Benjamin Daniel Sipprell

University of Wyoming

Dept of Atmospheric Science

Fine-scale observations of a pre-convective convergence line

in the central Great Plains on 19 June 2002

Little is known about how low-level convergence alters the thermodynamic properties and depth of the convective boundary-layer, and how these changes lead to the initiation of deep convection. In the afternoon of 19 June 2002, a radar “fine-line” developed through Northwestern Kansas. This line was associated with a synoptic wind shift. As part of the the International H₂O Project (IHOP_2002), numerous observation platforms, including the University of Wyoming King Air aircraft (UWKA), sampled this line as it became more marked and triggered thunderstorms.  The UWKA conducted several perpendicular traverses and one parallel traverse of the fine-line during 19-21 UTC. Data collected illustrates a progression of a tighter gradient of mixing ratio and equivalent potential temperature (q_e) at the fine-line along with a wind shift and across-line convergence.  In addition, the UWKA found higher virtual potential temperature (q_v) values at the edge of the fine-line boundary compared to all other locations. 
Several WCR reflectivity and vertical velocity profiles of the fine-line boundary were sampled.  During the later-half of the time period of interest, WCR Doppler velocities within the CBL showed vertical velocity values greater than 5 m/s at the fine-line, driven by anomalously high values of q_v. The convergent flow deepened the CBL sufficiently to erode the remaining convective inhibition, as demonstrated by a dropsonde cross-section. In short, the confluence of air with high mixing ratio and q_e air led to deep convection along the fine-line.