A method for estimating high spatial resolution total precipitable water in all-weather condition by fusing satellite near-infrared and microwave observations

Total precipitable water (TPW) is a key parameter in the study of water cycle and climate change. High resolution TPW is critical for understanding regional water cycle processes. Besides, high-resolution TPW is also an important input parameter for atmospheric correction which helps to eliminate atmospheric impact and improve surface parameter retrieval accuracy. However, obtaining high-resolution TPW in all-weather condition from a single satellite sensor still has many limitations and challenges. This paper proposed a new fusion method to obtain all-weather TPW with a high spatial resolution of 0.05 degrees x 0.05 degrees by taking advantage of various water vapor related variables derived from microwave and near-infrared (NIR) remote sensing. At the same time, the contribution of variables in the fusion algorithm was analyzed to find the best fusion scheme. There are two major improvements in the newly developed fusion method. First, a new water vapor sensitive parameter - ratio of brightness temperature polarization difference between frequencies 89 and 36.5 GHz (Delta Tb89/Delta Tb36.5) was first introduced in the fusion algorithm to provide more high spatial resolution TPW information in both clear and cloudy sky conditions, and the accuracy of the fused TPW was significantly improved when the parameter Delta Tb89/Delta Tb36.5 was introduced. Second, a two-steps fusion method was proposed to produce a fused high spatial resolution TPW by fully taking advantage of high quality TPW information from AMSR2 TPW and Delta Tb89/Delta Tb36.5 in cloudy condition in the first step, and high quality TPW from MODIS in clear sky condition in the second step. The TPW derived from global navigation satellite system (GNSS) stations of Crustal Movement Observation Network of China (CMONOC) was used to validate the fused TPW and original TPW from MYD05 and AMSR2 at spatial resolution of 0.05 degrees x 0.05 degrees. According to the validation, the correlation coefficient, root mean square error, and relative root mean square error between the fused TPW and the GNSS TPW are 0.96, 4.23 mm and 23.67% respectively. The root mean square errors were improved by 19.96% and 27.32%, respectively, compared to the original MYD05 TPW and AMSR2 TPW.