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Weather and Climate Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/wcd-2020-41
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wcd-2020-41
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  03 Sep 2020

03 Sep 2020

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A revised version of this preprint is currently under review for the journal WCD.

Polar Lows – Moist Baroclinic Cyclones Developing in Four Different Vertical Wind Shear Environments

Patrick Johannes Stoll1, Thomas Spengler2, Annick Terpstra2, and Rune Grand Graversen1,3 Patrick Johannes Stoll et al.
  • 1Department of Physics and Technology, Arctic University of Norway, Tromsø, Norway
  • 2Geophysical Institute, University of Bergen, and Bjerknes Centre for Climate Research, Bergen, Norway
  • 3Norwegian Meteorological Institute, Norway

Abstract. Polar lows are intense mesoscale cyclones that develop in polar marine air masses. Motivated by the large variety in their proposed intensification mechanisms, cloud structure, and ambient sub-synoptic environment, we use self-organising maps to classify polar lows. The method is applied to 370 polar lows in the North-East Atlantic, which were obtained by matching mesoscale cyclones from the ERA-5 reanalysis to polar lows registered by the Norwegian Meteorological Institute in the STARS dataset. ERA-5 reproduces 93 % of the STARS polar lows.

We identify five different polar-low configurations, which are characterised by the vertical wind shear vector relative to the propagation direction. Four categories feature a strong shear with different orientations of the shear vector, whereas the fifth category contains conditions with weak shear. The orientation of the vertical-shear vector for the strong shear categories determines the dynamics of the systems, confirming the relevance of the previously identified categorisation into forward and reverse-shear polar lows. In addition, we expand the categorisation with right and left-shear polar lows that propagate towards colder and warmer environments, respectively.

Polar lows in the four strong shear categories feature an up-shear tilt in the vertical, typical for the intensification through moist baroclinic processes. As weak-shear conditions mainly occur at the mature or lysis stage of polar lows, we find no evidence for hurricane-like development and propose that spirali-form PLs are most likely associated with a warm seclusion process.

Patrick Johannes Stoll et al.

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Patrick Johannes Stoll et al.

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Latest update: 03 Dec 2020
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Short summary
Polar lows are intense meso-scale cyclones occurring at high latitudes. The research community has not agreed on a conceptual model to describe polar-low development. Here, we apply self-organising maps to identify the typical ambient sub-synoptic environments of polar lows and find that they can be interpreted as moist-baroclinic cyclones.
Polar lows are intense meso-scale cyclones occurring at high latitudes. The research community...
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