2.2.2. Satellite Datasets

IMERG is NASA's level-3 multi-satellite GPM product. After the GPM mission, four different versions (V03, V04, V05, and V06) of the IMERG algorithm were developed. IMERG combines retrievals from PMW and microwave-calibrated infrared (IR) to produce a high-resolution global SBP product. Compared to earlier versions (V03 and V04), IMERG V05, and V06 inter-calibrate all individual PMW

satellite estimates from Goddard PROFiling (GPROF2017) scheme [57]. Until V05B, the Lagrangian time interpolation scheme was computed from IR data, while in V06, a new model-based morphing scheme in which motion vectors are retrieved from Modern-Era Retrospective Reanalysis 2 (MERRA-2) and Goddard Earth Observing System Model (GEOS) Forward Processing (FP) data using total column water vapor (TQV) [58]. For the first time, IMERG V06 used Precipitation Retrieval and Profiling Scheme (PRPS) Sounder for Atmospheric Profiling of Humidity in the Intertropics by Radiometry (SAPHIR) only to the combined Ku-swath DPR/GMI product (CORRA) [20]. These intercalibrated estimates were then merged into a single PMW SBP estimate and used to calibrate the IR-based precipitation. IMERG uses a Kalman filter-based method to combine the observed PMW, propagated Pulse Width Modulation (PWM), and IR estimates into a single, best estimate. IMERG provides three different products, Early, Late and Final runs, with a latency of ~4 h, ~14 h, and ~3.5 months, respectively. The only forward direction of the cloud motion vector propagation algorithm was adopted in the Early run. In addition to Early, backward morphing was added in the Late run. Meanwhile, in the Final run, climatological calibration coefficients were added on the basis of Late run [33]. The rain-gauge data were not assimilated in Early and Late run, while the Final product was adjusted using 1◦ GPCC gauge analysis which is interpolated to 0.1º and applied equally to every half hour in the month. The Final run is mostly recommended for research purposes. IMERG applies the wind-loss correction scheme [59] to the GPCC gauge analyses during the calibration process. The latest IMERG version 06 products were released to the public in March 2019. The Final version of the IMERG product includes two precipitation fields, precipitationUncal and precipitationCal. The Early and Late precipitation products are identical to the Final precipitationUncal since the gauge correction is only applied to the precipitationCal field in the Final product. IMERG V6B had an upgrade of full intercalibration to GPM combined instrument datasets (2BCMB), also in this version the input precipitation rates were increased from 50 to 200 mm/h to adjust fractional coverage [44]. For this study, data from precipitationUncal (hereafter IMERG-UC) and precipitationCal (hereafter IMERG-C) of 0.1◦ spatial resolution from the Final run IMERG version 06B between March 2014 and December 2016 were obtained from the PMM website (https://gpm.nasa.gov/data-access/downloads/gpm). A detailed description of the IMERG algorithm can be found in Huffman et al. [60].

GSMaP is a SBP product developed by Japan Science and Technology (JST) under the Core Research for Evolutional Science and Technology (CREST) program [61,62]. To provide high-precision precipitation products, GSMaP combines various available PMW and IR sensors [63]. In the development of the GSMaP precipitation products, the instantaneous precipitation rate is first archived from the PMW radiometers based on various satellite platforms, such as GMI, advanced microwave sounding unit-A (AMSU-A), advanced microwave scanning radiometer-2 (AMSR-2), TRMM Microwave Imager (TMI), microwave humidity sounder (MHS), and special sensor microwave imager/sounder (SSMIS). Further, the gaps between the PMW-based estimates are propagated using atmospheric moving vector (cloud motion vector) calculated from successive IR images. In addition, a new Kalman filter model is applied to refine precipitation rates after the propagation [61]. Finally, forward and backward propagated precipitation estimates are weighted and combined to produce the GSMaP-MVK product. GSMaP-MVK also uses IR to correct satellite estimates but adopts different PMW imagers and sounders. In addition to PMW and IR, GSMaP-MVK estimates are adjusted using daily 0.5º NOAA/CPC gauge-based analysis to develop GSMaP-Gauge precipitation product; this reduces precipitation biases and has a latency of 3 days [64]. In addition to the GPM/DPR database, orographic rain correction classification also introduced in the algorithm of GSMaP-V07. In the current study, the Version 07 satellite-only (GSMaP-MVK) and gauge adjusted (GSMaP-Gauge) hourly datasets with a 0.1◦ spatial resolution were used. An overview of the selected datasets in the current study is provided in Table 1.


**Table 1.** Details of the datasets used in this study.
