Wednesday, 1 August 2001
Inertial instability: Climatology and possible relationship to severe weather predictability
The spatial and temporal occurrence of inertial instability in the upper-troposphere is explored, as is the hypothesis that regions of inertial instability may be related to the occurrence of severe convective storms. The first part of the study presents a climatology of 250-hPa inertial instability in the Northern Hemisphere. Inertially unstable conditions occur most frequently in the low latitudes and where anticyclonic shear and curvature in the subtropical jet is strongest in the mid latitudes. Inertial instability is most common in the winter months when the subtropical jet is at its strongest and farthest south. A study of 74 cases where high-risk outlooks for severe weather were issued reveals no definite relationship between upper-tropospheric inertial stability and severe weather occurrence. The upper-level absolute vorticity advection in these events is also examined, and cyclonic vorticity advection (CVA) is present in most of the cases. However, the data do not show a definite relationship between the strength of the CVA and the occurrence of severe weather. Four cases are examined in further detail, emphasizing the wide range of conditions that are present during severe weather outbreaks. While inertial instability is shown to exist above some severe weather events, the exact function that it serves or the possibilities for its use as a forecasting tool are unclear.