Environmental shear and upper-level features associated with derecho-producing convective systems

A type of widespread convective windstorm (a derecho) is an occasional occurrence east of the Rocky Mountains, particularly during the late spring and summer months. This study focuses on the progressive derecho, which is produced by a large, long-lived mesoscale convective system, often taking a broad, bow-shaped configuration. Recent numerical modeling studies suggest that, for typical convectively generated cold pools, a significant amount of environmental low-level shear is needed to sustain the severity of the convection over long time periods. However, a detailed climatological examination of the relevant shear profiles in the vicinity of derecohs has yet to be found in the literature.

This study examines the environmental low-, mid- and upper-level shear attributes from progressive derecho events between the years 1993-1999. These shear values are then compared to the climatological shear values at several mid-western U.S. locations that typically are contained within the highest frequency of annual derecho events and are also categorized temporally and spatially in relation to the convective system. In addition, the source of the environmental shear and the related forcing mechanisms are examined in an attempt to understand the importance of low-level baroclinic zones and jet streams, as well as upper-level jet streaks and potential vorticity anomalies to the maintenance of the convection.