Assimilation of INSAT AMVs in NCMRWF NWP System: An Evaluation of the Indian Summer Monsoon Onset Features
The Indian summer monsoon (ISM), a steady phenomenon that happens every year and extends from June through September, marks its onset over Kerala approximately on 1 June. A large scale atmospheric and oceanic circulation changes are observed associated with the onset of the ISM. Accurate prediction of the ISM and the associated circulation features are still under constant evolution. High resolution Atmospheric Motion Vectors (AMVs) provide a near-complete picture of the monsoon circulations, and their assimilation in the NWP models also helps to a large extent to understand the monsoon circulations. There are many geostationary satellites stationed over the Indian monsoon region, providing continuous observations. INSAT-3D (820E) and INSAT-3DR (740E) are two of such satellites operated by Indian Space Research Organization (ISRO). The combined INSAT system (INSAT-3D and 3DR) generates AMVs every 15 minutes. This high temporal resolution AMVs is a valuable data set to explore the ISM, particularly the onset features. National Centre for Medium Range Weather Forecasting (NCMRWF) receives the AMVs from the INSAT satellites through Global Telecommunication System (GTS) via India Meteorological Department (IMD). NCMRWF also validates these observations before their assimilation into the NWP system. The objective of this study is double fold, (i) validate the relatively new INSAT-3DR AMVs against the unbiased in-situ observations and AMVs from other satellites over the same geographical location, and (ii) assimilation of INSAT-3D and 3DR AMVs in the NCMRWF NWP system to evaluate the ISM features compared to a baseline experiment which denies the INSAT AMVs. The validation and assimilation experiments are carried out during May-June 2020. Validation shows that the INSAT-3DR AMVs at different atmospheric levels are comparable with the AMVs from other satellites over the same area. Preliminary analysis shows that the assimilation of INSAT AMVs improves the ISM onset features compared to the baseline experiment. The formation of tropical cyclones over the North Indian Ocean (NIO) is rare during the ISM period, mainly due to the strong vertical shear associated with the monsoonal flow. ISM-2020 witnessed a cyclone, “Nisarga,” formed over the Arabian Sea just after the monsoon onset. This study also analyses the impact of the INSAT AMV assimilation in predicting the characteristics of the cyclone Nisarga and associated progression of onset scenario.
