This novel study explains a physical mechanism for the rainfall initiation over the southeast peninsular India (SEPI, referred to be a ‘rain shadow’ region) during the southwest monsoon season. Further, the contrasting rainfall patterns between the rain shadow region and central India (CI,) are also elucidated through the response of the maritime continent (MRC) convection. The MRC is found to be a prominent source for the initiation of wet spells over the rain shadow region (during which CI is in dry phase), with the rainfall anomalies over MRC leads SEPI by ~5-7 days. Evolution of convective anomalies resembles a classical Gill-type response, with a pair of Rossby waves on the poleward side of the convection center and Kelvin waves on the eastern side. Thus, the combined effect of large-scale circulation and moisture anomalies over MRC contributes to the preconditioning and subsequent positive rainfall anomalies over the rain shadow region.

Aim of this study is to assess the fidelity of National Center for Medium Range Weather Prediction Unified model’s (NCUM) global (12km) and regional (4km) versions in representing the monsoon sub-seasonal variability over Indian region by applying the process-oriented diagnostics. Moisture budget analysis is performed on the model’s forecast fields for a typical extended monsoon break event occurred during July 2019. The exercise is repeated by using the fifth generation of ECMWF atmospheric reanalysis (ERA5) and the relative roles of the budget terms are quantified. We also tested the budget diagnostics onto the newly generated Indian Monsoon Data Assimilation and Analysis (IMDAA) product. The results obtained here are consistent with our understanding that moisture advection acts a leading term in inducing the break conditions over central Indian and adjoining oceanic regions. Specifically, dry air advection from the northwest regions strongly dries the atmospheric column nearly 7-10 days before and the peak dry phase over Indian subcontinent. Movement of this dryness, with time, towards central India and Arabian Sea is consistent with anomalous total precipitable water content seen from satellite observations. Preliminary results are encouraging and one of the direct implications of this work is that the lead times obtained in the moisture budget assessment can be used for understanding and improvement of the model forecasts.