A Study of SSM/I-Derived Columnar Water Vapor over the Global Oceans

Four years of columnar water vapor (CWV) data from July 1987 through June 1991 derived from a satellite-based physical retrieval method are analysed using microwave observations from the Special Sensor Microwave/Imager. This retrieval along with three statistically based retrievals were compared to radiosonde data compiled for the GEWEX Water Vapor Project. It is shown that the root-mean-square (rms) difference (R) between the radiosonde data and these satellite retrievals ranges from 4.66 kg m−2 to 5.08 kg m−2. The rms difference was found to have a seasonal variability of up to 1.0 kg m−2 with the highest R in JJA. It was also found to significantly depend on the specified time and spatial coincidence of the satellite pixels with the radiosonde observation. The rms difference decreased by about 0.65 kg m−2 when the time constraint was reduced from 2 h to 0.5 h, but R increased by about 0.3 kg m−2 when the spatial coincidence was reduced from 50 to 20 km. The relationship between 4 yr of CWV and SST data was found to vary significantly with season and hemisphere. The correlation between 4 yr of monthly mean, globally averaged SST and CWV was approximately 0.95 for the hemispheric means and 0.65 for the global means. The 4-yr correlation of SST and CWV was found to be strongest in the subtropical regions and weakest in several tropical regions.