Embedded Convection in the Warm Conveyor Belt of a North Atlantic Cyclone and its Relevance for the Mesoscale and Larger-scale Dynamics

Abstract:

Warm conveyor belts (WCBs) are important coherent airstreams in extratropical cyclones with considerable impact on the evolution of clouds, precipitation and flow dynamics. The strong cloud-diabatic processes during WCB ascent from the boundary layer to the upper troposphere modify the potential vorticity (PV) distribution, which in turn can influence the large-scale flow evolution by inducing perturbations on the extratropical waveguide. According to the classical concept, WCBs rise continuously with moderate ascent rates below 50 hPa/h and thereby form mainly stratiform clouds. Recent studies, however, identified smaller-scale convective ascent embedded in WCBs with relatively faster ascent rates. Still, its dynamical significance has not yet been analyzed systematically. We present a case study of embedded convection in the WCB of a North Atlantic cyclone that occurred during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) campaign. Satellite observations, airborne radar measurements and online trajectories from a convection-permitting simulation consistently suggest that convection, with ascent rates exceeding 100-200 hPa/h, is embedded in extended areas within the slower large-scale WCB ascent region. Three-dimensional Lagrangian composites along the convectively ascending WCB trajectories show locally enhanced surface precipitation rates. On the mesoscale, the composites show the formation of diabatically-produced horizontal upper-level PV dipoles that are centered around the convective WCB ascent, whereby one pole reaches negative PV values. These mesoscale PV dipoles are associated with a coherent cyclonic and anticyclonic circulation anomaly around the positive and negative PV pole, respectively. The superposition of these circulation anomalies locally decelerates the wind in the center of the dipole. On a larger scale, these convectively-produced mesoscale PV dipoles aggregate to elongated PV dipole bands aligned with the aggregated convection and the vertical wind shear vector. Thereby, the negative PV bands can interact with the upper-level waveguide and lead to jet streaks downstream, emphasizing the relevance of embedded convection in WCBs not only for the mesoscale environment but also for the large-scale circulation and predictability.