Recent Trends in Four Common Stability Indices Derived from U.S. Radiosonde Observations

Abstract

Daily sounding-derived atmospheric stability indices are typically employed for short-term severe weather forecasts. Over longer time periods, these indicators may convey changes in the potential for severe storm development over the United States. Daily (0000 UTC) observations from 48 radiosonde stations in the contiguous United States are extracted to assemble a ?50 yr record of four common stability indices: the Lifted Index, the K-Index, convective available potential energy (CAPE), and the Air Force Severe Weather Threat Index. Because of radiosonde data inhomogeneities, the 1973?97 period is the focus of the analysis. Trends in the mean and extreme values of daily index observations are calculated for spring and summer seasons. In addition, climatological mean indices, as well as the mean frequency of index extremes, are determined for all U.S. regions. At stations free of obvious data discontinuities, the early part of the record (1948?65) is compared with more recent periods. In spring, few significant changes in either index means or extremes are reported. However, in summer, widespread trends toward enhanced instability appear, particularly in the Lifted Index, the K-Index, and CAPE. Stations in the Plains and southern states show the most consistent increases in severe weather potential, while significant trends in index extremes are uncommon in the western region, possibly due to the rarity of severe weather there.