IMERG卫星降水产品在中国的干旱监测效用评估

GPM（Global Precipitation Measurement）时代，高时空分辨率的IMERG（Integrated Multi-satellitE Retrievals for GPM）是最主流的卫星遥感反演降水产品。该研究以地面网格CPAP（China Gauge-based Monthly Precipitation Analysis Product）数据为基准资料，选用标准化降水指数（Standardized Precipitation Index，SPI）、标准化降水蒸散指数（Standardized Precipitation Evapotranspiration Index，SPEI）和综合气象干旱指数（Composite Index of Meteorological Drought，CI），评价了最新版回顾性IMERG降水产品在干旱监测方面的应用能力及比较了其计算的干旱指数间的差异性。结果表明：1）IMERG能较好地捕捉中国月均降水量的空间格局；其与CPAP数据具有较高的一致性，相关系数高于0.9的区域占大陆面积的73.7%。2）基于IMERG的干旱指数具有良好的可靠性，它与CPAP计算结果的区域平均相关系数在大部分地区超过0.8。总体上，SPEI较SPI和CI的适用性更好。3）在西南地区，IMERG可准确再现干旱随时间的变化过程并捕获典型干旱事件的空间特征，同样SPEI表现优于SPI和CI。总之，回顾性IMERG降水产品在中国干旱监测中表现出很好的应用潜力，但不同干旱指数会影响它的精度。在全球变暖条件下，基于IMERG的SPEI指数应用于干旱监测与评估整体上优于仅考虑降水因子的SPI指数，以及考虑降水和气温因子的CI指数。

Evaluation of drought monitoring in China using IMERG satellite precipitation products

Abstract: In the Global Precipitation Measurement (GPM) era, the Integrated Multi-satellitE Retrievals for GPM (IMERG) with high spatiotemporal resolution (spatial resolution of 0.1° and the maximum temporal resolution of 0.5 h) has become one of the most mainstream satellite precipitation products.This study aims to primarily evaluate the ability of latest retrospective IMERG Final Run precipitation product for drought monitoring in China. The in situ China gauge-based monthly precipitation analysis product (CPAP) gridded data was selected as the reference. Two dimensionless and commonly used meteorological drought indices, including the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI), and a comprehensive drought index, the Composite Index of meteorological drought (CI), were calculated using IMERG and CPAP data at conventional scale, respectively. The performance of IMERG product was verified for drought monitoring, where three drought indices were compared over China in the period of June 2000 to December 2017. The results showed that: 1) The IMERG product can well capture the spatial pattern of mean monthly precipitation in China (spatial correlation coefficient of 0.96 and relative bias of 0.02), indicating high consistency with the CPAP data, especially in humid and semi humid areas. The regions with a correlation coefficient over 0.9 were occupied 73.7% of the continental area. However, there was relatively high spatial heterogeneity in the Xinjiang and Qinghai Tibet Plateau regions. 2) The SPI and SPEI were calculated using IMERG product, where the regional mean correlation coefficient was higher than 0.8 in most sub-regions, indicating high reliability and good consistency with the CPAP data at multiple temporal scales (1, 3, 6, and 12 months). Specifically, the IMERG-based SPEI and SPI presented the best performance on the 12-month scale. Moreover, the CI was calculated using IMERG product, showing good usability with the regional mean correlation coefficient over 0.75. Generally, the IMERG-based SPEI had better applicability than the SPI and the CI in most regions. 3) In southwestern China, the IMERG can accurately reflect the spatial and temporal variations of drought (drought intensity and drought area), particularly for the IMERG-based SPEI. Temporally, the correlation coefficient between the IMERG-based and CPAP-based SPEI was over 0.96 in this region. Spatially, the IMERG-based SPEI precisely captured the spatial characteristics of typical drought disasters on the specific month (March 2010) as well. In short, the retrospective IMERG precipitation product performed a great potential for drought monitoring over China, where the accuracy was influenced by the selection of different drought indices. If the precipitation factor was considered only, the IMERG-based SPI can be applied to identify and evaluate meteorological drought at large scale. However, under the background of global warming, a suggestion can be that the IMERG-based SPEI has higher potential than the SPI and the CI (considering precipitation and temperature factors) to be utilized in drought monitoring, particularly in semi-arid and arid areas. The IMERG-based CI was suitable for coastal areas, such as Northern China, the middle and lower reaches of the Yangtze River, and Southeastern China.