Polar Lows – Moist Baroclinic Cyclones Developing in Four Different Vertical Wind Shear Environments
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 pola...
| Main Authors: | , , , |
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| Format: | Text |
| Language: | English |
| Published: |
2020
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| Subjects: | |
| Online Access: | https://doi.org/10.5194/wcd-2020-41 https://wcd.copernicus.org/preprints/wcd-2020-41/ |
| Summary: | 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. |
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