Prediction of Indian summer monsoon onset with high resolution model: a case study

Abstract

Abstract India is an agro-based country, receives nearly 80% of the annual rainfall during the summer monsoon season, and has a vital socioeconomic security link. The start of the Indian rainy season is observed over the country's southern tip (Kerala) and is referred to as the monsoon onset. The Indian Summer Monsoon (ISM) onset is one of the key aspects and unique for each year. This paper aims the prediction of ISM onset for different years by using the operational numerical weather forecasts at medium-range time scales by using different atmospheric variables from UKMO, and NCMRWF deterministic (NCUM) and ensemble model (NEPS) weather forecasts. For predicting the 2018 onset, we have adopted objective methods like the rainfall criteria, daily circulation index, potential temperature, moisture transport etc. The results for an early onset year (2018) show that the models could predict the onset date agreeing with the observed India Meteorological Department (IMD) onset date of 29 May 2018 from both UKMO 5-day and NCUM 10-day forecasts. This study also emphasizes on the utilization of the medium-range numerical weather forecasts, providing enough time to plan the farming activities. We also present the models’ performance/ skill for assessing the onset dates for the delayed onset (2019) and timely onset (2020) monsoons with NCUM 10-days lead time, and the results agree with IMD dates. From the UKMO, NCUM, NEPS models’ rainfall forecast show that the onset date on 29 May 2018 as IMD. Similarly, the analysis from circulation index, potential temperature, vertically integrated moisture transport analysis, outgoing long wave radiation, tropospheric temperature index clearly shows the onset date agreeing with IMD observations. Similarly, for the late onset (2019) and timely onset (2020) years, the daily circulation index agrees with IMD observed onset dates. Finally, these results from the deterministic and probabilistic forecasts are consistent with the observed onset dates, and these medium range forecasts are highly helpful to compute the monsoon onset date with higher lead times (10 days) within the error of one day.

Keywords

• Indian summer monsoon
• Onset
• Unified model
• Medium range forecast
• Ensemble forecast
• Monsoon indices