Validation of MODIS integrated water vapor product against reference GPS data at the Iberian Peninsula |
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| Institution: | 1. State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences. No.20 Datun Road, Chaoyang District, Beijing 100101, China;2. Satellite Environment Center, Ministry of Environmental Protection. No. 4 Fengde East Road, Yongfeng Industrial Base, Haidian District, Beijing 100029, China;1. College of Marine Sciences, Shanghai Ocean University, Shanghai, China;2. The Key Laboratory of Sustainable Exploitation of Oceanic Fisheries Resources, Ministry of Education, Shanghai Ocean University, Shanghai, China;3. Engineering Research Center on Estuarine and Oceanographic Mapping, Shanghai Municipal Ocean Bureau, Shanghai, China;4. National Engineering Research Centre for Oceanic Fisheries, Shanghai Ocean University, Shanghai, China;5. Key Laboratory of Oceanic Fisheries Exploration, Ministry of Agriculture, Shanghai Municipal Ocean Bureau, Shanghai, China;6. School of Earth Sciences and Engineering, Hohai University, Nanjing, China;7. Climate Change Research Centre, University of New South Wales, Sydney, New South Wales, Australia;8. College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai, China |
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| Abstract: | In this work, the water vapor product from MODIS (MODerate-resolution Imaging Spectroradiometer) instrument, on-board Aqua and Terra satellites, is compared against GPS water vapor data from 21 stations in the Iberian Peninsula as reference. GPS water vapor data is obtained from ground-based receiver stations which measure the delay caused by water vapor in the GPS microwave signals. The study period extends from 2007 until 2012. Regression analysis in every GPS station show that MODIS overestimates low integrated water vapor (IWV) data and tends to underestimate high IWV data. R2 shows a fair agreement, between 0.38 and 0.71. Inter-quartile range (IQR) in every station is around 30–45%. The dependence on several parameters was also analyzed. IWV dependence showed that low IWV are highly overestimated by MODIS, with high IQR (low precision), sharply decreasing as IWV increases. Regarding dependence on solar zenith angle (SZA), performance of MODIS IWV data decreases between 50° and 90°, while night-time MODIS data (infrared) are quite stable. The seasonal cycles of IWV and SZA cause a seasonal dependence on MODIS performance. In summer and winter, MODIS IWV tends to overestimate the reference IWV value, while in spring and autumn the tendency is to underestimate. Low IWV from coastal stations is highly overestimated (∼60%) and quite imprecise (IQR around 60%). On the contrary, high IWV data show very little dependence along seasons. Cloud-fraction (CF) dependence was also studied, showing that clouds display a negligible impact on IWV over/underestimation. However, IQR increases with CF, except in night-time satellite values, which are quite stable. |
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| Keywords: | MODIS Water vapor Validation IWV GPS Satellite |
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