Evolution of squall line variability and error growth in an ensemble of large eddy simulations

Abstract

A chain of processes is identified that regulates much of the spread in an ensemble of squall lines in large eddy simulations with tight initial conditions. Patterns of gravity wave propagation de-correlate and restructure the initial condition spread until a second phase of convective initiation is taking place, i.e. after 30 min of simulation time. Subsequently, variability in this convective initiation and mass overturn is associated with differences in cold pool propagation within the ensemble (propagation at 2–4 m s−1. An ensemble sensitivity analysis reveals that anomalies in squall-line-relative flow with respect to the ensemble mean are also associated with the secondary convective initiation. Downdraughts are fed with extra air by a convergence zone on the rearward flank of the updraughts. An analysis of difference growth within the ensemble shows that a substantial proportion of variability is explained by cold pool propagation contrasts during this stage (30–80 min), which is partly removed when a feature-relative perspective is taken. The patterns of coherent variability exist on the timescale of an hour and dissipate subsequently (80–100 min).