This study introduces two refined rainfall anomaly indices-the Modified Rainfall Anomaly Index (MRAI) and the Standardized Rainfall Anomaly Index (SRAI)-to address limitations in the traditional Rainfall Anomaly Index (RAI). The existing RAI struggles to effectively capture extreme wet and dry rainfall conditions and relies on a simplistic formulation. To evaluate these indices on a continental scale, data from the Integrated Multi-Satellite Retrievals for GPM (IMERG) was used for the Conterminous United States (CONUS), enabling scalability to ungaged locations and beyond. Additionally, daily annual maximum series (AMS) from 2,360 NOAA stations provided reference data to enhance the robustness of the analysis. IMERG data informed the MRAI and SRAI, while station data validated the RAI. Findings showed MRAI's kurtosis ranged from 0 to +0.9, consistently higher than SRAI and RAI, while SRAI exhibited a kurtosis range from -0.9 to -0.1. Median index ranges were -2 to 0 for SRAI, 0 to +2 for MRAI, and -1 to +1 for RAI. Comparing MRAI and RAI, performance metrics revealed average PRB of -23.5, RMSE of 0.93, MBR of 0.74, NSE of 0.82, and KGE of 0.53. For SRAI versus RAI, metrics showed PRB of -14.5, RMSE of 1.7, MBR of 0.8, NSE of 0.41, and KGE of 0.46. Spatially, MRAI effectively captured positive anomalies leaning toward wet extremes, while SRAI identified negative anomalies indicating dry extremes. These indices demonstrate significant potential for climate change studies, especially when applied together.
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