Abstract
Prediction of heavy/extreme rains is still a challenge, even for the most advanced state-of-the-art high-resolution Numerical Weather Prediction (NWP) modelling systems. Hydrological models use the rainfall forecasts from the NWP models as input. This study evaluates the performance of the UK Met Office Unified Model (UM) in predicting the rainfall exceeding 80th and 90th percentiles. Such high rainfall amounts occur over the Western Ghats (WGs) and North East (NE) India mainly due to the forced ascent of air parcels. Apart from the significant upgrades in the UM's dynamical core, the model features an increased horizontal grid (40–10 km) and vertical resolution (50–70 levels). The prediction skill of heavy rainfall events improves with an increased horizontal resolution of the model. The probability of detection (POD), false alarm ratio (FAR), and critical success index (CSI) are the verification metrics used. As per these metrics, model rainfall forecasts have improved during 2007–2018 (increase in CSI from 0.29 to 0.38, POD from 0.45 to 0.55, and decrease in FAR from 0.55 to 0.45). Additionally, to verify extreme and rare events, the symmetric extremal dependence index (SEDI) is also used. SEDI also shows an increase from 0.47 to 0.62 and 0.16 to 0.41 over WGs and NE India during the study period, suggesting an improved skill of predicting heavy rains over the mountains. The improved forecast performance is consistent and relatively higher over WGs than over NE states.
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The authors are thankfull to anonymous reviewers for their critical reviews and comments. Thanks are also due to our collegues at NCMRWF for their interactive discussions and positive feedback.
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Kuldeep Sharma: Computation, graphics, analysis of results and initial draft; Raghavendra Ashrit: Conception of the idea, planning, execution; Sushant Kumar: Data compilation and processing; Sean Milton: Analysis of results and revision of the manuscript; Ekkattil N Rajagopal: Supervisory, guidance and revision of the manuscript and Ashis K Mitra: Supervisory, guidance and revision of the manuscript.
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Sharma, K., Ashrit, R., Kumar, S. et al. Unified model rainfall forecasts over India during 2007–2018: Evaluating extreme rains over hilly regions. J Earth Syst Sci 130, 82 (2021). https://doi.org/10.1007/s12040-021-01595-1
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DOI: https://doi.org/10.1007/s12040-021-01595-1