The impact of the wet tropospheric correction on the interpretation of altimeter‐derived ocean topography in the northeast Pacific

Abstract

Atmospheric water vapor data derived from the special sensor microwave/imager (SSM/I) are used to make time-coincident, wet tropospheric range corrections to Geosat altimeter data in the northeast Pacific. The original and corrected sea surface height residuals along numerous tracks are examined to determine the impact of water vapor on the altimeter signal. Mesoscale feature analyses of corrected and uncorrected altimeter data are used to assess the impact of water vapor path lengthening in areas of low sea surface height variability. Results indicate that the horizontal spatial variations in the water vapor height corrections are frequently similar to true oceanographic gradients. interpretation of altimeter data is affected in several ways. The unaccounted-for presence of atmospheric water vapor may mimic or mask the true ocean features, and even small changes in the water vapor over short spatial scales can enhance a partially obscured feature. Analyses of all Geosat tracks crossing the area of interest in September 1987 clearly illustrate that water vapor frequently contaminates the ocean topography measurements, making the water vapor adjustment critical before the altimeter data can be successfully used to locate and identify mesoscale ocean features. Furthermore, the SSM/I and Geosat data must be closely matched in both space and time, a difficult task since it takes 3.5 days to obtain global SSM/I coverage with one operational sensor. To optimize the mesoscale oceanographic application of altimeter data, a bore-sighted radiometer should be included on all altimeter spaceborne platforms.

The estimation of geoid profiles in the northwest Atlantic from simultaneous satellite altimetry and airborne expendable bathythermograph sections

“…We ascribe an estimate of 6-10 cm rms for the total of both higher-order water vapor and ionospheric range error. However, the special sensor microwave imager (SSM/I) based studies of Phoebus and Hawkins [ 1990], while carried out in a different region, indicate that individual instantaneous passes of Geosat ERM altimetry (such as the simultaneous pass) can be contaminated with even larger episodic water vapor signatures.…”

Precipitable water and surface humidity over global oceans from special sensor microwave imager and European Center for Medium Range Weather Forecasts

“…The accuracy of W retrieved from scanning multichannel microwave radiometer (SMMR) on Seasat and Nimbus 7 were found to be equal to that derived from radiosonde soundings [e.g., Alishouse, 1983;Chang et al, 1984]. The impact of various methods of estimating water vapor as a path length correction on sea level measurements by the Geosat altimeter was discussed by Monaldo [1990], Emery et al [1990], Phoebus and Hawkins [1990], and Zimbelman and Busalacchi [1990].…”

Influence of Tropospheric Water Vapor Corrections on Geosat Altimetry in the North Atlantic Ocean

“…No significant effect of SSM/1 corrections on SSH wavenumber spectra and inferred eddy scales was found at midlatitudes (20°-40°N), where the correlation scales of WV and SSH are well separated. It should be kept in mind, however, that at mid-and high latitudes small-scale WV structures of high frequencies are not resolved in the interpolated SSM/I corrections ( Phoebus and Hawkins 1990;Minster et al 1992) and may still be present in the corrected SSH data. In the tropics (0°-20°N) the SSM/1 WV energy level at wavelengths exceeding 600 km is high enough to reduce the uncorrected SSH variability and to change the spectral slope in this low-wavenumber range.…”