Role of Space-Borne Sea Surface Winds on the Simulation of Tropical Cyclones Over the Indian Seas

The impact of space-based sea surface winds on the simulation of tropical cyclones that formed over two distinct basins of the northern Indian Ocean during October–November 2019 is investigated in this study. Observing system experiments (OSEs) using the National Centre for Medium Range Weather Forecasting (NCMRWF) Unified Model (NCUM) assimilation and forecast system were designed to simulate the characteristics of the cyclones “Kyarr”, which formed over the Arabian Sea (24 October to 3 November 2019), and “Bulbul” over the Bay of Bengal (4–11 November 2019) by including the sea surface winds derived from the scatterometer on board the MetOp-A, MetOp-B, ScatSat-1 and microwave radiometer aboard WindSat satellites. Approximately 3% of the sea surface winds received were assimilated in the NCUM system during both cyclone cases, and the two MetOp scatterometers contributed the most significant fraction. Assimilation of sea surface winds improved the cyclone characteristics in the analysis compared to a baseline experiment, which denied all the sea surface winds. Sea surface wind assimilation improved the surface wind analysis by 2–7% and reduced the root mean square differences by ~ 10% when compared against ERA5 reanalysis. Better simulation of cyclone track in the higher lead time suggests that the sea surface wind information is critical in the analysis during the cyclone's initial stage. The estimated track and intensity errors were larger for the Kyarr than the Bulbul cyclone. This could be due to the nature of the cyclones, Kyarr being a super cyclone which dissipated over the ocean, whereas Bulbul was a severe cyclone that dissipated after landfall. An improvement in the post-landfall track of Bulbul due to the assimilation of sea surface winds is also noted.