Effective Buoyancy in Squall Lines

Abstract Squall lines consist of a buoyancy discontinuity with positive buoyancy extending hundreds of kilometers behind their leading edge. Because of this structure, conceptual models for isolated deep convective updrafts, which have a comparatively limited horizontal extent, fail to explain squall-line thermodynamics. The present article addresses this knowledge gap by forming analytic solutions for effective buoyancy using simplified density distributions that mimic squall-line structure and by examining accompanying numerical simulations. It is shown with both analytical analysis and simulations that effective buoyancy along most squall-line updraft trajectories is less than half of the value predicted by parcel theory, implying that squall lines are fundamentally incapable of realizing all of their convective available potential energy as kinetic energy. This scaling factor is generally unaffected by the system’s horizontal extent, the width of the deep convective updrafts along the system’s leading edge, and the curvature of bowing segments. When the cold pool and low-level shear are close to balanced, there is an increased prevalence of “hot towers,” whose local buoyancy exceeds their immediate surroundings, allowing the effective buoyancy-to-buoyancy ratio along some trajectories to slightly exceed one-half. As the rearward slant of updrafts increases, the dilution of updraft buoyancy increases, the ratio of effective buoyancy to buoyancy decreases, and hot towers become less prevalent, leading to weaker updrafts. Dilution of updrafts, along with system slant and the associated reduction in effective buoyancy, is the primary control on updraft intensity. These results provide an underlying foundation for future theories that predict squall-line updraft speeds. Significance Statement Squall lines are organized clusters of thunderstorms that span hundreds of kilometers, producing prolific lightning, heavy rainfall, and damaging winds. Unlike isolated thunderstorms, their structure includes a wide area of buoyancy behind the leading edge, making traditional theories of storm updrafts insufficient to explain their behavior. This study addresses the aforementioned knowledge gap by using theory and computer simulations to improve our understanding of squall-line updrafts. The findings reveal that squall-line updrafts are inherently less efficient at converting environmental energy into updraft motion than isolated storms, with efficiency generally less than half of what conventional theory predicts. The study also shows how factors such as the balance between shear and buoyancy along the system outflow and updraft slant influence storm strength. These results lay the groundwork for improved understanding and forecasting of squall-line dynamics.