Ocean salinity observations with SMOS mission

Ocean Salini y Obse a ions wi h SMOS Mission M. Ma in-Nei a"' , J. Fon o', M. S okosz@', I. C~ bella'~', A. Camps"' Eu opean Space Agency ESTEC, Keple laan 1,2200-AG Noo dwijk, The Ne he lands Phone: +31-71-565 4052; Fax:+31-71-565 4596; E-mail: [email p o ec ed] @) Ins i u o de Ciencias del Ma - ICWCSIC (Spain) "'Sou hamp on Oceanog aphy Cen m - SOC (UK) (4'Poly echnic Uni e si y o Ca alonia - UPC (Spain) E-mail: [email p o ec ed]; [email p o ec ed]; [email p o ec ed]; [email p o ec ed] INTRODUCTION The pu pose o his pape is o p esen he capabili ies o SMOS (Soil Mois u e and Ocean Salini y mission) o he global mapping o ocean salini y om space. SMOS has been selec ed by he Eu opean Space Agency as he second Ea h Explo e Oppo uni y wi h a launch da e in June 2005. The senso emba ked on SMOS is MIRAS, a Mic owa e Imaging Radiome e wi h Ape u e Syn hesis. MIRAS wo ks a L-band, in he wo-pola isa ions, and has ull pola ime ic capabili y. The measu emen o sea su ace salini y (SSS) is one o he challenges o SMOS. This pape p esen s i s he scien i ic equi emen s o a numbe o oceanog aphic applica ions. The scien i ic equi emen s a e hen ansla ed in o ins umen accu acy, sensi i i y, s abili y and spa ial esolu ion Majo sou ces o e o in he e ie al o ocean salini y will be add essed h ough an expe imen al campaign which is desc ibed. SCIENTIFIC OBJECTIVES Knowledge o he dis ibu ion o sal in he global ocean and i s annual and in e -annual a iabili y, a e c ucial in unde s auding he ole o he ocean in he clima e sys em. Ocean ci cula ion is mainly d i en by he momen um and hea luxes h ough he a mosphe e-ocean in e ace, which can be aced by obse a ion o SSS. In addi ion, salini y also de e mines ocean densi y and hence he mohaline ci cula ion. In some egions (e.g. he A c ic), salini y is he mos impo an a iable as i con ols p ocesses such as deep wa e o ma ion which is a key componen in he ocean he mohaline ci cula ion ((con eyo belb). Ocean salini y is also linked o he oceanic ca bon cycle, as i plays a pa in es ablishing he chemical equilib ium, which in u n egula es he CO2 up ake and elease. The e o e, he assimila ion o SSS in o global ocean biogeochemical models could imp o e es ima es o he abso p ion o CO2 by he oceans. Moni o ing SSS could also be used o imp o e he quali y o ENSO (El Niiio - Sou hem Oscilla ion) p edic ion by nume ical models. P esen ly he models assimila e empe a u e andlo al ime e de i ed sea le el da a only. The lack o salini y nxasu emen s esul s in majo disc epancies be ween modeled and obse ed &ice cu en s. Fo example, a 0.5 psu i@ ac ical salini y uni s) e o accoun s o a 3.8 cm/s e o in geos ophic elociw a 1 km dep h calcula ed om he co esponding su ;ice alue. This is paaicula ly impo an in he Wes em Equa o ial Paci ic whe e he e is a s ong ENSO- ela ed nea -su ace salini y signal and whe e zonal ad ec ion is o majo impo ance o ENS0 mechnisms. SSS is co ela ed wi h es ima es o he ne e apo a ion minus p ecipi a ion (E-P) balance. (E-P) is di icul o measu e accu a ely o e he ocean, so global maps o SSS would p o ide a cons ain on es ima es o (E-P) on he global scale. This would gi e insigh s in o he phenomena d i ing he he mohaline Ci culi3 iOn and also allow alida ing la en hea lux es ima es The wa e lux h ough he sea su ace laye s is c i ical o densi y s a i ica ion and s ongly in luences he mixed laye dep h and he in ensi y o su ace cu en s. The dielec ic cons au o seawa e is de e mined, among o he a iables, by salini y. In p inciple i is possilde o e ie e SSS om mic owa e mieasu emen s as long as a iables in luencing he b igh ness empe a u e (TB) signal (SST, oughness, oam,) can be accoun ed o e.g. by lhe use o di e en iewing angles, pola isa ions and equencies. The sensi i i y o TB o SSS is maximum a low mic owa e equencies and he good condi ions o salini y e ie al a e ound a L-band (1.4 GHz, 21cm wa eleng h). Howe e , i mus be s essed ha a his equency he sensi i i y o TB o SISS is low (0.5K pe psu o an SST o 2O9C, dec easing o 0.25K pe psu o an SST o OO), placing demanding equi emlen s on he pe o mance o he ins umen ( e e o Fig. 1). Due o his low sensi i i y, and he spa ial esolu ion ha can be expec ed wi h a spacebo ne mic owa e in e e ome ic adiome e , we a e !s ill no in a posi ion o ob ain SSS da a o mesoscale o egonal s udies. Howe e , se e al phenomena ex eimely ele an o la ge- 0-7803-6359-O/OO/$lO.OO 0 2000 IEEE 2552 scale and clima ic s udies can bene i om such an obse a ion app oach: ba ie laye e ec s on opical Pacl ic hea lux, halos e ic adjus men o hea s o age om sea le el, No h A lan ic he mohaline ci cula ion, su ace eshwa e lux balance, e c. Conside ing he esolu ion cons ains he o e all goal o SSS e ie al om SMOS da a is o mee he GODAE (Global Ocean Da a Assimila ion Expe imen ) op imised equi emen o open ocean SSS: a esolu ion o 0.1 psu o e 200 km boxes e e y 10 days. The emissi i i y o he sea su ace is a unc ion o salini y, empe a u e and wind speed as a mino pa ame e . The bes sensi i i y o he OS is ob ained a 1.4 GHz, howe e , he in luence o sea empe a u e can no be neglec ed e en a hese equencies. Hence, he measu emen o he ocean salini y equi es he use o ano he spacec a , which allows he e ie al o SST. MIRAS CONFIGURATION The ins umen o SMOS mission is MIRAS, a 2D in e e ome ic adiome e designed o mee he main equi emen s o he mission [l]. I consis s o a o al o 81 small an enna ecei e s uni o mly a anged on a Y-shape s uc u e, and a o al powe adiome e in i s cen e o absolu e calib a ion. To a oid alias, he minimum an enna spacing o his con igu a ion is hld3=0.57h, being h he wa eleng h a he cen al equency. Due o he high b igh ness empe a u e con as be ween he Ea h and he sky, a limi ed amoun o aliasing is pe mi ed, as long as only a ec s he sky, so he selec ed an enna spacing is se o 0.89h. Fo 27 an ennas pe a m, his gi es a o al a m leng h o abou 5.2 m, co esponding o an angula esolu ion o less han 2’. The ins umen will be pu on a low Ea h sun-synch onous o bi wi h he an enna bo esigh il ed wi h espec o nadi , so he angula esolu ion ansla es o a g ound spa ial esolu ion be e han 50 Km. 1.41 GHz B ghimss Tempe a u e s T,S a $ midmm angle 1O2[-,..pu.u~c --_ .- 3 88 i _._. I I I 25 30 35 40 Mimi y (€4 8L- Figu e 1 B igh ness Tempe a u e s SSS o Di e en SST (cou esy om Ga y Lage loe ) “E9 DOMAIN 111 Figu e 2 Typical MIRAS Field o View (cou esy CASA and GMV) The on-g ound ield o iew (FOV) o he ins umen has a dis o ed hexagon shape wi h cu ed sides, ha ing each pixel di e en sizes and incidence angles (Fig.2). A snapsho b igh ness empe a u e map o his ield o iew is ob ained each 0.78 s, wi h an a e age esolu ion o abou 5K o e 200K [2]. Indi idual esolu ion o a gi en pixel is in ac weigh ed by he an enna pa e n. Due o he mo ion o he pla o m, each pixel is measu ed se e al imes wi h di e en spa ial and adiome ic esolu ion and incidence angle. p o ided good models exis o accoun o hese a ying measu ing condi ions, he salini y map can be ob ained by a e aging ou all he e ie ed alues a each snap-sho . The minimum FOV along- ack dimension is abou 800 Km, which a 7 km/s co esponds o 114 s, o 146 ~nap-sho s o 0.78 s each. The equi alen adiome ic esolu ion ums ou o be hen 0.4 K, co esponding o a single-pass salini y esolu ion o 0.8 psu a 20°C o 1.6 psu a O’C. Howe e , in he cen e o he FOV, he o al along ack dis ance is abou 1400 km, gi ing 0.6 and 1.2 psu salini y esolu ions. To imp o e hese igu es, a e aging o di e en pixels o sa elli e passes is needed, which can be done due o he slow ime- space a ia ion o salini y in open ocean. The ins umen will be pe iodically calib a ed using a noise injec ion mechanism in combina ion wi h a highly s able noise injec ion adiome e . I is expec ed ha his mechanism will p o ide an absolu e adiome ic accu acy below 2 K. THE WISE CAMPAIGN The ange o incidence angles o MIRAS goes om 0’ a nadi o app oxima ely 60°, o pixels in he sa elli e’s g ound- ack. This ansla es in o a b igh ness empe a u e 2553 ange a e ical and ho izon al pola iza ions om 50 o 150 K wi h a small dependence on wind speed (Fig. 3). Figu e 3. Simula ed e ical and ho izon al b igh ness empe a u es o wind speed 0 and 10 m/s @ 15°C &36 psu. Fu he , he scanning con igu a ion o SMOS p esen s new challenges: - Two-dimensional imaging o he scene, wi h a ying incidence angles and pixel esolu ion as he pixel a els ough he alias- lee ield o iew; - No ye well unde s ood azimu hal dependence o he i s wo S okes pa ame e s (T , Th) wi h wind di ec ion; - Unknown signa u e o he 3 d and 4* S okes pa ame e s (U, V) and hei azimu ldele a ion dependence wi h wind speed; - E ec o sea oam a L-band; - Pola iza ion mixing be ween e ical and ho izon al pola iza ions due o he ela i e o ien a ion be ween he an enna ame and he pixel's local e e ece ame; - Feasibili y o accu a e e ie al o U, and e en ually V, assuming ha Fa aday o a ion due o he ionosphe ic e ec s has been co ec ed o by o he means. A Wind and Salini y Expe imen (WISE) is being p epa ed o answe he i s ou challenges abo e. The campaign will ake place a Cusubluncu an oil pla o m (owned by Repsol) 40 km away om he Eb o i e mou h. In his si e high wind in ensi ies o 90 km/h (25 m/s) a e no uncommon du ing Oc obe and No embe when he campaign is o eseen o ake place ( his yea 2000). The equi ed measu emen s will be ob ained lom he ollowing ins umen a ion: - Two L-band adiome e s: a ully pola ime ic adome e T , U and V) om he Poly echnic Uni e si y o Ca alonia a Ba celona (Spain), and a dual pola iza ion adiome e (Th and T ) om he Uni e si y o Massachuse s a Amhe s (USA); - Th ee oceanog aphic buoys om he ICM (Spain) and LODYC (F ance), ha will measu e sea su ace salini y, sea su ace empe a u e, wind speed and wind di ec ion; - Po able me eo ological s a ion o a mosphmic p essu e, empe a u e, ela i e hu midi y and ain a e; - S e eocame a lom LODYC ha will p o ide 3D images o he sea su ace o measu e sea su ace ms slopes - Video images o he an enna bo esigh om a ideo came a moun ed on UPC adiome e ; - In a ed adiome e ha will p o ide SST es ima es. The adiome e s will be placed. in he No h side o he pla o m a abou 32 m abo e he sea le el, which will also a oid sun glin e ec s. The oceanog aphic buoys will be moo ed in he No h side so as o collec g ound u h da a. Typical measu emen sequences a e: - Scan in ele a ion ( ixed azimu h angle), ypically +=Oo (No h), s a ing a an incidence angle 8i=20°, and ending a an incidence angle 3i = 65 ', in angula s eps o 5" o 79; - Scan in azimu h a a ixed incidence angle a a pa icula alue (Qi = 20'-65"), s a ing a an azimu h angle app oxima ely I+= 120' Wes and ending a an azimu h angle app oxima ely 4~75' Eas in angula s eps o 10' o 15". The down-welling b igh ness e:mpe a u e (a mosphe ic, cosmic and galac ic noises) will ble de e mined &e each measu emen by looking a 3 di e en ele a ion angles: nadi , ho izon and zeni h. CONCLUSIOlNS Fo many yea s i has been possible o measu e sea su ace empe a u e (SST) using in a- ed sa elli e senso s. These measu emen s ha e p o ided a g ea deal o in o ma ion abou he physics o he oceans. Ye ocean ci cula ion is dependen on wa e densi y - de e mined by bo h empe a u e and salini y. Measu ing he sal con en o he oceans is now becoming possible o he i s ime. ?'his will lead o an imp o emen in ou knowledge o he global ocean ci cula ion. REFERENCES [l] Y. Ke , J. Fon , P. Wad eu el, A. Camps, J. Ba & I. Co bella, F. To es, N. Dao, M. Vall.llosse a, G. Caudal, "Nex Gene a ion Radiome e s: SMOS. A Dual Pol L-baud 2D Ape u es Syn hesis Radiome e s 'I, 2000 IEEE Ae ospace Con e ence, Ma ch, 2000 Mon ana, USA. [2] Camps, A., I. Co bella, J. Ba a F. To es, "Radiome ic Sensi i i y Compu a ion in Ape u e Syn hesis In e e ome ic Radiome y", IIZEE T ansac ions on Geoscience and Remo e Sensing, h4a ch 1998, GRS-36, No 2, pp 680-685. (Co ec ion on GIG-36 No.5 Sep . 1998). 2554