Cold pool and Gravity Waves Drive Secondary Convective Initiation Ahead of Squall Lines

Secondary convective initiation (SCI) ahead of mesoscale convective systems, such as squall lines, is a globally observed phenomenon. This study employs idealized numerical simulations to investigate the spatiotemporal characteristics of SCI and its connection to squall line evolution. In a typical mid-latitude environment, SCI frequently develops ahead of the squall line, subsequently modifying squall line’s morphology and intensity through processes like merging or shifting the leading edge, depending on their relative distance.

Over an 8-hour simulation, SCI becomes increasingly frequent and exhibits periodic explosive growth (outbreaks), primarily driven by distant SCI events (≥ 5 km from the leading edge), while the number of close SCI events (< 5 km) remains stable. Distant SCIs also extend progressively farther over time, with some forming over 100 km ahead. These SCIs are more likely to occur ahead of regions with locally stronger cold pools and higher radar reflectivity within the squall line. In contrast to close SCI events governed by spatial cold pool variability, SCI outbreaks consistently lag behind recurrent surges in cold pool intensity and are closely linked to the passage of n=2 gravity waves. These waves are characterized by upward motion in the lower troposphere and downward motion aloft. Their formation is primarily driven by strong evaporative cooling and the melting of hydrometeors within the squall line, which concurrently enhances the cold pool. As each n=2 wave propagates forward, its ascent induces adiabatic cooling and enhances low-level moisture, significantly humidifying and destabilizing the lower troposphere, thereby promoting each SCI outbreak. Moreover, the repeated wave generation and long-distance wave propagation (>300 km) amplify these effects, increasing SCI frequency and expanding its reach.