Organization of convective ascents in a warm conveyor belt

• Main Authors: Blanchard, Nicolas, Pantillon, Florian, Chaboureau, Jean-Pierre, Delanoë, Julien
• Format: Text
• Language: English
• Published: 2020
• Subjects: North Atlantic
• Online Access: https://doi.org/10.5194/wcd-2020-25; https://wcd.copernicus.org/preprints/wcd-2020-25/

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 and the production of mesoscale bands of negative potential vorticity (PV), 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. The simulation reproduces well the mesoscale structure of the cyclone shown by satellite infrared observations, while the location of trajectories rising by 150 hPa during a relatively short 12 h window matches the WCB region expected from high clouds. One third of those trajectories, categorized as fast ascents, further reach a 100 hPa (2h) −1 threshold during their ascent and follow the cyclonic flow mainly at lower levels. In agreement with radar observations, convective updrafts are found in the WCB and are characterized by moderate reflectivity values up to 20 dBz and vertical velocities above 0.3 m s −1 . Updraft objects and fast ascents consistently show three main types of convection in the WCB: (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; (iii) mid-level convection below the upper-level jet. Mesoscale PV dipoles with strong positive and negative values are located in the vicinity of convective ascents and appear to accelerate both low-level and upper-level jets. Both convective ascents and negative PV organize into structures with coherent shape, location and evolution, thus suggesting a dynamical linkage. The results show that convection embedded in WCBs occurs in a coherent and organized manner rather than as isolated cells.