Impact of Aeolus horizontal line of sight wind observations in a global NWP system

Abstract

This study evaluates the impact of Aeolus horizontal line-of-sight (HLOS) wind observations in the National Centre for Medium Range Weather Forecasting (NCMRWF) Unified Model (NCUM) assimilation and forecast system from 20 May to 22 June 2020. The quality of both Rayleigh clear and Mie cloudy Aeolus HLOS winds are validated during the study period and found to be useful for assimilation. Observing System Experiment (OSE) and adjoint-based Forecast Sensitivity to Observation Impact (FSOI) methods are used to assess the impact of the Aeolus HLOS wind observations in the global NCUM system. The Indian Summer Monsoon onset and the life cycle of a severe cyclonic storm Nisarga formed over the eastern Arabian Sea are investigated. The FSOI study clearly shows that HLOS wind observations significantly impact the model forecast, especially over the Tropics and Southern Hemisphere extra-tropics. The impact of HLOS winds in the upper troposphere and above is more marked. Forecasts with the assimilation of the HLOS winds enhanced the circulation during the onset of the Indian summer monsoon and weakened the Monsoon Low Level Jet (MLLJ) when the Nisarga cyclone formed over the Arabian Sea. The high-resolution wind observations from Aeolus reduced the position error of the tropical cyclone Nisarga in the NCUM analysis.