Tropical Cyclogenesis in the Western North Pacific Improved by GNSS RO Data Assimilation Using Hybrid 3DEnVar with a Nonlocal Excess Phase Operator

Accurate tropical cyclogenesis predictions are crucial for typhoon forecasting and disaster mitigation. Satellite observations are valuable for improving tropical cyclogenesis over the ocean, where there are few traditional radiosonde observations. Global Navigation Satellite System (GNSS) Radio Occultation (RO) data, mostly from FORMOSAT-7/COSMIC-2, provides extensive coverage of tropical regions with valuable atmospheric boundary-layer information due to its deep penetration into the lower tropical troposphere. This study assessed the impact of GNSS RO data assimilation (DA) on the predictions of tropical cyclogenesis using ten typhoon cases in the Western North Pacific from 2020 to 2022. Conventional data and GNSS RO data were assimilated for each case using the WRF hybrid 3DEnVar system. The results show that including RO data with the nonlocal excess phase operator considerably improves the accuracy of cyclogenesis predictions both in terms of location and timing. Detailed case studies for Typhoons Chanthu (2021) and Hagupit (2020) reveal enhanced mid-tropospheric moisture by RO data assimilation, highlighting the important role of water vapor in tropical cyclogenesis. Although satellite radiances are also assimilated for both typhoons, the impacts of RO data are still evident in providing improved initial conditions that are more favorable for capturing genesis. For ensemble forecasts, more ensemble members successfully detect Chanthu’s genesis and give a higher probability of detection for Typhoon Hagupit when RO data are included. The positive ensemble impacts highlight the fact that the RO data with the nonlocal operator may give steady improvement for operational forecasting of tropical cyclogenesis.