Intercomparison of MODIS- and GNSS-Based Precipitable Water Vapor Over Manila, Philippines in 2015

Precipitable water vapor (PWV) plays a crucial role in the climate system and its impact on atmospheric activities, weather, and climate changes. MODIS-PWV products are often compared to ground-based retrieved PWV data over different areas around the globe. However, there haven’t been as frequent studies similar to it over any areas of Philippine territory. This study assessed the accuracy of MODIS-PWV against GNSS-derived PWV using correlational and linear regression analysis. It provided results that showed a very high positive correlation of R = 0.91 and an accuracy of RMSE = 4.83 mm, R2 = 0.83 between MODIS-PWV and GNSS-derived PWV in 2015 over Manila. MODIS-PWV is found to be significantly underestimating the GNSS-derived PWV; which could be due to their disparate spatial positioning, the location of the region, and surface elevation. During the dry and wet season, there is also an underestimation but a very high positive correlation. The PWV peak values at the site in 2015 ranged from 50 to 70 mm for both its daily and monthly mean. In the dry season, there was a greater variability between MODIS-PWV and GNSS-derived PWV, with a range of 5.31–9.55 mm. To the contrary, during the wet season, the variability was lower and ranged from 5.48 to 7.63 mm; which agree well with the climate type of Manila. The study infers that the results are significant in the Philippines, where the understanding of MODIS-PWV accuracy against GNSS-derived PWV and its limitations helps provide outright improved PWV observations.