On the use of GPS measurements for Moderate Resolution Imaging Spectrometer precipitable water vapor evaluation over southern Tibet

[1] Precipitable water vapor (PWV) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) near-infrared measurements from Terra and Aqua satellites have not been evaluated over southern Tibet, which has an elevated and complex topography. This study uses ground measurements from a network of 22 GPS receivers to evaluate the accuracy of MODIS PWV over this region. The evaluation shows that both the Terra and Aqua MODIS PWV have a high correlation with GPS measurements, but they generally tend to overestimate water vapor under clear-sky conditions, with scale factors from 1.06 to 1.19 and root mean square errors from 2.1 to 3.19 mm. The overestimation is mainly attributed to the systematic bias of MODIS PWV at sites below 3000 m, which increases with increasing water vapor, especially during the monsoon season, when the summer monsoon brings a large amount of water vapor from the tropical ocean to southern Tibet. In contrast, MODIS PWV at sites above 3000 m matches well with the GPS PWV. A linear fit model that was proposed and employed in previous studies is used to correct the errors of MODIS PWV at low sites. The performance of MODIS PWV is improved at each low site after the correction. The corrected MODIS PWV can represent the monthly variation of daily average GPS PWV within a 2% error. This evaluation study should facilitate the use of MODIS-derived water vapor in the models that calculate radiative forcing and simulate climate change over Tibet and its surrounding areas.