The NOAA Earth System Research Laboratory (ESRL) has been comparing GOES water vapor products with derived water vapor estimates from global positioning system (GPS) signal delays since 2002 (Birkenheuer and Gutman, 2005). Data from the GPS and other Global Navigation Satellite Systems (GNSS) allows us to objectively intercompare water vapor estimates derived from different observing systems over long periods of time. An outgrowth of this work has been an hourly bias correction scheme that is comprised of a scaling and power term applied independently on an hourly basis. After plotting bias correction coefficients for GOES 12 and GOES 11 spacecraft, a striking similarity was observed during daylight hours. The correction coefficients demonstrate near-identical trends, phase shifted by roughly the solar time-difference between the two spacecraft. We also detect day and night differences in MODIS total water products where night data demonstrates a definite dry bias. In the case of MODIS, the only real variables are solar effects, since the same platform is involved for both passes, and the same GPS ground stations are used in the comparison, thus eliminating those dependencies. Causes for this behavior are far ranging and are discussed. Unlike GOES, MODIS does not use a model first guess in the retrieval process and this complicates the potential cause. Cloud effects may be more prevalent during day lit hours since the tendency for convective processes increases; this would tend to moisten daytime products. Night scenes might be drier if solar heating of the spacecraft increases the photon flux to the detector during those hours. This is clearly an effect that is not understood and could potentially be a new research area. It does demonstrate the value of using GPS data to validate satellite moisture measurements, an issue that remains a necessity as we move into the GOES R and NPOESS era.
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