Mergers as the Maintenance Mechanism of Cutoff Lows: A Case Study over Europe in July 2021

A cutoff low that covered Central Europe in the middle of July 2021 brought heavy rainfall and severe flooding, resulting in more than 200 fatalities. This low was formed by a trough on 11 July and merged with another cutoff low around 12–13 July. Analysis of the energy budget and potential vorticity suggests that the main cutoff low was maintained through the merger with another cutoff low; this was the dominant contributor to maintenance of the main cutoff low around 12–13 July. The results of Lagrangian trajectory analyses support this conclusion. Analysis of diabatic PV modification during the merger indicates that radiation acts mainly to enhance the potential vorticity of the parcels when they move from another cutoff low into the main cutoff low, especially in the upper layer (~ 350 K). However, that effect is not pronounced in the lower layer (~ 330 K). These results demonstrate that cutoff lows can be maintained through the merger with another cutoff low and underline the need to consider diabatic processes when investigating mergers.