AC: response to RC1 and RC2

Abstract

Squall lines represent an organized form of atmospheric convection that link processes occurring at the small end of the mesoscale and processes ocurring at the large end of the mesoscale. This study analyses the initial condition sensitivity of idealized squall lines in an LES ensemble. The ensemble spread of the squall lines is evaluated using passive tracers, an ensemble sensitivity analysis, other statistical tools and an error growth metric. Analysing gravity wave dynamics, convective initiation, squall line relative motion and updraft/downdraft characteristics and transport, a chain of interacting processes is identified. From the convective point of view ensemble spread is rooted in a secondary phase of convective initiation (30–35 min) a few km ahead of the squall line. Contrasts in the amount secondary initiation arise within the ensemble, as vertical velocity varies at the location of convective initiation within the ensemble due to differences in gravity wave amplitude and phase. Immediately after the secondary phase of initiation (30–45 min), the cold pool accelerates to velocities of 2–4 m/s (ensemble envelope). With the spread in secondary convective initiation, upward mass transport is disturbed, which also affects downdraft mass fluxes. Furthermore, once accelerated (30–40 minutes), the cold pool nearly maintains its propagation speed in each ensemble member. It is shown that part of the errors occurring after 45–85 minutes are explained by the cold pool velocity and a correction for cold pool velocity removes a substantial fraction of the spread. A coherent anomaly of the circulation within the squall line, which is consistent with extra upward mass transport, exists during this phase of the evolution. It is proposed that the identified chain of interactions may be explained by a common mode of variability, which determines a substantial portion of the ensemble spread in the stage after 30–85 minutes in many diagnostics. Based on a non-monotonic relation between initial conditions and local vertical velocities that cause secondary initiation, one can argue that an intrinsic limit of predictability exists, as Melhauser and Zhang (2012) do.