The absence of a radiometer on Geosat to provide simultaneous measurements of tropospheric water vapor makes it necessary to have the best possible estimates of coincident water vapor corrections from other satellite-borne sensor systems. In this study, tropospheric water vapor contents are computed from two different satellite sensor systems. These water vapor estimates are intercompared and are compared with other coincident estimates of tropospheric water vapor from radiosondes and from the standard weather analyses of the Fleet Numerical Oceanography Center (FNOC). Statistical comparisons are also made with a climatological water vapor data base computed from the scanning multichannel microwave radiometer (SMMR). The primary data sources for this study are the TIROS operational vertical sounder (TOVS) flying on the NOAA series of polar orbiter weather satellites and the special sensor microwave imager (SSM/I) flying since July 1987 on Defense Meteorological Satellite Program (DMSP) satellite number 8. A period of overlapping TOVS and SSM/I data from the summer of 1987 is used for statistical comparisons to determine the utility and reliability of altimetric path length corrections computed from these data. Because of its all weather sensing capability the SSM/I is shown to be capable of providing reliable global water vapor maps over periods as short as 4 days, while the TOVS system can be reliably used only for compositing periods of a week or more because of cloud contamination.

In terms of magnitudes, both systems provide water vapor estimates consistent with a global set of island-ship radiosonde to within 0.5--0.9 g/cm2. The SSM/I provides the most detailed global water vapor maps, closely approximated by the 8-day TOVS composites. The larger spatial resolution of the FNOC analysis and the SMMR climatology fails to properly resolve many of the important spatial details of the global water vapor distribution. The FNOC analysis systematically underestimates tropical water vapor contents. Seasonal means and standard deviations of both the TOVS and SSM/I global water vapor maps clearly demonstrate the importance of the annual cycle in atmospheric water vapor. The relative success of the SMMR climatology in matching some of the statistics emphasizes the importance of the annual cycle. Converted to altimetric path-length corrections for typical satellite tracks in the North Atlantic and North Pacific, the water vapor errors are maximum just north of the equator in both oceans. SSM/I- and TOVS-derived path length corrections are both significantly higher in the equatorial zone than those derived from the other water vapor maps. ¿ American Geophysical Union 1990