Preprints
https://doi.org/10.5194/wcd-2021-17
https://doi.org/10.5194/wcd-2021-17

  29 Mar 2021

29 Mar 2021

Review status: this preprint is currently under review for the journal WCD.

A global analysis of the dry-dynamic forcing during cyclone growth and propagation

Philippe Besson1, Luise J. Fischer1,2, Sebastian Schemm1, and Michael Sprenger1 Philippe Besson et al.
  • 1Atmospheric and Climate Science, ETH Zürich, Switzerland
  • 2Institute for Environmental Decisions, ETH Zürich, Switzerland

Abstract. Mechanisms driving the intensification and propagation direction of extratropical cyclones are an active field of research. Dry-dynamic forcing factors have been established as fundamental drivers of the deepening and propagation of extratropical cyclones, but their climatological interplay, geographical distribution and relatedness to the observed cyclone deepening and propagation direction remains unknown. This study considers two key dry-dynamic forcing factors, the Eady Growth Rate (EGR) and the upper-level induced quasi-geostrophic lifting (QGω), and relates them to the surface deepening rates and the propagation direction during the cyclones' growth phase. To this aim, a feature-based cyclone tracking is used and the forcing environment is climatologically analyzed based on ERA-Interim data. The interplay is visualized by means of a forcing histogram, which allows one to identify different combinations of EGR and QGω and their combined influence on the cyclone deepening (12-hour sea-level pressure change) and propagation direction. The key results of the study are: (i) The geographical locations of four different forcing categories, corresponding to cyclone growth in environments characterized by low QGω and low EGR (Q↓E↓), low QGω but high EGR (Q↓E↑), high QGω and low EGR (Q↑E↓) and high QGω and EGR (Q↑E↑), displays distinct hot spots with only mild overlaps. For instance, cyclone growth in a Q↑E↑ forcing environment is found in the entrance regions of the North Pacific and Atlantic storm tracks. Category Q↓E↑ is typical found over continental North America, along the southern tip of Greenland, over parts of East Asia and the western North Pacific. In contrast, category Q↑E↓ dominates the subtropics; (ii) the four categories are associated with different stages of the cyclones' growth phase: large EGR forcing occurs typically earlier, during the growth phase at genesis, while large QGω forcing attains its maximum amplitude later towards maturity; (iii) poleward cyclone propagation is strongest over the North Pacific and North Atlantic, and the poleward propagation tendency becomes more pronounced as the deepening rate gets larger; zonal, or even equatorward propagation, on the other hand, is characteristic for cyclones developing in the lee of mountain ranges, e.g., to the lee of the Rocky Mountains. The exact location of maximum QGω forcing relative to the surface cyclone center is found to be a good indicator for the direction of propagation, while no information on the propagation direction can be inferred from the EGR. Ultimately, the strength of the poleward propagation and of the deepening are inherently connected and the two dry-dynamic forcing factors allow cyclone development in distinct environments to effectively be identified.

Philippe Besson et al.

Status: open (until 10 May 2021)

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Philippe Besson et al.

Philippe Besson et al.

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Short summary
The strongest cyclone intensification is associated with a strong dry-dynamical forcing. Moreover, strong forcing and strong intensification correspond to a tendency for poleward cyclone propagation, which occurs in distinct regions in the Northern Hemisphere. There is a clear spatial pattern in the occurrence of certain forcing combinations. This implies a fundamental relationship between dry-dynamical processes and the intensification as well as the propagation of extratropical cyclones.