Ocean -Tropical Cyclone Interactions Modified by Assimilating CYGNSS Surface Winds in Coupled HAFS

For the analysis and forecasting of tropical cyclones (TCs), the main benefits of data from the NASA Cyclone Global Navigation Satellite System (CyGNSS) constellation of satellites are the increased revisit frequency compared with polar-orbiting satellites and the ability to provide ocean surface wind observations through convective precipitation. Consequently, CyGNSS delivers an improved capability to observe the structure and evolution of ocean surface winds in and around tropical cyclones. This study quantifies the impact of assimilating Level 2-derived 10 m (near-surface) winds from the latest version of CyGNSS, on 6-hourly analyses and 5-day forecasts of weak tropical cyclones, using a recent version of NOAA's Hurricane Analysis & Forecast System (HAFS). In the present study, we find first, that for many cases, assimilation of CyGNSS near-surface winds improves storm track, intensity, and structure statistics in the analysis and early in the forecast. Second, we find that assimilation of CyGNSS observations provides additional insights into the evolution of air-sea interaction in intensifying TCs: In effect, the ocean responds in the coupled model to modifications in the initial 10 m wind field, thereby impacting forecasts of intensity, storm structure, and sea surface height, as demonstrated by several case studies. We also analyze some forecasts where assimilating CyGNSS appears to degrade performance for either intensity or structure, examining likely mechanisms for this.