PNNL

The Science 

Tropical cyclones (TCs) cause damage not only through peak winds but also through how far their winds extend from the storm center. This outer size influences rainfall coverage, wind duration, and storm surge, yet long-term regional trends in storm size have remained uncertain. The study examined changes in TC outer size in the western North Atlantic from 1979 to 2022 using observational best track records, atmospheric reanalysis, and numerical simulations using the Energy Exascale Earth System Model (E3SM). In this study, outer size was defined as the average radius of gale-force winds. The analysis found a statistically significant increase in storm outer size over the western subtropical North Atlantic, with an area-average increase of about 7.5 percent. The largest increases occurred offshore of the U.S. East Coast. The results also showed that a larger outer size was closely associated with stronger storm intensity and faster intensification rates across multiple independent datasets.

The Impact 

The findings demonstrated that TC wind fields have expanded regionally over recent decades, adding a new dimension to documented changes in storm behavior. Prior studies focused mainly on storm intensity, rainfall, or frequency, but regional outer-size trends were not well established. This work showed that TC outer size increased in concert with storm intensification and with more favorable ocean–atmosphere thermodynamic conditions. 

E3SM model simulations reproduced the observed linkage between storm intensification and size expansion, providing mechanistic support for the results. The study also showed that warmer sea surface temperatures relative to the tropical mean contributed to environments that favored storm expansion. These results underline the value of improved storm size variability representation in hazard and risk modeling and highlight that assuming fixed or basin-wide storm size distributions can miss impact-relevant regional changes.

Summary 

TC damage depends strongly on storm size as well as peak intensity, yet long-term regional changes in storm outer wind structure have been difficult to quantify. This study evaluated trends in TC outer size across the North Atlantic over the period 1979–2022. Outer size was measured using a wind radius of 17 meters per second, a standard operational metric representing the extent of gale-force winds. Researchers analyzed multiple independent datasets, including the National Hurricane Center’s best track data, ERA5 atmospheric reanalysis, NOAA’s extended best track wind radii records, and high-resolution E3SM model simulations. Across these datasets, results consistently indicated increasing TC outer size over the western subtropical North Atlantic, especially near the U.S. East Coast. Area-averaged increases reached about 7.5 percent between early and later multi-decadal periods and were statistically significant.

Statistical modeling showed that storm intensity and initial storm size explained much of the variability in outer wind radius, with intensity emerging as a dominant factor. Observations and model output both demonstrated that storms undergoing intensification also tended to expand in size on average. Environmental analysis showed increasing relative sea surface temperature and increasing potential intensity in the region, indicating more favorable thermodynamic conditions for storm strengthening and expansion. A high-resolution E3SM simulation reproduced the observed relationships between intensification, convection, and outer size growth. Multi-model simulations further indicated that both natural variability and aerosol forcing contributed to the observed environmental trends. Together, the results provided regionally resolved evidence that storm wind footprints have expanded alongside intensification under non-stationary environmental conditions.

Contact 

Renu Joseph, Earth and Environmental System Modeling, renu.joseph@science.doe.gov

L. Ruby Leung, Pacific Northwest National Laboratory, ruby.leung@pnnl.gov

Karthik Balaguru, Pacific Northwest National Laboratory, Karthik.balaguru@pnnl.gov

Funding 

This research was supported by the U.S. Department of Energy, Office of Science, Biological and Environmental Research (BER) program as part of the Regional and Global Model Analysis (RGMA) program area through the Water cycle: Modeling of Circulation, Convection, and Earth system Mechanisms (WACCEM) scientific focus area and as part of the RGMA and MultiSector Dynamics program areas through the collaborative, multiprogram Integrated Coastal Modeling project. This research was also supported as part of the E3SM project, funded by the U.S. Department of Energy, Office of Science, BER. The research used computational resources from the National Energy Research Scientific Computing Center, a U.S. Department of Energy User Facility supported by the Office of Science under contract DE‐AC02‐05CH11231. Pacific Northwest National Laboratory is operated for U.S. Department of Energy by Battelle Memorial Institute under contract DEAC05‐76RL01830. Contributions from Paul A. Ullrich were performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE‐AC52‐07NA27344. For CMIP5 and CMIP6, the U.S. Department of Energy’s PCMDI provides coordinating support and led the development of software infrastructure in partnership with the Global Organization for Earth System Science Portals.