| Despite advances made in forecasting isolated supercell thunderstorms, mergers involving supercells remain a challenge for severe thunderstorm forecasting. In this study, mergers between supercells and ordinary cells are investigated. A series of numerical experiments are performed using an idealized, homogenous environment supportive of cyclonically-rotating, right-moving supercells. Convective is initiated by introducing warm bubbles at t = 0 and t = 3300 s, resulting in two storms of different maturity. The placement of the second bubble is used to control the location of the merger and the relative maturity of the second storm. From these experiments, simplified conceptual models for the long-term (>40 minute) outcome of merger are developed. The simplest mode of merger is one in which outflow from the new cell cuts off inflow to the original. If the new cell is not producing a cold pool 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. If the minimum separation of the updraft maxima is less than 10 km, an updraft collision occurs. If the minimum separation of the updraft maxima is greater than 20 km and the new cell merges into the forward flank of the original cell, a dual-cell system results. If the minimum separation of the updraft maxima is between 10 and 20 km, the combination of precipitation produced in the updrafts leads to a cold pool surge and subsequent formation of an updraft bridge, joining the updrafts. The outcome in this type of merger is controlled by the distribution of precipitation in the merging system. These conceptual models are then tested against real-world observations from a tornado outbreak in central Illinois on 19 April 1996 and high-resolution observations of multiple mergers on 11 June 2010 during the second Verification of the Origin of Rotation in Tornadoes Experiment (VORTEX2). |
