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 (CBL) and how these changes lead to the initiation of deep convection. In the afternoon of 19 June 2002 a radar ‘fine-line' developed through NW Kansas. This line was associated with a synoptic wind shift and had a distinct and sustained humidity gradient; hence it is referred to as a dryline. As part of the International H2O Project (IHOP) numerous observation platforms, including the University of Wyoming King Air aircraft (UWKA), sampled this line as it became more marked and triggered deep convection. The UWKA conducted several perpendicular traverses and one parallel traverse of the dryline prior to convective initiation (CI). Data collected illustrates a progression of a SE-ward progressing dryline becoming stationary and ultimately regressing. The buoyancy, or virtual potential temperature anomaly, changed sign during this evolution suggesting that the propagation, at this scale, was controlled by density current dynamics.

Vertical transects of the airborne Wyoming Cloud Radar (WCR) reflectivity confirm this reversal through the changing slope of the echo plume. During the stationary phase, the convergent flow gives rise to a deep, erect echo plume, resulting in near-surface equivalent potential temperature values reaching well above the CBL depth on either side of the boundary. During this phase the WCR recorded updraft values greater than 5 m/s at the dryline. These updrafts are convective since they are associated with anomalously high values of virtual potential temperature. While such convective updrafts may have triggered the penetration of some thermals through the level of free convection, on a very small scale, what really allowed CI is the deepening of the CBL and thus the elimination of convective inhibition along the convergence line, as demonstrated by numerous soundings collected in and around the fine-line. In short, sustained confluence of air with high mixing ratio and e led to the deepening of the CBL, allowing some entraining thermals, previously confined to the CBL, to penetrate and accelerate in the free troposphere.