Long-Term Morphological Changes in Simulated Supercells Following Mergers with Nascent Supercells in Directionally Varying Shear

Mergers involving supercells remain a challenge for severe thunderstorm forecasting. In this study, mergers between supercells and ordinary cells (e.g., cells forming in a similar environment but too young to be fully developed supercells) are investigated. A series of numerical experiments are performed using an idealized, homogenous environment supportive of cyclonically rotating, right-moving supercells. Warm bubbles are introduced at different times, resulting in two storms of different maturity; their placement is used to control the location of the merger and the relative maturity of the second storm. Simplified conceptual models for the long-term outcomes of mergers are developed. In the simplest mode of merger, outflow from the new cell cuts off inflow to the original. If the new cell’s cold pool is not sufficiently strong to cut off the inflow to the original cell, the minimum separation of the updraft maxima during the merger becomes a key controlling factor in the outcome. If it is less than 10 km, an updraft collision occurs, resulting in a classic supercell. If it is greater than 20 km and the new cell merges into the original cell’s forward flank, a dual-cell system results. If it is between 10 and 20 km, the enhanced precipitation produced during the merger leads to a cold pool surge and an updraft bridge, joining the original updrafts and developing into either a small bow echo (with forward-flank mergers) or a supercell on the classic high-precipitation spectrum (with rear-flank mergers), depending on the distribution of precipitation in the merging system.