Impact of Aeolus horizontal line-of-sight wind observations on tropical cyclone forecasting in a global numerical weather prediction system

The satellite Aeolus, carrying a Doppler Wind Lidar instrument, was launched in August 2018. It is the first mission of the European Space Agency to provide wind profiles globally with high vertical resolutions. The significant positive impact of assimilating the horizontal line-of-sight (HLOS) wind data from Aeolus on numerical weather prediction has been reported in many works. In this study, in terms of tropical cyclone (TC) forecasting, the impact of the assimilated data on the Japan Meteorological Agency's global model was investigated in detail. Especially, the direct effect given by the increment in each analysis was studied and validated using the reinitialised analysis experiment. The 23 TCs that occurred in the western North Pacific during the summer of 2020 were focused on in the validation period. The largest improvement of track forecasting was found for cases of the NEAR group in which Aeolus passed within 5 degrees of latitude and longitude from a TC centre. The cases of the EAST or WEST groups in which Aeolus passed east or west of the TC centre also indicated some improvements, suggesting that the prediction accuracy of wind upstream of a TC was important for track forecasting. In the NEAR group cases, the increase in positive bias of the TC central pressure was also observed in the cases wherein Aeolus passed inside the maximum wind radius of the TCs. This is explained by the inability to represent the vorticity field in the upper levels in the analyses. One of the possible reasons for this is the insufficient horizontal resolution of observation. We also include some case studies to assess how Aeolus data improved track forecasting and affected intensity forecasting of TCs, specifically.