Satellite-Observed Water Vapor Boundaries in Association with Clear-Air Turbulence

Water vapor boundaries (WVBs) are recognized indicators of clear-air turbulence (CAT), yet their relationship remains insufficiently explored due to the lack of automated detection methods. In this study, an improved multi-scale “2-out-of-8” neighborhood criterion recognition algorithm is adopted to automatically identify mid- to upper-tropospheric WVBs based on water vapor observations from the Advanced Geostationary Radiation Imager onboard the Fengyun-4A satellite. By integrating high-altitude CAT reports, identified WVBs, and the ERA5 reanalysis data, the spatiotemporal relationship between CAT and WVBs, along with their associated environmental characteristics, is systematically analyzed over China and its surroundings in 2019. A total of 341 WVBs and 752 CAT events were identified, with 45% of the CAT events exhibiting a pronounced association with the WVBs. It is found that the risk of encountering CAT increased markedly for flights operating above 5000 m and located along the leading edge or within 100 km downstream of dry surge or jet-parallel jet WVBs, particularly when the flight path intersected the WVB at angles greater than 30º. Environmental field analysis reveals that WVB regions exhibited enhanced potential vorticity and potential vorticity gradients, higher wind speed, and pronounced wind shear, all of which favor the enhancement of Kelvin-Helmholtz instability and the triggering of CAT events. These findings highlight the indicative role of WVBs in CAT occurrence and provide valuable references for CAT detection and forecasting.