Evaluation and integration of the top-down and bottom-up satellite precipitation products over mainland China

The satellite precipitation products (SPPs) obtained from the satellite soil moisture data through a bottom-up approach (i.e., the SM2RAIN algorithm) have been developed and released in recent years. However, the assessments and the integration of them with the conventional top-down SPPs remain absent over China. This study evaluated and integrated, for the first time, the bottom-up SM2RAIN-ASCAT (SM2RASC) product and the top-down Integrated Multi-satellite Retrievals for Global Precipitation Measurement (IMERG) Early Run product over mainland China. The evaluations were conducted at multiple temporal scales using both the continuous and categorical metrics; and the integration was implemented through a nudging scheme. Results indicate that IMERG outperforms SM2RASC in terms of CC at the daily scale. Meanwhile, it achieves a better performance than SM2RASC in detecting precipitation events. However, interestingly, IMERG performs worse than SM2RASC in terms of CC and NRMSE at the monthly scale. Both IMERG and SM2RASC show noticeable seasonal variability regarding the performance, with better qualities in the wet seasons than in the dry seasons. Spatially, IMERG performs better in the humid subregions than in the semi-arid subregions in terms of CC, while SM2RASC performs better in the semi-arid region than in the other subregions. Both of them perform relatively worse in the complex mountainous subregion (i.e. the Qinghai-Tibetan Plateau) and the arid subregion. The performances of the SPPs are closely correlated to the elevations and precipitation magnitudes. The correlations and their significances, however, vary between IMERG and SM2RASC, and between different time scales and different evaluation metrics. The integrated product could increase the median CC by up to 25.86% (or 7.23%), and reduce the median NRMSE by up to 14.72% (or 24.62%), at the daily (or monthly) scale in the validation period (i.e., 2012-2017), compared to the parent products. This study demonstrated the overall good performance of SM2RASC over mainland China, particularly in the semi-arid region, and meanwhile, highlighted the strong benefits of integrating the bottom-up and top-down SPPs.