## 2 Materials and Methods
### Satellite Retrievals and Gauge Data  
High-resolution IMERG precipitation products have been released since mid-March 2014. It is necessary and urgent to conduct a comprehensive assessment all over the world, especially in areas with complex terrain and climates, such as the TP. A detailed description of IMERG is provided in [6; 18]. In this research, we use the very early Day-1 stage of the IMERG \"Final run\" product. The IMERG data are downloaded from the GPM website [19]. The IMERG algorithm is designed to intercalibrate, merge and interpolate all possible satellite microwave precipitation estimates, accompanied with microwave-calibrated infrared satellite estimates, gauge precipitation analyses and other potential precipitation estimators at reliable temporal and spatial scales [18]. In order to retrieve precipitation from the GPM constellation, the 2014 version of the Goddard Profiling Algorithm (GPROF2014) was used and then gridded, intercalibrated to the GPM Combined Instrument product with a 0.10 degree scale [18]. The CMORPH Kalman Filter Lagrangian time interpolation scheme [20] and PERSIANN-CCS re-calibration scheme [21] were also provided in the computation routines. As the predecessor to GPM, the TMPA system produces estimates of quasi-global rainfall (50\({}^{\circ}\) N-50\({}^{\circ}\) S) at relatively fine resolutions (0.25\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\), 3 h) [22]. TMPA provides two standard products, i.e., near-real time and post-real time, which have been widely utilized in hydrological and climatic studies, e.g., [23; 24; 25]. In this study, we use the latest TMPA 3B42V7 product. The rain gauge network used for validation comprises 258 automatic gauges over the whole TP (Table S1). The used hourly gauge data are available from China's hourly gauge network, which consists of a total of \(\sim\)2000 automatic weather stations distributed across China [15]. Hourly precipitation is observed by siphon or tipping-bucket rain gauges and recorded automatically. Hourly precipitation reports from these stations are transferred to China Meteorological Administration (CMA). Because all the stations used are automatic observation systems, and we do not have the bias-adjusted precipitation procedure for automatic rain gauges, the precipitation undercatch is not considered in this study. However, all of the gauge data have undergone strict quality control in three levels by CMA, including: (1) the extreme values' check; (2) internal consistency check; and (3) spatial consistency check [26]. The quality-controlled hourly gauge data are used widely for satellite precipitation estimation and high spatiotemporal gauge-satellite precipitation integration in mainland China [10; 15; 27]. To our best understanding, this is the best hourly rainfall observational dataset over the study region. We have some other gridded rainfall datasets, for instance, China Gauge-based Daily Precipitation Analysis (CGDPA). However, this is based on a daily scale that does not meet our requirement for evaluation at the sub-daily scale. Figure 1: Overview of the geographical and topographical location of the Tibetan Plateau (TP) and three main river basins, where the red, blue and black polygons indicate the basin of the Yellow River, Yangtze River and Brahmaputra, respectively; the black dots show rain gauges, and the red rectangle shows the field experimental site for snowfall observation. In terms of the independence of the intercomparison between gauge- and satellite-based data, there are 19 GPCC stations used for bias adjustment or the update of TRMM/GPM research level products over the TP, accounting for a very small portion of the total gauges (-7%) (Table S1). Therefore, the evaluation and comparison of GPM and TRMM are based on -93% independent gauge stations. In addition, both TRMM and GPM products combine GPCC gauge data at the monthly scale [18,22], while the analysis of two satellite products are performed at the sub-daily scale.