Validation of TMI derived total precipitable water vapour with operational soundings

This study presents a validation exercise of the total precipitable water vapour (TPWV) obtained from microwave passive radiometry over oceans from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). This study reports the results of the comparison of the TMI derived TPWV against the one obtained from radiosoundings from operational meteorological sites. A TMI-radiosounding comparison dataset has been created using semi-objectively defined matching criteria. Screening of low quality observation from both dataset is performed to include in the comparison low quality observations. Applying the match-up criteria for the period Dec.1997-Jan.2012 gives a set of 141679 comparison cases from 232 sites. The overall resulting BIAS is 0.90 mm with a RMS of 4.84 mm. Detailed analyses are performed to discuss and to interpret the comparison results in terms of error derived from: the criteria adopted to match the observations (space and time), instrumental errors from both TMI and radiosonde (solar illumination, radiosonde type), algorithms error (by correlating the comparison results against the value of the other geophysical parameters derived from the TMI, vertical integration scheme for the radiosounding).