When tiny convective spread affects a midlatitude jet: spread sequence

Abstract

We investigate the evolution of spread over three days in a numerical 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. The spread evolution is compared 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 conceptual stage 2. One exemplary convective system follows the conceptual evolution of Baumgart et al., i.e., convective uncertainty initially projects onto the jet by upper-tropospheric outflow, which further amplifies spread through 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. Subsequently, this anticyclonic perturbation hardly (if at all) projects convective uncertainty onto the particularly straight jet stream, which truncates the conceptual evolution. For the third system, negligible fingerprints of second and third stages are identified. Alongside convective heating, longwave radiation jointly dominates the spread evolution near the convective systems (as opposed to earlier studies). Longwave-radiative tendencies of convective anvils outlive the accompanied heating tendencies and extend spatially. Furthermore, we link convective variability of the exemplary system directly to longwave-radiative tendencies. Therefore, longwave radiation appears to contributes substantially to stages 1 and 2 here. Finally, we identify flow dependence of the impact of convection on the jet. (Truncated abstract)