Thermal is one basic model of cumulus convection, describing a rising buoyant air current embedded in the ambient atmosphere. While isolated rising thermal is easily dissipating, successive thermals tend to be more favorable for sustained updraft growth. Each thermal possesses an independent life cycle, acting as a localized updraft that detaches from the boundary layer and rises continuously to the cloud top (Scorer, 1957; Woodward, 1959). When an old thermal weakens, a new thermal continues to ascend. These successive thermals organize into a “chain” structure, which is defined as “thermal chain” by Morrison et al. (2020). Past studies indicate such phenomenon is quite favorable for the shallow-to-deep convection transition, because older thermals can moisten the atmosphere and thus make subsequent thermal reach higher altitudes. Nevertheless, despite its potential importance for CI, the precise manner in which thermal chain exactly affects convection development remains unclear. In particular, the detailed picture of how the in-cloud thermals interact with one another, including how new thermals specifically replace old ones in thermal chains, and how this succession drives the rapid ascent of cloud top and ultimately leads to deep convection—remains poorly understood, especially in severe storms such as supercells and hailstorms.
       In this study, the role of thermal chains in driving convection initiation (CI) was explored in a large-eddy simulated severe storm. For the first time, thermals are classified into bottom-up and middle-up. Bottom-up thermals originate in the boundary layer, while middle‑up thermals originate at mid‑levels, out of the boundary layer. Based on thermal tracking and a five‑minute thermal development analysis, a more complete picture of thermal‑chain behavior during CI is established. In contrast to the traditional perspective that successive thermals are primarily bottom‑up, our results show that middle‑up thermals actively participate in thermal chain process and are essential for sustaining convective growth. For severe storm, wake entrainment on the downshear side of the updraft induces thermal splitting, which can lead to the formation of middle-up thermal. The middle-up thermal can also form a “chain” with bottom‑up thermal, replacing the weakening old thermal, thereby sustaining the continued intensification of vertical motion (Fig. 1). This process supports a rapid transition from shallow to deep convection. Middle-up thermals emerge in regions with high buoyancy, and therefore require stronger vertical acceleration to detach from their surroundings. Notably, the updraft splitting contributes substantially to middle-up thermal generation.
        Unlike past studies focusing on the statistical features of thermals, this study reveals more thermal–thermal interaction characteristics within severe storms. It may help to elucidate how the inhomogeneous dynamic and thermodynamic structures within convective clouds influence CI, and moreover, provides important implications for cumulus parameterizations. Most current parameterization schemes still rely on a single entraining plume that exchanges mass exclusively with the large‑scale mean state, omitting any interaction between clouds or between thermals. The absence of these in‑cloud heterogeneity prevents these schemes from capturing rapid shallow‑to‑deep convective transitions, posing considerable difficulties for global climate models (GCMs) in convective prediction.

deep convection,thermal chain,updraft split,wake entrainment