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Satellite Altimetry: Observing Ocean Variability from Space

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Satellite Altimetry: Observing Ocean Variability from Space FEATURE SATELLITE ALTIMETRY: OBSERVING OCEAN VARIABILITY FROM SPACE By Lee-Leung Fu, Dudley B. Chelton and Victor Zlotnicki DIRECT measurement of large-scale ocean determination techniques, the height of the sea sur- currents with current meter arrays is difficult and face relative to the same ellipsoid (the sea level) can costly on basin scales. Since large-scale currents are be obtained by subtracting the altimeter range very nearly in geostrophic balance, their velocity can measurement h from H (Fig. 1). The dynamic sea be calculated from the pressure gradient on an surface elevation hj is then the difference between equigeopotential surface. The surface geostrophic the sea level and the geoid height h,. The magnitude current therefore can be calculated from the devia- of dynamic sea surface elevation ranges from 10cm tion of sea level from the equigeopotential at the (gym-scale currents) to over l m (western boundary ocean surface (the marine geoid). Measuring sea currents and eddies), with variance spread over a level from space by satellite altimetry thus offers a wide range of wavenumbers and frequencies. The unique opportunity for determining the global sur- required measurement accuracy is thus dependent on face geostrophic ocean circulation and its variability. the phenomenon of interest. Generally, an overall Coupled with knowledge of the geoid and the ocean accuracy of better than 10cm is required for observa- density field, satellite altimetry provides the only tions of the dynamic sea surtace elevation to be feasible approach for determining absolute geostro- useful. Present knowledge of the global geoid is phic currents in the global ocean. Repeated altimetric accurate to this level only at scales longer than about observations of sea level at the same locations can 5,000km. At shorter scales, present applications of resolve the variability of surface geostrophic cur- altimetry are therefore limited to the study of tempo- rents, without the requirement for an accurate geoid ral variability of ocean currents obtained from re- model. In this paper, we briefly introduce the tech- peated altimeter measurements at the same locations, nique of satellite altimetry, highlight progress made thus allowing removal of the time-invariant geoid. in the past, assess the challenges that lie ahead, and The accuracy of the sea level measurements then discuss future plans. becomes the primary issue. Uncertainty in the al- The Technique timetric measurement of sea level results from errors A satellite altimeter is a radar that precisely in the satellite range measurement h and the orbit measures the range from the radar antenna to the height H. ocean surface. A schematic summary of altimeter Errors in h can be ascribed to three sources as measurements and the corrections that must be ap- indicated in Fig. 1: instrumental errors, atmospheric plied to obtain dynamic sea surface elevation is refraction, and the interaction between the radar shown in Fig. 1 (p. 6). The altimeter transmits a short pulse and the air-sea interlace. A thorough discus- pulse of microwave radiation with known power sion of the elements of instrumental error is given by toward the sea surface at satellite nadir (the point Chelton (1988). Briefly, instrumental errors consist directly beneath the satellite). This pulse interacts of a random part and a systematic (long-wavelength) with the sea surface, and part of the incident radiation part. The random component is often referred to as reflects back to the satellite, giving the range from the the instrument precision. Since the earliest space- 2-way travel time of the pulse. Near-surface wind borne altimeter flown on Skylab in 1973, the instru- speed and significant wave height also can be deter- ment precision for 1-sec averaged data has improved mined from the power and shape of the returned from about 60cm, which barely resolved the sea signal. surface slope across the Gulf Stream, to better than If the orbit height H of the satellite relative to a 4cm, which is sufficient to detect even small ampli- reference ellipsoid is known from precision orbit tude eddies in the ocean (Table l, p. 10). Unlike random errors, the systematic component of instru- Lee-Leung Fu, Jet Pr~qmlsi(m Laboratoo'. Col(l?,'nia Instttute mental error cannot be reduced by along-track aver- ,!f Te('hmdogy. Pasadena. CA 9ll09: Dudley' B. Chelton, Col- aging of the data. These systematic instrumental lege of Oceano,waphy, Oregon State University: Corvallis, OR errors are still poorly understood and remain a chal- 97331: Victor Zlotnicki, .let Propulsion Laboratory: California lenge to altimeter engineers. Institute of Technolow. Pasadena, CA 91109. 4 OCEANOGRAPHY.NOVEMBER.1988 g r • % i • u ~. A. .. 5:' f : ~" Figure 3. Upper Panel: Difference bem'een mesoscale sea level variability during the northern hemisphere winter (December. January, February) and the annual average shown on the back cover. Differences are displayed in terms of the square root of the 1 °gridded variance differences, smoothed by a 9 ° square median filter. Positive (negative) values correspond to anomalously high (low) wintertime eddy energy. Lower Panel: Same as the upper panel, except for the southern hemisphere winter (June. July. August). Data for both panels are from the first year of the Geosat exact repeat mission (December 1986-November 1987). OCEANOGRAPHyoNOVEM BER. 1988 5 Uncertainty in h introduced by atmospheric re- )~ Satelhte fraction result from decreases in the speed of light mainly due to dry gases (primarily oxygen), water vapor, and ionospheric free electrons along the propa- I I H ~nStr urnent Car rectlons gation path of the radar pulse. The range error due to refraction by dry gases is large (approximately • j''.:r,,~ ]j ~jmr, Atmospheric Refrochon • L ,, ,m,, J,:~, 230cm), but can be accurately corrected with knowl- Correchons ~ , " 'r't'mr') r'l [ 'rt"' edge of sea level atmospheric pressure obtained from • ^l' ' .'l ' I External Geophysical Corr_ct,ons meteorological models. The atmospheric water va- por content, however, cannot be predicted by mete- A~r sea / • ,r ' ,, ." orological models with sufficient accuracy for cor- [nterfoce ~[orrecth}ns 1 recting the water vapor induced range error, which • ,i¢,,,, : , .L ~ hd= H-h-hg has a magnitude of 5-40cm. Accurate concurrent ~.~.~_......._..~.--~1~, ~-'~'Z~%..~ ~-"-~-.~ .Sea Surf .... measurement of atmospheric water vapor from a passive microwave radiometer is required for this n :' .... correction (Tapley et al., 1982b). Similarly, present Topocjrophy ionospheric models are not sufficiently accurate for correcting the ionospheric range error, which has a Fig, 1 magnitude of 0.2-20cm. Direct measurement of the Figure 1. ,4 .schdttldtid ,~llt?ltlltll~V ¢d'ell[llllCtel DIC<ISItIeIHcIII.~ <lily total electron content in the ionosphere is required. thv t Ol'r6dliolls [lid! tllltA[ he applied tn ohtailz the dvmztlli~ Sed Since the ionospheric range error varies with fre- ,Sltl't}lt'd eldvdtioII Ill. The altinldter rtlll,k,d ItldtlSltl't'tllelll ix h, tllld quency of the transmitted signal, radar altimeter H and lie are thd orbit heivht and ,v,eoid hei,ehl, rd,vwctivdly, An improvement measurements at two frequencies can be used to IdldflVe tO d *'qf~'l'dlldd elJip.~old <l/l[~l'(~.villltlgltlll IH gild ddlth',v estimate the ionospheric range correction (Topex Slll't~ldC, from 10m for the Science Working Group. 1981, Chelton, 1988). Figure 2. Sea level at Majuro m the westdrn tropical Pacific Skylab mission to Additional errors in h result from the complex 17.1': N. 171.4" E). Line without svmh.ls i.s a Iow-pas.v fihered interaction of the altimeter pulse with the sea surface. (sm,,,,thed over a 2-week pel'iod~ time seriev ohtai/ted .fiom the 50cm recently The signal received by an altimeter is the radiation re- Geusat altimeter d~lttl. Lille with plus synlhuls is ohtdined /}onl a achieved for Seasat flected by those specular scatterers within the pulse Cldfic-sphnd tit of monthly nlean values [rnm did Mqjm'o title footprint. The range measurement is therefore re- ,~age. <A/?er C/whey and Milh'r. 19,~,~). by Marsh etal. (1988) lated to the distribution of the specular scatterers, Figure 4. Wavcnumher-l)'eqlwn O' sprit tlUm :)/'sea h'vd[ variabil- rather than true mean sea level, The difference be- is another major ity e.slimaled fi'om Geu.sat ahimcter t&ta m tile" .4~ulha.s retr:glec- tween mean sea level and the mean scattering surface lion reA,ion south of A/)'ica. Thd .S'lR'l'H•ldql is pl'('SClltt'd Ill till achievement• is referred to as the electromagnetic (EM) bias. The e'ndr,,..,y-presdl•vill),, fi~rm: thd .g~ectral enerKv is i)rup~ntiunal tu power backscattered from wave troughs is greater the Yolltllle IllldeF tile { t*lllt)ltrdd surtilud. 7Jle ('()llgOlll illiCIT<l/ IS than that from wave crests, which biases the range ]Ocm" and elrt'<lS ovt'l fi()l'DIJ (-Ire stippled. measurements below true mean sea level. Since EM bias tends to increase with wave height, it is generally expressed as a percentage of significant wave height error resulting from uncertainty in the orbit height H (SWH), defined to be 4 times the standard deviation is due to the lack of continuous direct observations of of wave height. Estimates of EM bias derived empiri- the satellite position. The orbit height is usually de- cally from in-orbit measurements range from 1-4c/c termined by statistical estimation based on assimila- of SWH, but with uncertainty as large as the correc- tion of sparse tracking measurements into a dynami- tion (Born etal., 1982). cal orbit model. The main elements of the model Because of the pulse compression technique used include the earth's gravity field, solar and terrestrial in the present altimeter design (Chehon, 1988), the radiation pressures, and atmospheric drag.
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