Moderate-Resolution Imaging Spectroradiometer (MODIS) Global atmospheric column water vapor amount product (MOD05_L2) released by NASA Goddard DAAC was evaluated using 1 year of observations obtained by the Global Positioning System (GPS) installed at Gaize (32.3°N, 84.06°E, 4420 m) and Naqu (30.48°N, 92.06°E, 4518 m) on the Tibetan Plateau. It was found that the MODIS retrieved atmospheric column water vapor amount is much lower than that retrieved by GPS at the two sites. The results show that there is a systematic bias in precipitable water between MODIS and GPS, with root-mean-square errors (rms) of 2.93 mm at Naqu and 3.48 mm at Gaize. Mean relative differences in PW between GPS and MODIS at Naqu and Gaize are 57.1% and 51.1%, respectively. The reason for the bias is found to be an error in the coding of operational water vapor retrievals at the NASA Goddard DAAC for data processing. After correction for the MODIS water vapor product (MOD05_L2), the RMS difference between GPS-PW and MODIS-PW drops to 1.9 mm at Naqu and 1.68 mm at Gaize.
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