Traditional satellite-based retrievals of the integrated water vapor (IWV) content (or precipitable water) over the oceans utilize passive microwave observations such as from the Special Sensor Microwave Imager (SSM/I). Validation of these retrievals is hindered by the lack of direct measurements over the ocean, and much of the past validation has been limited to use of coastal or island radiosonde sites. New satellite-based GPS radio occultation soundings from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission provide an important new source of all-weather, open ocean IWV retrievals.

In an initial regional intercomparison, IWV retrievals over the ocean from four different SSM/I algorithms are collocated and compared with new retrievals derived from the COSMIC data. The study bounds extend between 10-60° N latitude and 70-180° W longitude for the periods of November 1-16 and December 8-12, 2006. The retrievals exhibit strong agreement and demonstrate that the COSMIC data can be an important new independent validation source for traditional passive microwave oceanic IWV retrievals. Three of the SSM/I retrieval algorithms have essentially no overall bias relative to the COSMIC retrievals and rms differences of less than 0.3 cm. Differences among the retrievals are shown to be functions of position (latitude), cloud liquid water content, rain rate, and effective spatial resolution. Different dependencies on these parameters make it difficult to identify a single SSM/I algorithm that agrees most closely with the COSMIC data. Comparisons with SSM/I data averaged over different spatial domains demonstrate the effective average spatial resolution of the COSMIC retrievals and show the contribution of spatial IWV gradients to the differences. The results suggest a new method of quantifying the uncertainty in individual IWV retrievals as functions of coincident environmental parameters for application to data assimilation and numerical weather prediction applications.