When tiny convective spread affects a midlatitude jet: Spread sequence

Abstract We investigate the spread evolution over 3 days in an ensemble experiment starting from tiny initial condition uncertainty. We simulate a real event during which three mesoscale convective systems occur in close proximity to the midlatitude jet. Combining ensemble sensitivity analysis with a spread‐growth diagnostic based on piecewise potential‐vorticity tendencies, we compare the spread evolution with an existing conceptual three‐stage model. Each system follows the first stage, characterised by development of convective variability. Nevertheless, we find significant variation among the systems in their propensity to interact with the jet stream, which characterises the second conceptual stage. One exemplary convective system follows the conceptual evolution of Baumgart et al.: convective uncertainty initially projects onto the jet by upper tropospheric outflow, which further amplifies spread through balanced nonlinear growth as it propagates downstream. Rossby‐like dispersion in the downstream spread is strongly associated with the convective variability. In contrast, for another convective system, convective variability projects onto the local anticyclonic flow aloft only. Subsequently, this anticyclonic perturbation does not project considerable convective uncertainty onto the straight jet stream, which truncates the conceptual spread evolution. For the third system, negligible fingerprints of distinct spread‐growth stages (beyond the initial stage) are identified. Alongside convective heating, long‐wave radiation jointly dominates the spread evolution near the convective systems, whereas earlier studies suggest convective heating by the deep‐convective parametrisation dominates. Long‐wave radiative tendencies of convective anvils outlive the accompanied heating tendencies and extend spatially. Furthermore, we link convective variability of the exemplary system directly to long‐wave radiative tendencies. Therefore, long‐wave radiation appears to contribute substantially to stages 1 and 2 here. Finally, we identify flow dependence of the impact of convection on the jet, which may relate to the wave‐relative location of convective systems. Hence, we advocate to improve understanding of particularly favourable conditions for downstream propagation of convective variability.