A 25-Year Climatology of Air Stagnation Events in North America

Air stagnation events (ASEs) can cause problems for the wildland fire community and those affected by wildland fires, especially regarding smoke transport and dispersion. However, ASEs historically have not been studied as rigorously as strong-wind events, and there isn't a standard definition of ASE criteria used across the country. Some previous definitions of ASEs include parameters for mid-tropospheric wind speed thresholds and/or synoptic patterns/flow regimes, and there is no consistency between ASE definitions for a (minimum) duration criterion.

For this study, we defined an ASE to be surface-based, with observed wind speeds no greater than 6 kt for at least 24 h to eliminate purely diurnal calm periods. Then, we collected surface observations from the Integrated Surface Dataset from January 1997 to December 2021 for stations across North America. After processing all available stations for a sufficiently complete data record, we found ~850 stations for which we could analyze ASEs. Preliminary results show that ASEs tend to start in the late afternoon or early evening and end just after sunrise, peak in the late summer and early fall, and are more likely to persist in mountain valleys. Further analysis of individual stations and local/regional spatial and temporal patterns will be presented.