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- Title
Mergers as the Maintenance Mechanism of Cutoff Lows: A Case Study over Europe in July 2021.
- Authors
Yamamoto, Koryu; Iga, Keita; Yamazaki, Akira
- Abstract
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. However, that effect is not pronounced in the lower layer. These results demonstrate that cutoff lows can be maintained through a merger with another cutoff low and underline the need to consider diabatic processes when investigating mergers. Significance Statement: This study examines an upper-tropospheric cyclone called a cutoff low, which caused a high-impact weather event over Europe in the middle of July 2021, and investigates its maintenance mechanism. This cutoff low merged with another, suggesting a contribution to the maintenance. Diabatic processes during the merger are also investigated. The results of this study suggest that not only do cyclone regions merge, but diabatic modification of the vortex structure can be seen when two cutoff lows merge, and the modification process may differ in different vertical layers of the cutoff low.
- Subjects
EUROPE; MERGERS &; acquisitions; RAINFALL; VORTEX motion; ENERGY budget (Geophysics); POTENTIAL energy; CYCLONES
- Publication
Monthly Weather Review, 2024, Vol 152, Issue 6, p1241
- ISSN
0027-0644
- Publication type
Article
- DOI
10.1175/MWR-D-23-0024.1