk Irrll,,.+, ,..r7(10 I+L,:.,elc-,Sl, Norll-i York, ()rll&no, (3~.u'lad.i %131 I F'3 R,'celr,.d .17 ( )ctol.,I I!.1,~¢, tf,r Jsed ?3 F'ebruanl I~:~l!.) 00,'},1 I'J,%' ,,,",Lig ,,,"$ ,'I 50 '"El~,'r,.'r S,'.'nc,, Pultlishme. Co I., , I!.I,S9 6,5,', ,4f,,,m., ot II., A n . , r . o~, Ni.u, )'ork. ,~r~, I0010

C,ollJns aJrd Emery, 1988: Vesecky et al., 1988) and the advanced very h i l l resolution radionleter (AVHRR) from N(-)AA's polar orbiting metet,rological satellites (Nmnis et al.. 1986; C,.,llins, 1987) ()f these the f,,nner is l_lle overwhelming favorite The SAR e,,mt,ines a line resolution (2(10 m and less) and au 'all weather 2,-I-h mlaging capability with startlingly det',uled scenes ol" the tee to pro duee a higtdy desirable dat;tset. I.Iwk,rtunately, SA R mmgery e,,mes in relatively thin strips (about 100 krrl) and the overlap of sequenti',.d ,,rbits does not ',dh.,w fi,r snyopt~c measurernents of sea ice mot,re. The AVHRR, whde hmderecl by fls fair weather, da.~inle imaging, can deliver twice daily snyt,ptit' vaews of the polar ,,ceans and is s,,metimes capable ,,f pr,,ducing clear unagery i.,[ the Beaulort Sea [or tip t,, ,5 eonsecutwe days. Mea.suremerfls ['rOrTi AVHRR are thus well stated f,,r providing devel(,ping regtorral sc.de it'e models with v'.dlmble, if sporadic, v',didatlon data. Th~s paper presents an infli'.d quahtatwe comparison between aut,,mated ice rue~su.rements arid a smlple wincl-driverl theory (.,I tt'e nlotl(m. DATA The AVHRR data was received and processed at the Satellite t.)ceanography Lab m d~e Department ,,I (.)ceanography at the Un,verslty of British C,olumbia. The raw data was earth-h.)cated and geometrically c,,rrected act'or&rlg to Emery and Ikeda (1984), where a 1 pLxel accuracy was re ported. The resolutlorl o1 the navigated ptxel is

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appu)xJmately i.7 krn at nadir arid changes slowly away [r()nl this point. The riawgated chgitM images cover 87( ) '.,' ,"470 km (512.,.512 puxels) and are t.'entered at 72"N 127"W The image pmr used ill Ihns study is showtl m Figs. I and 2 They are from the visible channel ol the AVHRR, wFHch is sellsi I,ve tc~ hgbt ,n tire I).N4-0 fig/J,m wavelength rai,ge (red) COM

PUTATIONAL

M ETHODS

A briel d e s c n p t . m o1' the mea.smement te,'hri..lue is pr,)v]ded here whde the details may Ire fom,I elsewhere (Collins. 1987). We begin with twl~ ml

ages, t ( t , ! l ) mlaged at t. and g(t,y) imaged at I I. We assume that the rnovemeilt of the pat'k it'e may I_)e n.~deled as a nu.rnbet of tratlslations. "the ,rnage paJr is slibchwded irlh~ smaller segments, 1, I and 'g,l' wl-lele tl I is slna.[ler than g,I The pn)ll lern i,,, to fit,I all estmm|e qJl' the location of 1, t w~thm g,~, where g,t is assumed lid t'ontaJl| tile first segment plils a sample ot adchhve stationary (_;aussian r,..se, iI( t. !t) Therefore, 'all tenlp(rvaJ ~.'hanges m the ice within l, I between t. and t I are modeled as addit.ve noise Ig,,( t, .q) = t,t( ~,, y) + al( t, y)] In the lang-uage of linear filter theory we wish t() lind the filter h( t. !t) such that when tt is vonvl)lved wtth l;/, the nla.ximum value of the

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F'lgurc 2 AVHRR Baad I (O~~-06Flp, m), 7 J,me 1984 ,,utput will .yield the best esl,mate of the displa,.'ement of tl I within g,l' F'urtJler we wish sut'h a Filter to be c,ptim'a] with respect t,, s,)me performant'e eriten,,n, in this ease the mean squared error between the estimated and true displacement. The s,,luti,_,n to this optmlizati,,n problem is commonly ca]Jed the matched filter wldch is a gelleral resldt placing no restrk'tion on the color, or frequency distribution, ol" the additive tloise. Samples ,.if th,s noise may be obtained by ,.'omputing g , i - /il at sever'a] correct match Iot'aUons. The autot'orrelation funt'tion of noise samples I'rom tile AVHRR imagery tndi,.'ated that the noise was

approximately white and the npUmM filter reduces to a simple c,)rrelator (this was not the case for the SAR whose opLim'M filter comprised a high-pa.ss filter followed hy a correlator) The displacement measu.renlents obtaJned frorn this method are shown ,n Fig. 3

WIND-DRIVEN THEORY

Thonldike arid Colony (19~2) investigated the Iollowing linear relationship between the ice velocity u, the geostr,~phic wind C, and the rneasl t..'ean

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the shore at least 75% of the ice vel~'ity was explmned by the geostrophie wind while closer to die shore this figure fell to 5(~, and less It was pc~st-tilated that intem'M ~ce stress, which tends to dissipate energy and oppose the ,ee velocity, plays a larger role near the coast ~ d tends t() wea.ken the hnearity of Eq. (I). For tlus study surface anMysis charts were obtained horn the Atmo spheric Environment Servk'e in Edmontor, mid the contours of 24-h averaged atmospheric pressure were reduced to 10 m winds in the ILsual way. The values of the st,aling factor and turning angle used were ()(K)77 and 18", respeutively, to agree w~th the correspond,ng spnng values quoted ,n Th~rndike and (-',oh.my (1982) The pred..'ted wind
COM PAR ISON Tile m, mleled ,ee velocities were sut)tra<.'ted from the chsplaeement mea.suremerRs and the residual dJsplacements are shown in Fig. o. =' The isobars s.penmposed ou both the modeled ice veloenties

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'lilts stud| I u,as t drrtcd mtt .,/.h' tlu' author u,tts a sludct~t tn tit,' f)~7~arfrruml o t Occano/~.raphtj at tl.. I tmt,~'rsrttl o/ Rnlish Cohnrll.a und(~" II.' '~'U~9('rl'/SH,.'~'I tl I Wdham I En.'rt/, .,ha is Is~ah'tulhl acl~nou,lrdg~',l tar /its sttp,ppt~rl arid gutdalltw FI tlmu'tal sulrl,ltrrl during I/its l~'n~l was ~tm~t,t&'d by Dr Em,'rtt thtctugh g~r~tttls ~ont tl.' Natural Science anti Engine|ring Rcscan'h (.'tmncd i,/ (?art|ida arid the (~artadian Atmo~'ph,'rtl Ent,trlmmt'nl ,v,ct~,m'c Th(, autlurr ts cttrr(~nlh/ supptmh'tl bt/ tit(' Radar I')ota De{,Htrpn.'nt Program ot Thc Canada (¥nh'c /or Rmltoh, Sensing

tll_,l ill the Beatd',)rt Sea with speeds away I'nml the coast e, mlparallle t(t thi)se (d_)served (3 -5 rn,,"s) (MePhee,

I,qS()). Siilee d i e area lirir, l e d l a t e l v west

Itl Banks Islalld is ill)l a regl(.)ll ~)1 mean I]()w mtensitit'ati~.l (Newt(l,i, l.-J~3: Serntller, 1986), tile large ~esidij'al Illagntludes irl this area are lll()St likely a illarinl'eslatntt;i (il' the ilitelllaJ nee stress The ~tbserved ice r,,~tn,tll may I~e res,~lved hUllO a ('(l,II p, m e l i t par',JJel h) the t',)ast ( l l ~ r l h w a l d ) arid a t.'i)lilpOl'lerit p e r p e n t h c l d a r hi the t'l)ast ( w e s h v a r d ) II is weU kr..)wll that I(.'e t'anl..)t s l l p p o r t tensile

stress (MeU~r, 1!-)73); hence the pel'pell(Jleldal (.{inlp~.lenl ~il ice rl.tt,lll in largely ul.~pposed. "rile par'.dlel t'l)ii]pi)nlellt gnves rise h) a shear stress which ns opposed lie V . o (the term representinlg Ihe efl'e¢'l (~1' the intenv.d ice stress a nrl the hdl r , i o r i w t l t u r r l baJarlee) T h e d i r e c t , . i oi' the resldwals then is t'oiislstent with the r.|{~ttele~l physlt's .'~.li Irlspectl(tll {)[ the irnagerv sl-.~ws I_hal Ihe ~ltirthwesl {'.)nler ol Balff, s Isla.d is lutting out lilts) the it'e pack al.i hir.lenng Ow llorthward c(llnponeld of It'e Ill(it|till. This regl(.)ll in ',.dso c h a r a e t e n z e d I_ty the largest lesldual magnlhldes. There is a large band ,tt {=pen water separating Lhe pack Iroln the s.)uth t~ni (.'{)asthne al,.I the reslduaJs il, Ihls area are likely (.hie t~) the ,~,Ela(.'e elrcLLlalloll ()1' the ,,.'ean

CONCLUSIONS This note demonstrates the .tillty ~)1 AVHRR's synoptic inzage scale. In spite ol its disadvaldages it remailzs a lisehal souree ~| data Ior studying larger scale sea ,.'e behavior More quanlJtative x,,,~)rk In t u l d e r w a y int'hJding s t u d y i n g |_he b e h a v l i ) r ~)I veloeity denvatives arid r e l a t e d llivarialits ( w . ) r t i e l t y . d i v e r g e r i e e and shear} as has been (l()rie

by (Ri~throck, 1988) ~s well as eomparisor, ,)I mea st|red velocities t~) nit)re corllplex models.

REFERENCES (~oLGn.s. M J (1987). The e,JnTialiorl ill p~t'k ice ni, lhorl Irl dlb~|aJ ,~aleLLlle llTiagery LW m.'i|i-hed ~tel-lrl~,, M.'i.,,te, l-~ lii,~ M.S, r)~.p.,.l.l~Hierl| ill ()cea.riugraphy, UIiIVerM|y ol Bnh.sh

(_;ohJmi.a. Va.nc.uver. (Sa.riada, I:.~'.~pp (.:,dhri.s. M I . arid Enlerv. W I (1988). A COiTipulahii, rlaJ nlethlld lot estirriahrl K ,sea ice rrlotlorl trl sequerlh.'d ,~'~al ~,yTItht"|lL' .'lp~l-tlue radu.r imagery by rllalt'hed ~J|enrlg, I t.'(rph=ls R~'s !-t3 9241-92,."51 DeRyt.ke. R I (1973). ,~ea It'e II'l()|i(Irl~, Oil Arl|a.rllt,.i irl I.Jle Vll,lrilly i)l the E~,|PrTI Ros.s Sea a.s observed by ~,atelhle,

I th'oph¢/.~ R,'s 7,u,'~'173-,"~7!-) EITieI-$', ~,~" I ;Hid Ikeda. %t (1!)~4). A c, mlpa.nsoii ,ll ge,.llel |"It Vqll'let'iliIfl rrteLhql(.l.,J Ill ,A,'¢HRFL ii-li.,l~erv, ('arl ] R,'

mob' Sclt.s Lql 4H-.r~'; File, M..'rod R.|lm,,k. D % (19~7), Sea n~'e |rat'km~ I_w IiP..,|ed correlaliorl.s Ib./~;/', '/"tans ( .~'osci Re.lob' S, vl.~' | ;E 25 570 580 H~.dJ. B arid R,ithr,,.'k. D ~. (1981). Sea ice dl.splacernerl| Irorn Sea,,a| WTllhe|l,,' apertiirt-, radar. I [.;,'(~ph/i.~' Rcs H6 11,07H- 11 ,()82 '~lcPh,~e. M (; (l!.},"i:~). ~r, .',,l.,Iv.sl~ r~l pack ice drill ill ',urnmer. li= Sea h'c I'rl,'c.~s,'s am/Moth'}.~ (R S F'rltl'hu.td, Ed ). tlrliver'~llv ,d W.,'J'liriKt,m Pre.s,,. Sealtle. WA, pp 62- 75 Nlelh~r. M (19~:h, M~l'h;.uii,',.d ilehavl.r ,ll wa ,ce. CRREL .M,.,,~aph ~;.I I. (;,lid Re~,~rP, Resea.rt'h aJld Eri~neenri~, l..ai,.al.rv. H=mover. NH Nev,'tilrl, I L (l!'~'7;J), 'Tile |iJ.llada [-t.'l.~,lrl' mea.rl ('lr(,lila|lilll &rid irileFrrledlah = .sL"a.le I'lo~.' leahxtes Dilt'lor'JJ cLl.s.serta|li=ri, i lrilverM|y (ll WLl..shlli~,,1Oli, 15H pp NlrlriP, F{ M , Ertiery, ~, J, .lli(J |_',,iL[l.ri,'~,, ~| J (198~), .'~'.ll|llrtl{.l|e(J e~|rat'llllrl ill pack i~,e ilil.,lll)[i [rr~rn adva.nt'ed very h i l l re,,ohJlllm radlortleler IITlag~ery, ] C.t'l*itlllt.IS' I~'.~ !41 1(),725- [(I,'7:H [ll=|llrili'k

I-) A (l!-)H~i). Aiih~llialed l,'e ITieLLMI/enit-'rlh Irlffn a

dt-'t'ade ill SAR ,magerv-- H,~w w~ll we get tllerrl ',~ How w'lll Wl-, USe IhPITI '~, M T S / / I E E E

Work.,.,hilp (irl IriMn.Lrrierlla|lor|

S.tlntrptic Mcastm'mtvits of &'a Ic+, Motion 8 5

and Mea.stuurnenl in P.ku Re.on,s, 27-29 I~uiuary 1,988, Monleray, CA Semtr, er. A J (1,t}87). A nurrlencal ,',l'udy I}1'sea it'u IJ.rld o{.'e~Lrl cirt'ulaliorl Irl U'lv A.rt'ht', I ['/ills (.)r't'tdrl 17 11)77-1099 Thorr.bke, A, S. asld C.hmy, R, (1982), Sea ice rnotion trl

re,',p~mse .[

geoslr, phnc winds,

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87 ,"~45- :~52, Ve,',ecky, I F , Sa.ntadasn, R , ,qnH|h, M P, r)aida, J h , and Bracewell, R N (198g), Observation nl .sea lOP dyTlaJTIl(.'S iri .byTiLbt'll(.'apel-l'u£e ra 'da.r Irriagtq"y Llulomaled LLI'|d.lyAI.% IEEE Tran,s (?.*'osri R+'mot+'._~'ns G E 2 6 ' 3 8 - 4 8