Evaluation of Subseasonal-to-Seasonal (S2S) precipitation forecast performance from NMME-2 over Indonesia

Abstract

Abstract Global Subseasonal-to-Seasonal (S2S) precipitation forecast was known to bridge the gap between weather and seasonal forecast, can be utilized as supporting information for decision-making related to hydrometeorological disaster mitigation activities. However, uncertainty in global models caused forecast performance can be different across regions and time periods. Therefore, it is important to evaluate forecast performance before utilizing the prediction result in any region. In this study, performance of probabilistic precipitation forecast from Multi-model Ensemble (MME) of three models in The North American Multi-Model Ensemble phase 2 (NMME-2) project was evaluated in Indonesia region based on two evaluation metrics: continuous ranked probability score (CRPS) and reliability diagram. The evaluation was conducted during the boreal summer (May–October) and boreal winter (November–April) periods, as well as during the active period of subseasonal climate variability phenomenon Madden-Julian Oscillation (MJO). Our result shows that S2S precipitation forecast from MME of CFSv2, CanCM4 and GEOS5 models are sufficiently accurate and reliable during the boreal summer period for Central Sumatra, Southern Sumatra, Southern Kalimantan, Java, Southern Sulawesi, and Southern Papua regions, with a range of CRPS values between 4-16 mm/7 days and a ‘perfect’ reliability category. There is no notable distinction in the performance of S2S precipitation forecast between the active and inactive events of the Madden-Julian Oscillation (MJO). The difference in CRPS values between these two periods is only around 0.8-1.2 mm/7 days, and there is no difference in reliability categories across Indonesia as a whole, nor significant spatial pattern differences.