Sunday, 11 January 2009
Comparison of Water Vapor Measurements from GPS Atmospheric Remote Sensing Techniques
Phoenix Convention Center
In this study, we compare precipitable water vapor (PWV) values from two different observing techniques over the Caribbean Sea, Gulf of Mexico, and United States regions, including ground-based GPS water vapor sensing and COSMIC radio occultation (RO) measurements, as well as global analyses from NCEP and ECMWF models. The results show good overall agreement; however, the PWV values estimated by ground-based GPS receivers tend to have a slight dry bias for low PWV values and a slight wet bias for higher PWV values, when compared with GPS RO measurements and global analyses. This may be a consequence of missing low altitude data from RO in areas where the water vapor is more concentrated, locations of the RO soundings with respect to ground-based stations, time and distance differences of either GPS techniques, or simply the difference of these two measurement techniques.
An application of a student T-test to this 910 matched profiles sample gives a retrieved t-value of 2.35, which is larger than the critical value of the sample, 1.96. This means there is a significant difference between both ground-based and space-based GPS techniques datasets with a 0.01% chance of observing a difference as large as it was observed in other random samples. The dry bias associated with space-based GPS is attributed to the missing low altitude data, where the concentration of water vapor is large. The close agreements between space-based and global analyses are due to the fact that these global analyses assimilate space-based GPS RO data from COSMIC, and the retrieval of water vapor profiles from space-based technique requires the use of global analyses as the first guess. As a result, there is mutual dependence between them.