Organization of convective ascents in a warm conveyor belt

Abstract

Warm conveyor belts (WCBs) are warm, moist airstreams of extratropical cyclones leading to widespread clouds and heavy precipitation, where associated diabatic processes can influence midlatitude dynamics. Although WCBs are traditionally seen as continuous slantwise ascents, recent studies have emphasized the presence of embedded convection, the impact of which on large-scale dynamics is still debated. Here, detailed cloud and wind measurements obtained with airborne Doppler radar provide unique information on the WCB of the Stalactite cyclone on 2 October 2016 during the North Atlantic Waveguide and Downstream Impact Experiment. The measurements are complemented by a convection-permitting simulation, enabling online Lagrangian trajectories and 3-D objects clustering. Trajectories rising by 150 hPa during a relatively short 12 h window are identified as ascents and examined in the WCB region. One-third take an anticyclonic turn at upper levels, while two-thirds follow the cyclonic flow at lower levels. Identified trajectories that reach a 100 hPa (2 h)−1 threshold are further categorized as fast ascents. They represent one-third of the ascents and are located at lower levels mainly. Both radar observations and simulation reveal the presence of convective updrafts in the WCB region, which are characterized by moderate reflectivity values up to 20 dBZ. Fast ascents and updraft objects with vertical velocities above 0.3 m s−1 consistently show three main types of convection in the WCB region: (i) frontal convection along the surface cold front and the western edge of the low-level jet, (ii) banded convection at about 2 km altitude along the eastern edge of the low-level jet, and (iii) mid-level convection below the upper-level jet. Frontal and banded convection result in shallow ascents, while mid-level convection contributes to the anticyclonic WCB outflow. The results emphasize that convection embedded in WCBs occurs in a coherent and organized manner rather than as isolated cells.