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An altim etric m ean sea surface o f the Arctic Ocean has been com puted by averaging individual collinear tracks from several repeat cycles. A pproxim ately tw o years o f ER S-2 data w ere used for this purpose (Cycles 001 to 021), as it is desirable to use data from an integer num ber o f years so as to reduce the effects o f any seasonal biases

in the means. The iterative m ethod o f Snaith [1993] was used to generate the m eans,

w hich involved the rem oval o f long w avelength errors by detrending the data along- track (section 4.2.8). C rossover differences o f the resulting m ean profiles w ere gen erated for the entire region covered by E R S-2 north o f 6 0 “N. T he standard d e v ia tio n o f the d ifferen ces w as 9.8 cm , w h ich in clu d es v a ria tio n s due to oceanographic signals and residual geophysical correction errors.

In d iv id u al m ean track s are also co m bin ed into a single g rid d ed d ata set for

co m p ariso n w ith an existing A rctic m ean sea surface. F igu re 4.18 show s the

difference betw een the altim etric mean surface and the Ohio State U niversity m ean sea surface, O SU M SS95, on a 0.25°x0.25° grid. O SU M SS95 is com puted from one year averages o f sea surface height from TO PEX /PO SE ID O N , G eosat and ERS-1

(35-day repeat), and incorporates data from the ERS-1 168-day rep eat phase \Yi,

1995; R app and Yi, 1997]. The resolution o f the m ean sea surface is 1/16° x 1/16°, but

has been sub-sam pled here to aid the comparison.

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Figure 4.18 Difference between the ERS-2 mean sea surface and OSUMSS95 in the Arctic, on a 0.25° x 0.25° grid.

Figure 4.19 shows the short wavelength features in the Arctic mean sea surface. The long wavelength components have been computed from a low order expansion of the N ASA /N IM A Earth G ravity M odel, EGM 96, and rem oved from the altim etric surface. The model is com plete to degree and order 360, but only the spherical harmonic components with degree and order less than or equal to 2 0 were used in the

geoid undulation computation. This retains only the long wavelength geoid features

(> 2000 km). Rapp [1993] discusses the spherical harmonic representation o f the

Figure 4.19 Difference between the ERS-2 mean sea surface and the BGM96 geoid

undulations, expanded to degree and order 2 0.

4,7 Summary

• Several strands o f evidence have been presented w hich support the

hypothesis that the specular waveforms generally observed in sea ice data originate from regions of calm water between ice floes, very new ice or meltponds. We consider the latter to be a source of noise in this work.

• Diffuse open ocean-like waveforms have been observed in areas o f 100%

ice concentration. Comparison with coincident ATSR-2 imagery shows that these w aveform s originate from the surfaces o f first-year and multiyear ice floes.

Results from a full sim ulation o f the ERS-1 tracking system show that the noise level on tracker estim ates o f sea surface height w hen diffuse echoes

are received by the altim eter is around 8 cm. The corresponding value for

specular echoes is 192 cm, w ith a reduction to 54 cm after retracking. A noise level o f 54 cm was obtained from retracked data in the S alar de U yuni region.

V alues o f the H TL error signal, e, for sea ice data have a m uch greater range than for open ocean data, and are frequently saturated at ±4.54 m.

For open ocean data, e rarely exceeds ±0.5 m.

Results from the tracker sim ulations for the ERS-1 and 2 system s reveal a linear trend between the estim ated height error and £, for e less than zero. Sim ilar observations have been made in the Salar de U yuni dataset and in com parisons o f elevation m easurem ents in areas o f seasonal ice cover. The trend has been ascribed to the large m ovem ent o f the range w indow during the w aveform averaging sequence, w hich results in variations in the position o f the w aveform retracking point relative to the telem etered range.

A linear height correction was applied to the sea ice data on the basis o f the value of e, using the above results. This correction reduced the noise level in the sim ulated sea ice data from 54 cm for retracked w aveform s to 25 cm. A sim ilar reduction from 54 cm to 19 cm w as noted in the Salar de Uyuni dataset.

C om parisons betw een data in areas o f seasonal ice cov er and further sim ulations have revealed a retracking bias w hich depends prim arily on the w aveform specularity. A correction has been devised on the basis o f these observations.

An error budget for sea ice altim eter data has been com piled, resulting in a total error o f 9.4 cm. This excludes the error associated w ith the inverted barom eter correction, and the effects o f neglecting sea state bias.

An A rctic m ean sea surface has been generated w hich reveals substantial differences w hen com pared to an existing global m ean sea surface, OSU M SS95, in the Arctic region.