PNNL

The Science

Hailstones are a natural hazard that cause considerable economic losses and property damage each year in the United States. Researchers at the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL) have led a team studying yearly and geographic variations in severe and significantly severe hail over the Southern Great Plains states during a 13-year period. They found that aerosol loading, the El Niño-Southern Oscillation, and sea surface temperature anomalies over the northern Gulf of Mexico influence the observed yearly variation. They also found that the significant increasing trend of hail occurrences in southern Oklahoma and northeastern Texas is associated with the increasing trend of sea surface temperature warm anomaly years in the recent decade, suggesting an increasing risk of losses due to hail in the changing climate for this area.

The Impact

Previous studies of hail in this region have focused on century-long trends, not yearly variation. The researchers studied hail from two sources along with environmental conditions from 2004 to 2016 during the spring season, when hail is most frequent in the Southern Great Plains. Their work provides information about hail variation and its causes that can help mitigate hail damage, for example, by implementing extra preventive measures to reduce hail damage in the years when the three factors discovered in this study are among the conditions favorable for severe hail occurrences. The three contributing factors to hail activity in this region may also help forecast spring hail activity in the U.S. using statistical models.

Summary

The researchers studied severe and significant severe hail in the U.S. Southern Great Plains during March to May each year from 2004 to 2016. First, they gathered data reported in hail reports from the National Oceanic and Atmospheric Administration Storm Prediction Center and the newly developed radar-retrieved maximum expected size of hail (MESH). They noticed hail frequency in the Southern Great Plains varied yearly and geographically in the past decade. Then the researchers looked at changes in environmental conditions associated with the interannual variation of hail occurrence, and they found three factors that appeared to affect the hail variability: aerosol loading, the El Niño-Southern Oscillation (ENSO), and sea surface temperature anomalies over the northern Gulf of Mexico. The interannual variabilities corresponded strongly with sea surface temperature variation over the northern Gulf of Mexico without any outlier. 2016, however, was identified as an outlier for both the aerosol and ENSO correlations. Statistical analysis without this outlier showed that (1) aerosols that may be mainly from northern Mexico have the largest correlation with hail interannual variability among the three factors, and (2) meteorological covariation does not significantly contribute to the high correlation. These analyses warrant further investigation of aerosol impacts on hail occurrence. Geographically, the researchers also found that the increasing trend of hail occurrence in spring is statistically significant in southern Oklahoma and northeastern Texas and is associated with the increasing trend of sea surface temperature warm anomaly years.

PNNL Contacts

Jiwen Fan, Pacific Northwest National Laboratory, Jiwen.Fan@pnnl.gov

Funding

This study is supported by the U.S. Department of Energy Office of Science Early Career Award Program. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy (DOE) by Battelle Memorial Institute under contract DE-AC05-76RL01830. One of the authors, C. Homeyer, was supported by NSF under Grant AGS-1522910 and NASA under Award NNX15AV81G. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility operated under contract DE-AC02-05CH11231.