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

• Published in: Quarterly journal of the Royal Meteorological Society 2024-04, Vol.150 (760), p.1447-1472
• Main Authors: Okabe, Izumi, Okamoto, Kozo
• Format: Article
• Language: English
• Subjects: Aeolus; Cyclone forecasting; Cyclones; data assimilation; Doppler sonar; Doppler Wind Lidar; Forecasting; Global weather; Hurricanes; Lidar; Maximum winds; Predictions; single‐perspective instrument; tropical cyclone; Tropical cyclone forecasting; Tropical cyclones; Vorticity; Weather; Weather forecasting; Wind; Wind data; Wind observation; Wind profiles
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.4653

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 in each analysis resulting from the increment was studied and validated using a 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. Aeolus is the first satellite mission to provide global wind profiles with high vertical resolutions. Our study revealed that the assimilation of Aeolus wind data has a significantly positive impact on track forecasting, not only when the satellite passed near the centre of tropical cyclones (TCs) (considered as NEAR group cases) but also when the data improved the accuracy of predicting the environmental wind upstream of TCs (considered as EAST or WEST group cases). However, we also discovered that assimilating Aeolus wind data potentially degrades (weakens) the cyclone's intensity prediction when the satellite passed near the centre of TCs.