Particularly, the IMERG products (0.1° and 30 minutes), which are built on the advances of many previous satellite precipitation algorithms, have gained strong appeals to the community since its release in April 2014. Many studies show that IMERG is superior to other precipitation datasets in many cases (Tang, Ma, et al., 2016; Tang, Zeng, et al., 2016; Jiang & Bauer‐Gottwein, 2019; Tang et al., 2020). Recently, IMERG completed the retrospective processing using the updated V06 algorithm and covers the period starting from June 2000. This reproduction process makes IMERG more suitable for long‐term hydrological and meteorological studies, such as flood simulation and monitoring (Zhang et al., 2015).
Flash floods are often triggered by intense precipitation in a short period, such as a few minutes to a few hours. Flash floods are among the most destructive natural hazards in the world and can cause huge economic losses and casualties. For example, the National Weather Service (NWS) of the USA reported that 278 people died from flash floods from October 1, 2007, to October 1, 2015 (Gourley et al., 2017). Moreover, on average, 984 people die per year due to flash floods in China since 1950 (Guo et al., 2018). Therefore, there is a strong demand for flood forecast and monitoring, particularly considering the accelerated processes of climate change and land degradation. Near‐real‐time satellite precipitation products provide new opportunities to flood monitoring. This is extremely important for alpine regions where the ground‐based measurements are usually scarce or absent. Meanwhile, it is known that satellite precipitation products could underestimate extreme precipitation due to the limitation of sensors and retrieval algorithms.
To improve the accuracy of early warning, appropriate flash flood early warning methods are one of the key factors. The current classic methods include soil moisture index (SMI), flash flood guidance (FFG), rain trigger index (RTI), etc. (Tang et al., 2017). FFG, among the most widely applied methods, was proposed by the US River Forecasting Center in the early 1970s. FFG defines precipitation that is required to generate bank‐full flood conditions related to flash floods within a given time and area (Shen et al., 2014). However, FFG has high data requirements due to the consideration of many influencing factors. SMI can accurately describe the soil moisture trend in the aeration zone, which is mainly obtained from the total water depth of a three‐layer tank model with fixed parameters; but its parameters related to the infiltration time are difficult to obtain. Taking into account the effective cumulative rainfall and rainfall intensity, the RTI method focuses on antecedent conditions and has been proved to be effective in predicting flash floods (Tang et al., 2017). However, this method is dependent on rainfall stations, lacks consideration of intermittent rainfall, and is greatly affected by rainfall field segmentation. Furthermore, since the RTI model uses the deductive coefficient of previous days to calculate the previous rainfall, this will induce a higher false alarm rate. An improved RTI method was proposed by Chen et al. (2017) to calculate the antecedent rainfall and effective accumulated rainfall, and it performed well in practical applications.
Overall, the objectives of this work are twofold: (1) evaluating the quality of existing satellite precipitation products in China with special attention paid to IMERG and (2) applying IMERG near‐real‐time data to flash flood warning in a typical region, Yunnan Province, China, based on the improved RTI method. The materials (figures and analyses) in this chapter are reorganized based on the latest research work from the authors, i.e., Tang et al. (2020) and Ma et al. (2020). The results can provide some guidance on the selection of satellite precipitation products according to their characteristics and application of the latest satellite products in flood hazards.
2.2. STUDY AREA AND DATASETS
2.2.1. Study Area
The evaluation of satellite precipitation products is conducted between 73°–135°E and 18–53°N in China (Figure 2.1a). The terrain is characterized by decreasing elevation from west to east (Figure 2.1b). China can be further divided into eight subregions according to topography differences and climatic conditions (Wang et al., 2018). The three typical subregions highlighted in Figure 2.1 are the Qinghai‐Tibet Plateau (TP) with extremely complex climate and topography, Xinjiang Province (XJ) with an arid climate, and the northeastern region (NE) in higher latitudes. Previous studies have shown that satellite precipitation data sets in these regions tended to have large uncertainties (Tang, Ma, et al., 2016). Besides, these regions have relatively uneven distribution of rain gauges. Gauges are much denser in the eastern and southern regions comparing to TP, XJ and NE, due to terrain and climate constraints (Figure 1a and b).
Figure 2.1 (a) Spatial distributions rain gauges in China and (b) DEM and the division of eight subregions. The Tibetan Plateau (TP), Xinjiang (XJ), and northeast China (NE) are highlighted.
Source: Based on Tang et al. (2020), Figure 01B, p 03 / Elsevier.
The area where satellite precipitation is applied to flood warning is Yunnan Province, China (Figure 2.2). This work is not implemented in the whole of China due to the limitation of hazard data availability. Yunnan Province is a low‐latitude plateau in southwest China, with an area of 390,000 square kilometers. The topography in Yunnan is relatively complex, with about 84% of the total area being mountainous. The average elevation of Yunnan is around 1980 m and most areas have elevation between 1000 and 3500 m. Another feature of Yunnan is the wide distribution of karst landforms characterized by a low water retention rate, which accounts for about 29% of Yunnan’s area. From the climate point of view, Yunnan is mainly affected by the East Asian monsoon and southwest monsoon, and about 90% of precipitation falls between May and October. Owing to the above‐mentioned factors, Yunnan experiences frequent flash flood hazards. For example, 72 people died from flash floods in 2014 in Yunnan, accounting for 22.2% of the total deaths from flash floods in China. Previous studies show that critical rainfall levels (i.e., the threshold of precipitation triggering floods) are 35–200 mm in northwestern Yunnan, 50–200 mm in southwestern Yunnan, and 100–300 mm in eastern Yunnan.
2.2.2. Datasets
Rain gauge data
A gridded station‐based precipitation product, the Chinese Rain Gauge‐based Daily Precipitation Analysis (CGDPA), is used to evaluate the quality of different precipitation products. CGDPA is developed by the China Meteorological Administration (CMA) (Shen & Xiong, 2016). It uses the optimal interpolation to obtain a 0.25° continuous precipitation map and daily resolution based on around 2400 national rain gauges (Figure 2.1a). CGDPA also implicitly considers the topographic effect on precipitation. Due to the high density of stations and well‐designed interpolation methods, CGDPA has high accuracy and has been widely used as the reference for satellite/reanalysis precipitation assessment (Tang, Ma, et al., 2016). Noted that the evaluation only involves grids that contain at least one rain gauge to ensure the reliability of results. The period is from 2000 to 2018, which covers