An Expanded Conceptual Model for Dryline Convective Initiation

Despite recent advances in data assimilation and numerical weather prediction, the forecasting of convective initiation (CI) continues to be a challenging problem. However, very dense, high-resolution observational networks should provide for a greater situational awareness of these scenarios, and possibly much improved understanding of the physical processes involved. To test this idea, data from three dryline CI events were collected during 2010 as part of the Center for Collaborative Adaptive Sensing of the Atmosphere (CASA) Integrated Project 1 (IP1) testbed. Data from weather radar (8 WSR-88Ds and 4 CASA X-band radars) and surface networks (including Oklahoma and West Texas mesonets) were assimilated using a two-step process including three-dimensional variational analysis (3DVar) and the Advanced Regional Prediction System (ARPS) Data Analysis System (ADAS). Three-dimensional analyses were generated at 400 m resolution across a domain including much of Oklahoma and west Texas.

Three dryline CI cases were reviewed – 7 April 2010, 10 May 2010, and 19 May 2010. In all three cases, a thin surface boundary was noted ~50 to 100 km ahead of the primary dryline, indicated by a sharp gradient in equivalent potential temperature (θ_e). Dubbed “the mixing line”, this boundary was marked by a subtle westerly wind shift and slightly drier air behind it. The area between the mixing line and dryline, referred to as the “dryline zone”, was a region of enhanced vertical motion and frontogenesis. For all three cases, CI occurred within the dryline zone, and usually very near the mixing line. An expanded conceptual model of the dryline is proposed in light of the association between the primary dryline, the mixing line and dryline zone. Indeed, this revised conceptual model appears to explain well dryline propagation and CI development.