Validation of MODIS Total Precipitable Water Over the North American Monsoon Region Using Surface GPS Technology

In this research we validate estimates of atmospheric total precipitable water (TPW) from the MODIS (Moderate Resolution Imaging Spectroradiometer) instruments onboard the Terra and Aqua satellites using surface Global Positioning System (GPS) derived TPW collected at ten stations across northwest Mexico during the 2013 North American monsoon (NAM) season. The MODIS Level 2 products provide TPW estimated from both the infrared (IR) and near-infrared (NIR) spectral bands and are available over the NAM region approximately twice per day for the IR and once per day for the NIR. Our comparisons indicate that both the MODIS IR and NIR TPW estimates capture the overall behavior of TPW over the region, with decreasing mean values and variance seen with increasing site elevation. In addition, the correlations of Terra and Aqua IR TPW with the GPS observations are all significant at the 95% confidence level, indicating the MODIS IR captures temporal variability across the season well. On the other hand, the MODIS NIR correlations show little or no significance. Both the MODIS IR and NIR also show significant differences in the seasonal means with respect to the GPS at several locations, with a greater number of sites in agreement for the NIR estimates than for the IR estimates but with higher standard deviations. The dependence of the errors on spatial averaging, elevation and time of overpass will be discussed to help identify contributing factors to the observed differences.