Читать книгу Remote Sensing of Water-Related Hazards - Группа авторов - Страница 29

The precipitation phase can be divided into rainfall, snowfall, and a mix of both. Snowfall measurements often contain larger uncertainties compared to liquid rainfall, and thus evaluation of snowfall based on CGDPA may not be suitable. To break through this limitation, we used an objective analysis method, i.e., the triple collocation (TC; Stoffelen, 1998). TC can estimate the RMSE of three independent inputs without knowing the “truth.” The inputs are precipitation data sets from different sources in this study (i.e., gauges, satellite sensors, and reanalysis models), sources that are independent of each other. Alemohammad et al. (2015) developed the multiplicative TC (MTC), which is more suitable to estimate the errors of precipitation. The MTC has proved superior to the additive model used by the original TC. The formulas are as below:

(7)

(8)

where C _{i, j} (i = 1,2,3; j = 1,2,3) is the covariance between two inputs in the triplet, σ _i is RMSE, and is the CC between the truth and product i. This study employs the MTC method to estimate the CC and RMSE of gauge, IMERG, and reanalysis snowfall products. The three data sources are independent of each other and satisfy the requirement of MTC.