A Lagrangian climatology of coupled extratropical cyclones and mesoscale convective systems

This study focuses on the co-evolution of synoptic extratropical cyclones (ETC) and mesoscale convective systems (MCS) by comparing databases of Lagrangian tracks for both storm types and locating points which are co-located to identify "coupled" systems. We find that these coupled tracks occur at the southward edge of the regions with the most ETC points, and on the northward edge of the MCS points. Since both of these regions have strong seasonal cycles, the coupled points also show a strong seasonal cycle. During all seasons however the coupled points tend to be concentrated over warm ocean waters in the Kuroshio, Gulf Stream, and over central North America. We also show that ETC systems that contain MCS deepen approximately 50% faster than systems without MCS. Most of the coupled points occur at the initial coupling time for both systems, indicating that for the coupled systems the ETC and MCS are forming at very similar times, for all regions and seasons. To investigate the dynamics behind this, we used ERA5 data around the time of initial coupling and find that the coupled systems are occurring in regions of particularly strong initial frontal conditions, which is followed by a strong intensification of the ETC. The MCS are typically located to the north east of the cyclone center, in a region of uplift surrounding the frontal zone. These results suggest that understanding the distribution of strong fronts is key to understanding the coupling between the different storm types.