Factors that Influence the Growth of Supercells into MCSs after Sunset

This research investigates the mesoscale atmospheric factors that cause tornado-producing afternoon supercells to transition to Mesoscale Convective Systems (MCSs) with sunset. At night, the Planetary Boundary Layer (PBL) cools and stabilizes, surface convection changes to elevated convection and the low level jet has a maximum at night (and as a result subsequently low-level vertical wind shear and ascent). Numerical weather prediction models currently have difficulty forecasting the timing and occurrence of MCS transition, as this transition is not present with every incidence of discrete supercells. MCSs that grow from supercells also present a substantial flash flooding threat, as localized precipitation maximums are often within close proximity of tornadoes.

            MCS cases are days with supercells that made the overnight transition to an MCS based on radar reflectivity imagery, whereas null cases had supercells, but the absence of a defined MCS. For 21 MCS cases and 17 null cases, hourly rapid refresh (RAP) model data was averaged out over subjective coordinates centered at the transition, with a grid created for each, and averaged over all x and y. Composites of atmospheric fields from 18Z to 9Z were constructed separately for MCS and null cases. Comparing the two, it is evident that the low level jet is stronger in MCS cases, as indicated by the stronger change in meridional wind component. Furthermore, the divergence and potential temperature advection changes indicate greater ascent in the low levels of the nocturnal environments of MCS cases. Although CAPE is similar between MCS and null cases, null cases exhibited higher Delta Z LFC heights, whereas MCS cases had slightly greater wind shear, particularly in the low levels.