# Evaluation of IMERG Precipitation Products in the Southeast Costal Urban Region of China
## 1 Introduction  
Precipitation is one of the key components of the hydrological cycle, connecting the ocean, land and atmosphere [1]. In recent decades, the intensification of extreme precipitation events has aggravated the frequency and intensity of floods [2], which have caused extensive economic and social damage worldwide, especially in populous and economically active urban areas [3]. Timely and reliable precipitation information is urgently needed for urban flood monitoring and warning to reduce casualties and economic losses. However, due to the strong heterogeneity of precipitation processes [4], the accurate acquisition of precipitation data with fine spatial and temporal resolution still remains a great challenge for the world [5, 6]. The rain gauge is considered the traditional and direct method for measuring precipitation at any given point. This approach is limited by the uneven distribution of stations and high maintenance costs [7, 8, 9, 10]. To capture the spatial variability of precipitation, several techniques have been developed, such as ground-based weather radar [11, 12] and numerical weather prediction and reanalysis [13]. Satellite remote sensing is another effective means to obtain high-resolution precipitation data with wide coverage as well as spatial and temporal continuity [14]. Based on the massive data accumulated by the global satellitenetwork, continuous precipitation can be derived by visible/infrared inversion [15; 16; 17; 18], active or passive microwave inversion [19; 20; 21], and multi-sensor joint inversion [22; 23]. There are many satellite precipitation products (SPPs) that have been developed and used, such as the TRMM Multi-satellite Precipitation Analysis (TMPA) [23], Global Satellite Mapping of Precipitation [24], Climate Prediction Center Morphing Technique [22] and Integrated Multi-satellite Retrievals for GPM (IMERG) [25]. Among the various SPPs, TMPA has contributed nearly two decades of high-quality tropical and subtropical precipitation data [26], but is no longer available after 2019. As the successor to TMPA, IMERG offers precipitation estimates starting from 2014, by inverting atmospheric signals emitted or scattered by hydrometeors [27], with the improved detection of weak and solid precipitation [28]. Benefiting from advanced equipment, IMERG provides wider coverage, finer spatial and temporal resolution, and more reliable precipitation records than its predecessor TMPA. In recent years, as one of the state-of-the-art SPPs, IMERG products have been widely used in hydrological simulation [29], flood prediction [30], and extreme event analysis [31]. Since the release of the IMERG products, they have been evaluated in different regions and at various spatial and temporal scales to explore their accuracy and reliability. The superiority of IMERG over TMPA and its ability to capture spatial patterns of precipitation have been noted by studies conducted in mainland China [32], the United States [33; 34], Iran [8], India [35], and Brazil [9]. Evaluations at different timescales demonstrate that the performance of IMERG is more effective at larger timescales [36; 37; 38; 39]. For different climatic regimes, the ability of IMERG to estimate precipitation is stronger in humid areas than in semi-arid and arid areas [40; 41; 42]. Several studies have reported that shortcomings of IMERG include the overestimation of weak precipitation [43; 44], underestimation of high intensity precipitation [45; 46], and the inability to accurately represent precipitation over complex terrain [47; 48]. In addition, evaluations on different surface types show that estimating coastal precipitation is particularly challenging for IMERG, which is attributed to the joint contribution of contrasting radiation from ocean and land over the coastline [49; 50; 51; 52]. Some evaluations have demonstrated that IMERG typically has better performance in the summer than in the winter [53; 54; 55].